Cortical magnification and peripheral vision

Abstract

In a generalized form, the cortical magnification theory of peripheral vision predicts that the thresholds of any visual stimuli are similar across the whole visual field if the cortical stimulus representations calculated by means of the cortical magnification factor are similar independently of eccentricity. Failures of the theory in spatial vision were analyzed, and the theory was tested with five visual acuity tasks and two hyperacuity tasks. Almost all increases in thresholds with eccentricity were explained by the theory in five of these tasks, which included the two-dot vernier hyperacuity test, the measurement of visual acuities with gratings, the Snellen E test, and two acuity tests that required either separation between dots or discrimination between two mirror-symmetric forms. The two-dot vernier thresholds could be explained as a special case of orientation discrimination, and orientation discrimination at different eccentricities was in agreement with the cortical magnification theory. The increase of thresholds in peripheral vision was larger than predicted by the theory in the Landolt visual acuity and bisection hyperacuity tests, possibly because of retinal undersampling.

Contrast thresholds for identification of numeric characters in direct and eccentric view

“…The linear extent in millimeters of visual cortex corresponding to each degree of the visual field is called the cortical magnification factor M. Anatomical studies show that the inverse of M decreases approximately as a linear function of retinal eccentricity (Cowey & Rolls 1974;Daniel & Whitteridge, 1961;Van Essen et al, 1984). Scaling the size of peripherally presented targets by the inverse of M has been proposed to compensate for the lower peripheral performance (Rovamo, Virsu, & Näsänen, 1978;Virsu et al, 1987;cf. Koenderink et al, 1978, p. 854).…”

“…This group includes several hyperacuity measures, such as vernier acuity (Bourdon, 1902;Hering, 1899;Levi, Klein, & Aitsebaomo, 1985;Weymouth, 1958), single-line vernier (Westheimer, 1982) and two-dot vernier (Westheimer, 1982; but see Virsu et aL, 1987Virsu et aL, , p. 1574, as well as contrast of small uniform fields (Harvey & Poppel, 1972, or Poppel & Harvey, 1973, apparent movement of counterphased gratings (Hilz, Rentschler, & Brettel, 1981), stereoacuity (Fendick & Westheimer, 1983), grating orientation (Spinelli, Bazzeo, & Vicario, 1984; but see Virsu et al 1987Virsu et al , p. 1574, numerosity judgment (Parth & Rentschler, 1984), bisection of a straight line (Levi & Klein, 1986;Virsu et al 1987), Landolt acuity (Virsu et al, 1987), pattern symmetry (Rentschler & Treutwein, 1985;Saarinen, 1987), spatial phase quantization sensitivity (Harvey, Rentschler, & Weiss, 1985), masking by spatially correlated noise (HUbner, Rentschler, & Encke, 1985), and localization (Burbeck & Yap, 1990). (For partial reviews see Pointer, 1986 andVirsu et al, 1987. ) Since the beginning of this century, it has been known that the readability of character groups cannot easily be deduced from individual character recognition (Wagner, 1918; for an overview of the older literature see Townsend, Taylor, & Brown, 1971).…”

Size scaling and its effect on letter detection

“…M-scaling has been found to be effective in a variety of studies (Bijl, Koenderink, & Kappers, 1992;Kitterle, 1986;Levi, Klein, & Aitsebaomo, 1985;Virsu et al, 1987). In general, the findings indicate that, compared with the processing ofconstant-sized (N-scaled) stimuli, processing of scaled stimuli shows a minimal effect of presentation location.…”

“…Kitterle (1986) reports that M-scaling equates visibility in brightness and color discriminations, but for spatial tasks such as vernier thresholds, phase discriminations, and detection of sinusoidal gratings, performance is better with foveal than with peripheral stimuli. Virsu et al (1987) tested the application of cortical magnification theory to seven visual tasks and found a slight foveal superiority in everyone of them. Moreover, foveal/peripheral processing differences were found to increase slowly with eccentricity.…”

Deviations from strict M scaling

“…They proposed an elegant hypothesis that predicts equal thresholds for visual stimuli across the visual field if the calculated cortical representations of these stimuli are equivalent. This hypothesis, which was later called the cortical magnification theory of peripheral vision, 6 has been verified for a wide variety of psychophysical tasks, although failures have also been reported (for an overview see, e.g., Pointer 7 and Virsu et al 6 ; also see…”