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Disentangling signal from noise in visual contrast discrimination

Abstract

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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Figure 1: Signal detection theory1 framework and the trial sequence.
Figure 2: Threshold increments versus contrast functions for four observers (different symbols).
Figure 3: Response criteria expressed in Z-scores of false alarms (ZFA) as a function of the sensitivity index, d′.
Figure 4: Criterion (ZFA) ratio and α-ratio as a function of the normalized pedestal-contrast ratio for all possible pairwise combinations of the four baseline contrasts used in the main experiment.

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Acknowledgements

We thank E. Ahissar, H. Barlow, D. Foster, C. Lorenzi, Z.-L. Lu, M. Morgan, A. Reeves, J. Solomon, J. Thomas and C. Tyler for their comments.

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Correspondence to Andrei Gorea.

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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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