Biology Reference
In-Depth Information
that the cone system, in contrast to the rod system, made extensive
contacts with amacrine cells that distributed inhibition in the retinal
pathways.
The ERG response of the E-retina, on the other hand, was
characterized by a marked b-wave, assumed to reflect the underlying
slow-reacting excitatory electrical potential PII that excited the nerve
fibres and produced their 'on' responses.
The c-wave (the slow secondary rise of the ERG) was supposed
to be determined by the PI component, but Granit (
1947
) presumed
that this component either did not affect the discharge of the nerve
fibres at all or did so indirectly by altering the threshold level of the
discharge.
Granit found further support of his explanation of the E- and
I-retinas when he measured the change in the ERG response with
light adaptation both in an E-retina containing rods and cones and in a
pure I-retina containing only cones. Thus, while the E-retina showed
a decrease in the b-wave (indicating a reduction of the excitatory PII
underlying component) and a relative increase in the a- and d-waves
(indicating a relative increase of the inhibitory PIII component), the
pure I-retina showed no appreciable change in the form of its ERG.
He obtained another supporting finding with important theore-
tical implications under conditions where the light stimulus was
flickering. In the E-retina the flickering light stimulus produced a
sequence of b-waves (b-b-b-b-b), while the I-retina elicited alternating
a- and b-waves (a-b-a-b-a-b). Also, the I-retina was able to follow a
much higher flicker rate than the E-retina, presumably due to the
rapidly reacting inhibitory PIII component that dominated the
I-retina response. Accordingly, while the pure E-retina, containing
no cones, would tend to elicit simple, relatively slow 'on'-responses
in the nerve fibres, the I-retina would be able to produce complex,
rapidly changing 'on-off' discharges.
Granit (
1947
) presumed that this characteristic of the I-retina
gave extreme efficiency in transforming a visual field into frequency
codes that could carry information about the physical world to the
