speed discrimination results in ASD compared to TD group using drifting grating stimuli

Published: 29-03-2019| Version 1 | DOI: 10.17632/pkgf4kbw99.1
Asmaa Bakroon


This data represent the threshold of speed discrimination performance in participants with ASD compared to control group. this threshold was varied upon the change of spatial and temporal parameter of a drifting grating stimuli. when drifting grating stimuli moved at fast speed for short duration, ASD group showed an elevation in speed-discrimination threshold similar to the TD group. However, for long presentation, the ASD performance was enhanced in response to slow speed compared to TD group, for which task information was maintained for an extended period of time. Yet, these threshold differences were not statistically significant with any of the study parameters. We also compared the time taken for each participant to response to the stimulus. the results showed that both groups responded similarly to the increase/decrease of stimuli duration more than the speed parameter. However, an overall average RT shows no statistical differences in the group performance for any of the stimulus parameter in spite of the fact that enhanced RT was observed in the autism group when slow speed stimuli were presented at long duration.


Steps to reproduce

The stimuli consisted of a pair of gratings with a spatial frequency 2 cycle/ degree, oriented vertically and drifting perpendicular to the direction of orientation. The stimulus occupied 4.08 deg of visual angle at a viewing distance of 1m. Stimulus contrast was kept at 100%, and both stimuli were presented in two circular windows. To generate the drifting grating; where the temporal envelop was a hybrid- Gaussian, where edges are half-Gaussians and the central portion is set to maximum contrast. Fine temporal precision was obtained by adjusting the SD of half-Gaussian edges, and transferring “excess” contrast to the flat central portion. This hybrid envelope allowed fine temporal precision of brief stimuli and avoided protracted fade-in/fade-out periods associated with prolonged temporal Gaussians. Each circle of the drifting subtended 0.5° radius at the viewing distance of one meter and separated from each other by 2.5°. A fixation cross was presented midway between the two apertures. The subject had to fixate binocularly on the fixation cross, then cross mark disappear to prevent image after-effect, and to allow perception of the moving drifting. Two reference speeds were used: 2deg/sec and 6 deg/sec, hence, the stimulus had two temporal frequencies of 4 Hz and 12Hz, (temporal frequency equals spatial frequency times speed). The two drifting grating were presented simultaneously for either 500ms or 250ms. The inter-stimulus interval between trials was set to 500 ms, during which time participants had to respond. The task was conducted in a dark room, with only a side light ( room luminance ~ 0.56 cd/m2). The observer’s task was to indicate which of the two moving gratings presented simultaneously was faster. One of the drifting grating circle was the “standard”, which moved at the reference speed of 2 deg/sec or 6 deg/sec (depending on the block of trials that was run). The other grating “test” drifted at speeds controlled by an adaptive staircase method. Both standard and the test grating were randomized in position throughout the task. A 3 down / 1 up staircase controlled the speed differences between trials to home in on threshold. At the beginning of each trial the test grating speed was 75% faster than the standard grating, and then decreased by 25% after three consecutive hit responses. However, around threshold the step-size was set to decrease by 10 % after 3 consecutive hits, and increased by 5% after one miss response. The estimated performance criteria based on the relative step size of up/ down, and increasing/ decreasing ratio of speed discrimination task was equal to 87.5% based on the criterion equivalent estimation calculated by the software. Eight reversals were collected before the staircase terminated and the threshold was taken as the mean of the last six reversals.