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MiYoung Kwon, Fang Fang, Allen M.Y. Cheong, Gordon Legge, Sheng He; The impact of prolonged contrast reduction on visual contrast coding. Journal of Vision 2007;7(9):358. doi: 10.1167/7.9.358.
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How does prolonged reduction in retinal-image contrast affect visual-contrast coding? Rubin and Legge (Vision Research, 29, 79–91, 1989) proposed that vision through cataracts and some other ocular-media opacities can be modeled by a contrast-reducing “filter” in front of an otherwise normal visual system (contrast attenuation model). Recent evidence, however, indicates that some forms of long-term visual deprivation result in compensatory perceptual and neural changes in the adult visual pathway. The goal of our study is to investigate the impact of prolonged contrast reduction on contrast coding with both behavioral and neuronal measures. If long-term contrast reduction results in a form of neural compensation, we might expect to see improvement in behavioral contrast discrimination and an increase in the gain of neuronal contrast responses compared with the unadapted condition. In order to test this hypothesis, normally sighted participants adapted by viewing the world through contrast reducing goggles (approximately three fold reduction of image contrast) for four hours. During the adaptation period, participants went about their usual daily activities. Participants' contrast-discrimination functions (psychophysics) and neuronal contrast response functions (fMRI BOLD) were measured before and after adaptation. Data from two participants showed that after adaptation, there were significant decreases in contrast discrimination thresholds (improved contrast discrimination) and increased BOLD responses in the early visual cortical areas of the brain (V1 and V2) compared to unadapted values. There was also evidence that these changes in contrast coding were more pronounced for contrasts below about 30%, i.e., within the range of contrasts available during adaptation. These findings indicate that the early visual pathway adapts to prolonged exposure to a reduced-contrast world. The adaptation appears to be compensatory, such that the precision of contrast coding is improved for low retinal-image contrasts.
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