Discrimination and detection thresholds: the effect of observer criterion on the spatial properties of chromatic and achromatic mechanisms.

Spectral sensitivity functions were determined for structured test targets presented on a white background field. Data obtained for threshold detection of the targets are consistent with results obtained from previous studies which used unstructured test fields showing functions, with three maxima, dependent on opponent colour mechanisms. Data obtained from threshold discrimination measurements show a marked reduction in the blue sensitivity when 1 degree test targets are used. This effect decreases significantly when larger targets are used. A change of the observer's criterion can significantly alter the relative contributions of the achromatic and chromatic channels at threshold.