Biology Reference
In-Depth Information
b
0.6
c
d
PI
0.4
PII
0.2
0
a
PIII
-0.2
0
0.5
1.0
Seconds
1.5
2.0
Fig. 8.1 A typical ERG curve as obtained by Granit (1938) with the
multi-phasic fluctuation of potential; the characteristic a-, b-, c- and
d-waves. These electrical changes have been analyzed by Granit into
three components, PI, PII and PIII, as shown in the figure.
was varied, for example, by light- and dark-adapting the eye, by using
retinas of different species where the relative number of rods and
cones varied, and by using blue and deep red test lights. The most
convincing evidence in favour of the hypothesis was obtained under
conditions where the adaptation level of the eye changed. Thus,
during long-term dark adaptation, for example, the ERG of a mixed
rod-cone retina underwent both a Purkinje shift (the maximum
spectral response changed from about 560 nm to about 500 nm) and a
change from the I- to the E-ERG response.
The ERG of an E-retina containing both rods and cones
is illustrated schematically in
Fig. 8.1
where both the different
waveforms (a, b, c and d) and the presumed underlying components
(PI, PII and PIII) are shown. All these underlying components
were found to be present in both types of retina, but to different
degrees.
The ERG of the I-retina was characterized by marked a- and
d-waves, assumed to be determined by the underlying PIII component,
representing a rapidly reacting negative electrical potential. To
account for this characteristic of the I-retina, Granit (
1947
) assumed
