Human peripheral spatial resolution for achromatic and chromatic stimuli: limits imposed by optical and retinal factors.

Abstract

SUMMARY1. The aim of this study was to determine whether optical, receptoral or higherorder neural properties limit spatial resolution (acuity) in human vision, especially in the peripheral regions of the visual field.2. Both achromatic and chromatic stimuli were used, and measures were taken to ensure that the resolution estimates were not contaminated by the detection of spatial sampling artifacts. Spatial contrast sensitivity functions were measured at retinal locations from 0 to 55 deg along the naso-temporal meridian for: (i) discriminating the direction of drift of luminance-modulated (black-white) sinusoidal stimuli drifting at 8 Hz (achromatic task); and (ii) for detecting isoluminant red-green sinusoidal stimuli drifting at 0-4 Hz (chromatic task). Achromatic contrast sensitivity functions were also measured along the vertical meridian for eccentricities of 8 and 40 deg. Each achromatic function was extrapolated to a contrast sensitivity of one (100 % contrast) to estimate achromatic acuity. Chromatic acuities were obtained by expressing chromatic contrast in terms of cone contrasts and using the same method of extrapolation. We compared the results with recent data on human optical properties and retinal anatomy.3. Both achromatic and chromatic acuity decline with distance from the fovea, but at a faster rate than that dictated by the known optical and/or receptoral properties of the human eye. We conclude that, for stimuli -of either achromatic or chromatic contrast, peripheral spatial resolution is limited by post-receptoral mechanisms. Also, chromatic acuity declines more steeply than luminance acuity with eccentricity suggesting that there are additional post-receptoral limitations on colour resolution in the periphery.4. A clear naso-temporal asymmetry is seen in the resolution whose dependence is qualitatively, but not quantitatively, similar to the Nyquist limits imposed by the asymmetric density of human retinal ganglion cells. We discuss the possibility that

Efficiency in detection of isoluminant and isochromatic interference fringes

“…Despite similar experimental conditions and techniques, the data from Mullen'? and Anderson et al 33 differ from each other by -0.5 log unit at high spatial frequencies. The reasons for this difference are unknown.…”

“…Second, avoiding blurring allows us to extend our red-green-isoluminant contrastsensitivity measurements to 20-27 cycles per degree (c/deg), depending on the observer, substantially higher than has been measured previously. [15][16][17][28][29][30][31][32][33] This allows us to compare performance for isochromatic and isoluminant stimuli at high spatial frequencies, which is the focus of this paper.…”

Why rods and cones?

“…[1][2][3] On the other hand, our spatial acuity in the peripheral field is very low, despite being cone mediated. As shown by Anderson et al, 4 this has nothing to do with poor optics, because the off-axis optical quality is remarkably good, and nor indeed is it a direct reflection of the low density of cones in the periphery. Instead, this poor spatial acuity in the periphery results from the pooling by peripheral retinal ganglion cells of photopic signals across extensive retinal areas (see Figure 5 of Anderson et al 4 ).…”

Aberration-free measurements of the visibility of isoluminant gratings

“…Mullen7,8 and Anderson et al 9 separated the positions of the different wavelength components of their stimulus to compensate for both axial and transverse chromatic aberrations. Even with this compensation, their measurements did not extend beyond 10 cycles per degree (c/deg),…”