Control of Vision
G. The Function of the On and Off Channels in Vision
We need to add one complexity to this picture.
Curiously, in the primate retina, there are only ON rod bipolar cells.
This means that if they are blocked as it can be done with APB, the rod
system is eliminated entirely. This is exactly what happens when APB is
administered to the eye. The result is that the animal becomes night blind.
These findings suggest that the reason the ON
and OFF channels have emerged is to be able to send signals to the central
nervous system rapidly with excitatory signals for stimuli made visible
either by virtue of light increment or light decrement. This enables the
organism to rapidly process both kinds of information. If this were not
the case organisms could not respond quickly enough to stimuli seen by
virtue of light increment. This processing capacity comes about by virtue
of the ON bipolar cells that have the mGluR6 neurotransmitter receptors
that invert the photoreceptor signal and hence excite the ON ganglion
cells when stimuli are seen by virtue of light increment.