Increment spectral sensitivities for spatial periodic grating patterns: evidence for variable tuning of the chromatic system.

Increment spectral sensitivities were measured for spatial periodic grating patterns. First, the increment threshold was determined as a function of wavelength, for various spatial frequencies and white-background intensities. Second, the additivity of test mixture was tested. Thirdly, the threshold vs intensity curves were determined for various spatial frequencies of test monochromatic stimuli. Finally, the increment threshold was determined as a function of spatial frequency. The following results were obtained: the background intensity and the spatial frequency affected the increment spectral sensitivity functions in different manners; the result of test mixture showed subadditivity for high background intensity and approximately linear additivity for low background intensity; the resultant t.v.i. curves converged toward Weber's law at high background intensity; and the sensitivity vs spatial frequency curve showed a loss of the sensitivity at low spatial frequency for high background intensity. These results called in question the existing hypothesis of separate chromatic and achromatic systems whose spectral tunings are invariant with a spatial parameter, but favored the hypothesis of variable tuning of the chromatic system.