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Batsheva Hadad, Sivan Schwartz, Orit Nizri, Nof Harel; Atypical Basic Psychophysics in autism: Violation of Weber's law in vision and haptic. Journal of Vision 2018;18(10):35. doi: https://doi.org/10.1167/18.10.35.
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© ARVO (1962-2015); The Authors (2016-present)
Perceptual atypicalities are widely acknowledged but poorly understood features of autism. The underlying assumption in prevailing models is that atypicalities result from reduced top-down influences, but sensory processing is intact. Changes in bottom-up factors, if they exist at all, are considered quantitative, mostly involving changes in noise levels. Consequently, testing has been limited to mid- and high-level processes, with little consideration of potential underlying constraints of basic sensory-perceptual processing. We show here that the widely-documented modulated context effects in autism reach deeper than hitherto suspected, with reduced inferential perception in stimulus encoding, during which the system constructs and continuously updates a generative model of the sensory inputs it receives. Specifically, we tested the adherence of vision and haptic to Weber's law, a fundamental principle of transient plasticity, whereby the output of processes depends not only on the absolute change but also on its calibration based on the immediate standard stimulation. According to Weber, sensitivity along intensities changes based on a rule of DI/I=C, where DI is the increase in intensity to a stimulusI that is required to produce detectable changes. Weber fractions (C) should thus remain constant. We measured JNDs for size visual judgments (Exp. 1), and for weight haptic discrimination (Exp. 2), based on the best-fitting individual psychometric functions. Results for the TD group confirmed Weber's law, demonstrating a linear increase in JNDs with intensity, resulting in constant fractions (DI/I) across intensities. The results for ASDs, in contrast, showed no scaling of JNDs with intensities; instead, fractions decreased linearly with intensities. In a striking contrast to its consistency in typical perception, Weber's law does not hold in visual and haptic perception in autism. This general, low-level altered mechanism may account for atypical perception demonstrated in higher-level processing and for sensory symptoms in autism.
Meeting abstract presented at VSS 2018
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