Biomedical subjects
Publications and source records attributed to H S Smallman.
Through unique observations of an adult case of bilateral congenital cataract removal, we have found evidence that retinal photoreceptors will swiftly realign towards the brightest regions in the pupils of the eye. Cones may be phototropic, actively orientating themselves towards light like sunflowers in a field.
Research on when and how to use three-dimensional (3D) perspective views on flat screens for operational tasks such as air traffic control is complex. We propose a functional distinction between tasks: those that require shape understanding versus those that require precise judgments of relative position. The distortions inherent in 3D displays hamper judging relative positions, whereas the integration of dimensions in 3D displays facilitates shape understanding. We confirmed these hypotheses with two initial experiments involving simple block shapes. The shape-understanding tasks were identification or mental rotation. The relative-position tasks were locating shadows and determining directions and distances between objects. We then extended the results to four experiments involving complex natural terrain. We compare our distinction with the integral/separable task distinction of Haskel and Wickens (1993). Applications for this research include displays for air traffic control, geoplots for military command and control, and potentially, any display of 3D information.
The masking of a sinusoidal test grating by contrast-modulated (CM) gratings could, in principle, be attributable to the presence of a distortion product, injected into the stimulus during some nonlinear transformation at an early level of visual processing (e.g. Nachmias, 1989 Vision Research 29 137-142). If so, CM gratings and luminance-modulated (LM) gratings of similar effective contrast and spatial frequency should mask the detection of sinusoids in a similar fashion. We compared the effects of masking by 1 cycle deg-1 CM gratings [both simple beats (8 + 9 cycles deg-1) and amplitude-modulated gratings (8 + 9 + 10 cycles deg-1)], with those of masking by 1 cycle deg-1 LM gratings of low contrast. We found that: (i) CM and low-contrast LM grating masks yielded similar spatial-frequency tuning functions around the modulation frequency of 1 cycle deg-1; (ii) low-contrast LM gratings masked the detection of test sinusoids in a highly phase-dependent fashion, while masking by CM gratings did not vary systematically with relative spatial phase. The results suggest that masking produced by CM gratings cannot simply be explained by the presence of a distortion product at the beat or modulation frequency.
We investigated over what central area disparity in a random dot stereogram is integrated to stimulate an initial vergence response. Vergence was measured subjectively, with a forced choice dichoptic nonius vernier task following a brief (230 msec) stimulus presentation. Stimuli were random-dot stereograms showing a central circular disc of 12.5 min arc crossed retinal disparity in front of, and occluding, a same density fixation plane surround. The size of the disc was varied. All ten observers responded to the brief stimulus. Initial vergence increased with increasing disc diameter and, for nine out of ten subjects, reached a maximum with the disc ca 6 deg, suggesting this is the extent of the spatial integration region. Below 6 deg diameter, surround and target disparities were averaged together. Initial horizontal vergence responds automatically to a cyclopean target presented in the centre of gaze by pooling disparities within a limited but surprisingly large area.
It has been suggested that breaking camouflage is one of the major functions of stereopsis (Julesz, 1971). In this study, we found that stereopsis is less effective in breaking camouflage for moving targets than for static ones. Observers were asked to detect a single dot moving on a straight trajectory amidst identical noise dots in random motion. In the three-dimensional (3D) condition, the noise dots filled a cylindrical volume 5.7 cm in height and diameter; the trajectory signal dot moved on an oblique 3D trajectory through the center of the cylinder. In the two-dimensional (2D) control condition, observers viewed one half-image of the 3D cylinder binocularly. Surprisingly, trajectory detection in the 3D condition was only slightly better than in the 2D condition. Stereoscopic tuning for motion detection was also measured with a novel target configuration in which the random motion noise was presented in two depth planes that straddled the fixation plane where the trajectory target was presented. As the disparity between the noise planes and the fixation plane was increased, trajectory detection improved, reaching a peak between 6 and 12 arcmin, and then declining to the 2D level at larger disparities, where the noise became diplopic. Similar tuning measurements were made for detecting a static pattern, a string of five aligned dots presented in the fixation plane between two planes of static noise dots. Adding disparity to the noise planes produced a far greater improvement in static detection than in motion detection, for a comparable range of disparities (1.5-12 arcmin). We speculate that the temporal characteristics of the stereo system are not well suited for responding to moving targets, with the result that stereo does not greatly enhance motion detection in noise.
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How are binocular disparities encoded and represented in the human visual system? An 'encoding cube' diagram is introduced to visualise differences between competing models. To distinguish the models experimentally, the depth-increment-detection function (discriminating disparity d from d +/- delta d) was measured as a function of standing disparity (d) with spatially filtered random-dot stereograms of different centre spatial frequencies. Stereothresholds degraded more quickly as standing disparity was increased with stimuli defined by high rather than low centre spatial frequency. This is consistent with a close correlation between the spatial scale of detection mechanisms and the disparities they process. It is shown that a simple model, where discrimination is limited by the noisy ratio of outputs of three disparity-selective mechanisms at each spatial scale, can account for the data. It is not necessary to invoke a population code for disparity to model the depth-increment-detection function. This type of encoding scheme implies insensitivity to large interocular phase differences. Might the system have developed a strategy to disambiguate or shift the matches made at fine scales with those made at the coarse scales at large standing disparities? In agreement with Rohaly and Wilson, no evidence was found that this is so. Such a scheme would predict that stereothresholds determined with targets composed of compounds of high and low frequency should be superior to those of either component alone. Although a small stereoacuity benefit was found at small disparities, the more striking result was that stereothresholds for compound-frequency targets were actually degraded at large standing disparities. The results argue against neural shifting of the matching range of fine scales by coarse-scale matches posited by certain stereo models.
Many studies have demonstrated that the human visual system is sensitive to very small differences in relative binocular disparity. It is not known over what monocular regions information is spatially integrated to mediate performance in such tasks. In this study we present psychophysical observations that define the smallest spatial scale involved in disparity processing, and we indicate the nature of the computations performed by the units mediating that disparity discrimination. We show that human observers can identify the sign of disparity of a single target dot when it is embedded in a row of identical dots, with these noise dots presented either in the fixation plane or with a proportion binocularly uncorrelated. In conjunction with the psychophysical data, we explore how a class of simple correlator models of stereopsis must be constrained in order to account for human performance for the same fine-scale tasks. Such models can perform the task only when the correlation is carried out over a very small region of the image, for a very small range of disparities. Our results demonstrate that there is a fine-scale input to the stereo system, mediated by foveal mechanisms that spatially integrate visual signals over a region as small as 4-6 arcmin in diameter.
It is widely held that in human spatial vision the visual scene is initially processed through visual filters, each of which is responsive to narrow ranges of image spatial frequencies. The physiological basis of these filters are thought to be cortical neurons with receptive fields of different sizes. The grain of the neural representation of spatial vision is much finer than had been supposed. Using laser interferometry, which effectively bypasses the demodulation of the optics of the eye, we measured discrimination of, and adaptation to, high spatial frequency laser interference fringe patterns. Spatial frequency discrimination was good right up to the visual resolution limit (average Weber fractions of 0.13 at 50 c/deg). Both contrast and spatial frequency matches made after adapting to extremely fine interference fringes strongly suggested that there existed even finer, relatively unadapted, filters (mechanisms with small receptive fields). The smallest cortical receptive fields processing spatial information in human vision are so small that they can possess receptive field centers hardly wider than single cone photoreceptors.
Stereopsis employs differences in the location of features in the two eyes to reconstruct their relative depths. Computational models largely ignore the contrast of these features; they simply require them to be visible in each eye and to possess the same contrast polarity. With a competitive matching paradigm we show that only features with a certain ratio of contrasts in the two eyes match. Increasing the contrast in one eye requires proportionally more contrast in the other eye for matching. This contrast relation exactly parallels the relation found for dichoptic masking, which behaves unlike any other form of contrast masking. A strange consequence of this contrast ratio constraint is that a feature may be monocularly visible, yet unmatched because the contrast ratio has not been satisfied. In this case features are seen as faint unmatched 'ghosts' near the plane of fixation. In a competitive matching situation then, stereopsis acts as if it imposes a contrast threshold on matches; features failing to exceed the threshold remain unmatched. This is a simple and biologically plausible way for the system to eliminate false matches and to reduce matching ambiguity.
Spatial frequency selectivity has been incorporated into various theories of stereo matching, along with spatial scale interactions operating from coarse-to-fine spatial scales. We concentrate here on the role of fine scale information in the stereo matching process and show that fine scale information is capable of disambiguating matches made at coarser scales. An ambiguous coarse scale stimulus was created by presenting a low frequency (2 c/deg) sine wave in anti-phase to the two eyes, whose endpoints betrayed no information about which way the sine waves should be matched. It could be seen with crossed or uncrossed disparity equally validly and at chance from trial to trial. To this was added a fine scale (8 c/deg) filtered random dot stimulus specifying unambiguously a certain disparity. Observers judged the apparent depth of the two stimuli as the disparity of the fine scale stimulus was varied. The sine wave was usually perceived to have the same sign disparity as the fine scale stimulus. Depth matching with the two superimposed stimuli confirmed that the coarse scale stimulus was actually disambiguated, and seen with disparities equal to half its spatial period. The results suggest the operation of a cross-spatial scale matching disambiguation process, which can operate in a fine-to-coarse fashion.
In stereo-matching algorithms, the 'uniqueness constraint' requires that a feature in one stereo half-image be matched to, at most, one similar feature in the other half-image. Experiments are reported in which binocular contrast thresholds and depth-discrimination judgments have been used to determine whether the human stereo system makes unique matches. A single high-spatial-frequency target in the left eye was paired stereoscopically with two identical targets, presented near retinal correspondence (+/- 3.5 min of disparity), in the right eye. Contrast-increment thresholds were measured for each of the targets in the right eye, and it was found that the target in the left eye masked both. Indeed, the amount of binocular masking for each member of the double target nearly equaled the masking observed when only a single target was presented to the right eye. Depth judgments confirmed that the target in the left eye had been matched to both targets in the right eye. It is concluded that uniqueness is not an absolute constraint on human stereo matching.
Contrast thresholds for 75% correct depth identification in narrow-band filtered random dot stereograms were determined for different center spatial frequencies and binocular disparities. Rigorous control over vergence was maintained during testing, and a forced-choice procedure was used. The resulting contrast sensitivity function for stereopsis revealed sensitivity over a greater range of disparities at low than at high spatial frequencies. Sensitivity peaked for large disparities at low spatial frequencies and for small disparities at high spatial frequencies. When disparities were converted to effective binocular phase differences, the variation of contrast sensitivity with phase followed a consistent pattern across spatial frequencies, with peak sensitivity occurring mainly for binocular phases of between 90 degrees and 180 degrees. These results have implications for the extent of spatial integration at the input to the disparity sensing mechanism. A model postulating a spread of positional disparities independent of the spatial frequency selectivity of disparity-sensitive units cannot account for the results. But the size-disparity correlation strongly evident in our data is predicted by certain models of stereopsis, such as phase disparity encoding. An ideal observer analysis is developed that demonstrates that our results were not forced by the nature of the stimulus employed; rather, the quantum efficiency for stereopsis at contrast threshold follows the size-disparity correlation.
Previous studies of the role of color in visual search have shown efficient coding for as many as six colors in a high-density display. In an effort to increase this limit, we established an optimal basic color code from extensive surface-color-naming data. This code yielded excellent segregation in a visual search task: The time required to find a critical target of a cued color increased only marginally as up to nine groups of different colors were added to the display. It made no difference whether the cue was provided by name or by example. Significant color differences in this task triggered a second experiment, which examined the detectability of the critical target feature in the periphery. A close correlation was found to exist in the order of color performance between the two experiments. Color segregation was tested again in a third experiment, in which subjects were required to count the number of targets of the cued color. The colors again segregated well. A final experiment tested the proposition that it was the basic nature of the colors that was responsible for the good segregation. When seven basic colors were pitted against seven equally discriminable nonbasic ones in a modified version of the visual search task, no significant difference was found between the two groups. It is concluded that basic colors segregate well not because they are universally named but because they are well separated in color space.