Paradigms of Consciousness
Session 4, February 22, 2002
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
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
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 ýconsciousness as
a property of graded representation.ţ 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.