Purchase this article with an account.
Rick Peterson, William McIlhagga; In search of the early nonlinearity: no luck yet. Journal of Vision 2003;3(9):355. doi: 10.1167/3.9.355.
Download citation file:
© ARVO (1962-2015); The Authors (2016-present)
Most models of contrast discrimination consist of a linear filtering stage followed by a nonlinearity and a decision rule. It may be, however, that a local nonlinearity precedes the linear filtering stage. Kingdom and Whittle (1994) suggested an early decelerating nonlinearity (e.g. cone compression) to explain contrast discrimination for low spatial frequency gratings, where discrimination thresholds at high pedestal contrast were lower than at medium pedestal contrasts.
Here we report on an attempt to find direct evidence for such a nonlinearity. We looked for a nonlinearity by measuring the detection threshold of a thin vertical line (1′ arc gaussian cross-section) superimposed on a 1 cycle/degree high-contrast (80%) vertical sinusoidal grating. The grating could be at 0 degrees phase relative to the line (the line appeared at the grating peak) to 180 degrees phase (the line appeared at the grating trough). If contrast discrimination data can be explained by a local nonlinearity, we would expect the line detection threshold to be smallest at 180 degrees phase, rising smoothly to a maximum at 0 degrees phase.
Instead, we found the smallest thresholds when the sinusoid was placed at 45 and 135 degrees phase. This pattern of thresholds is not consistent with a simple early nonlinearity. It may be explained by assuming there are two mechanisms, which measure contrast from two operating points at about 50% and −50% contrast (for an 80% grating), and each of which follows Weber's law for contrast discrimination. These mechanisms may be the retinal on and off channels, and the two-mechanism model may also explain Whittle's (1986) data on increment and decrement contrast discrimination.
KingdomF.A.A.WhittleP(1994) Contrast Discrimination at High Contrasts Reveals the Influence of Local Light Adaptation on Contrast Processing, Vision research 36 817–829
WhittleP(1986) Increments and decrements: Luminance discrimination, Vision esearch 26 1677–1691.
This PDF is available to Subscribers Only