Human ability to detect stimulus changes (ΔC) decreases with increasing reference level (C). Because detection performance reflects the signal-to-noise ratio within the relevant sensory brain module, this behavior can be accounted for in two extreme ways: first, the internal response change ΔR evoked by a constant ΔC decreases with C (that is, the transducer R = f(C) displays a compressive nonlinearity), whereas the internal noise is independent of R; second, ΔR is constant with C but the noise level increases with R. A newly discovered constraint on human decision-making helps solve this century-old problem: in a detection task where multiple changes occur with equal probabilities, observers use a unique response criterion to decide whether a change has occurred. For contrast discrimination, our results supported the first account above: human performance was limited by the contrast transducer nonlinearity and an almost constant noise.
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Gorea, A., Sagi, D. Disentangling signal from noise in visual contrast discrimination. Nat Neurosci 4, 1146–1150 (2001). https://doi.org/10.1038/nn741
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