Abstract

The threshold increment spectral sensitivity for a foveally presented 0.5-s raised cosine having blurred edges exhibits a Sloan notch which coincides with the wavelength of the adapting field and is typically 0.6 log units deep on a 578-nm field of 0.10 log quanta deg⁻² s⁻¹.¹ In a previous abstract² it was reported that the long-wave side of the Sloan notch pairs of suprathreshold stimuli of different wavelengths could be found that were indiscriminable. We have repeated and extended those results. A 650-nm standard test stimulus 0.3, 0.5, or 0.7 log units above threshold was paired with a randomly selected, variable intensity light of another wavelength between 600 and 670 nm, and the observer was forced to select the standard. For two observers it was possible to obtain complete action spectra of lights that could not be distinguished from the standard at the noted units above threshold. Truncated indiscriminability spectra were also obtained on the midwave side of the Sloan notch. In all cases the indiscriminability spectra were identical in form with the threshold action spectrum. Because the Sloan notch action spectrum is manifestly cone-antagonistic, it can be concluded that the indiscriminable stimuli are rendered so not at the quantum catch level, but rather at a proximal site where the antagonistic signals converge.

© 1989 Optical Society of America