Control of Vision
I. Adaptation and Afterimages
The response range of retinal ganglion cells
is limited. Their maximum discharge frequency is not much more than 500
action potentials per second. Consequently, to provide the high level
of sensitivity we have for luminance changes, adaptation needs to occur
earlier in the visual system. It occurs in fact to a large extent at the
level of the photoreceptor molecules typically referred to as photopigment.
Photopigments serve to transduce electromagnetic energy into electrochemical
signals. Rods carry rhodopsin that has its peak sensitivity at 500 nanometers.
The three cones we have peak at 400, 540 and 560 nanometers. Quantal absorption
of photons causes a change in shape in these molecules called isomerization.
Once in the isomerized state, the molecule is no longer available to absorb
light quanta. The changes that occur in these molecules generate electrochemical
signals that activate subsequent stages in the retina. The relative number
of quanta in the isomerized state at any moment is proportional to the
rate at which quanta are delivered to the eye. Thus a tenfold increase
in quanta entering the eye reduces the number of pigment molecules available
to absorb these quanta ten fold (Hood). Herein lies the crux of adaptation.
The retinal ganglion cells respond predominantly the dynamic changes
that occur in the ratio of pigment molecules that are in one state or