INFLUENCE OF THE CONTRAST SENSITIVITY FUNCTION ON THE REACTION-TIME

The reaction time (RT) vs spatial frequency (SF) curve is determined, using gratings from 1 to 40 c/deg, at seven different contrast levels between 0.95 and 0.02. The form of the RT/SF function: (a) replicated the inverse of the contrast sensitivity function (CSF) at near thresho ld contrast levels; (b) behaved differently at higher contrasts, exhib iting two branches at contrast close to 1. The interpretation is that there are two factors determining this function: (1) the transition fr om the operation of fast transient channels at low SF to the operation of slow sustained channels at high SF, the transition taking place wi thin a narrow SF band close to 6 or 8 c/deg (depending on the subject) and (2) the contrast attenuation by the optical and neural transfer f unction, operating throughout the SF range. At high contrasts, the eff ect of the first factor can be clearly observed, because the effect of the second factor does not change with spatial frequency except in a region where the RT/SF function changes rapidly. At lower contrasts, h owever, the second factor becomes increasingly relevant while the firs t becomes less and less observable.