Paradigms of Consciousness 
Session 4, February 22, 2002
Blindsight

Articles discussed (led by Hillary Rodman)
Farah, M.J., and Feinberg, T.E. (1997) Consciousness of perception after brain damage. Seminars in Neurology 17: 145-152.

Weiskrantz, L. (1996) Blindsight revisited. Current Opinion in Neurobiology 6: 215-220.

Lamme, V.A.F. (2001) Blindsight: the role of feedforward and feedback corticocortical connections. Acta Psychologica 107: 209-228.

Blindsight is an interesting topic since it is believed to shed some light on the subject of visual awareness. Individuals that are blindsighted are not consciously aware of visual stimuli in part of what used to be their entire visual field. Although consciously unaware, these individuals do seem to "perceive" stimuli when tested by forced response experiments. An example of a forced response experiment is one in which the subject is asked to choose whether or not a stimulus was given even though they may not be consciously aware of that stimulus. Results from these types of experiments indicate that subjects retain some form of visual awareness in the "blind" part of their visual field.

Initial discussion centered on whether these experiments could be interpreted given that repair may have taken place. In one case the subject received lesions at a young age and it is likely that repair mechanisms may have managed to restore some sort of vision to the affected part of his visual field. The appropriateness of using primate studies was also questioned as they relate to human circuitry.

Given these objections we discussed a synopsis of the three papers we read. The papers discussed a dorsal pathway that included information from the V1 through to the V3 and MT centers of the brain. The ventral pathway depends more on subcortical centers like the lateral geniculate nucleus, thalamus and the temporal lobe.

There appears to be a feedforward path that is very rapid. It was suggested that this feedfoward path is used to get an approximate understanding of visual stimuli. Possible biovalue would be to move out of the way of half recognized moving objects. The feedback path may bring information from memory storage centers. Feedback information returns all the way back to the primary occipital cortex (V1) where fairly primitive forms of processing are believed to take place. The memory information of the feedback pathway may be used to adjust input from the feedforward and result in object recognition.

This proposed mechanism is similar to Edelman's concept of consciousness arising from the cross checking of various maps. Here we have raw visual information being compared to memory registers. Lamme proposed that it is the feedback pathway that is required for visual awareness. In the author's interpretation a certain amount of visual information is still able to progress in a feedfoward fashion through intact subcortical structures and remaining striate cortex. In this case it would be the lack of an intact feedback mechanism that results in loss of visual awareness. 

We then discussed the three main models currently being considered for visual awareness. The first being that a local center determines visual awareness. In this scenario all appropriate brain centers report to this final center where awareness is created. The second scenario is the integrative interpretation. Several centers are believed to integrate their information, perhaps in a mechanism similar to that suggested by Crick.(synchronization). The final mechanism is ìconsciousness as a property of graded representation.î That is, when enough visual information is moving through the visual processing pathway a threshold is reached that can be defined as awareness. Degradation of structures important for visual processing limits information flow to a point below threshold, and thus we have loss of visual awareness.

The point was raised that  it is important point that none of these models explains what visual awareness is. He suggested that, like Zeki, visual awareness results from the action of processing itself and cannot be separated from it.