Online research in philosophy

What are the neural correlates of conscious visual awareness? Tackling this question requires contrasting neural correlates of stimulus processing culminating in visual awareness with neural correlates of stimulus processing unaccompanied by awareness. To contrast these two neural states, one must be able to erase an otherwise visible stimulus from awareness. This paper describes and critiques visual phenomena involving dissociation of physical stimulation and conscious awareness: degraded stimulation, visual masking, visual crowding, bistable figures, binocular rivalry, motion-induced blindness, inattentional blindness, change blindness and attentional blink. While no single strategy stands above the others, those producing changing visual awareness despite invariant physical stimulation are clearly preferable.

The relationship between brain activity and conscious visual experience is central to our understanding of the neural mechanisms underlying perception. Binocular rivalry, where monocular stimuli compete for perceptual dominance, has been previously used to dissociate the constant stimulus from the varying percept. We report here fMRI results from humans experiencing binocular rivalry under a dichoptic stimulation paradigm that consisted of two drifting random dot patterns with different motion coherence. Each pattern had also a different color, which both enhanced rivalry and was used for reporting which of the two patterns was visible at each time. As the perception of the subjects alternated between coherent motion and motion noise, we examined the effect that these alternations had on the strength of the MR signal throughout the brain. Our results demonstrate that motion perception is able to modulate the activity of several of the visual areas which are known to be involved in motion processing. More specifically, in addition to area V5 which showed the strongest modulation, a higher activity during the perception of motion than during the perception of noise was also clearly observed in areas V3A and LOC, and less so in area V3. In previous studies, these areas had been selectively activated by motion stimuli but whether their activity reflects motion perception or not remained unclear; here we show that they are involved in motion perception as well. The present findings therefore suggest a lack of a clear distinction between ?processing? versus ?perceptual? areas in the brain, but rather that the areas involved in the processing of a specific visual attribute are also part of the neuronal network that is collectively responsible for its perceptual representation.

Among psychologists and vision scientists,binocular rivalry has enjoyed sustainedinterest for decades dating back to the 19thcentury. In recent years, however, rivalry''saudience has expanded to includeneuroscientists who envision rivalry as a tool for exploring the neural concomitants ofconscious visual awareness and perceptualorganization. For rivalry''s potential to berealized, workers using this tool need toknow details of this fascinating phenomenon,and providing those details is the purpose ofthis article. After placing rivalry in ahistorical context, I summarize major findingsconcerning the spatial characteristics and thetemporal dynamics of rivalry, discuss two majortheoretical accounts of rivalry ( eye vs stimulus rivalry) and speculate on possibleneural concomitants of binocular rivalry.

Cognitive functions like perception, memory, language, or consciousness are based on highly parallel and distributed information processing by the brain. One of the major unresolved questions is how information can be integrated and how coherent representational states can be established in the distributed neuronal systems subserving these functions. It has been suggested that this so-called ''binding problem'' may be solved in the temporal domain. The hypothesis is that synchronization of neuronal discharges can serve for the integration of distributed neurons into cell assemblies and that this process may underlie the selection of perceptually and behaviorally relevant information. As we intend to show here, this temporal binding hypothesis has implications for the search of the neural correlate of consciousness. We review experimental results, mainly obtained in the visual system, which support the notion of temporal binding. In particular, we discuss recent experiments on the neural mechanisms of binocular rivalry which suggest that appropriate synchronization among cortical neurons may be one of the necessary conditions for the buildup of perceptual states and awareness of sensory stimuli.

We investigated binocular rivalry in the twocerebral hemispheres of callosotomized(split-brain) observers. We found that rivalryoccurs for complex stimuli in split-brainobservers, and that it is similar in the twohemispheres. This poses difficulties for twotheories of rivalry: (1) that rivalry occursbecause of switching of activity between thetwo hemispheres, and (2) that rivalry iscontrolled by a structure in the rightfrontoparietal cortex. Instead, similar rivalryfrom the two hemispheres is consistent with atheory that its mechanism is low in the visualsystem, at which each hemisphere conducts asimilar analysis of its half of visual space.

The neural basis of binocular rivalry has beenthe subject of vigorous debate. Do discrepantmonocular patterns rival for awareness becauseof neural competition among patternrepresentations or monocular channels? In thisarticle, I briefly review psychophysical andneurophysiological evidence pertaining to boththeories and discuss important new neuroimagingdata which reveal that rivalry is fullyresolved in monocular visual cortex. These newfindings strongly suggest that interocularcompetition mediates binocular rivalry and thatV1 plays an important role in the selection ofconscious visual information. They furthersuggest that rivalry is not a unitaryphenomenon. Interocular competition may fullyaccount for binocular rivalry whereas aseparate mechanism involving patterncompetition likely accounts for monocular andstimulus rivalry.

A midbrain neural basis for the perceptualoscillations of binocular rivalry is suggestedon the basis of fMRI studies of rivalry andinferences from the properties of rivalry thatcannot be explained from the known propertiesof primary visual cortical (V1) neurons. Therivalry switch is proposed to activatehomologous areas of each cerebral hemispherealternately, by means of a bistable oscillatorcircuit that straddles the midline of theventral tegmentum. This bistable oscillatoroperates at the same slow rate that ischaracteristic of perceptual rivalryalternations. Whilst attempting to divert thepresent preoccupation with cortical mechanismsfor rivalry, the new proposal integrates manycortical areas, in keeping with recent evidencethat binocular rivalry involves widespreadareas of the hemispheres. By linking rivalry tointerhemispheric switching mechanisms in thisway, the new proposal for the switch makes theprediction that binocular rivalry will besubject to high level influences such as moodand motivation. These predictions are beingfulfilled, with rivalry playing an increasingrole in the diagnosis and understanding if mooddisorders, schizophrenia and other psychiatricconditions.