Showing papers by "Robert F. Hess published in 1987"
TL;DR: It is concluded that raised spatial uncertainty due to metrical scrambling is a suitable model for anisometropic amblyopia.
80 citations
TL;DR: The finding that these suprathreshold compound stimuli are discriminated on the basis of the local spatial features, and not on differences in their phase spectra as previously thought, allows a reinterpretation of the importance of phase coding in normal vision and of the selective loss of these discriminations that have been previously reported for peripheral vision and in amblyopia.
33 citations
TL;DR: The contention that the typical and complete achromat is a functional rod monochromat is supported and hence can be used to explore the sensitivity of the isolated rod post‐receptoral mechanism under mesopic conditions where its sensitivity is optimal.
Abstract: 1. Detection thresholds for two-dimensional Gabor functions of varying spatial and temporal frequency were used to investigate the post-receptoral sensitivity across the retina of the typical and complete achromat. 2. Under photopic conditions there is no evidence for post-receptoral cone function at any retinal eccentricity investigated. Sensitivity saturates in a way consistent with known psychophysical and electrophysiological measures of rod saturation. This occurs in a unitary fashion across the retina. 3. Under scotopic conditions the regional fall-off in spatio-temporal sensitivity is similar for the achromat and duplex retina. This suggests that the rods in the achromat make normal neural connections. 4. Taken together this supports the contention that the typical and complete achromat is a functional rod monochromat and hence can be used to explore the sensitivity of the isolated rod post-receptoral mechanism under mesopic conditions where its sensitivity is optimal. This is where its contribution is most difficult to isolate in the duplex retina. 5. For the human rod mechanism, mesopic post-receptoral sensitivity for all spatio-temporal stimuli is optimal in the central region of the retina and falls off as a function of eccentricity. 6. For localized stimuli, peripheral spatial sensitivity is reduced evenly at all spatial frequencies compared with that of the central retina. A similar displacement of the spatial sensitivity function of the rod mechanism occurs as illuminance is reduced. 7. For localized stimuli, temporal acuity of the rod mechanism is around 20-25 Hz irrespective of retinal position. As the illuminance is further lowered dynamics of the rod pathway are reduced irrespective of retinal position and the sensitivity function maintains a bandpass shape. 8. The regional distribution sensitivity of the rod mechanism changes as illuminance is reduced from mesopic to scotopic levels.
28 citations
TL;DR: There is no evidence from these results to suggest that the central foveal projection is more likely to be affected following an attack of optic neuritis than the projections of other eccentricities within the central 10 degrees as far as mechanisms subserving luminance vision are concerned at these spatial frequencies.
Abstract: Contrast sensitivity was measured at nine locations within the central 10 degrees of the visual field in cases of recovered optic neuritis having varying degrees of residual deficit. A sample of 82 patches of visual field was obtained in 14 cases. Circular patches of vertically orientated sinusoidal gratings, 2.5 degrees in diameter, were used. The gratings were modulated in time at 8 Hz and the effect of spatial frequency on the threshold loss determined at each visual field location. As anticipated from what is known of visual field changes in the disorder there was considerable variation in the magnitude of the contrast threshold elevation at different locations in the visual field in any one case. The variability was more marked in cases with greater overall deficit. Three types of spatial loss were encountered. The most common was a loss which increased at higher spatial frequencies, found in 65 of the 82 patches of visual field examined. In 11 the loss was unaffected by spatial frequency and in the remaining 6 the loss was maximal at an intermediate spatial frequency. There was no instance of a loss maximal at low spatial frequencies. Overall the results indicate that sensitivity to higher spatial frequencies is more likely to be impaired following an attack of optic neuritis. In the combined results the effect of spatial frequency on the threshold elevation was statistically significant at all eccentricities (P less than 0.001). Analysis of the combined results revealed no difference in the mean contrast sensitivity loss at eccentricities of zero, 3.75 degrees or 7.5 degrees for intermediate and low spatial frequencies. There is no evidence from these results to suggest that the central foveal projection (papillomacular bundle) is more likely to be affected following an attack of optic neuritis than the projections of other eccentricities within the central 10 degrees as far as mechanisms subserving luminance vision are concerned at these spatial frequencies. Overall there was slightly greater reduction in acuity within the central 5 degrees than at 7.5 degrees eccentricity (P less than 0.05). This may be accounted for by the finding that higher spatial frequencies are more affected, rather than being related to eccentricity per se.
22 citations
01 Jan 1987
TL;DR: The origins of contrast sensitivity approach can be traced back to Selwyn, who worked for Kodak in the United Kingdom, and in the 1940’s was involved in reconnaisance photography and developed the use of sinewave stimuli.
Abstract: The origins of contrast sensitivity approach can be traced back to Selwyn, who worked for Kodak in the United Kingdom, and in the 1940’s was involved in reconnaisance photography. He was keen to measure the transfer function of cameras and films and recognised the advantage of using sinewave patterns - that is, because they produced a known image distribution. He used human subjects but only to calibrate his cameras and films (SELWYN [1]). In the late 1940’s and early 1950’s, Otto Schade, a television engineer, carried this approach forward (SCHADE [2]) and concentrated mainly on the transfer properties of human vision, using initially squarewave gratings and an optical projection (onto a television raster display) technique. In the 1960’s, Fergus Campbell and Gerald Westheimer working together in Cambridge were interested in measuring optical quality and accommodation and further developed the use of sinewave stimuli. John Robson, who was then a graduate student built the first electronic means of spatial sinewave generation which had good time and space resolution using a commercially available oscilloscope display. Apart from a refinement of the contrast linearity which was also later developed by John, this system is essentially what is used today.
3 citations