Blue Cone Signals in the Extra Striate Cortex: Explanation for Blind Sight?
Our perception of vision is largely as a result of the signals conveyed from the eye to
the brain via the retino-thalamo-cortical pathway.
Visual signals within this pathway originate from three cone photoreceptors
responsible for day vision and one scotopic receptor, the rods.
The cones are classified according to their spectral sensitivity peaks as Long, Medium
and Short wavelength sensitive cones. Our chromatic perception is dependent upon how the
brain processes variations in the activity among these photoreceptors.
Chromatic signals are generally thought to be processed by two parallel streams,
the red-green system by the parvocellular system and the blue-yellow system
by the koniocellular system.
A large body of work has already been documented to identify these streams within
the retina and the main thalamic visual nucleus, the Lateral Geniculate Nucleus.
However, our knowledge about the processing of the chromatic signal within the visual brain,
particularly the pathways taken by the blue-yellow colour signals within the brain is at its infancy.
Majority of relay cells within the LGN project to the primary Visual cortex
and hence is considered vital for our visual perception.
However, more recent studies have shown that this area
receives robust chromatic signal not only via V1 but also bypassing V1.
Other studies have also shown that in subjects with blindsight
found that within the hemianopic field, S-cone modulating stimuli
were very effective in eliciting visual performance, and in fact in one patient
presentation of narrow band blue stimuli led to excellent performance but not red stimuli
Thus, there is increasing evidence that S-cone signals reach area MT via
a more direct route that bypasses V1 and providing an alternate explanation of blindsight in humans.
Neuro Open J. 2015; 2(1): e2-e3. doi: 10.17140/NOJ-2-e002