Fig. 1.

Schematic diagram of the four functional stages in the process of colour discrimination. Stage 1 receptor signals arise from quantum absorption (r) in the S, M and L-receptors for each of two different stimuli (reference and test stimulus). Quantum catches are transduced into graded voltage signals giving rise to receptor signals: {r _S r _M r _L} for the reference stimulus and (r′_S r′_M r′_L) for the alternative test stimulus. This stage may also include any postreceptoral processing if the segregation between different receptor types is preserved. Stage 2 neural coding of the receptor signals into achromatic and/or chromatic signals. Three coding units are depicted, the output of which is given by f ₁ f ₂ f ₃ and f′₁ f′₂ f′₃, for the reference and test stimulus, respectively. The units depicted here by a single symbol may represent several morphological and/or physiological layers. Stage 3 the two sets of signals are compared and evaluated. The output of this stage is the computed difference between stage 2 signals using a particular algorithm. Metric models will calculate a perceptual distance ΔS for the stimulus pair. Stage 4 the behavioural response, which is represented by the probability p _corr that one of two stimuli is selected. According to the metric approach p _corr depends on ΔS alone. Reprinted from Brandt and Vorobyev (1997), with permission from Elsevier