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[First, some considerations concerning a general neural process.]
The Myth of Synaptic Efficacy
This is a widely spread belief that has probably its origin in Shannon's information theory indiscriminately applied to neural processes. Once this view is rejected, the idea that  "[t]he extent to which synaptic activity can signal a sensory stimulus limits the information available to a neuron" (Arenz et al "The Contribution of Single Synapses to Sensory Representation in Vivo", 2008) loses any plausibility. (my emphasis)
What can be rejected for the brain as a whole (see the entry "Do we get too much information?" in my thread Retina: Miscellanious) can certainly be put in doubt when dealing with (individual) neurons.
Many concepts related to synaptic efficiency are likewise taken as dogmas, one of them being the probability of secretion of neurotransmitters that is supposed to be enhanced or reduced according to the circumstances. Such a concept, which is obviously a statistical instrument in ... (read more)
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A first intuitional approach

Movement is a very old metaphysical problem that divided the great ancient Greek thinkers.
Zeno's paradoxes, as discussed by Plato, Aristotle and others, found their way in modern thinking in the form of calculus, relativity, and last but not least, in the neurological underpinnings of motion perception. I will confine myself in this thread to the latter aspect, with only brief remarks concerning the others. Both the physical and philosophical traditions are rich in debates that would take many volumes to treat properly. 

The idea that we are always looking at a stationary picture of reality (in which stationary objects remain in the same place relative to the visual scene they are part of), and that the sensation of movements comes from the differences between two consecutive images is, very often, the implicit assumption of the 'scientific" approach to motion perception. 
I need to be very clear on this point. In my view, it is not the calculation of these d ... (read more)
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Inhibition is a typical homunculus concept. It creates no problem when considered as an active reaction of the organism (stopping a movement to change directions for instance) since it would have a definite neuronal target. But how can the brain decide which neurons to inhibit when (primarily) engaged in the excitatory stimulation of other neurons? To go back to our previous example, how can the brain stop the stimulation of all memories that have something red in them?
Obviously it cannot. We have no control over which associations are activated at any time. We have all experienced moments when we were really grateful that nobody could read our mind because of the embarrassing thoughts or images that would just pop up in our consciousness. 
That does not mean that there is no (unconscious) inhibition at all. Maybe the embarrassing associations have been let through for reasons irrelevant to our problem now, but that at the same time many other memories were stopped from popping up. Stil ... (read more)
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I cannot ever hope to treat of all important issues concerning the retina, so there will always remain things to add and others to reconsider. I propose to use this thread for just that, as a container of unresolved questions that need more work. I will try to use no more than a single entry for a single issue.

What does convergence in the retina mean?
Assuming I am on the right track, and that neurons do not hide any mysterious, computational, codes, then it is obvious that converging inputs can only affect the intensity of the original input, either by enhancing it, or by reducing it. It also means that rods nor cones can contribute their spectral influence (including gray shades) on the receiving cell. After all, color sensitivity has already disappeared from view, making room for mere intensity related effects, including changes in membrane conductance.
A simple scenario would make it so that receptors, via the intermediary neuronal layers, are already wired together in spatial config ... (read more)
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I would like to present a short and general review of a book treating of low-level processes concerning neurons. This book raises many questions concerning the nature of sensation, and its philosophical, theoretical and methodological consequences. I propose to to leave the discussion of the questions to after the analysis of these low-level processes, and the assessment of their general significance. I think it is very important to lay down a scientific foundation for the discussion, and that can only with a thorough understanding of the chemical processes that determine the neurons's behavior, and ultimately, that of the brain as a whole.
I am assuming that this book is representative of the current scientific view on the relevant chemical processes in the brain, and does not present a controversial, or obsolete, interpretation of said processes.
As will come clear by reading the following lines, this book present quite a challenge for the conceptions I have been developing, concerning ... (read more)
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I was (re)reading Weiskrantz' 2009, "Blindsight", trying to make sense of the different areas that were involved in this peculiar case of vision. I had tried to do the same earlier with Prosopagnosia 
[too many articles to mention, but it started with Gross, a graduate student of (who else?) Weiskrantz, Gross et al, 1972 "Visual properties of neurons in inferotemporal cortex of the macaque", and certainly did not end with the objections of the Tarr group against the specificity of this ailment, Gauthier, Behrmann&Tarr, 1999, "Can face recognition really be dissociated from object recognition?". The debate is still alive and kicking: Richler et al, 2012, "Holistic Processing Predicts Face Recognition".]
also to no avail. My frustration had almost reached a boiling point when I realized that it really did not matter where those phenomena are situated in the brain. Even if I believed in computational modules (or even computational neurons), which I most certainly do not, then I would still o ... (read more)
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The idea that the results of Hubel and Wiesel are flawed (see last two entries in my thread "Lateral Inhibition and Receptive Field") will certainly evoke a lot of resistance. After all, I am not talking of inconsequential details that can be corrected by following research. It concerns results and thoughts that have become dogmas in the field: different receptive fields at different stages; hierarchical organization of visual pathways; feature sensitivity; etc. 
So, why does the fact that the results of Hubel and Wiesel are based on retinal stimulations, and not on optic fibers (via the Lateral Geniculate Nucleus) make them flawed?

Imagine the following hypothetical, and unrealistic, example:

Retinal Stimulation :
Optic Fiber Response:
Visual Cortex Response:

[I could not get them all on one line, the symbols got scrambled.]

Such a crude drawing shows that neurons in the visual cortex rotate retinal stimulation, after it has been processed by the ganglion cells, 90 degrees upwards ... (read more)
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The role of sensation is practically inexistent in the majority of articles based on traditional scientific methods imported from the exact sciences. The danger of artificially obtained data has been recognized lately, and more and more emphasis is being put on the necessity of studying visual processes in situ, or at least in situations as close as possible to the natural environment of animals and humans alike. The authors of "Do We Know What the Early Visual System Does?", ( by Garandini et al, 2005), recognizing this trend, leave no room for misunderstanding:
"We can claim that we know what the visual system does once we can predict neural responses to arbitrary stimuli, including those seen in nature."  Using videos or images of natural scenes poses new challenges to neuroscience. It is a far step away of clearly defined parameters that can be easily quantified. This could be interpreted as an admission of failure of the traditional scientific methods, based on the v ... (read more)
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A very popular text book, Sensation and Perception by Goldstein, states the following:
"Light reflected from objects in the environment enters the eye through the pupil and is focused by the cornea and lens to form sharp images of the objects on the retina."
This is a common view that I have found explicitly expressed in any book or article I have read on vision.
The assumption of a retinal image poses at least two problems:
1) 2D array vs 3D world. How come we see objects in 3D while the retinal image is 2D?
2) The Inverse Problem: different objects have the same projection on the retina, but still we have no difficulty distinguishing between them.
I would like to add a third one. The retinal blind spot.
The explanations I have found have me baffled. The ingenious tests by Ramachandran and others, that purport to prove the existence of the blind spot, only add to my confusion. I have tried to find an answer to a similar problem, Tunnel Vision (Retinitis Pigmentosa), but I did not get any far ... (read more)
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