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.
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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In order to induce stereoscopic depth, retinal images in the right and left eyes must differ. Our experiments demonstrate that these uniocular differences for simple three-line configurations at stereo-threshold cannot usually be distinguished one from the other. We confirm that depth values are associated with individual features by virtue of their disparity, rather than by a comparison of their image separations in the two eyes. In ordinary stereoacuity patterns, no prior identification of monocular relationships is needed to produce discriminable depth differences. For certain patterns, disparity of the positions of the averages of the targets' internal light distributions in the two eyes can substitute for disparity of pattern contours in stereoscopic depth discrimination.
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To determine how vestibular and optokinetic eye movement signals share the central oculomotor neural organization, optokinetic afternystagmus was superposed on vestibular nystagmus in the monkey. To a first approximation there was algebraic additivity in the velocity domain. This result suggests that vestibular and optokinetic eye movements are integrated at a level of neural organization above the ocular motoneurons, at a level in which neural signals are coded in terms of eye movement velocity rather than eye position.
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Stereoscopic acuity in the human fovea remains unimpaired with retinal image motions of up to 2 deg/s. These findings apply to lateral motion of the test target alone, and to simultaneous lateral motion of both test and comparison targets. For good steroscopic acuity, depth motion can be tolerated only so long as the configuration does not move outside the disparity zone for optimal stereoscopic acuity, within 2-3 arc min of either side of the fixation plane. The presence during a short exposure of some stimulus components lying outside this zone leads to an overall reduction of stereoscopic acuity, either by dilution through summation, or by active inhibition of the best achievable depth resolution.
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Horizontal, vertical, and over-all size differences were introduced in the retinal images of the two eyes of normal subjects during brief presentations of simple foveal targets. Horizontal disparities, whether accompanied by vertical disparities or not, induced the appearance of a rotation of the target around a vertical axis out of the frontal plane, according to the expectation from geometry, but vertical disparities had no effect. Over-all size changes in one eye induced the effect of the horizontal component. Threshold experiments showed that even with practice and error feedback, vertical disparity detection has at most only one-tenth the sensitivity of horizontal disparity detection. Although at variance with findings on the induced size effect obtained under more complex observation conditions, these results confirm that the processing of horizontal disparity plays a special role in the integration of the signals coming from the two eyes.
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