The foveal point spread function as a determinant for detail vision

Abstract

Abstract-A point spread function, chosen to link contrast sensitivity and stimulus dimensions. can be obtained from measured thresholds by assuming small-signal-linearity and peak detection for the visual system. To that end a special case of summation of subthreshold signals (perturbation) is used. taking specific measures against the effect of sensitivity drift. The basic assumptions are tested simuitaneousl~-and confirmed. Other provisional assumptions tike radial symmetry and homogeneity were evaluated along a horizontal and a vertical meridian through the fovea. In the fovea no deviation from radial symmetry was found. The effect of inhomogeneity within the central fovea. seems to be too small to cause a significant change in the point spread function. The validity for predicting thresholds of stimuli exposing larger areas is tested. Annuli with varying radii show no significant aberration if probability summation is taken into account. Predicted disk thresholds, however, show a large discrepancy with experiment for radii larger than 2min arc. A possible extension of the model with multiple-detection units having tuned sizes is evaluated. INTRODUCfIONIn order to investigate characteristics of detail vision stimuli like Landolt C's, lines or gratings are generally used. Sometimes, radially symmetrical stimuli like disks and annuli are used to investigate lateral processing over short distances (Fiorentini and Maffei, 1970;Westheimer, 1967). The spatial processing of the visual system is rather inhomogeneously over the retina. This is demonstrated by data on visual acuity (Le Grand, 1967) and thresholds for disks (Kishto, 1970;Wilson, 1970). A review is made by van Doorn et al. (1972). The effect is also found with thresholds of lines (Hines, 1976;Wilson and Bergen, 1979; Limb and Rubinstein, 1977). Wilson and Bergen, ibid, stressing the importance of local measurement of the inhomogeneous system. used line sources instead of gratings as stimuli. However, line sources of the usual type still stimulate retinal parts with different properties. The use of point sources is an obvious further step, the more so, since it is not clear how much the discrepancy between measured thresholds and prediction based on a line spread function (e.g. Wilson and Bergen, 1979) is disguised by effects of the mentioned extension of the lines.A point spread or weighting function can be used to characterize the effect of a near-threshold-pointsource on the contrast sensitivity of neighbouring retinal points (e.g. van Meeteren, 1973;Kelly, 1975). However, the threshold changes which have to be measured to obtain such a function are very small. Consequently special measures have to be taken to eliminate the effects of unavoidable sensitivity drift in order to obtain the required precision.In this report the results of a technique to obtain a point spread function from threshold measurements is investigated and the predictive power is tested. This point spread function should represent the simple processing of the visual system, thus ...

Spatial Frequency Range in the Detection Process—1. Narrow Bars

“…It was stated above that retinal inhomogeneity was ignored in the present model since the stimuli were confined mainly to the fovea, However, the extent of the filtered stimuli will obviously be greater than the filtered stimuli, and Blommaert and Roufs (1981) have shown that even relatively small variations in retinal inhomogeneity can have a significant effect on threshold model predictions. This point will be discussed in a later work.…”

“…However, as was shown for example by the work of Hubel and Wiesel (Hubel and Wiesel, 1962, 1968, bars and edges are the basic functions for the visual system. Although it is possible, using Fourier analysis (Goodyear, 1971), to describe any spatial variation in spatial frequency terms, the models which are based on the spatial characteristics of the visual system work best in the spatial domain (Blommaert and Roufs, 1981;Hines, 1976;Limb and Rubenstein, 1977;Wilson and Bergen, 1979). A spatial domain analysis would not be appropriate in the present case, since the spatial characteristic of the test stimulus varies with the type and cut-off frequency of the filter used.…”

Quantitative Analysis of Illusory Movement: Spatial Filtering and Line Localization in the Human Visual System

“…A Gaussian with σ = 1.1 arcminute has a full width at half height (FWHH) of 2.6 arcminute. Meeteren & Vos, 1972;Blommaert & Roufs, 1981). At a sufficiently high luminance, the LSF and PSF have negative side bands, yielding a sensitivity profile that resembles h' '(x).…”

P.43: Model Development for Cell‐Gap Induced Mura to Improve Quality of Glass Substrates to Display Manufacturers

“…In this model, the human eye is assumed to have a linear response to received signals -it acts as a linear filter characterized by a certain line spread function (LSF). Such LSF mimics the performance of an average human eye at best and is usually obtained by fitting the experimental data [4][5][6][7][8] that involve human subjects. Due to a high variability among the population, such LSF can have rather large margins of error associated with it.…”

“…We next postulate that the perceived contrast is the determining factor for the visibility of a visual stimulus. We assume that the visible threshold of the perceived contrast is a fixed value PC th above which the visual stimuli become visible, independent of the shape and form of the stimulus [4][5][6]. It can be normalized such that PC th = 1, i.e.…”