Abstract
Abstract: :
Purpose: To assess effect of spatial frequency (SF), temporal frequency, velocity, contrast and eccentricity on a speed discrimination task, using simultaneously presented grating stimuli. Method: Experimental subjects were presented with a pair of drifting, vertically oriented gratings. The task was to determine the faster moving grating. One of the standard grating moved at a set reference speed and the other, the test, was varied using a staircase procedure. Each staircase was made to terminate after 6 reversals were obtained. Speed discrimination threshold was estimated as the mean of the last four reversals. The stimulus was generated on a monitor using the Cambridge vision system VSG2/5. The test distance was of 1.5 m. Speed discrimination thresholds were assessed for 3 speed conditions (2,4 and 8 deg/sec), for different spatial frequencies (0.1-16cycles/deg), different contrast levels (100%-20%), for free viewing and for an eccentricity of asymp 4 degrees from the center of fixation. Results: The test revealed an optimal spatial and temporal frequency for which optimal thresholds were obtained. The optimal SF shifted to higher SF as the speed of target increased. For the grating moving at 2 deg/sec and 4 deg/sec the optimal SF was found to be around 1 cycle/deg while for the 8deg/sec grating velocity the optimal SF was found to be around 2 cycles/deg. Results also showed that the Weber fractions were lower for low SF gratings at 8deg/sec then for low SF grating moving at 4 or 2 deg/sec. Test results was also compared for a free viewing task and for gratings presented asymp 4 degree from the center of fixation. Results show that subjects did comparatively better or similar to that of free viewing condition for low SF targets and significantly worse than free viewing condition for high SF gratings, for all speed conditions. Contrast of the grating was varied from 100% to 20%. Speed discrimination tasks were not affected by changes in contrast for grating with spatial frequency in the optimal range. Conclusion: The results strongly support with the information processing of the magnocellular pathway. Magnocellular pathway processes information dealing with motion and are sensitive to low SF and high temporal frequency. This results in the better performance for low SF grating with high temporal frequency as seen in our study. The same is true for better test performance obtained for low SF presented at the periphery (4 degree from fixation) and a decrease in the test performance for high SF. This study has implication for developing a speed discrimination test that can be used as a part of battery of vision tests for assessing human driving performance.
Keywords: motion-2D • spatial vision • physiological optics