Biology Reference
In-Depth Information
8
The duplicity theory
of R. Granit
The spikes recorded by Hartline and Kuffler with the microelectrode
technique represented end products in a series of consecutive
events in the retina, starting with absorption of photons. To gain
information about these extremely complex events that preceded
the discharges of the optic nerve fibres, the research workers had to
rely on the measurements of the electroretinogram (ERG). Indeed, it
was generally held that mechanisms underlying the ERG response
directly determined the impulse pattern of the optic nerve.
The ERG technique was first employed in 1865 by Frithiof
Holmgren, a Swedish physiologist. He applied a pair of electrodes to
an eye and found that the galvanometer connected to the electrodes
gave a marked deflection both when the eye was illuminated and
when the light was turned off. (For a description of the development
of this technique, see introduction section of Granit,
1963
.)
8.1 S u p p o r t i n g e v i d e n c e f o r t h e d u p l i c i t y
theory from the ERG technique
A very extensive review of the research literature on ERG was made
by Granit (
1947
). Presuming that the ERG response represented an
average reaction, reflecting the processes of activated photoreceptors,
the evidence reviewed was found to support the duplicity theory,
suggesting that there were two quite different ERG response patterns
of the retina, the so-called E- and I-ERG responses - the former
characteristic of rod-dominant and the latter of cone-dominant
retinas.
The hypothesis that the ERG response of rod and cone retinas
differed was based on evidence obtained by measuring the ERG under
conditions where the relative contribution of the two receptor types
