Threshold and mean response time taken from psychophysical optic flow motion experiments to determine eccentricity discrimination and contrast sensitivity in adults with autism compared to control group.
In Eccentricity task: threshold, is the minimum eccentricity angle measured by degrees from the central heading line. It indicates that participant was able to identify accurately the direction of heading of optic flow motion. A higher threshold means low sensitivity. This threshold was measured as a dependent value for each participant at each parametric value using adaptive staircase method. In Contrast task: threshold presents the minimum dots luminance that participant was able to identify accurately the direction of optic flow motion. Different contrast levels were achieved by changing the luminance of dots with respect to the uniform gray background using an adaptive staircase method. Higher threshold also indicate low sensitivity. Response time, is the time taken for the participants to response to the direction of optic flow motion at each parameter level and in both tasks. Time was taken from the stimulus end until the participant press the keypad to response. Time input is in millisecond.
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Stimuli were computer generated using the psykinematix software and displayed on a gamma corrected MacBook Pro-laptop (15.4-inch, 2880 x 1800 pixels, 60 Hz refresh rate). We used a random dot kinematogram to design the optic flow motion. Motion quality was degraded in two experiments: (1) Eccentricity threshold measurement: dot contrast was held constant at 100% and eccentricity angle was varied. OF direction randomized in two directions (right , left) to measure the minimum eccentricity angle. (2) Contrast sensitivity: here, the OF angle was fixed above threshold and direction of heading randomized to right and left of a fixation line. The Michelson contrast of the dots were varied to home in on threshold. In both tasks sensitivity for direction detection (inverse of thresholds) was measured as a function of stimulus speed and number of dots. Both parameters had two values, dots speed (4, 10 deg/s), and number of dots (15, 80 dots) and they were all randomized throughout the tasks. The stimuli were presented within a borderless gray square window (19º × 19º) presented at the center of the display. The dots were white and subtended (0.118°) with limited lifetime (200 ms) and updated after 18 frames. The OF motion moved inward to the right or to the left of a center line. The fixation line appeared before the presentation of the stimuli and disappeared during the optic flow motion allowing the perception of heading of direction of the dots only by processing the global motion. The stimulus was presented for (500 ms) followed by an interstimulus interval time of 500 m), and viewing distance was 56.5 cm. In both conditions, participants indicated whether the direction of optic flow motion was heading towered the right or the left of the center line as soon as the stimuli disappear. All participants had to complete 2 blocks with two different speed setting and two different dot densities, all of which randomized at 50% for each condition. Response time (RT) was collected throughout the experiment as the time taken by the subject to respond to stimulus.