Cat colour vision: one cone process or several?

Abstract

SUMMARY1. Peripheral mechanisms that might contribute to colour vision in the cat have been investigated by recording from single units in the lateral geniculate and optic tract. Evidence is presented that the input to these cells comes from a single class of cones with a single spectral sensitivity.2. In cats with pupils dilated a background level of 10-30 cd/M2 was sufficient to saturate the rod system for all units. When the rods were saturated, the spectral sensitivity of all units peaked at 556 nm; this was true both for centre and periphery of the receptive field. The spectral sensitivity curve was slightly narrower than the Dartnall nomogram. It could not be shifted by chromatic adaptation with red, green, blue or yellow backgrounds.3. In the mesopic range (0 1-10 cd/M2), the threshold could be predicted in terms of two mechanisms, a cone mechanism with spectral sensitivity peaking at 556 nm, and a rod mechanism with spectral sensitivity at 500 nm. The mechanisms were separated and their increment threshold curves measured by testing with one colour against a background of another colour. All units had input from both rods and cones. The changeover from rods to cones occurred at the same level of adaptation in both centre and periphery of the receptive field. Threshold for the cones was between 0 04 and 0-25 cd/M2 with the pupil dilated, for a spot covering the centre of the receptive field.4. None of the results was found to vary between lateral geniculate and optic tract, with layer in the lateral geniculate, or with distance from area centralis in the visual field.5. The lack of evidence for more than one cone type suggests that colour discrimination in the cat may be a phenomenon of mesopic vision, based on differences in spectral sensitivity of the rods and a single class of cones.

The Purkinje shift in cat: extent of the mesopic range

“…We cannot satisfactorily account for the fact that Barlow & Levick's figure agrees so closely with measurements made by ourselves and by Daw & Pearlman (1969), using an enormously larger pupil. The mesopic range defined for a 7 mm2 pupil should lie well over one log unit above the range for a fully dilated pupil.…”

“…Earlier work suggests that the overwhelming majority of cat retinal ganglion cells, cells in LGN and primary visual cortex, receive a dual input from rods and a single class of cones, maximally sensitive to wave-lengths 507 nm and 556 nm respectively (Daw & Pearlman, 1969;Pearlman & Daw, 1970;Andrews & Hammond, 1970a, b;Hammond, 1971). Thus each cell exhibits a comparable Purkinje shift, the approximate magnitude of which is predictable at threshold from absorption spectra derived from the Dartnall nomogram for the two visual pigments involved (Dartnall, 1953).…”

“…Granit, 1943Granit, , 1945Granit, , 1955Gunter, 1952Gunter, , 1954Weale, 1953;Barlow, Fitzhugh & Kuffler, 1957a;Barlow & Levick, 1968). It is established that the shift is primarily dependent upon a single class of cones (556 nm) and a single class of rods (500 nm) in the retina (Daw & Pearlman, 1969;Pearlman & Daw, 1970;Andrews & Hammond, 1970 a, b;Hammond, 1971). Rarely, single visual cells are wired to rods and in opponent fashion to two different classes of cone with sensitivity maxima at 450 nm and 556 nm respectively (Pearlman & Daw, 1970); occasionally they are wired only to rods (Andrews & Hammond, 1970 a, b).…”

“…According to Daw & Pearlman (1969) the mesopic range for a sample of LGN and optic tract units extends from 0.1 to 10 cd/M2 in cats with dilated pupils. Rod saturation occurs at adaptation levels above 10-30 cd/ m2.…”

The visual system of the cat

“…The scope of this review is limited to data which bears upon the spatial properties of receptive fields in normal, adult cat and to data dealing with the importance of early visual experience in determining these receptive field properties. No coverage is given to the meager work on the chromatic properties of cat visual neurons (e.g., Daw & Pearlman, 1969) or those studies of the effects of eye movements on the excitability of cat visual neurons (e.g., Adey & Noda, 1973). Also not included is the recent work on the visual system of the Siamese cat (e.g., Guillery & Casagrande, 1977).…”