Electrophysiological and psychophysical measures of spectral sensitivity obtained with an alternating barred pattern

The spectral sensitivity of the human visual system was determined through phase alternation of a barred pattern. Psychophysical determinations of absolute and increment threshold were obtained by the method of limits under six different stimulus conditions. It was found that curves derived from stimulating with the barred pattern were quite similar to those elicited with more conventional techniques. When the barred pattern was horizontally alternated so that the bars were displaced a single bar width at a rate of 10 Hz. it was found that there was a slight (approximately 0.3 log units) decrease in sensitivity. A stimulus pattern subtending 20° of visual angle superimposed upon a 300 td. background was chosen to attempt the determination of photopic spectral sensitivity using the amplitude and latency of the visual evoked cortical response (VECP). Curves were derived from 13-14 wavelengths selected from across the spectrum (440-660 nm.) by means of criterion amplitude and latency measures. The resulting functions were compared to psychophysical data obtained during the electrophysiological recording sessions and to curves reported by other investigators. The electrophyslologically obtained curves were found to be photopic in nature with an increase in sensitivity in the long-wavelength end of the spectrum. This lowered threshold for red took the form of a shift in the peak sensitivity for one subject and a prominent submaximum in the curves of the other. The relationships between some of the other parameters of the stimulus complex and the VECP were also examined. Increasing rate of alternation, increasing background luminance and decreasing area of the barred pattern were found to yield reductions in VECP amplitude. Size of the grating (bar width) and stimulus radiance were found to bear curvilinear relationships to VECP amplitude. It was concluded that phase alternation of a barred pattern in appropriate circumstances yields spectral sensitivity curves which are photopic in nature. Psychophysical curves agreed well with the C.I.E. photopic standard. Curves derived from latency and amplitude measures of the VECP were in general agreement with the electrophysiological results of other investigators. It also appeared that a number of other characteristics of foveal receptors could be successfully isolated using this technique.