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Disentangling signal from noise in visual contrast discrimination

Presents a psychophysical method to discriminate between changes in signal and changes in noise as a source of variation in psychophysical sensitivity measures. The key feature of this method is that it permits the gauging of the noise factor across the sensory (i.e., internal response) continuum. 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 ( Delta R) evoked by a constant stimulus change ( Delta C) decreases with C (that is, the transducer R = f(c) displays a compressive nonlinearity), whereas the internal noise is independent of R; second, Delta R 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 were asked to report the presence of a short contrast increment presented on either one or two suprathreshold, 3 cycle-per-degree vertical Gabor patches (baselines or pedestals). For contrast discrimination, the results supported the first account above: human performance was limited by the contrast transducer nonlinearity and an almost constant noise. (PsycINFO Database Record (c) 2005 APA, all rights reserved)



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