Psychophysical estimate of extrafoveal cone spacing

Abstract

In the extrafoveal retina, interference fringes at spatial frequencies higher than the resolution limit look like twodimensional spatial noise, the origin of which has not been firmly established. We show that over a limited range of high spatial frequencies this noise takes on a striated appearance, with the striations running perpendicular to the true fringe orientation.A model of cone aliasing based on anatomical measurements of extrafoveal cone position predicts that this orientation reversal should occur when the period of the interference fringe roughly equals the spacing between cones, i.e., when the fringe spatial frequency is about twice the cone Nyquist frequency. Psychophysical measurements of the orientation reversal at retinal eccentricities from 0.75 to 10 deg are in quantitative agreement with this prediction. This agreement implies that at least part of the spatial noise observed under these conditions results from aliasing by the cone mosaic. The orientation reversal provides a psychophysical method for estimating spacing in less regular mosaics, complementing another psychophysical technique for measuring spacing in the more regular mosaic of foveal cones [D. R. Williams, Vision Res. 25, 195 (1985); Vision Res. (submitted)].

Cone spacing and the visual resolution limit

“…A brief description of this apparatus and the modifications required for these experiments can be found in Ref. 6. The test field consisted of a 2-deg disk of 632.8-nm coherent light at 500 trolands (Td).…”

“…Attempts to relate visual acuity to the anatomical spacing of photoreceptors have treated the Nyquist frequency of the cone mosaic as a theoretical upper bound on the visual resolution of the entire visual system. 3 -5 Coletta and Williams 6 introduced a psychophysical technique for estimating cone spacing outside the fovea, complementing another technique for measuring cone spacing in the living fovea. 7 ' 8 These techniques make possible a comparison of cone spacing with measurements of visual resolution in the same retinal locations of the same observers.…”

In vivo investigation of human cone photoreceptors with SLO/OCT in combination with 3D motion correction on a cellular level

“…While in the first two frames no ring can be observed a decrease of the radius of the ring can be observed in the rest of the movie. The maximum of the ring corresponds to the spatial frequency of the cone rows or modal frequency of cone mosaic [26,27]. This frequency has to be multiplied by a factor (1/cos 30°) to convert the modal frequency into closest neighbor spacing.…”

The effects of temporal modulation and spatial location on the perceived spatial frequency of visual patterns

“…The available comparison sinusoidal gratings ranged from 0.63 to 6.17 cpd in equal log-unit steps of 0.004 and were always stationary. (These spatial frequency values are low enough that, even at 8°in the superior visual field, no aliasing of visual patterns should occur ;Coletta & Williams, 1987. ) Contrast for all stimuli is defined as follows:…”