Low-Spatial-Frequency Channels and the Spatial Frequency-Doubling Illusion

Abstract

This study examined the number and nature of spatiotemporal channels in the region where the frequency-doubling (FD) illusion would be expected to occur at eight locations spanning the central 30 degrees of the visual field. The probability of seeing the FD illusion was examined in 17 subjects. Stimuli were presented at 5 octaves of low spatial frequencies, at each of seven flicker frequencies in the range 5.65 to 27.95 Hz. In a single trial, subjects matched the apparent spatial frequency of the flickering test pattern using a two-alternative, forced-choice method. Thirteen subjects were examined for stimuli presented at contrast 0.95. Three or four subjects were examined at each of the contrasts 0.2, 0.4, and 0.8. A factor analysis was conducted on the psychometric functions, quantifying the number and possible spatiotemporal tuning of neural channels present. At contrast 0.95, three factors were able to explain 79.3% of the total variance in the psychometric responses to the 35 test conditions. This simple form of three broad spatiotemporal channels was also found at the other contrasts and in different subjects. The factor scores showed differential distribution of the factors onto the eight different visual field locations. Thus the expression of the three channels differed somewhat across the visual field. The results support earlier reports, that there are several low-spatial-frequency channels below 1 cyc/deg in the periphery. The results may have implications for the FDT and matrix perimeters.