Biomedical subjects
Publications and source records attributed to Y L Yap.
Retrospective analysis of 174 operative cholangiograms done over a 3-year period confirms its substantial benefit when used selectively in patients with clinical criteria for common bile duct exploration. A low incidental ductal stone rate of 1.6%, together with a 3.3% false positive cholangiogram rate and a case of bile duct injury resulting from the procedure makes its routine application appear superfluous.
Spatial-interval discrimination thresholds were measured for a pair of bars in the presence of other parallel bars placed far enough from the targets as to be outside the range of neural and optical blurring. Thresholds were elevated when the targets were embedded in an array of four parallel bars (two between and two flanking the targets), but not when there were only two parallels, whether the parallels were between the target bars or flanking them. The threshold elevation was larger with a 100-msec than with a 500-msec exposure duration. Attenuating the high spatial frequencies magnified the threshold elevation. The data indicate that the process responsible for spatial-interval discrimination automatically selects which spatial filters to use; it does not have to scan through all ranges of spatial filters.
The Weber function for separation--i.e. delta s as a function of separation s--is typically measured using a pair of targets presented roughly symmetrically relative to the fovea. With this paradigm, as the separation increases, the eccentricity of the individual targets increases also. To disentangle the effects of separation and eccentricity on the Weber function for separation, we systematically examined each of these variables and also examined the effects of target size and exposure duration. Separation discrimination thresholds were measured for average separations from 3 to 6 deg across a wide range of eccentricities, and for eccentricities of 2.5-10 deg for a range of separations. The dependence of threshold on target size was measured by varying the length of the stimuli from 1 to 120 min arc; the dependence on exposure duration was measured using durations of 100 and 500 msec at 10 deg eccentricity for comparison with data collected previously at smaller eccentricities. We found that for separations less than the eccentricity of the targets, thresholds depend primarily on separation; for larger separations, thresholds depend solely on eccentricity. In general, unless the targets are very small or quite brief, the spatial and temporal characteristics of the targets are not major contributors to the slope of the Weber function. Two mechanisms are proposed to account for thresholds in the two regions, one separation-dependent and one separation-independent.
Exposure duration was found to have a different effect on bisection thresholds than on separation-discrimination thresholds. Bisection thresholds were higher than separation discrimination thresholds between 33 and 150 msec but equal to or lower than them at longer durations. Experiments in which stimulus contrast was manipulated showed that the effect of exposure duration on separation-discrimination and bisection thresholds could not be attributed primarily to temporal contrast integration. The data could be accounted for by a model in which bisection is done by encoding the two separations in bisection sequentially.
This study evaluated to what extent inaccurate and asymmetric smooth pursuit in strabismic amblyopic eyes is attributable to abnormally high-velocity eye drifts that these eyes exhibit during monocular fixation. Smooth pursuit gains (peak eye velocity/peak target velocity) were determined in the amblyopic and nonamblyopic eyes of 11 strabismics for nasalward and temporalward motion; the target oscillated across 6 degrees of the horizontal meridian at frequencies ranging from 0.0625 to 1 Hz. In general, pursuit gains were higher for nasalward than temporalward motion, for both amblyopic and nonamblyopic eyes. Correction for each eye's mean velocity of fixational drift eliminated this nasal-temporal pursuit asymmetry for most of the nonamblyopic eyes, but not for the amblyopic eyes. Compared to the nonamblyopic eyes, corrected pursuit gains of the amblyopic eyes averaged about 0.2 lower nasalward and about 0.4 lower temporalward, but substantial variation occurred among individuals. We suggest that the overall reduction of pursuit gain in strabismic amblyopic eyes (after correction is made for fixational drift bias) stems from the use of a nonfoveal (eccentric fixation) locus for tracking; the further reduction of temporalward gain may result from a nasal-temporal asymmetry in processing motion signals.
The precision of discriminating the separation of two dots was measured as a function of separation for eccentricities of 0-10 deg under photopic and scotopic conditions. At each eccentricity, the 2-dot separation discrimination thresholds showed a V-shaped dependence on separation. For separations less than approximately twice the resolution threshold, performance deteriorated from photopic to scotopic conditions and appeared to be limited by ganglion cell receptive field size or spacing. For separations smaller than 10% of the effective eccentricity (eccentricity + 0.6 deg), the photopic 2-dot separation discrimination thresholds were significantly better than 3-dot bisection thresholds previously measured under similar experimental conditions, supporting the hypothesis that 3-dot bisection suffers from spatial interference for these separations. Interestingly, under scotopic conditions, 2-dot separation discrimination thresholds were better than resolution for a range of separations at each eccentricity, implying that cone input was not necessary for hyperacuity performance. 2-dot separation discrimination thresholds for large separations were little changed from photopic to scotopic luminance conditions.
Bisection thresholds are approximately proportional to the separation/eccentricity of the targets. This "Weber's law" for position has been invoked over the past century. It is the separation of the reference targets, or their eccentricity which determines the threshold? In previous studies separation and eccentricity are confounded. In the present report we have pitted separation against eccentricity. Bisection thresholds were measured for stimuli presented on an isoeccentric arc, so that separation could be varied while holding the eccentricity of the test lines constant. We used a 5-fold range of separations from 2-10 deg. In this regime, the present results provide strong evidence against Weber's law. When separation is varied but eccentricity held constant, there is no Weber's law. Rather the thresholds are approximately constant. Our results suggest that the judgement of the separation of widely separated objects is similar to a distance measurement using a ruler on the cortex, in that the error of measurement is independent of the separation between objects. The results imply that when we attempt to gauge the distance between widely separated objects it is unlikely that we do so on the basis of the outputs of large spatial filters; rather it appears that we make such judgements by estimating the cortical distance which separates the targets of interest.
Three experiments were performed to examine positional acuity and the role of spatial sampling in central, peripheral and amblyopic vision. In the first experiment, 3-line bisection acuity was compared to grating acuity. In normal foveal vision bisection acuity represents a hyperacuity. In anisometropic amblyopes, bisection acuity is reduced in rough proportion to their grating acuity. In strabismic amblyopes, and in the normal periphery, bisection acuity is reduced to a greater extent than grating acuity. This result implies that reduced contrast sensitivity of the spatial filters is not sufficient to account for the increased positional uncertainty found in peripheral vision and in strabismic amblyopia. In order to test the hypothesis that the high degree of positional uncertainty evident in these visual systems is a consequence of sparse spatial sampling, bisection thresholds and width discrimination thresholds were measured with stimuli comprised of discrete samples. The results showed that normal foveal vision and the vision of anisometropic amblyopes show little benefit from adding discrete samples to the stimulus. In contrast, the normal periphery, and the central field of strabismic amblyopes demonstrate marked positional uncertainty which can be efficiently reduced in proportion to the square root of the number of samples (up to about 10) comprising the stimulus in the direction orthogonal to the discrimination cue. In aggregate the results suggest that anisometropic and strabismic amblyopia are fundamentally different. The positional uncertainty in anisometropic amblyopia is consistent with the reduced sensitivity of the spatial filters. The data of the normal periphery and of the central field of strabismic amblyopes suggest that the cortical sampling grain imposes a fundamental limit upon their positional acuity.
Performance of three-dot bisection was determined as a function of the feature separation at eccentricities of 0-10 deg along the inferior vertical field meridian by using dot stimuli scaled in size to compensate for eccentricity. For these briefly flashed dot stimuli, the entire function of three-dot bisection acuity against dot separation worsens away from the fovea with a single scaling factor that is compatible with the change of the cortical magnification factor in area V1. When the presentation duration of the stimulus was lengthened from 150 msec to 1 sec, the improvement was much greater in the fovea than at 10-deg eccentricity for closely separated stimuli. We attribute this difference to a luminance cue (detection of a brightness change) that is present for a long stimulus duration at small separations in the fovea, but not in the periphery.
Performance of three-dot bisection was determined as a function of orientation for a variety of feature separations and field meridians at eccentricities of 0-10 deg for two observers. The dot stimuli and separations were scaled in size to compensate for eccentricity. The precision of three-dot bisection was found to depend on the direction of test-feature offset. In the fovea, horizontal and vertical bisections were better than oblique bisections, while at eccentricities of 5-20 deg, isoeccentric (on a tangent to a circle of a given eccentricity) bisection was better than radial bisection. The direction of offset was more important than the orientation of the stimulus. Large separations showed a stronger effect than small separations. The anisotropy of bisection appears different from the meridional effect for resolution and is unlikely to be simply related to a local anisotropy of the cortical magnification factor.