The impulse response of an S-cone pathway

Purpose: We sought to measure the chromatic impulse response function (IRF) of an isolated human S-cone pathway. Previously measured chromatic IRFs were obtained using short-wave stimuli to which not only S-cones, but M- and/or L-cones may have been sensitive. Method: 10 normal trichromats served as subjects. Their IRFs were measured with a double-pulse method in which the pulses were chromatically modulated at constant luminance (based on 18 Hz heterochromatic flicker photometry). Chromatic stimuli were presented as a Gaussian patch (1 SD = 2.3 deg) in one of four quadrants around a central fixation cross on a CRT screen. Each of the two pulses (6.67 ms) was separated by an interstimulus interval from 20 to 360 ms. Chromaticity of the pulses was changed from the equal-energy white of the background to a bluish color along individually determined tritan lines (based on color matching under strong S-cone adaptation from a 420 nm background superimposed in Maxwellian view). Chromatic detection thresholds were determined by a four-alternative forced-choice method with staircases for each SOA in one session. Measurements were repeated in at least four sessions for each observer. Results and Discussion: IRFs were calculated by varying four parameters of an exponentially-damped sinewave (Burr and Morrone, JOSA A 10, 1993). S-cone IRFs have only an excitatory phase and a much longer time course compared with luminous IRFs measured by the same apparatus and on the same observers. The duration of S-cone IRFs is ∼200 ms whereas the duration of the excitatory phase of luminous IRFs is ∼40-60 ms. Individual differences in the S-cone IRF are much greater than for luminous IRFs.