Colour-coding properties of sustained and transient channels in human vision

Abstract

WHEN the colour of a visual stimulus is repetitively alternated between say red and green, the percept evoked depends upon the temporal frequency of repetition. At very low frequencies (for example, 1–2 Hz), the repetitive change in colour is readily apparent. But at frequencies above about 10 Hz, no changes in colour are perceived; the stimulus in this case would appear a uniform yellow but would still seem to be flickering in brightness. Observations of this kind suggest that the colour of a stimulus may be analysed by a set of channels different from that analysing the brightness or flicker. The flicker or brightness channels respond to higher temporal frequencies of flicker than do the colour channels. This idea was confirmed by King-Smith¹ who has described how the sensitivity to coloured flashes of light changes with flash duration. He further described the subjective appearance of the flash, and concluded that a stimulus was not necessarily detected by the same channels as those that detected its colour. When the flash was brief, the stimuli usually looked achromatic, except when the wavelength was below about 490 nm. The intensity of the flash had to be raised above the detection threshold before the colour could be perceived. At longer flash durations, the colour was apparent as soon as the flash was detectable. The channels signalling the colour of a stimulus have a longer ‘integration time’ than those giving an achromatic percept. This kind of experiment examines differences in the high temporal frequency behaviour of the two sets of channels. It is of interest to examine the low temporal frequency behaviour as well because recent neuro-physiological investigations of the monkey's visual system make use of this behaviour in classifying neurones^2–5. Some neurones give a sustained response to prolonged presentation of a stationary stimulus; others respond only transiently to the stimulus onset. I show here that human colour channels give sustained responses while the flicker or brightness channels give transient responses.