PSYCHOPHYSICAL SIGNATURES ASSOCIATED WITH MAGNOCELLULAR AND PARVOCELLULAR PATHWAY CONTRAST GAIN

Physiological data have revealed characteristic contrast gain and temp oral integration signatures of the magnocellular (MC) and the parvocel lular (PC) pathways. The goal in this study was to find psychophysical correlates of these signatures. Psychophysical forced-choice, luminan ce pedestal discrimination data were collected with a stimulus-surroun d display. A 2.05 degrees four-square stimulus array was varied from 7 3 to 182 trolands (Td) in a larger 115-Td surround. When the stimulus array was pulsed briefly, discrimination thresholds showed a minimum a t the surround retinal illuminance, increasing in a V shape when the s timulus array was incremental or decremental to the surround. When the stimulus array was presented continuously as a steady pedestal within the constant 115-Td surround, discrimination thresholds increased mon otonically with stimulus array retinal illuminance, obeying a slope of unity. Exposure duration variation showed temporal summation to exten d to longer durations for the pulse increments and decrements than for the steady pedestal condition. Discrimination thresholds for pulsed m edium-sized contrast steps showed the contrast gain signature and the temporal signature of the PC pathway. Discrimination thresholds for th e steady pedestal paradigm showed the temporal signature of the MC pat hway. Discrimination thresholds for small pedestal steps of the stimul us array from a steady pedestal showed the contrast gain signature of the MC pathway. The data suggested a difference in the spatiotemporal control of adaptation of the two pathways: The MC pathway adapted loca lly to the stimulus array, while the PC pathway showed little evidence of local adaptation. The experiments show that characteristic signatu res of MC-and PC-pathway processing can be demonstrated by use of psyc hophysical procedures. (C) 1997 Optical Society of America.