An analogy between colour and spatial coding

The early stages of colour coding are well established in that the trichrom atic receptor stage is followed by a set of opponent colour channels. One i nterpretation of the sequence is that opponent channels carry unrelated asp ects of the colour stimulus, unlike the cone channels. The overlap of the c one channels can be removed by decorrelating their spectral-sensitivity fun ctions, and this procedure has been found to give opponent colour channels which match those found psychophysically. Since the known spatial-frequency channels also show considerable overlap, the question arises which aspects of the spatial stimulus are captured by decorrelating the spatial-frequenc y channel. The results of decorrelating the spatial-frequency channels are that the first decorrelated spatial filter acts as a broad bandpass filter which has a peak sensitivity at 7.9 cycles deg(-1), and that the second dec orrelated spatial filter acts as an opponent spatial-frequency channel, wit h a minimum output at a low (4.1 cycles deg(-1)) spatial frequency and a ma ximum output at a high (15.1 cycles deg(-1)) spatial frequency The characte ristics of the first decorrelated filter closely resemble the properties of the foveal perceptive field which have been used to explain the Hermann gr id illusion. Thus, the decorrelation analysis produces a model for the func tional organisation of the channel implementation at the neural and psychop hysical levels, but which directly relates to the subjective appearance of the visual stimuli.