Thursday, May 7, 2009
Northwest Hall (Chicago Hilton)
Background: Individuals with autism spectrum disorder (ASD) exhibit unusual performance in visual motion discrimination tasks. It is not clear whether the observed impairments are attributable to abnormalities in the early stages of visual processing or in later stages related to attention or decision-making. Hypothesizing that they are due to the latter, we measured reaction times and choices of 6 ASD-diagnosed and 7 typically developing adolescents in a 2AFC motion discrimination task. In this “self-paced” task, subjects fixated a central spot, presented together with a stochastic motion patch (8º diameter, centered 8º above fixation) of 250 ms and two response dots placed 8º to the left and right of the fixation spot. The direction of motion was equally likely to be rightward or leftward, and the strength of motion was randomly selected from 9 values between 0 and 40% coherence. Subjects were asked to fixate the central spot until they could judge the direction of motion, and to indicate their choice by moving their eyes to the response dot in the same direction as the judged motion. Subjects received auditory feedback about their choice after each trial. We measured saccades on each trial to examine the subjects' reaction times as well as their choices. We found that in comparison to control subjects, ASD-diagnosed subjects had higher thresholds for discriminating the direction of visual motion. In addition, their reaction times were shorter than those of control subjects. Moreover, in contrast to control subjects, the reaction times of ASD-diagnosed subjects did not vary with the strength of the motion signal. This suggests that ASD-diagnosed subjects may perform worse in motion discrimination tasks because of abnormal temporal control of the decision processes guiding the perceptual choice or controlling the behavioral response.
Objectives: To further investigate the role of temporal control in these effects on visual discrimination, we designed a “delayed-response” version of the task that controlled how long the subjects viewed the visual motion stimulus before giving their answer.
Methods: In this version of the task, subjects were instructed to maintain fixation on the central spot until the two response dots appeared (750 ms). The onset of the response dots indicated to the subjects that they should indicate their choice, in the same way as in the previous experiment.
Results: The ASD-diagnosed subjects performed better in discriminating the direction of visual motion in the “delayed-response” task (thresholds between 7 and 29%, 15% mean) than in the “self-paced” task (thresholds between 13 and 32%, 22% mean). Delaying the response time resulted in ASD-diagnosed subjects performing similarly to control subjects (thresholds between 9 and 28%, 16% mean).
Conclusions: These findings challenge the idea that the observed impairments in motion processing of ASD-diagnosed individuals are due to abnormalities in the early stages of visual processing, and instead suggest that the impairment is related to abnormalities in the temporal control of the decision process underlying the behavioral response.