The luminosity curve as affected by the relation between rod and cone adaptation

The electroretinogram evoked by the excitation of human foveal cones

1. A 2 degrees test stimulus foveally fixed and viewed against a blue background (40 degrees in extent and producing 2.0 x 10(4) scotopic td of retinal illuminance) evokes a small voltage which can be recorded from the human eye with a conventional contact lens electrode if the test stimulus is flashed at a rate of 15 c/s, and the responses to at least several hundred flashes are averaged.2. The action spectrum of the response obtained in this way agrees reasonably well with the observer's psychophysical foveal luminosity curve.3. For the peripheral retina, the action spectrum is similar to that of the fovea when allowance is made for differences in screening macular pigment.4. Such responses diminish when the test stimulus is focused on to the peripheral retina and disappear when the test light is focused on the blind spot.5. Therefore, the response to the test light fixated centrally is the result of the excitation only of cones mainly, if not exclusively, in the fovea.6. When the intensity of the background is reduced by a factor of 10, the action spectrum shows evidence of the effect of excitation of rods in the blue part of the spectrum and of cones in the red. These red and blue responses add linearly when combined together, provided they are adjusted to coincide in phase.

Mesopic luminous-efficiency functions

In order to establish the luminous-efficiency function for mesopic vision of the standard observer, luminous efficiency functions were measured with the direct heterochromatic brightness-matching method at retinal illuminance in the range of -2-2 log photopic trolands in order to cover the scotopic, mesopic, and photopic conditions. A steady visual field of 10 degrees arc was presented foveally. The functions underwent the usual complicated change from a rod type of luminous-efficiency function close to V'(lambda) to a cone function that had a wide and almost double-peaked shape. A simple formula to represent the mesopic luminous-efficiency functions was derived in which log sensitivities of scotopic and photopic vision were linearly added after being multiplied by coefficients that were dependent on the luminance level. Saturation functions were also obtained at the various luminance levels for which luminous efficiency was investigated. In spite of a great variation of luminance level, the saturation function remained more or less the same, which indicates that the chromatic channels retain their contributions to the brightness sensation at low retinal illumination.

Cat colour vision: one cone process or several?

1. 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/m(2) 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/m(2)), 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/m(2) 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.