The problem of conscious experience has been one that
has been difficult for neuroscience to study, but recently
new approaches have been devised that make the study of
consciousness open to experimental investigation. One
such approach has to do with where in the brain the signals
responsible for perception arise. Dr. Dan Pollen, in a
recent review, summarized the evidence that conscious
experience appears to occur in primary visual cortex,
Vl; to the extent that higher cortical areas can influence
elementary perception, they may do so by the backprojections
from these areas to Vl.
To explore this issue, the methodology of transcranial
magnetic stimulation is used. This is a noninvasive procedure
in which a magnetic coil is placed on the scalp. Activation
of this coil produces electrical activation of the underlying
brain region. Using two such coils, it is possible to
separately stimulate VI and the region, V5, that has been
implicated in movement detection. When VI alone is stimulated
the subject generally reports a stationary phosphene.
When V5 alone is stimulated the subject generally reports
a moving phosphene.
To address the issue of whether V5 stimulation produces
perception by sending a signal to VI, the following experiment
was done. A few milliseconds after V5 was stimulated,
VI was stimulated weakly. The weak VI stimulation did
not by itself cause a phosphene. However, it did interfere
with the normal perception produced by V5 stimulation:
specifically, the response to V5 stimulation often appeared
to be non-moving. This surprising result suggests that
some signal must go from V5 to VI where its processing
can be interfered with by weak VI stimulation; the alteration
in this processing affects perception.
