Biomedical subjects
Publications and source records attributed to G Westheimer.
Based on the distinction between uniocular vertical magnification and vertical disparity, the induced size effect experiments were reinterpreted and new experiments done to show that vertical disparity signals can produce other stereoptic depth effects. The direction and efficiency of utilization of vertical disparity signals depend on the quadrant of the visual field and the stimulus position within it.
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In an attempt to design a new stereo vision test we found that the spatial uncertainty effect had substantial impact on the measured stereo acuity thresholds. This effect is present whenever there is uncertainty of where the critical information occurs in a visual target. We studied the spatial uncertainty effect for stereo acuity and for line length estimation. In addition, we determined the temporal uncertainty effect in a stereoacuity test. In foveal vision, uncertainty effects increased visual thresholds, sometimes dramatically. For example, for an uncertainty factor of 25 (i.e., the critical information occurred randomly in any of 25 positions) stereo thresholds increased more than four-fold compared to those obtained without uncertainty. Although the uncertainty effect has been well described theoretically, we think that it is not always appreciated in the design of clinical or experimental visual tests. In tests aimed at determining visual thresholds, spatial and temporal uncertainty factors should be minimized.
1. The effect of spatial uncertainty on line orientation thresholds was studied in normal observers. Vertical lines, 5 min of arc long, built up a matrix in which one of the line elements could be tilted to the left or right. The orientation thresholds depended strongly on the number of alternative test positions. There was a linear relation between log (threshold) and log (P), where P is the probability that a particular line element was the one being tested. 2. The uncertainty effect was shown to be time dependent. The effect was more marked for the shortest stimulus duration (1 s). However, even with a 6 s stimulus duration, allowing several re-fixations, the thresholds were significantly higher in the presence of uncertainty, compared to the situation in which the test position was fixed and known to the observer. 3. When the measurements were restricted to the centre line in a matrix, thresholds were more than twice as high when the test line could be in any of the centre 3 x 3 positions, compared to the case in which there was no uncertainty as to the test position. Foreknowledge of location of the test line within the matrix improved the threshold further, even if the whole matrix was displaced to different retinal positions. 4. It is concluded that the physiological mechanism mediating threshold improvement probably operates on a cortical processing apparatus more central than V1.
The ideas of fractals and fractal dimension are here translated into the realm of visual psychophysics. Borders between two fields of different luminance were used. Because of the finite grain of the visual system, fractal dimension need be defined only within a certain size range. For a fractal dimension of 1.15, the just-detectable difference in fractal dimension was found to be about 0.0085, rising to about 0.015 for a fractal dimension of 1.25. Reducing exposure duration from 1 s to 0.33 s decreases sensitivity to differences in fractal dimension, but there was no gain in increasing the exposure duration. Good visual observers who are naive to the task require some training before reaching optimal performance. The ability to discriminate fractal dimension differs between fractal edges of the same fractal dimension that were generated with differing statistical programs. Even after considerable training, an observer makes 29% errors when asked to distinguish a fractal edge generated with a Gaussian random walk from one with a rectangular random walk. Gaussian random walk fractals can be more easily distinguished from Poissonian and Cauchy ones.
A briefly flashed pattern never appears quite sharp, and visual acuity is reduced with short exposures. These observations led to an examination of the effect of exposure duration on sharpness discrimination. A foveally seen edge appears just-discriminably blurred when its edge light distribution is changed from being sharp to conforming to a ramp whose width (increasing from 0% to 100% of maximal luminance) is approximately 1 arcmin. When the exposure duration is reduced, the ramp width for minimal blur increases, rising by factors of 2-4 for exposures as short as 30 msec. This change is not due to a shortage of light. Threshold blur discrimination is not affected by retinal image motions of up to 1 deg/sec. Temporal combinations of sharp and blurred borders always worsen performance, and multiple brief presentations do not give so good a threshold as a single longer one.
The effect of training on an observer's ability to detect the misalignment of three points, a hyperacuity, and to resolve a six-line grating was studied in a transfer-of-training design with observers (4 in each of two experiments) who were experienced in making psychophysical judgments of other visual stimuli. The transfer-of-training design enabled us to look for any training-based improvement. Long periods of training produced no statistically significant improvement in performance under any condition. There were small practice-based improvements, but the primary patterns indicated threshold fluctuation rather than improvement. We interpret the results to indicate that the neural mechanisms underlying three-point alignment and grating discrimination, like those for gap bisection (Klein & Levi, 1985), are not malleable to any significant extent.
Although stereoacuity and vernier acuity both yield comparable thresholds well below the eye's resolution limit, the neural circuits for these two classes of visual responses do not process the signals in an identical manner. It had previously been demonstrated that hyperacuity is more resistant to image blur than stereoacuity and that the zones within which two targets must be placed to achieve the lowest thresholds differ quite radically. Two further differences are reported here: reduced contrast affects stereoacuity more severely than hyperacuity, as also does shortening of exposure into a range of tens of milliseconds, even when the Bunsen-Roscoe-Bloch law has been factored out.
Vernier acuity was measured by finding the just discriminable offset for an edge separating fields of different luminances. The contrast of this stimulus is easily specified by the formula c = (Lstim - Lsur)(Lstim + Lsur). Vernier thresholds are about 4-5 sec of arc for contrasts 0.22 and higher, but increase exponetially with decreasing contrast. By comparison, the presence of the stimulus could be detected at a contrast of 0.016. The possible role of the magnocellular and parvocellular pathways in carrying the input signals to the fine localization process is discussed.
When it is surrounded by lines of a differing orientation, a test line changes its apparent orientation in a direction away from that of the surround lines. Using a nulling technique to arrive at numerical values, the properties of this simultaneous orientation contrast have been analyzed: it diminishes with distance of the surround lines; rises and then falls off as a function of surround line orientation; decreases with exposure duration; is sharply tuned (+/- 100 msec) for synchrony of test and surround line presentation; is robust to differences between test and surround line disparity but not intensity; and is reduced with dichoptic presentation of test and surround lines. Orientation contrast can be induced in a variety of oriented features, including illusory contours, an ellipse, a moving dot and a row of dots or lines, but two dots alone don't suffice. The results are taken as evidence that orientation is a domain sui generis, in which simultaneous contrast is exhibited in the same manner as in the domains of color, brightness and disparity.
A single-line stimulus in a context-free visual environment presented for 250 msec needs to move with a disparity velocity of 25 to 45 min of arc/s before the direction of depth motion can be correctly identified. Presence of a pair of flanking line stimuli improves this value severalfold, but even then the threshold is 18 to 28 times higher than for the detection of a static disparity difference.
Thresholds for temporal-order detection of two successively displayed vertical stripes were determined for monocular and binocular stimulation. Thresholds are lower for binocular stimulation by roughly a factor of 1.4 compared with those found under monocular stimulation. This corresponds to quadratic summation. This is found for angular separations of the two stripes amounting to 5 and 40 min of arc.
Binocular summation was studied with two different hyperacuity tests, a bisection test and a vernier-type test. The summation factor differed between the two tests and depended on the test configuration. In the bisection test, binocular summation increased with decreasing line separation, and for the largest separation binocular inhibition was found. In the vernier test summation was essentially more constant for different separations. It is argued that, for a bisection test with a small intertarget distance, the hyperacuity thresholds involve a component of intensity discrimination that is due to overlapping retinal line-spread functions. An additional, unexplained finding was that in a vertically oriented three-dot alignment test the monocularly perceived alignment always occurred while the center dot was displaced toward the nasal side.
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The signals from the two eyes must be routed to allow either eye to have access to the processing mechanisms for position, shape, colour, etc.; at the same time, information as to the eye of origin must be retained for the purposes of stereoscopy. The study of this confluence of signals from the two eyes was approached psychophysically by studying induced position and depth changes of adjacent binocular and monocular stimuli in the human fovea. It was demonstrated that a monocular visual stimulus located near a binocular one acquires a depth signal, according to a scheme originally proposed by Panum. The effect is unspecific as regards feature shape and brightness, and falls off with a length constant of about 15 minutes of arc in the fovea. A monocular stimulus also affects the apparent depth of its binocular neighbour in a centre-surround manner; disparity pooling changes to disparity repulsion when features are separated by distances of about 3 minutes of arc in the fovea. The findings led to the development of a scheme of uniocular connectivity to a matrix of depth units. Excitation patterns here would depend on the state of the input lines, the intrinsic neuronal interaction properties, and contextural configuring influences from other parts of the nervous system. Experiments showing the spatial extent of pooling and repulsive interaction within the disparity domain help to characterize the stimulus processing in this neural ensemble.
Results of psychophysical experiments are reported showing that synchrony, appropriate relative placement, and absence of standing disparity are important conditions to be met by members of a target configuration if they are to participate in the cooperative neural processes leading to the best disparity discrimination. Consecutive binocular presentation of the members of a stereo target decreases stereoacuity by a factor of about 10, and a step disparity displacement of a single line target needs to be larger still to be detected as a depth stimulus. A standing disaprity of even one minute of arc at least doubles the disaprity disxrimination threshold. It is postulated that a differencing mechanism operates on the depth signal of individual features; the temporal and spatial optima of target presentation for stereoscopic acuity outline the character of the concerned operations.
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