The Purkinje shift in cat: extent of the mesopic range

Abstract

SUMMARY1. The mesopic range, defined for cats with fully dilated pupils, extends from -1.0 to + 1-4 log cd/M2. The technique employed involves measurement of the relative shift in threshold with adaptation level, between two wave-lengths respectively selective for cones and rods at threshold. The magnitude of this shift is compared with that predicted from the Dartnall nomogram for visual pigments 556 and 507 (the in situ cone and rod absorption maxima).2. The result is based on monochromatic increment threshold determinations for 149 on-centre or off-centre fibres, isolated in the optic tract posterior to the chiasma, under a variety of adaptation levels. All units included receive dual input -from 500 nm rods and from a single class of cones with maximum sensitivity at 556 nm. No units receive input from 450 nm cones; six units with input mediated purely by rods are excluded from the sample.

Chromatic sensitivity and spatial organization of LGN neurone receptive fields in cat: cone‐rod interaction

“…Except at short and long wave-length extremes of the spectrum they could be matched rather well by nomogram curves for pigments maximally sensitive at 556 nm (cones) and at about 507 nm (rods) (Dartnall, 1953). Moreover, the relative sensitivity to wave-lengths 452 nm and 578 nm was invariably close to the value expected for mid-mesopic adaptation (Hammond & James, 1971).…”

“…Only the A-laminae were here investigated. All analyses were conducted in the mid-mesopic range, with monochromatic stimuli set against an adapting field with a luminance of 0-18 log cd/M2 (Hammond & James, 1971). The significance of this is threefold, as clearly shown by the threshold spectral sensitivity function for the LGN unit illustrated in Fig.…”

“…In all other cases the'types of receptor involved were quickly and reliably established from estimates of relative sensitivity to short and long wavelengths alone (Hammond & James, 1971), from suprathreshold responses to spectral stimuli of equal quantum flux (Andrews & Hammond, 1970b), or from considerations of latency of responses under near-threshold (Gouras & Link, 1966;Gouras, 1967) or suprathreshold conditions (Hammond, 1968;Andrews & Hammond, 1970b).…”

“…2, which is for a single LGN cell, typifies all six cells for which detailed measurements were made. The combined criteria used to establish threshold have been described and discussed earlier (Andrews & Hammond, 1970a;Hammond & James, 1971). In this case the stimulus spot was positioned over the receptive field centre and of optimal size, predetermined from suprathreshold measurements with a test spot at 525 nm.…”

Indoleamine‐mediated reciprocal modulation of on‐centre and off‐centre ganglion cell activity in the retina of the cat.

“…Within the mesopic range of luminances used in the present study (see Methods), both rod and cone inputs contribute substantially to the response of most ganglion cells (Hammond & James, 1971). A closer look at the wiring of the indoleamine-accumulating neurone suggests a specific relation to the rod pathway (Holmgren-Taylor, 1982).…”