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Norma V. Graham, S. Sabina Wolfson; Complex channels become more complex: Modeling a contrast adaptation process. Journal of Vision 2006;6(6):694. doi: 10.1167/6.6.694.
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© ARVO (1962-2015); The Authors (2016-present)
We have found a new (at least to us) kind of contrast adaptation in which differences between contrasts near the adapted level are harder to perceive than differences between contrasts further away. In one experiment, the observer is briefly adapted to a grid of Gabor-patch elements at some contrast level (C0). We then probe the adapted state using a striped element-arrangement pattern where the Gabors in alternating rows (or alternating columns) change contrast level to C1 while the Gabors in the remaining rows (or columns) change to C2. If C1 is an increment in contrast relative to C0 (that is, if C1=C0+ΔC) and C2 is a decrement in contrast relative to C0 (that is, if C2=C0-ΔC), the orientation of the striped pattern is very hard to identify. However, the orientation is easy to identify if the same size difference (2*ΔC) does NOT cross C0 (for example, if C1=C0+ΔC1 and C2= C0+3*ΔC).
We can model this contrast-controlled adaptation process by inserting an additional linear filter (F2) and rectification-type nonlinearity (N2) into a complex channel (F1N1F3), resulting in a F1N1F2N2F3 structure (which we call a Buffy channel). F2 is a small blur circle, so the output of it is a measure of local contrast. N2 is adaptable: the zero-point of N2 adapts to be equal to the recent time-averaged contrast (at that position), so the output is (approximately) zero if the current contrast equals the recent contrast (at that position).
Predictions from Buffy channels agree with experimental results.
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