Biology Reference
In-Depth Information
brain. Similarly, since PIII tended to increase relative to PII and also
to react faster when the retina became light adapted, Granit held,
in accordance with Lythgoe (
1940
), that light adaptation of a mixed
rod-cone retina transformed it from a rod to a cone system, resetting
the retinal instrument from slow to fast, and from integration to
differentiation (see Granit,
1963
, pp. 145-146).
Although the ERG measurement provided important informa-
tion about the frequency pattern of the optic nerve fibres, Granit
pointed out that the ERG was an average electrical response reflecting
various electrical events in the retina, and that there was no simple
relationship between the ERG measurements and the frequency of
discharge of the optic nerve. Indeed, Granit found that the c-wave,
produced by the PI component of the ERG, did not have any direct
relationship to the impulse pattern of the optic nerve, and that the
PIII component was chiefly concerned with inhibition of the optic
nerve impulses. Measuring the impulse pattern in single fibres, as
Hartline had done, might therefore provide a much better foundation
when it came to the interpretation of psychophysical facts. Granit,
inspired by this view, conducted a series of studies on various animal
species to correlate variation in light stimulus and impulse pattern
in single optic nerve fibres.
8.2 The dominator-modulator theory
The microelectrode technique employed was developed in the late
1930s by Granit and Svaetichin. It provided a supplement to Hartline's
micro-dissection technique and had the important advantage that it
could be applied to warm-blooded as easily as to cold-blooded species
(see Granit,
1963
, pp. 92-95). The cornea, lens and vitreous body
were removed and the microelectrode applied to the inner surface
of the retina with the aid of a micromanipulator. Spikes could then
be recorded from what Granit assumed to be single or well-synchro-
nized fibres. This assumption was based on the response characteris-
tics recorded, such as the 'all-or-none' law, simplicity of record and
duration of spikes.
