Perception

Frisby et al (1993 Perception 22 Supplement, 115) proposed that the visual system might make cyclovergent eye movements in order to minimise the overall pattern of both vertical and horizontal disparities when an observer views an inclined stereoscopic surface. Their measurements of cyclovergence, which used vertically oriented nonius lines, were found to be consistent with that proposal. In our experiment 1, we measured torsional eye movements objectively, using scleral coils, and found no evidence of a cyclovergent response to either a real inclined surface or to a simulated inclined surface in which the two stereoscopic images were related by a horizontal shear transformation. These results are inconsistent with the disparity minimisation hypothesis. In order to account for the discrepant findings of the two studies, we propose that vertically oriented nonius lines may not be a valid method for assessing cyclovergence because the lines can be seen as lying ‘within’ the inclined surface. In experiment 2, we tested the predictions of the cyclovergence hypothesis of Frisby et al against our own ‘within surface’ explanation, using both horizontally and vertically oriented nonius lines and dichoptic images related by either a horizontal or a vertical shear. If cyclovergence were the cause of the misalignment, both horizontal and vertical nonius lines should appear misaligned to the same extent. This was not found to be the case. We conclude that vertical nonius lines may not be a valid technique for measuring cyclovergence when the lines are seen against a background of an inclined surface.

When the corresponding horizontal meridia of the two eyes are aligned, the corresponding vertical meridia are tilted outwards in a temporal direction, a phenomenon first described by Helmholtz. However, it is not known if this effect is confined to the principal meridia or whether the same relationship exists between corresponding horizontal and corresponding vertical meridia at eccentric retinal locations. We sought to address this issue by exploiting the technique of Nakayama (1977 Proceedings of the Society of Photo-Optical Instrument Engineers 120 2–9) in which the positions of alternating dichoptic images that produce minimal apparent motion were used to measure the relative tilt of corresponding meridia at a range of eccentricities up to ±16 deg away from the fovea. Stimuli were composed of dichoptic images, one containing a blank field and the other a pair of dots, which alternated at a rate of 0.63 Hz and the relative tilt (binocular orientation difference) between the pairs of dots presented to the two eyes was varied between ±11°. Nonius lines were used to maintain vergence angle, which was varied between 28 cm and infinity. Subjects judged which pair of alternating images produced the smallest amount of apparent motion (position change). It was found that at all eccentricities examined the corresponding horizontal meridia were generally aligned but the corresponding vertical meridia were consistently offset (extorted) by about ±2°. The tilts of corresponding principal meridia were typically unaffected when vergence angle was varied, indicating that little or no cyclovergence accompanied changes in horizontal vergence. The results suggest that the binocular correspondence system appears to be mapped by a horizontal shear distortion that extends to retinal locations at least as far as 16 deg away from the foveae. The invariant extorsion of corresponding principal vertical meridia with vergence state is consistent with previous suggestions that the empirical vertical horopter becomes progressively inclined with respect to the vertical as viewing distance increases.

Previously (Popple et al, 1998 Vision Research 38 319–326) we found, using random-dot stereograms, that initial vergence increases with the size of a cyclopean disc. A corresponding improvement in stereoacuity within the disc was predicted, because disparities in the disc would be brought closer to the plane of current fixation.

In the present experiment, we looked at the effect of the spatial extent of a briefly presented (≤ 500 ms) cyclopean depth pedestal on stereoacuity thresholds. Observers were required to judge the depth of a small, 1.7 deg, central disc relative to a larger surrounding disc in a random-pattern stereogram. The larger disc was set, initially, at a pedestal disparity of ±24 min of arc against a fixation-plane surround. The size of the larger disc was varied from 2.6 to 8.0 deg. As predicted, stereoacuity thresholds fell significantly with increasing pedestal disc size. Next, the disparity of the pedestal disc was varied. When pedestal disparity was reduced to ±2.4 min of arc, a disparity too small to demand vergence, the size effect disappeared except when the pedestal boundary was within 30 min of arc of the test disc boundary. We argue from this result that the effect was largely due to vergence and not cyclopean integration alone. However, the effect of pedestal size was found to persist with stimuli too brief to permit vergence (≤ 100 ms) suggesting that factors other than vergence may also play a role.

Some recent studies on the extraretinal contribution to distance perception are reviewed. These experiments demonstrate that vergence can provide reliable information for judgments on the distance of proximal targets in the absence of all other cues. We argue that, although vergence is an unreliable cue at large fixation distances and is subject to a strong contraction bias when studied in isolation, these facts do not imply a minor role for vergence in near-space perception. When additional depth and distance cues are added, the contribution of vergence information becomes more complicated. We present results which indicate that the different cues to depth and distance are combined in a manner that can result in unexpected distortions of visual space. A simple heuristic model which can produce the observed distortions is outlined.

Recent physiological studies have established that cortical cells that are tuned for the direction of motion may also exhibit tuning for binocular disparity. This tuning does not appear to provide any advantage in discriminating the direction of global motion in random-dot kinematograms. Here we investigated the possibility that this tuning may be important in the perception of transparent motion. Random-dot kinematograms were presented which contained coherent motion in a single direction or in two opposing directions. A greater proportion of signal dots was required for the detection of transparent motion than of motion in a single direction. This difference vanished when the two opposite directions of motion were presented with different disparities. These results suggest that the direction of global motion can be computed separately for surfaces which are clearly segregated in depth.

Can the motion system selectively process elements at a particular depth? We attempted to answer this question using global coherence tasks in which signal and noise elements could be given different disparities. In experiment 1 we found that, if all the signal elements had a disparity different from that of the noise elements, performance was far better than when they had the same disparity (at least for stereo-normal observers). In a second experiment we found that adding additional noise elements to the motion task had no effect if they had a different disparity (however, they had a marked effect for stereo-blind observers). We conclude that stereo disparity can be used as a segmentation cue by the motion system.

Contrast thresholds for detecting sine-wave Gabor patches in two-dimensional externally added random-pixel noise were measured. Thresholds were obtained for monocular and binocular signals in the presence of spatial correlated (identical) and uncorrelated (independent) noise in the two eyes. Measurements were obtained at four different spectral densities of noise (including zero). Thresholds were higher for monocular stimuli than for binocular, and higher in the presence of correlated noise compared to uncorrelated noise. The magnitude of binocular summation, similar in correlated and uncorrelated noise, decreased with increasing noise strength. The independent contributions of internal noise and sampling efficiency to detection were analysed. Sampling efficiencies were higher for binocular than for monocular viewing for both types of noise, with values being higher with uncorrelated noise. Binocular stimuli showed a lower equivalent noise level compared to the mean monocular case for both types of noise.

When a horizontal or vertical magnifier is placed before one eye, a frontoparallel surface appears slanted. It appears slanted away from the eye with horizontal magnification (geometric effect) and toward the eye with vertical magnification (induced effect). According to current theory, the apparent slant in the geometric and induced effects should increase with viewing distance. The geometric effect does scale with distance, but there are conflicting reports as to whether the induced effect does. Ogle (1938 Archives of Ophthalmology 20 604–623) reported that settings in slant-nulling tasks increase systematically with viewing distance, but Gillam et al (1988 Perception & Psychophysics 44 473–483) and Rogers et al (1995 Perception 24 Supplement, 33) reported that settings in slant-estimation tasks do not. We re-examined this apparent contradiction. First, we conducted two experiments whose results are consistent with the literature and thus replicate the apparent contradiction. Next, we analyzed the signals available for stereoscopic slant perception and developed a general model of perceived slant. The model is based on the assumption that the visual system knows the reliability of various slant-estimation methods for the viewing situation under consideration. The model's behavior explains the contradiction in the literature. The model also predicts that, by manipulating eye position, apparent slant can be made to increase with distance for vertical, but not for horizontal, magnification. This prediction was confirmed experimentally.

Magnocellular-pathway deficits have been hypothesised to be responsible for the problems experienced by dyslexic individuals in reading. However, research has yet to provide a detailed account of the consequences of these deficits or to identify the behavioural link between them and reading disabilities. The aim of the present study was to determine the potential consequences of the magnocellular-pathway deficits for dyslexics in a comprehensive range of visual tasks. Dyslexics and nondyslexics were compared on their ability to (i) perform vernier-acuity and orientation-acuity tasks; (ii) perceive motion by using a range of measures common in the psychophysical literature (D_min, D_max, and global coherence); and (iii) perceive shapes presented in random-dot stereograms at a range of disparity pedestals, thereby dissociating stereopsis from vergence control. The results indicated no significant differences in performance between the dyslexic and nondyslexic subjects in terms of the visual-acuity measures. In general, dyslexics performed relatively poorly on measures of motion perception and stereopsis, although when considered individually some of the dyslexics performed better than some of the controls. The poor performance of the dyslexics in the stereogram tasks was attributable to a subgroup of dyslexics who also appeared to have severe difficulty with the motion-coherence task. These data are consistent with previous evidence that some dyslexics may have deficits within the magnocellular visual pathway.

Movement through an environment produces an optical spatiotemporal pattern, known as a flow field. When visually guiding movement using a flow field, do humans make use of information about the distance of constituent elements? Employing a novel active steering task, we examined the use of depth (height-in-scene and disparity) and the role of the retinal motion distribution in the perceptual control of heading from flow. We found that retinal motion distribution, rather than depth order, has the primary role in determining the accuracy of steering.

Displays consisted of orthographic and perspective projections of three collinear dots rotating rigidly around a fixed centre in a plane slanted 45° in depth. Observers were asked to decide whether the middle of the three dots was exactly centred in 3-D space between the other two dots. The visible rotation segments were 120°, 160°, or 200° and the displacements were 2%, 4%, or 6%. Our untrained observers performed more poorly overall than well-practised observers tested earlier by Lappin and Fuqua (1983 Science 221 480–482). Results of additional manipulations suggested that 2-D image properties played a more important role than acknowledged by Lappin and Fuqua. First, displacement size and the position of the rotation centre produced nonlinear effects. Second, the direction of displacement affected performance. Third, projection type and display type (static snapshots versus elliptic traces) were involved in complex interactions. Clearly, performance was affected by 2-D image properties that should be irrelevant to the calculation of 3-D projective invariants.