Atypical Basic Psychophysics in Autism: Violation of Weber’s Law in Vision and in Haptic

Background: Perceptual atypicalities are increasingly invoked as contributory causes of the fundamental characteristics of Autism Spectrum Disorders (ASD). The mechanism underlying these alterations remains unclear, as the literature mostly focuses on high-level perceptual processes without much recourse to potential underlying constraints of basic sensory-perceptual processing.

Objectives: Reduced perceptual inference by which processing is more sensitive to the absolute metrics of the environment has been generally demonstrated in ASD. However, despite the potential role of elementary inferential encoding of stimuli in this altered perception in autism, sensory and perceptual calibration, based on immediate stimulation, has hardly been tested. To characterize this basic psychophysics in autism we tested adherence to Weber, a principle subserving one of the most fundamental function of transient plasticity, where the output of perceptual processes depends not only on the absolute change but also on its calibration based on the immediate standard stimulation. Sensitivity along stimulus intensities changes according to a rule of DI/I=C, where DI is the increase in stimulus intensity to a stimulus of intensity I that is required to produce a detectable change in intensity. C (Weber fraction) should thus remain constant. We tested the adherence to this principle in both vision and haptic.

Methods: JNDs for size judgments were tested in the visual domain in Exp. 1 (20 high-functioning ASD and 20 TD), and JNDs for weight discrimination were tested in the haptic domain in Exp. 2 (12 high-functioning ASD and 11 TD). JNDs were calculated based on the actual psychometric functions of each individual, for each condition.

Results:

Exp. 1. Weber’s fraction (DI/I) computed for each disc size, for each subject, varied with disc size for the ASD but not for the TD group, F(3,135) = 3.31, p < .02, η²_p =.07. The fractions for the autistic group, which decreased linearly with disc size, F(3,75) = 6.93, p < .0001, η² =.22, demonstrated violation of Weber. In a clear contrast, these fractions remained fairly constant across disc sizes for the TD group, F(3,60) = 2.63, p > .10, η² =.09, demonstrating the expected adherence to Weber.

Exp. 2. As in vision, a significant interaction between group and weight was found, F(3, 69) = 2.62, p<0.03, η2p =.13, and further analysis of this interaction confirmed Weber law, with fractions remaining constant across the different weights for the TD group, F<1. For the ASD group, in contrast, Weber’s fraction again decreased linearly with increased weight, F(3, 36)=2.69, P<0.06, η2=.2.

Conclusions: In a striking contrast to its consistency in typical perception, results for the ASD group show that Weber’s law does not hold in visual and haptic perception in autism. The results identified a general mechanism underlying alterations in basic psychophysics that may account for sensory symptoms in autism, and for the atypical perception demonstrated for higher level perceptual processing.