Fig. 1.

Relationship between neuronal and psychophysical detection thresholds (NT and PT, respectively) under the cardinal mechanisms model. A: under the cardinal mechanisms model, primary visual cortex (V1) neurons are tuned to out-of-phase modulations of the long- and middle-wavelength sensitive cones (i.e., L−M) or to modulations of the short-wavelength sensitive cones (S-iso). The response of a neuron to intermediate stimuli is determined by the projection of the stimulus onto its preferred cardinal color direction. Thus NTs can be represented by lines in the isoluminant plane (red and blue dashed lines for a L−M and S-cone tuned neuron, respectively). PT is assumed to result from pooling across all responding neurons and is lower than the NT in either of the cardinal color directions. Due to probability summation, PTs trace out an arc in the interior of the square formed by the cardinal NTs. B: under the cardinal model, individual V1 neurons respond robustly to 1 of the 2 cardinal color directions but weakly to the intermediate color directions.