Using the equivalent-background transformation, Crawford has demonstrated the equivalence of adaptive states (dark and light adaptation) in scotopic vision. The critical feature of this transformation is that the threshold sensitivity of the eye at a particular time in the dark can be specified in terms of the steady-state, light-adapted condition necessary to produce the same threshold for all target diameters. This indicates that a single variable may control the spatial integration of light. Equivalence was investigated in the present study for long-term photopic dark adaptation using two target diameters (0.13° and 0.33°) and four chromatic combinations of red and green test and adapting stimuli (red on red, red on green, green on green, and green on red). Except under special conditions employing color-defective observers for which there was more complete isolation of cone mechanisms, equivalence between adaptive states was generally found not to exist. It is concluded that more than one process is needed to control the spatial integration of light in the photopic system, and these are hypothesized to be the wavelength-dependent excitatory and inhibitory processes of the visual receptive field.
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