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Robbe Goris, Felix Wichmann, Bruce Henning; A neurophysiologically plausible population-code model for human contrast discrimination. Journal of Vision 2009;9(8):1004. doi: https://doi.org/10.1167/9.8.1004.
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© ARVO (1962-2015); The Authors (2016-present)
The pedestal effect is the improvement in the detectability of a sinusoidal grating in the presence of another grating of the same orientation, spatial frequency and phase - usually called the pedestal. The pedestal, or “dipper” effect, as it is sometime called, has typically been attributed to contrast transduction and/or gain-control mechanisms operating within a single spatial-frequency-tuned channel. Recent evidence casts doubt on the single-channel assumption: the pedestal effect disappears in the presence of notched noise. Notched noise forces observers to use information near the spatial frequency of the grating they are trying to detect and thus prevents their using information carried in channels tuned to spatial frequencies that are much different from the signal frequency. Consequently, the disappearance of the pedestal effect in notched noise suggests that the pedestal effect stems from off-frequency looking. Here we consider a network consisting of units whose contrast response functions resemble those of the cortical cells believed to underlie human pattern vision and demonstrate that, when the outputs of multiple channels are combined by simple weighted summation - one implementation of off-frequency looking - the network produces contrast-discrimination data consistent with psychophysical observations: the pedestal effect is present without noise and in broadband noise, but almost disappears in notched noise. One important implication of these findings is that many channels with a wide heterogenity in spatial-frequency tuning are engaged even in processing low-contrast sinusoidal gratings.
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