Low spatial frequency channels and the spatial frequency doubling illusion

¹ DEPT. OF BIOMEDICAL SCIENCE, FACULTY OF ALLIED HEALTH SCIENCES, UNIVERSITI KEBANGSAAN MALAYSIA, KUALA LUMPUR, Wilayah Persekutuan, Malaysia; ARC CoE in Visual Sciences, RSBS, CVS, Australian National University, Canberra, Australia
² ARC Centre of Excellence in Vision Science, Centre for Visual Sciences, The Research School of Biological Sciences, The Australian National University, Canberra ACT, Australia; ARC CoE in Visual Sciences, RSBS, CVS, Australian National University, Canberra, Australia
³ ARC CoE in Visual Sciences & CVS, Australian National University, Canberra, Australia; ARC CoE in Visual Sciences, RSBS, CVS, Australian National University, Canberra, Australia

^* To whom correspondence should be addressed. E-mail: ted.maddess{at}anu.edu.au.

	Abstract

Purpose: This study examined the number and nature of spatio-temporal channels in the region where the frequency-doubling (FD) illusion would be expected to occur at 8 locations spanning the central 30 degrees of the visual field. Methods: 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 7 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 2 Alternative Forced Choice method. Thirteen subjects were examined for stimuli presented at contrast 0.95. Three or 4 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 spatio-temporal tuning of neural channels present. Results: 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 for different subjects. The Factor Scores showed differential distribution of the Factors onto the 8 different visual field locations. Thus the expression of the 3 channels differed somewhat across the visual field. Conclusions: The results support earlier reports, that several low spatial frequency channels exist below 1 cpd in the periphery. The results may have implications for the FDT and Matrix perimeters.

Key Words: psychophysics, spatial vision, temporal modulation, visual field