2. ELECTROPHYSIOLOGY OF THE RETINA
43
green off) are also found. The on and off components constituting the
receptive field of color-coded ganglion cells can be separately evoked by
choosing adequate wavelengths ( Wolbarsht et al., 1961). Figure 7 shows
the result of separation by applying green (500 mp) and red (650 mp)
light to such a unit. The sensitivity of the red component is generally
higher in the center of the receptive field, but it falls off more sharply in
the periphery than that of the green component.
According to the recent work of Daw (1967), each of the red and
green zones is very often surrounded by a zone of the opposite response
type, which is detectable only by the use of annular light patch. In
units such as illustrated in Fig. 7, for example, the organization of the
receptive field as mapped with annular light patch could be on centeroff periphery for green (500 mp) and off center-on periphery for red
(650 mp). Such arrangements explain the psychophysically known
phenomenon of the simultaneous color contrast. The size of the periphery
is very large, being at least 5 mm in diameter for both red and green.
IV. RESPONSE OF PHOTORECEPTORS
A. Early and Late Receptor Potential
As already mentioned, the distal PI11 is the earliest potential in the
ERG and is considered from its localization to correspond to the receptor
potential identified in mammals by Brown et al. (1965). While this
response has the latency of milliseconds and is easily evoked by moderate
intensity of light, another type of response which has no substantial
latency is elicited by very intense light flashes. This response was termed
the “early receptor potential” (early RP) to distinguish it from the conventional receptor potential which was accordingly termed the “late
receptor potential” (late RP) . The early RP was observed originally in
the monkey (Brown and Murakami, 1964a,b), but later this was found
to be universal to other animal forms including vertebrates and invertebrates. It is agreed that the early RP is a potential change associated with
some steps of bleaching of the photopigment and that this is not generated by changes in membrane permeability but most likely by the net
displacement of electric charge resulting from configuration changes in
photopigment molecules by light (Pak and Cone, 1964; Pak, 1965;
Brindley and Gardner-Medwin, 1966; Cone, 1967). For a net displacement of electric charge, a certain orientation of pigment molecules is
necessary. Arden and Ikeda (1966) and Cone (1967) have provided
43
green off) are also found. The on and off components constituting the
receptive field of color-coded ganglion cells can be separately evoked by
choosing adequate wavelengths ( Wolbarsht et al., 1961). Figure 7 shows
the result of separation by applying green (500 mp) and red (650 mp)
light to such a unit. The sensitivity of the red component is generally
higher in the center of the receptive field, but it falls off more sharply in
the periphery than that of the green component.
According to the recent work of Daw (1967), each of the red and
green zones is very often surrounded by a zone of the opposite response
type, which is detectable only by the use of annular light patch. In
units such as illustrated in Fig. 7, for example, the organization of the
receptive field as mapped with annular light patch could be on centeroff periphery for green (500 mp) and off center-on periphery for red
(650 mp). Such arrangements explain the psychophysically known
phenomenon of the simultaneous color contrast. The size of the periphery
is very large, being at least 5 mm in diameter for both red and green.
IV. RESPONSE OF PHOTORECEPTORS
A. Early and Late Receptor Potential
As already mentioned, the distal PI11 is the earliest potential in the
ERG and is considered from its localization to correspond to the receptor
potential identified in mammals by Brown et al. (1965). While this
response has the latency of milliseconds and is easily evoked by moderate
intensity of light, another type of response which has no substantial
latency is elicited by very intense light flashes. This response was termed
the “early receptor potential” (early RP) to distinguish it from the conventional receptor potential which was accordingly termed the “late
receptor potential” (late RP) . The early RP was observed originally in
the monkey (Brown and Murakami, 1964a,b), but later this was found
to be universal to other animal forms including vertebrates and invertebrates. It is agreed that the early RP is a potential change associated with
some steps of bleaching of the photopigment and that this is not generated by changes in membrane permeability but most likely by the net
displacement of electric charge resulting from configuration changes in
photopigment molecules by light (Pak and Cone, 1964; Pak, 1965;
Brindley and Gardner-Medwin, 1966; Cone, 1967). For a net displacement of electric charge, a certain orientation of pigment molecules is
necessary. Arden and Ikeda (1966) and Cone (1967) have provided