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Sarah Elliott, John Werner; Age-related changes in contrast gain related to the M and P pathways. Journal of Vision 2009;9(8):1071. doi: 10.1167/9.8.1071.
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The mechanisms responsible for the age-related reduction in contrast sensitivity are not fully understood, with evidence for the importance of both optical and neural origins. Whether there are differential age-related changes in the magnocellular (M) and parvocellular (P) pathways across the lifespan has not been studied extensively, but differences may reveal a neural locus of contrast sensitivity loss. The pedestal-delta-pedestal and pulsed-pedestal paradigms were used to evaluate psychophysically the contrast gain signature of the M and P pathways, respectively, for younger (mean age of 22) and older (mean age of 81) observers. A four-square array was presented as an increment or decrement to the background for 35 msec, with one test square presented at a slightly higher or lower retinal illumination compared to the other three. Using a four-alternative forced choice procedure, the observer's task was to choose the unique square. The two paradigms differ only in the pre-trial adaptation and inter-stimulus array, with adaptation to a uniform field or adaptation to a pedestal four-square array, respectively. Stimuli were presented in Maxwellian view, and heterochromatic flicker photometry was used to equate the illuminance for each observer. The contrast gain slopes obtained with young observers are consistent with previous reports. An overall increase in threshold discrimination was found with increased age using the pulsed-pedestal paradigm, but the contrast gain signature was similar to that of the young observers. The pedestal-delta-pedestal paradigm revealed a moderately reduced contrast gain slope in older observers. Both pathways appear to undergo age-related sensitivity losses, but the contrast gain signature is altered only in an M pathway.
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