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Eye movements

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1 What is the goal of vision, and does it need one?
The problem of the teleological approach is that it assumes that which still is not and cannot be known: clear vision. How can the brain have as goal the elimination of obstacles to clear vision, through, among other, all kinds of ocular movements, if clear vision is itself the result of these movements? As we shall see, many visual phenomena has been approached under this naive perspective, with many complex theories as a result.
If, like I claim, clear vision can never be the goal of the visual system, but only its effect, than we need a radical change of perspective in the study of visual and ocular phenomena, including eye movements.

I propose to follow critically the monumental work already mentioned, and try to keep in sight the links to vision in general, and the cerebellum in particular.

Leigh and Zee (eds) "The Neurology of Eye Movements", 2015.

2 Binocular vision is a field where the teleological assumptions show quite clearly.
The examples given (fig 1.11, video 1.17) seem to show coordinated movement between the two eyes, that are then thwarted by an artificial prism on one eye. What I personally see is that each eye could be independently focusing on its target, the vergence movements being the effects of such double independent fixations, instead of the results of a hypothetical coordinating center.
Binocular disparity is one of George's favorites. Imagine that, the brain comparing the visual inputs to both eyes, and then readjusting their respective focus! Do you hear him laughing?
The fact that voluntary and learned movements can be coordinated, does not mean that the automatic responses also are coordinated: we can decide to look at a specific object at a specific location. But each eye must execute this command as it can, without having to concern itself with how the other is doing.
The same way, when we get prescription glasses, we are not suddenly focusing differently on the external objects. More probable is that the same eye movements produce now better results because of the optical correction. What was blurry, looks now sharp. And that is not because we have changed the focus settings of our eyes! Otherwise we would never need glasses, and the eye movements would be under our control.
(See the entry Do we need an auto-focus mechanism? in Retina: Mischellanious)

This brings us to the accommodation problem. The eye seems to be straining itself to an object in clear focus with as a result muscular fatigue and headaches.
Why would it do that? Assuming that the muscle is reacting to the location on the retina, and not to the visual properties present at that location, there is no reason for the muscle to try for another position. 
The biggest hurdle in the study of phenomena concerning eye movements is their dual character: on one hand they can be considered as pure reflexes, on the other hand as voluntary movements. The difficulty is then to distinguish the effects of one (the reflex) from the other (the voluntary movement).
As a reflex, accommodation cannot give rise to any conflict whether the image is clear or blurry in one or both eyes. I would say that the reflex has done its job and has nothing left to do.
For the individual confronted with unfocused images that is where the problem starts. He wants and needs clear vision but is unable to voluntarily move his ocular muscles which would permit a better focus. The fact that optic devices like correcting lenses solve the problem is I think significant. When we put on eyeglasses, our eyes do not need to accommodate. The eye muscles do not need to shift to another position. In fact, our eyes do not need to do anything. They were already focused on the desired location, which came over as a blurry image, and now the image is clear. End of story.
When both eyes are out of focus, the brain does not seem to have any problem either. It is usually when both eyes show a different image that the troubles start. We could say that each eye, taken independently from the other, is probably functioning as it should. It is more of a coordination problem. The brain, for whatever reasons, have difficulty dealing with two different images at the same time, and that is most probably what leads to headaches and other stress symptoms.
The teleological approach makes us think that the eyes, apart or together, are aimed at a clear image of the retinal stimuli, and when that does not happen, they then attempt to remedy to the situation. I consider this as a George-friendly approach.
When we make the distinction between what the eyes do automatically, and what the brain tries to do with the eyes for its own goals, we are in a better position to distinguish between the effects pertaining to each process.

3 Frenzel Goggles are supposed to suppress fixation because of the exaggerated magnification of objects which make them unrecognizable. When looking at a subject wearing those glasses, one cannot help but notice the exploring eye movements which are obviously meant to map the strange landscape the subject is looking at. That makes me cautiously conclude that fixation is not so much suppressed as it is transferred to a magnified, strange world. There is nothing in the experience that would prevent a subject wearing those goggles to fixate on a random element in his field of vision. To us, it would look as though the subject is not looking at any object in particular. And we would be right. What the subject is seeing is entirely a different, almost microscopic, world that has little in common with ours. It does not mean that it is a non-visual world in which visual reflexes and habits would be magically abolished.
The use of the Frenzel goggles in the study of eye movements in general, and nystagmus in particular, is therefore questionable.

4 The Nature of the so-called Vestibular Ocular reflex or VOR

Here is the theoretical model I will try to defend in this thread:

1)There is a pure automatic aspect to eye movements that is related only to dynamical and gravitational effects.
2) While moving, passively or actively, we still want to be able to react to visual stimuli. Those reactions are learned and can therefore be changed through experience (by wearing reversing vision goggles for instance).
3) Saccades are not a reflex but belong to the second group of voluntary, even when unconscious, eye movements.
4) Vestibular sensations are not, necessarily, unconscious.

Let me start with the third point. 

[let it be clear that this entry is only introductory and cannot replace a full-fledged analysis.]

Saccades, including micro saccades
Fixating or tracking an object is a voluntary "decision". Movement attracts our gaze, but we can choose to ignore it and focus on another, stationary, object.
It does not matter whether we are stationary or moving. Imagine yourself jumping with a parachute from an airplane. Your eyes will tend to make movements dictated by dynamical and gravitational causes. Still, you will be able to counteract or completely suppress these movements and direct your gaze voluntarily. 
This situation is hardly different from that of the rotating devices favored by researchers in their experiments.
The two fundamental aspects of eye movements are present:
- the involuntary aspect because of physical laws;
- the voluntary aspect of gaze direction.

Vestibular Sensations are certainly different from all other sensations as touch, hearing or vision.
They also seem never to occur independently, but are always allied to other sensations, like muscular proprioception. When we move our head, we feel the muscles more than we feel any change in our inner ear.
Fluids moving in the canals and sacs do activate afferents neurons though. And we always seem to know whether we are lying on our back, our side, or standing. The same way we always seem to sense passive movements of our body.
Vestibular sensations are of an abstract nature that would more easily compare them to fleeting thoughts than to actual sensations. That does not make them any less real. Or unconscious. We may not feel what is happening in our ears, but then we dot feel the activation of photoreceptors either, we just see. The same way we could say of vestibular sensations that we just "know". Considered this way, these sensations are not any more conscious or unconscious than other sensations.

Concerning the first two points, the complexity of the mathematical models and calculations makes it quite hard for me to pierce through the numbers and symbols and glance at the unadulterated nature of eye movements. I will have to work quite hard at it to be able to show that those calculations are based on wrong theoretical assumptions and can therefore generally be ignored.
I will advocate a more Weberian approach where calculations can be used to understand the sensations that made them possible instead of replacing them. This is the promise I will try to fulfill.

5 Oscillopsia: why does it exist?
Our eyes are constantly moving, and still we experience the world as stable. Why should other kind of movements cancel this fundamental stability?
The general conclusion would seem to be that not movements per se, but the kind of movements are determinant for visual stability. But then, the types of movements eyes can make are quite limited: up-down, left-right, and rolling your eyes like female teenagers are prone to when confronted with a stupid or unwelcome remark.
There is though a kind of movement that is quite different from those three, a type that was already known by Helmhoz: press on your eye with a finger or blunt instrument, making the eye move in its orbit, and you will experience objects moving back and forth.
Nonetheless, nystagmus does not necessarily mean oscillopsia. [I would strongly advise the reader to watch some videos on Youtube concerning the personal experiences of people with nystagmus.] That means that the involuntary character of retinal slip does not cause movement sensation. At least, not alone.
The problem remains why some movements cause the sensation of the world moving, while others do not. Objectively it does not matter whether the movement comes from us or the object: imagine taking a picture, using a tripod, of a painting that somebody else is moving back and forth. The result could probably not be distinguished from a picture taken of the stationary painting, without a tripod and a (relatively) low shutter speed. 
If it is not the movement itself that causes the sensation of movement, what is then the cause of this sensation?
The distinction between voluntary and involuntary is, in itself, not an explanation, since the physical substrate would be the same in both cases: a changed spatial relationship between the viewer and the object.

[There is also a disease, akinetopsia or movement blindness, whereby the patients is unable to perceive movements. This ailment would certainly strengthen the idea of central mechanisms in the brain that process movements.]

Saccade suppression is supposed to eliminate blur. A teleological approach if there ever was one. Of course, not necessarily wrong because of that. But what if there was no ulterior motive to saccades except to bring the eye in(its next) position?

Can the brain keep track of what is happening while moving the eyes? It is already probable that we are never focusing our gaze from both eyes at the same time, but that we are, as it were, sometimes looking through one eye, sometimes through the other. That is certainly what birds or other animals with lateral eyes do. When they focus with one eye, the other one is put temporarily out of commission, or "defocused" (See Liversedge et al, eds,"The Oxford Handbook of Eye Movements", 2013, ch.1). So maybe the brain, against all odds, is not a parallel device but has to process information in a serial way, which does not preclude that some processes, like vision, could be parallel.

As Vallines and Greenlee put it, that certainly means 'Ruling out a pure retinal origin" of saccadic suppression ("Saccadic Suppression of Retinotopically Localized Blood Oxygen Level-Dependent Responses in Human Primary Visual Area V1", 2006). Does that mean that we also have to accept their conclusion that this is "additional evidence for the existence of an active saccadic suppression mechanism in humans"?
[See also, Thilo et al "The site of saccadic suppression", 2004. They show that phosphenes induced at the level of the cortex are not suppressed while retinal phosphenes are. Which supports the idea that the saccadic suppression takes place higher in the brain. That, of course, does not answer the following questions.]

What would such a suppressing mechanism look like, and how would it work?

Starting with the last question, the brain only needs to stop or ignore retinal stimulation while the eye is moving.
Which would mean that the brain has a way of ignoring visual stimulations, at least during saccades. But then, why not also at other times? Once the principle has been established, why limit this ability so drastically? It would certainly come in handy, for instance in situations where oscillopsia becomes active. If the brain could suppress the surplus of visual information it is getting through the eyes, it would certainly eliminate the unwelcome sensations of movement.
We must realize that we are dealing with a variation on the neural correlate of consciousness. Whatever mechanism we could point at would not answer the question why it makes saccadic suppression possible. It will be in last instance a choice imposed by empirical rather than theoretical considerations.
That does not seem to deter Thiele et al "Neural Mechanisms of Saccadic Suppression", 2002. They see no objection in speaking of "correlates of saccadic suppression" in the form of neurons that are activated or not, and that change their putative preferred direction of motion during saccades". It is nice to go though life unencumbered by any metaphysically reeking consideration!

Still, some candidates may seem more likely than others.

I would say, "blur" is a very unlikely candidate (put forward by all authors I read, and certainly by those mentioned, including the Oxford Handbook and Leigh and Zee (2015). Why would the brain go through all this trouble to suppress such a minor inconvenience that would hardly last more than a few milliseconds? Especially if such a blur were to be considered as a normal consequence of saccades. After all, when we we move our head very quickly, we are also experiencing something equivalent to a visual blur, but then in vestibular terms. Should there not be a vestibular suppressing mechanism also? And while we are at it, what about an auditive suppressor to counteract sound distortion when suddenly and abruptly moving our head?
Also, the suppression mechanism only works for a very limited time. If you keep moving your head left and right as fast you can (or just turn around like a dancing Derwish), you will not only start to feel dizzy (at least I do), you will also experience the world as moving. A sensation children really seem to enjoy.
I would also like to give the example of moving objects or characters in a film or video. We are able to recognize an object even at higher speeds than normal. And even when we do not, that does not prevent us from seeing where it is at any moment. So a blur, even if it did happen in real vision, would probably have no negative repercussions whatsoever!

We are obviously able to experience movement as well as visual stability. The problems arise when we get one while expecting the other. Sometimes movement is suppressed when it should not be (akinetopsia or movement blindness), and sometimes it is not suppressed when it should be (Oscillopsia). 

In both cases, we are unable to influence the process voluntarily.

Why believe that the brain has any influence at all on the physical substrates of those sensations?
We are unable to focus voluntarily on an object: either we have a clear vision or we do not. Also, we can choose, maybe unconsciously, to ignore a sound, but we cannot not hear it if it is loud enough.
Maybe, likewise, we either experience movement or we do not, without there being any neural mechanism that could change the outcome.
That would mean that any neural process that contributes to our sensation of movement (or visual stability), will also produce unwanted effects when malfunctioning.
If my analysis is correct, we would then have a very powerful investigating model: instead of looking for hypothetical neural mechanisms, we should concentrate on the way our sensations are created. That would also show us how they can possibly be abused.

6 Mach 
"Grundlinien der Lehre von den Bewegungsempfindungen", 1875. Translated as "Fundamentals of the Theory of Movement Perception by Dr. E. Mach". [The translation is out of print and I could not consult it.]

I will not try to analyze his epistemological conception (empirio-criticism, which was the object of a virulent attack by the famous Lenin, leader of the Russian Revolution of 1917), but will concentrate on this book that is directly related to the theme of this thread.

Some remarks Mach made along the way are quite instructive.
1) He is approaching the problem, sensations of motion, as a physicist. 
2) He came to his intuition, the existence of a specific organ for movement, by the study of fluids and suspended bodies.
3) He was reminded of the relationship between physical phenomena and sensations while traveling on a train. He noticed that while the train was on a long curve, the houses and trees looked like they were leaning to the side away from the vertical. He thought that such a phenomenon was easily explained if one assumed the existence of a specific sensation caused by mass acceleration. 
[He did not realize then, nor later, that what he was describing was in fact an optical phenomenon, and not, on the basis of the knowledge then available, any sensation necessarily or directly related to an "organ of balance". His intuition was apparently too strong to be ignored or put aside because of "objective" considerations.]
4) He wanted to find the source of those sensations of motion. ("Die Quellen dieser Bewegungsempfindungen aufzusuchen und ihre Abhängigkeit von den Bewegungen des Körpers zu finden, ist die Aufgabe dieser Schrift.")

Those remarks explain a lot about the way he sets on to solve the problem.
He gives a very detailed description of the laws governing the relations between two or more masses subject to gravity. Obviously, he intends to show that our body can be approached in the same fashion as all other natural objects.
The first conclusion he reached was also in line with this expectation. Assuming that physical processes leave their trace on a living body in the form of sensations (something our modern researchers methodically ignore!), he thought that the whole body was sensitive to motion. After all, the whole body, in all its parts, including the blood and other internal parts, is subject to the physical effects of movement and gravity.

He invents a very ingenious apparatus that will help him explore those effects on himself, other people and animals.

Before I go any further, let me state clearly that Mach is trying to prove the existence of an as yet unknown physiological organ that would produce all the sensations of motion we experience, and that he thinks he can do that by analyzing his own sensations while being rotated in all directions, armed only with his knowledge of physical laws!
[He admits to have never performed an autopsy.]

This is so different from the way science is done nowadays that we can hardly believe that somebody who contributed so much to science and in so many fields could actually believe in the soundness of such an approach.

Motion and Acceleration:
Mach believes that it is not so much motion that we feel, but acceration. He gives different examples to illustrate his idea. Going back to the train on the curve, he remarks that sometimes we also experience the train as leaning away from the vertical, and the houses and trees as straight. In both cases, he thinks that comes because we experience the effects resulting from gravitational and centrifugal forces the as a vertical force. Notice how, once again, he moves unconsciously from optical to vestibular sensations.

Gravitational and rotational Device:
The principle behind the idea is certainly not complicated, in hindsight. The external part, a large wooden frame which rotate around one vertical axis, contains two other parts which can rotate independently of each other. The inner part, what he calls the chair, can also be moved up and own, along the horizontal axis.
The most important part of the setup is something that has somehow disappeared from modern devices: the "observer", the poor soul being rotated) can st on the chair, which is covered by a surrounding paper wall, cutting any visual intrusion of the rotating movements.
In other words, the observer experiences all the movements applied directly or indirectly to the chair, while he remains in a visually stable environment!
I could not stress enough the theoretical importance of this part. Researchers nowadays when studying the vestibular and ocular reflexes seem not to make the distinction between the visual and the vestibular aspect of those phenomena. In fact, all the articles I have read point to the same approach: the visual and the vestibular cannot be separated. As Leigh and Zee (2015) put it so clearly, "The vestibular system helps to optimize vision during head movements". Eye movements are given this all-encompassing function of retinal stabilization, and can therefore never be considered apart from what they are subservient to: vision.

With this simple precaution, Mach has made it possible to find out the origin of his sensations of movements.
He soon arrives to the conclusion that they have to be localized in the head. He bases this on the fact that his sensations seem to be highly correlated with the position of his head during the different movements and rotations.
We see again the general paradigm: from physical phenomenon to physical laws to sensations.
George seems to be very agitated, I suppose he wants me to point to the fact that physical laws in themselves could never justify such a conclusion, but poor George does not realize that for once he is innocent of all charges. Mach, because he is analyzing his own sensations, can tell immediately whether a change in position has a different effect on him. He does not need to claim a myserious link between physical laws and brain processes. He is experiencing them. His conviction is of course a subjective one. Something he is certainly aware of. That is why he continues the same experiments with other people, and also with pigeons and dogs.
What I find particularly indicative of the scientific spirit dominating the 19th century, is that he apologizes for the fact that interpreting the sensations other people or animals can be feeling is always hazardous!

Animals reactions
More than the reactions of other human observers, it is the reactions of the animals that is the most telling. 
First let me stress the fact that Mach, even if he was aware of the nystagmus effects linked to passive rotation and motion, was foremost interested in the sensations themselves. He wanted to know if the same sensations could be attributed to animals as to humans. That is probably why he never stood still by one fundamental difference between the tests he subjected the poor animals to, and those he and his collaborators had undergone: the animal were visually unprotected!
They had to undergo the rotations and movements without the protection afforded by the paper bunker. No wonder they behaved so strangely afterwards. But all Mach was interested in was whether this could be attributed to a specific movement organ.

As history shows, he was right, and vestibular functions of the inner ear are not a mystery anymore. That is also how Mach is remembered nowadays. As one of the pioneers in the discovery of these functions. His work is often cited in articles treating of vestibular and ocular reflexes, without the authors realizing that in fact he, unwillingly, proved them wrong on a fundamental point: vestibular phenomena are unrelated to vision! That there are, strictly speaking, no vestibular reflexes, only sensations. [The fact that the observer is shielded from the visual movements inside his paper hut, eliminates any visual reflexes. The observer feels no need to follow with his eyes the movements of the rotating parts.]
In Mach's example we have a very clear distinction between sensations created by vestibular stimulations, and reactions created by visual stimulations. It shows that the concept of VOR is in fact an artificial construct of modern science.
We could therefore consider vestibular-ocular reflexes as scientific construct based on the confusion of visual reflexes and vestibular sensations. That such a confusion could last for so long is truly a wonder, since we, in this era of technological wonders, have so many opportunities to experience our vestibular sensations independently of any visual distraction. After all, who has never taken a lift or traveled in a plane before?

7 Retraction: I am afraid that the analysis of Mach's book has shown me that eye movements and vestibular sensations are not (necessarily) related. That would change my program accordingly. 
One consequence of this result would be extreme caution in attributing causes to ailments like benign paroxysmal positional vertigo (BPPV).  A medical sites states clearly: "Often, there's no known cause for BPPV." Lesions or malfunctions in the inner ear are given as possible causes among others. Not being a "medicus", I will certainly not try to establish a cause of disease. I will just remark that malfunctions in the inner ear could have indirect effects on eye reflexes. That would make them the final cause of the ailment, even if the direct causes are located elsewhere in the brain, and make of BPPV an indirect symptom.


Eye movements
This thread has been abusively deleted. The Philpapers Team offered me the opportunity to restore it.

"How many threads do you need to restore? Combining multiple posts into one would be a way to get around the limitation on 2 posts, and would also be less work for you. Since they were previously accepted, we'll make sure to accept them if you notify us ahead of time with the subject heading." The PhilPapers Team


8 Efferent nerves to the inner ear: a First Approach

Some references:

- Rasmussen "The olivary peduncle and other fiber projections of the superior olivary complex", 1946 ; 
- Rasmussen et al "Neural Mechanisms of the Auditory and Vestibular Systems", 1959/2011;
- Warr "Olivocochlear and vestibulocochlear efferent neurons of the feline brain stem: their location, morphology and number determined by retrograde axonal transport and acetylcholinesterase histochemistry", 1975 ; 
 Brown "Morphology of labeled efferent fibers in the guinea pig cochlea", 1987 ; 
- Wolff  "Efferente Aktivatät in den Statonerven einiger Landpulmonaten (Gastropoda)", 1970 ;
- Ryugo et al "Auditory and Vestibular Efferents", 2011
- Goldberg et al " The Vestibular System: A Sixth Sense", 2012.

The idea that the brain could influence how its sensory receptors work is very difficult to accept. We can understand that organisms can choose what to direct their attention on and what not, but just as we cannot not hear, we cannot not see or not feel something touching our skin.
Nonetheless, it is this principle which has been put in doubt since many decades, even if it seems to have had no practical consequences at all. At least, I do not know of any article that goes beyond establishing this "fact", accompanied by some platitudes about why Evolution saw it fit to develop such a mechanism. But how such efferent neurons could function is anybody's guess.

This seems like a perfect place for George to build a nest [okay, home, have it your way]. Stimulation of the hair-cells in the semi-circular canals and otholitic organs is supposed to be mechanical. Head movements result in fluid movement that finaly hyper- or de-polarizes the receptors.
I could understand the logic behind efferent neurons stimulating the afferent cells of the receptors. Modulation of the effects of the mechanical stimulation could,  with great difficulty I might add, be seen as an evolutionary precaution whose usefullness still has to be established.
But what could efferent neurons possibly have for effect on the receptor-cells themselves? The hair-cells represent translational and gravitational on one side, rotational effects on the other side. What would the efferents neurons represent? What would the modulation of these effects look like? Less rotation or less translation? Less gravity?!

The idea of sensory modulation is not as strange as it might appear in the first place. We can partially close our eye lids, or squint, to regulate the amount of light impinging on our retina. We sweat or shiver to counter the effects of temperature. The first type is purely voluntary while the second would be considered more as a biological reflex on which we have no direct influence.
Why not a modulation of cochlear and vestibular stimulations? In the case of auditive stimulation we can easily find all kind of logical arguments in favor of such a mechanism. A better use of attentional processes would be high on the list.
I honestly can think of no advantage of the modulation of vestibular stimulations, or at least, those that I can think of seem to be the wrong ones. It would be very advantageous for an organism to counter the effects of involuntary rotation. Falling in a swollen river and being tossed in all directions by the water is a situation where a'cool" head could mean the difference between life and death. And what about being shaken violently by a predator and slipping from its jaws? Would it not be a blessing if the prey could run away as soon as it fell on the ground, instead of feeling dizzy and disoriented?
How about going to the fare with your grandchildren and getting on one of those horrible rotating devices which they love so much? I would immediately sign for such a bio-enhancement!
But alas! It is not to be. Whatever these efferent neurons are doing to hair-cells, it is nothing so obviously useful.

Vestibular modulation would only make sense in situations where the brain can anticipate vestibular stimulation. Not only that, the brain would need to know in advance what kind of stimulation it will be getting: is it a simple head movement, or a stuntman salto?
Otherwise, the brain might be modulating stimulations that would be best kept unaldurated for the right reactions to follow.
But such a distinction seems very unlikely. If it were possible, nobody would ever get sea sick!
The fact that we finally can learn to cope with the boat movements could mean that the modulation process is itself a learning process that takes time to settle in and show its fruits. 
But modulating the way we experience gravity and movements would also have consequences on the way we run, walk or stand. The body needs those sensations to be authentic at all times. It would be like the brain deciding to modulate vision and reduce our sensitivity to blue in the summer because of the cloudless sky. Maybe it does just that, but certainly not by changing how visual cells react to different colors at the retina level!

9 saccadic suppression revisited

Change blindness occurs not only during eye movements, but during fixation. We would be looking straight at the change and not see it because it happened when we were not looking, and we are still relying on our memory.
If we always "see" the content of our memory and not the world itself, our vision does not need to be suppressed at any moment. A change attracts our attention, we fixate on it and record it in memory. And that is the moment we 'see it". 

[whether one happens before the other or simultaneously does not seem to be really important in this context. Anyway, that is a question I would not know how to answer: do we record it first and then see it, or vice versa? The first alternative would be more in line with the idea that we see what we remember, but maybe it does not apply to the first time a change is perceived.]

What we therefore need to explain is not a, probably, inexistent phenomenon (vision suppression during eye movement), but a real phenomenon: change blindness, or rather its positive correlate, change perception.

Mach has taught us that we can only be conscious of acceleration and not of motion itself. Maybe the same rule applies to vision: we can only see change.

That is why we can see the blur in movies, and not in real life. The first one is an objective phenomenon, the result of chemical (emulsion sensitivity), or mechanical processes (shutter speed). Whatever the reasons, it really exists independently from our perception processes. A blur in real life would mean that objects move faster than light, which physicists consider impossible, certainly for everyday processes. Such a blur can therefore only be produced by our own perception.

10 What is VOR? And does it really exist?
The relationship between eye and head movements is always considered in a so-called occulo-vestibular context. 

head--> stimulation of inner ear ---> vision constraints--> eye.

As you can see, the relationship is a very complicated one. We are very far from the doll'eyes paradigm which assumes a simple linear relationship between both organs. 
This is by the way what an online textbook from Dartmouth Medical School has to say on this subject:
"If the patient is awake [the brain] keeps the eyes from deviating from midposition and actually may drive the eyes beyond the midposition toward the direction of turning. If the patient is in a coma due to bilateral hemispheric suppression [...] the eyes deviate away from the direction of head rotation in an unchecked manner (the reflex response is not inhibited by cerebral cortical input)." (Reeves and Swenson "DISORDERS OF THE NERVOUS SYSTEM". Ch.6]
So, if there ever was a doubt as to the link between head and eye movements, the reaction of coma patients should take it away. What such examples do show is that, just like in the case of Mach's experiments, visual constraints play no role whatsoever in this model. 

We have though, instead of
head movement --> stimulation of inner ear---> NO eye movement. (Mach)

[A case of the so-called VOR cancellation. Remark that such a concept turns VOR into something like the Freudian concept of "resistance". It can never be falsified. Whatever the patient says the therapist does not like can be interpreted this way, and in all impunity. The same way, whenever a phenomenon does not seem to support the existence of VOR, just put it under the heading of VOR-cancelation, and you are good to go.] 

head movement --> stimulation of inner ear---> eye movement. (VOR)

It seems impossible to make the line any shorter since any head movement would have a stimulation of the inner ear as a consequence. 
A very hard proof, for those, like me, who doubt this trinity, is the so-called  Barany's caloric test for which he received the Nobel prize in 1914.
In fact, this test is considered so reliable that it is one of the tests used to establish brain death!
Its simplicity is equaled only by its beauty [or the other way around.]: pour some water, warmer or cooler than the body temperature, in one of the inner ears, and a nystagmus will be produced, its direction depending on whether it was cold or warm water.
(Following the COWS mnemonic: Cold Opposite Warm Same)

Remark that we did shorten the line which has now become:

stimulation of inner ear --> eye movement

And that seems like a definite proof that vestibular sensations and eye movements are causally related.

But here is "le hic", as the French would say. People undergoing the caloric test are subject to vertigo. And the problem with vertigo, or dizziness, is that it is, usually, not a very nice sensation. It is somehow comparable to pain.

Metaphysical Digression

The withdrawal reflex is said to come into action before we even have the time to feel pain. It is therefore purely mechanical. How about jumping up and down and bringing your hurt finger to your mouth? What is the neuronal cause of this behavior? You could look for pain in the brain as long as you want to, you will not find it.
Somebody armed with science-fiction technology that would allow him to follow the excitation of each individual nerve in situ, that is with full consideration of the context in which it is happening, would probably see two series of events follow each other, but with no apparent link between them.

action one - gap - action two

This paradigm would be a very clear representation of causal efficacy of a non-physical event.
It is indicative for the Zeitgeist that a concept like emergence has been accepted so easily because it could hitch a ride with modern concepts like "computer", "neuron" and "complexity" that could hide its metaphysical roots, while the concepts of spontaneous generation or mental causality are ridiculed.

action one - emergence of a link - action two
action one - spontaneous generation of a link - action two
action one - sensation/emotion - action two

I must say that of all three realizations of the same paradigm, the last one has at least the plausibility of psychological phenomena.

We are worried that the acceptance of causal efficacy for mental events would open the dam wide open to all kinds of mystical pretensions, which would of course certainly happen. But do not think for a second that the mystics would then get a free lunch.
The physical realm cannot solve the halting problem, it needs for that the mental realm. But does that mean that the mental realm is "computationally" or "logically" complete? [Whatever those terms may mean in that realm.]

Or will the mental realm also need the physical realm to be effective?

The previous paradigm would then become:

Sensation/Emotion - gap - Sensation/Emotion

And what could fill in the gap better than the physical realm? We would then have:

Sensation/Emotion - physical action - Sensation/Emotion

In other words, you cannot go indefinitely from sensation to sensation, you need the in-between stops in the physical realm to access other sensations. [we can go from pain to relief only if the effects of pain are taken away. And that is a physical process. Without that, we would probably witness something akin to inertia: the pain would never stop, just like an object put in motion can never stop out of itself!]

We would end up with what we already have, a physical and a mental realm. No harm done.

What makes the gap nigh invisible is its familiar nature. We see no gap between the behavior of someone burning his finger, withdrawing his hand, and jumping up and down. Why would we see it in the neural version of this series of events?
It would take a very thorough knowledge and mapping of the brain to discover the gap. Without this scientific certainty, that may never be attained, that two successive events are not causally related, we will always fill in the gap ourselves.
Of course, such speculative considerations may turn out to be superfluous.

A Simpler Explanation is maybe given by 
Baloh and Honrubia's "Clinical Neurophysiology of the Vestibular System", 2010:
"Presumably, the spontaneous afferent nerve activity increases and decreases because of heating and cooling of the afferent nerve, respectively." (P.179)
Which means in fact that the caloric test is nothing else but a surrogate vestibular stimulation. 
We are therefore back to the previous situation:

head movement --> stimulation of inner ear---> eye movement
or rather:
caloric stimulation --> stimulation of inner ear---> eye movement

The metaphysical digression was after all maybe not so superfluous?
Let us if we can find more down to earth arguments.

11 Are eye movements and vestibular stimulation causally related?
Not according to Mach'experiments, and certainly not according to our very own everyday experiences. We do not get a doll's eyes reflex when going up or down in an elevator, and air pilots seem to be very well capable or repressing any occulo-vestibular reflexes they may have. This was known even before World War II as vestibular habituation: (Griffith "The organic effects of repeated bodily rotation", 1920; Dodge "Habituation to rotation", 1923).
Furthermore, there is no unequivocal proof of a direct link between the inner ear afferents and the extra-ocular motoneurons, the topological concept of vestibular nuclei notwithstanding. All evidence is circumstantial and could be very easily interpreted differently. Which I certainly will.
All in All, there is no reason to blindly accept the existence of such a link.
In fact, doing away with VOR might turn out to be very beneficial for further research. After all, more than a century of experiments after Mach have still not given any final results that could satisfy everybody. Researchers are still playing with Barany chairs and arguing about meaningless numbers and other quantitative models. And it does not look like they will stop any time soon!

In his Nobel Lecture (1914) Barany spoke of different tests that showed the links of the inner ear neurons not only with the extra-ocular muscles, but also, via the cerebellum, with practically all body muscles.
The effect on vestibular malfunctions on balance and posture were already known, Flouren's predilection for methodical ablations of different layers of neurons in the cerebellum and elsewhere having given convincing results which had been confirmed by others.
[Flourens "Recherches Experimentales sur les Proprietes et les Fonctions du systeme nerveux dans les Animaux Vertebres", 1842.]

What I find particularly illuminating in the examples given is the complexity of the connections between vestibular neurons  and other parts of the nervous system.
Let us take the simple example of telling the patient to hold his right arm  straight and then "syringe" his right ear with cold or warm water. The arm, inexorably, starts deviating from its position.
The deviation is even more noticeable when the patient is asked to touch with his own fingers those of the doctor. His fingers will deviate to the right or the left, opposite to the nystagmus that had been caused by the caloric stimulation.

Here is "anozer 'ic":
The same stimulation can apparently have an effect on many muscles whatever their initial position. Barany speaks of the different joints at the wrist, elbow, hip [no George, not 'ip, hip. We must not abuse of a good thing!], etc.
That is a lot of branching, which must also involve a lot of logic to make sure that when the same vestibular neurons are activated, the right muscles, according to the situation and context, are innervated .[Shall it be the eye AND the cerebellum, or the eye alone? Would you like a gift wrapping?]

The conception of a direct connection between stimulation of the inner ear and so many different muscles is untenable. Other, more complex, neural mechanisms must be involved. And if that is the case, how can we still speak of "reflexes"?

12 Retinal Image and its Movements

I must admit that I have great difficulties at times following the argumentation of different authors, starting from the pioneers of the 19th century. This is my problem:
A retinal image can be compared to a drawing on the inside of a globe. however the globes moves, the image will move with it, whatever rotation the globe does, the image will rotate ccordingly... In space... If we make abstraction of the globe.
What about torsional movements. Can the retinal image turn around its own axis if the globe does the same thing? Again, if we make abstraction of the globe, we can only conclude that the image will have rotated about its own axis: it will have performed a torsional movement.
Here is the problem as I see it: the globe, that is the eye, can be said to rotate and even translate in space... If we make abstraction of the head! Otherwise, we can say that the eye can go left-right and up-down. Even the so called rolling of one's eyes is in fact a combination of the previous movements.
Listing's plane is the mathematization of this intuition. We cannot move or turn our eyes in 3D space, only in a 2D plane. 
Let us use the ancient Greek conception of the eyes being the origin of the light we direct on external objects to see them: the electric torch paradigm.
Do we have to imagine the torch fastened to a system of horizontal and vertical rails on a wall (Listing's plane), or a torch fastened to a mobile wall that can be moved up and down, but also be tilted forward or backward?
What about tilted to the left or right side? Can the eye do that?

I suppose that all tilted positions can be attributed to the eye in a stationary head, while all translations can be seen as the result of the head and or the body moving. [Helmholz about the way the eye is fixed in its orbit: "any displacement of the eyeball as a whole, that is, any displacement in which every point of the eyeball is moved in the same direction, is rendered impossible." Treatise, vol.3. par.27]
The problem is that the eye cannot be tilted from its place. It can only be rotated. So, instead of a wall, maybe a hand holding a torch would bring us closer to how the eye behaves.
Do we need different concepts for the eye movements in this second case. Can we say really that the eyes move from left to right or up and down, just because the head itself is moving?

In the after-image paradigm, what is calculated, the retinal image, or the visual sensation as after-image?
The retinal image can be said to perform any movement, and not only torsion, only in an abstract, geometrical way. As part of the eye globe it just follows passively the movements of the latter, while in fact remaining stationary.
What is calculated is not the position in space of an external, observable object, but that of an internal, private visual sensation.
We are somehow back to the Weberian paradigm, aren't we?
That certainly seems to be the case, at least until the calculations are suddenly transferred to another dimension.
The movements of the after image are not used to analyze how your sensations behave in different circumstances, but to justify calculations where they are completely ignored and neutralized. Optical laws in the modern version do not need to mention visual sensations at all. Even the use of after-images has made place for the neutral calculation of eye movements with magnetic and electrical devices. 


13 Efferent nerves to the inner ear: a possible explanation
The difference between vestibular sensations and sensory organs is that the first is oriented at signals from within the body, while the others warn the body about any external intrusion. 
I was sitting in my favorite, old but comfortable swivel chair which was, once again, leaning a bit to the side. Time to re-fasten the feet after shoving them back in the right position. But then I started to think about this unremarkable fact: I could feel that the chair was misaligned, and I could feel it everywhere but in my head. And, after all, why should i feel it in my head? My head was straight on my shoulders, it was the rest of my body that felt somewhat askew.
Then I understood, or thought I did, why the inner ear would need efferents from other parts of the body. It has to know when it is out of balance even if the head is straight, and therefore silent.
My comparison with vision was the wrong one. In this case, it seems that the brain was entitled to change the sensibility to blue (read balance) if blue was too dominant. And that had to happen at the source, in the inner ear. It would be indeed like changing the sensibility of the retina to a certain color, but then, in this case, the retina is directed directly and only to the own body. And the inner ear needs authentic sensations from every part of our body.
How does it work exactly? I have no idea.


14 Binocular vision and its myths
Wheatstone wrote his famous paper "On Physiological vision" in 1838, one year before Daguerre published his results on the ancester of photography and cinema. The steroscope Wheatstone described must have seemed as a wondrous instrument, capable of bringing to life dead objects! It also created quite a commotion among physiologists, especially in Germany, where the discipline was approaching its peak under giants like Messner, Mueller and others, all soon to be eclipsed by Helmholz and Hering.
At that time, the Theory of the Identity of Retinal Images was predominant. One assumed that the only way binocular single vision could be attained was by the fusion of two similar images falling on corresponding locations in both retinas. Albrecht Nagel ["Das Sehen mit zwei Augen", 1861; not to be confused with the American batman], quotes Volkmann who predicted the fall of the discipline and the repudiation of all the calculations made possible by Mueller, Panum and others. Nagel, more optimistic, and a fervent follower of Helmholz' empiricist approach, promised that the situation was not as dire as it seemed. We were witnessing a painful paradigm shift and Nagel was the savior.
Nowadays, Wheatstone's view of binocular disparity has become common knowledge, and whole industries (optics, cinema) are based on his analysis. Still, it looks more like a Pyrrhic victory than a technical knockout.
The Theory of the Identity of Retinal Images has not been abandoned, as one would expect, but has surreptitiously hidden behind its victor, thereby guaranteeing its survival.
Before I turn to that, I would like to look more closely at Wheatstone's approach.

Binocular disparity and its significance
Wheatstone gives two very interesting examples to introduce the stereoscope. The second one gets a very thorough analysis as it is at the base of his invention: two plane drawings, we would say 2D descriptions, are presented in the stereoscope with as unexpected result a life-like impression of a real spatial object.
More interesting is the first example of two identical, real three dimensional objects, viewed simultaneously through separate tubes, the way we would use binoculars, but then without the common point of fixation.
Wheatstone is of course intent on promoting his new device, and only uses this first example to illustrate the principle behind the main course. Strangely enough, and as far as I know, nobody since then has ever stood still by the significance of this first example. After all, it was the physical embodiment of a contradictory principle to what Wheatstone was trying to prove. Looking at two three-dimensional objects also gave a three dimensional sensation. So, where does that leave the theory of binocular disparity and Julesz' random-dot stereograms("Foundations of Cyclopean Perception", 1971)? 
The conclusion that two, minimally dissimilar, two-dimensional objects create the three-dimensional impression of a single object, is of course undeniable. It only becomes subject to doubt when used as the explanation of how 3D vison works. The assumptions on which it is based are quite strange, to say the least.
It assumes that each eye sees the world as two-dimensional. According to Nagel, and conform Helmolz's conception, when we look with one eye, we still see objects as three-dimensional, because that is the way we have learned to see them. A nice example of perception as inference. Also, a theory which one either accepts or rejects, because it can neither be disproved nor proven.
What is really interesting though is whether the third dimension is a real property of space or an illusion created by the brain for its own benefit. Do we see objects in three dimensions because there are three dimensions, or is the world in fact as flat as, or even more so than, ancient earth? The answer to this question should determine how seriously we take the theory of binocular disparity.
If space is three dimensional why would we need such a complicated detour to experience it as such? More to the point, why should the experience of the third dimension, if it does indeed exist, be considered as an illusion brought about by habits and learning?
In other words, the theory of binocular disparity, as advocated by Nagel (and not Wheatstone) and every author after him, only makes sense if space is two-dimensional, and depth an illusion!

The Theory of the Identity of Retinal Images had apparently to undergo a serious change, that might end up to be strictly cosmetic. It was saved by Panum's analysis of the horopter ("Physiologische Untersuchen über das Sehen mit zwei Augen",1858), first mentioned in the eleventh century by Ibn Al Haitham, and since then many times recalculated until the concept lost any meaning. Almost a century ago Danville in "The psychological significance of the horopter" (1933) drew attention to the fact that all the approaches to the horopter were geometrical, and giving different results because of different starting points. The difficulty in establishing a clear criterion had still not been solved in his time, as is also still the case today. He tried for a psychological approach where he tried to determine when humans experienced a single object as double. He found out that the theoretical delimitations of the horopter were of little use. Nowadays researchers speak of a theoretical and an empirical horopter. But the idea that there is one or more areas where both retinal inputs are seen as one, and others where a single object is seen as double. has not lost its attraction.
I have certainly no objection against such empirical assumptions that can be investigated scientifically. What I find less obvious are the theoretical prejudices that usually accompany such experiments.
The idea that the brain somehow not only needs corresponding images on both retinas, but also is capable of actively searching for them is, like I said elsewhere, a typical homunculus approach.
Let us take the example of two dissimilar images sent to each retina. Let us say, one object inclined to the left, the other to the right. Apparently, in such a case, the brain would try to somehow make fusion possible by rotating the eyes (cyclorotation) in an appropriate way, that is by limiting the disparity between both images. 

[Quiet George, Im' getting to it!]
The idea that the brain somehow knows what to do so that both images on the retinas could be considered as one is one of the most ludicrous assumptions that could ever be made. Still, the number of articles containing complex mathematical calculations and models is still growing. 
What is even more puzzling is that not a single article I read can in fact give an example of factual cyclorotation. All conclusions are based on geometrical models that predict eye movements and rotations (Hausen "Considerations on Listing’s law and the primary position by
means of a matrix description of eye position control", 1989). And even if they did somehow describe real cyclorotational movements, that would still not necessarily have to be explained by the need for the brain to bring two images together. It could as easily be explained by the fact that each eye is trying to keep focus of the image it is seeing, independently of what the other eye is doing. A kind of smooth pursuit as it were, around the own axis.

[Some references:
- Goodenough et al "Eye torsion in response to a tilted visual stimulus", 1979.
- Mok et al "Rotation of Listing’s plane during vergence", 1992.
- Van Rijn and Van Den Berg " Binocular eye orientation during fixations: Listing’s law to include eye vergence", 1993.
- Minken and Van Gisbergen "A three dimensional analysis of vergence movements at various levels of elevation", 1994.
- Howard and Rogers "Binocular Vision and Stereopsis", 1995
- Hooge and Van Den berg "Visually Evoked Cyclovergence and Extended Listing’s Law", 2000.]

Split brains, binocular disparity and cyclorotations Oh la la!
The last point (two disparate images presented to the brain and its reaction) reminds me of an urban myth that has surrounded Kim Peek for years, the model for Dustin Hoffmann's Rain Man. Many a writer on the web shamelessly affirms that Peek was capable of reading two books, or two pages, at the same time. Except the fact that neither Peek nor his father ever admitted to such a feat, a simple observation of Peek on the many videos on the web, while he is reading, shows him as looking (incredibly) rapidly at one page, and then the other. As somebody without a corpus callosum, Peek'brain was comparable to that of patients who had had the connection between both hemispheres disconnected to help them with their epileptic attacks.
Gazzaniga, the world's expert on splitbrains [even if his boss, Sperry, took all the credit with the Nobel Prize of 1981, at least that is the impression I got reading "Tales from Both Sides of the Brain", 2015, the author's declaration of loyalty notwithstanding] has a very interesting theory concerning the communication between both, isolated, hemispheres. Let us see if we can make use of it with our problem of binocular vision.
As formulated already in his article of 1969 “Cross-cueing mechanisms and ipsilateral eye-hand control in split-brain monkeys”, Gazzaniga is convinced that both hemispheres are still able to communicate with each other, even in the case by monkeys, where the ablation of the interconnections is much more radical than by humans.
A very specific way of communication is what he calls auto-ceuing. One hemisphere would initiate certain movements (head, eyes, body) to tip the other over what was going on.
The question is, how does the other hemisphere know what to do with the cues? Communication demands a common language and a common subject on which to communicate. He gives the example of one hemisphere having to push some lever but not knowing which, and the other hemisphere signaling by its head and eye movements which lever to push. Restraining the head made cueing as good as impossible, and the responses dropped to chance level.

What does that prove? 

To Gazzaniga, the existence of communication channels between hemispheres that are independent of the corpus callosum. Also, he is convinced that, even if he considers speaking of two different minds in the same body as going too far, the brain is made of a multitude of modules that work independently of each other, but that communicate somehow via this cueing process. A non-linguistic module will obviously need to get non linguistic cues to be able to decipher them, and other modules, just like in the example given, gladly comply.
This is a very interesting view that is I am afraid much too general to be falsifiable. So, I will leave it to those who can use it better than me. [better than I? George, you are not British, are you?]

Back to the main question. All modules must be "conscious" of the same situation and have the same goal. In our example, one hemisphere could cue the other if it knew what was expected from its twin. And the latter had to assume that the attempts to communication were related to the problem at hand, and not to the fact that its sibling was bored with the whole thing.
However you look at it, I would say that there was only one mind which had to make use of disconnected parts of his brain, and had somehow to improvise each time to get things done. If we consider the hemispheres as the repository of all the experiences the individual had had in his life, we could say that activating one hemisphere or the other brought different aspects of the same mind to the fore. A mind which we could, with exaggeration, consider as a kind of tabula rasa that gets written on separately each time by one of the hemispheres.

Okay, George, you're up!

George: in cases like this I have to be very careful. See, each time I am in a hemisphere, I am caught in its world and know only what it knows, and can do only what it can do. Then I get called by the other hemisphere, and I do not forget where  I had been, I even could remember what I was doing. Sort of. I mean, I still knew how I felt, I just could not remember how to do what I wanted to do.
me: you mean you had the know-what but not the know-how?
George: uh?
me: let us say that you hear a story from hemisphere A, sorry I can never remember which does what, you understand then the story, but when you get to B, you still know the sory, only this time, it is not in any language that B can understand, or even in the language that A told it in, so you have no way of telling it to B.
George: That's it! Are you sure you are not one of us?
me: you are one of me! And don't you forget it![mumble mumble].

In other words, there is common information to both hemispheres, and each hemisphere can be used by George separately, but George has no influence whatsoever on the content in each hemisphere. It can only make the best of it, wherever it is.
The link between both hemispheres can of course be put on the conto of sub-cortical connections, the question remains what these neural connections can convey. Gazzaniga speaks of emotions but he does that in the traditional way, as neural substrates in the form of a limbic system. I go beyond that and surmise that there is another dimension which is served by these neural substrates, and which make those emotions accessible to the whole organism. Such an assumption can, so far, easily be ignored.

binocular disparity again:
What does that mean for our problem? It would seem like the brain has a way of getting the information about one eye from one hemisphere, and combine it with the information from the other eye and hemisphere.
The problem is that it concerns the same kind of information in both hemispheres. No cueing would seem necessary. Also, even if one hemisphere could cue the other, not all eye movements, and certainly not torsional eye rotations, are under voluntary control. Furthermore, the cueing language would have to be very sophisticated to relay the need of rotating the eye around its axis until it fits the other image.
All in all, inter-hemispheral communication does not sound like a viable option.

A last remark: Gazzaniga often made the patients sit on their hands, or restrained their body movements because he considered the attempts of communicating one hemisphere with the other as a form of "kids cheating in a classroom". He was so convinced of his model, independent modules cueing each other, that he never stood still by the possibility that it was in fact one and the same mind that was trying to solve the problem as best as it could.

serial or parallel?
The stories relayed by the splitbrain tradition would certainly seem to favor the parallel view. The image of a patient trying to put his pants on while his other hand tries to take it off is a soap series worthy. Still, I would bet that even such a tragicomedy was in fact built out of two independent blocks: the hands, I surmise, were not working against each other at the same time, but alternately, however fast the change from one hemisphere to the other went. If I am right, that would mean that the same mind was indeed behind both series of actions, but each time following different experiential content.
Applied to binocular disparity, and the ubiquity of ocular dominance [King and Zhou "New Ideas About Binocular Coordination of Eye Movements: Is There a Chameleon in the Primate Family Tree?", 2000] I would say that we always see mainly through one eye. What the other eye, when it is open, adds to our vision is not fused with what we are already seeing, but added to it.
How can you blend images together? Fusion is something I would not know how to explain, neither in neuronal nor in supra-physical terms. 
What I can understand is the fact that we see not only what both eyes see, but then through the dominant or fixating eye, plus what each eye sees but the other does not.

Last but not least: Wheatstone stereoscope was aimed at foveal vision, the area which was declared free from binocular disparity by Panum, Nagel and others. Wheatstone has been neutralized, his invention taken as a gadget, but its theoretical implications ignored. 
Nonetheless the contradiction remains. You cannot have a working stereoscope and a disparity free fovea. One of them has to go.

15 Addendum: 
Regarding the contradiction between a working stereoscope and a disparity free fovea it is only active as long as we consider binocular disparity as the cause of depth perception. Once we accept that it is only one of the ways the brain has to experience 3D, the principle that corresponding points in the retinas make binocular single vision possible can also be accepted as another empirical rule. Just like viewing two plane images gives a depth impression. We do not need to explain those principles, just acknowledge them and study their consequences.

By doing that we avoid at the same time the fallacy of the neural correlate.

A special case
When applied to the case where both retinas get a different input, we can take inspiration of the split-brain paradigm.
1) fast alternating fixation: the brain tries to fixate with each eye independently of the other.
2) ocular dominance: it settles on one view.
This, if normal conjugate fixation is artificially made impossible.

As far as the controversy between Helmholz and Hering concerning common or differentiated innervation of both  eyes, I think that both were right in some respects, and wrong in others. A discussion for another time.


16 Cyclorotations: do they really exist?

They are of course mechanically possible [I am so glad I do not have to say "metaphysically possible" for once!], but the only time I have seen them were in computer animations on the web. None of the videos I have watched concerning eye movements has ever shown an actual cyclorotation. So I will admit that I am a bit at a loss here. 

Here is another reason why I do not believe in the reality of cyclorotations.
Imagine looking at an a somewhat inclined vertical that suddenly moves to a sharper angle. The line will impinge on different photoreceptors of the retina. Cyclorotations are supposed to prevent that.

Why should they? We see an object in one position, then in another. The idea that we would not know that it was still the same object, the so-called correspondence problem, does not really make much sense. When we are looking at an object, we are not only using our eyes or isolated areas of our brain. The whole brain is engaged. That is also what makes smooth pursuit possible. Nobody ever wonders how that is possible in the context of the correspondence problem, even though both deal with the same so-called problem: the same object on different locations in space and on the retina.

The assumption that a single object has to leave an impression on corresponding parts of the retina was infirmed by the stereoscope. So, unless we want to attribute to the brain the faculty of distinguishing the cases of binocular disparity which should be respected, because they create a 3D image, from those where the disparity creates a double image, then we have to accept the fact that the brain has no control on any of these situations.

Besides, if it did, how come we still see double images where there are single objects?
Back to the horopter? Which version?

After-image and eye movements
The concept of cyclorotation was deemed indispensable with the discovery that the after image on the retina changed position with the movements of the eye. Helmholtz gave even an example of when the after-image rotated, and when it did not. Nowadays afterimages are not used anymore to study eye movements because they are subjective sensations that the researchers in the 19th century tried to elevate to scientific data. Magnetic and electric devices are used instead, like the so-called search coils. I honestly do not know the technical details, but apparently their results are fully compatible with those given on the basis of after-images. That is why I will consider them as similar and hope that I am not wrong.
After-images are persistent visual stimulations which do not need fixation to be perceived, even if they were born in the foveal area. When we move our eyes intending to fixate our gaze on another object, the after-images remains imprinted on the same location on the retina where it originally came to life. Seeing them rotate is then the only reason why the German physiologists [Panum was Danish and Donders was Dutch] came to believe in cyclorotations. 

We cannot fixate on an after-image, for the simple reason that the eye cannot fixate on the retina itself! The fact that the after-images has rotated in its new position, relative to the original one, does not mean necessarily that our eye has rotated around its axis. That is only the case if you take the after-image as starting point.
That would mean fixating the after-image before, during and after the eye has finished its movement. But that is of course impossible.
If the eye movement is unrelated to the after-image then the rotation of the after-image must therefore have another 
cause, not necessarily mechanical. After all, we are speaking of a visual sensation.

But what about a retinal image? Can it be said to rotate around it own axis as the after-image is supposed to do?
Well, such a retinal image could be compared to a drawing on or inside a globe. How could it ever rotate around its own axis?