Biomedical subjects

Robert S Allison

Publications and source records attributed to Robert S Allison.

7 recordsLinked to original sources

PubMed2006

Illusory scene distortion occurs during perceived self-rotation in roll.

We report a novel illusory distortion of the visual scene, which became apparent during both: (i) observer rotation inside a furnished stationary room; and (ii) room rotation about the stationary observer. While this distortion had several manifestations, the most common experience was that scenery near fixation appeared to sometimes lead and other times lag more peripheral scenery. Across a series of experiments, we eliminated explanations based on eye-movements, distance misperception, peripheral aliasing, differential motion sensitivity and adaptation. We found that these illusory scene distortions occurred only when the observer perceived (real or illusory) changes in self-tilt and maintained a stable fixation.

Adult↗

PubMed2005

Effect of decorrelation on 3-D grating detection with static and dynamic random-dot stereograms.

Three experiments examined the effects of image decorrelation on the stereoscopic detection of sinusoidal depth gratings in static and dynamic random-dot stereograms (RDS). Detection was found to tolerate greater levels of image decorrelation as: (i) density increased from 23 to 676 dots/deg2; (ii) spatial frequency decreased from 0.88 to 0.22 cpd; (iii) amplitude increased above 0.5 arcmin; and (iv) dot lifetime decreased from 1.6s (static RDS) to 80 ms (dynamic RDS). In each case, the specific pattern of tolerance to decorrelation could be explained by its consequences for image sampling, filtering, and the influence of depth noise.

Adult↗

PubMed2004

Detection of the depth order of defocused images.

The sign of an accommodative response is provided by differences in chromatic aberration between under- and over-accommodated images. We asked whether these differences enable people to judge the depth order of two stimuli in the absence of other depth cues. Two vertical edges separated by an illuminated gap were presented at random relative distances. Exposure was brief, or prolonged with fixed or changing accommodation. The gap was illuminated with tungsten light or monochromatic light. Subjects could detect image blur with brief exposure for both types of light. But they could detect depth order only in tungsten light with long exposure, with or without changes in accommodation.

Accommodation, Ocular↗

PubMed2004

The stimulus integration area for horizontal vergence.

Over what region of space are horizontal disparities integrated to form the stimulus for vergence? The vergence system might be expected to respond to disparities within a small area of interest to bring them into the range of precise stereoscopic processing. However, the literature suggests that disparities are integrated over a fairly large parafoveal area. We report the results of six experiments designed to explore the spatial characteristics of the stimulus for vergence. Binocular eye movements were recorded using magnetic search coils. Each dichoptic display consisted of a central target stimulus that the subject attempted to fuse, and a competing stimulus with conflicting disparity. In some conditions the target was stationary, providing a fixation stimulus. In other conditions, the disparity of the target changed to provide a vergence-tracking stimulus. The target and competing stimulus were combined in a variety of conditions including those in which (1) a transparent textured-disc target was superimposed on a competing textured background, (2) a textured-disc target filled the centre of a competing annular background, and (3) a small target was presented within the centre of a competing annular background of various inner diameters. In some conditions the target and competing stimulus were separated in stereoscopic depth. The results are consistent with a disparity integration area with a diameter of about 5 degrees. Stimuli beyond this integration area can drive vergence in their own right, but they do not appear to be summed or averaged with a central stimulus to form a combined disparity signal. A competing stimulus had less effect on vergence when separated from the target by a disparity pedestal. As a result, we propose that it may be more useful to think in terms of an integration volume for vergence rather than a two-dimensional retinal integration area.

Adult↗

PubMed2003

Geometric and induced effects in binocular stereopsis and motion parallax.

This paper examines and contrasts motion-parallax analogues of the induced-size and induced-shear effects with the equivalent induced effects from binocular disparity. During lateral head motion or with binocular stereopsis, vertical-shear and vertical-size transformations produced 'induced effects' of apparent inclination and slant that are not predicted geometrically. With vertical head motion, horizontal-shear and horizontal-size transformations produced similar analogues of the disparity induced effects. Typically, the induced effects were opposite in direction and slightly smaller in size than the geometric effects. Local induced-shear and induced-size effects could be elicited from motion parallax, but not from disparity, and were most pronounced when the stimulus contained discontinuities in velocity gradient. The implications of these results are discussed in the context of models of depth perception from disparity and structure from motion.

Depth Perception↗

PubMed2003

Coherent perspective jitter induces visual illusions of self-motion.

Palmisano et al (2000 Perception 29 57-67) found that adding coherent perspective jitter to constant-velocity radial flow improved visually induced illusions of self-motion (vection). This was a surprising finding, because unlike pure radial flow, this jittering radial flow should have generated sustained visual--vestibular conflicts--previously thought to always reduce/impair vection. We attempted to ascertain the essential stimulus features for this jitter advantage for vection by examining three novel types of jitter display. While adding incoherent jitter to radial flow was found to impair vection, adding coherent non-perspective jitter had little effect on this subjective experience (contrary to the notion that jitter improves vection by reducing adaptation to radial flow). Importantly, we found that coherent perspective jitter not only improves the vection induced by radial flow, but it also appears to induce modest vection by itself (demonstrating that vection can still occur when there is an extreme mismatch between actual and expected vestibular activity). These results suggest that the previously demonstrated advantage for coherent perspective jitter was due (in part at least) to jittering vection combining with forwards vection in depth to produce a more compelling overall vection experience.

Adolescent↗

PubMed2003

Effects of scleral search coil wear on visual function.

PURPOSE: The scleral search coil is widely regarded as the gold standard measurement technique for eye movements. The effect of wearing scleral search coils on human vision has not been systematically studied. However, there are anecdotal reports of degraded visual acuity, mild eye irritation, and an increase rise in intraocular pressure (IOP). The current study was conducted to investigate the effect of scleral search coil use on visual acuity and ocular integrity. METHODS: Six subjects were examined; all had previously worn search coils. Two drops of topical anesthetic were administered before insertion of the coils. Coils were inserted by hand and secured by applying mild pressure. The coils were removed after 45 minutes or on request of either the subject or the clinician. Before, during (at 15-minutes intervals), and after the coil was worn, the following measurements were taken for both eyes: tonometry (noncontact), corneal topography, biomicroscopic examination, visual acuity (monocular Snellen), and an eye-discomfort rating. RESULTS: Scleral coils produced a variety of effects, including ocular discomfort, hyperemia of the bulbar conjunctiva, increased IOP, buckling of the iris, grade 2 and 3 corneal staining, and reduction in visual acuity. Effects appeared as early as 15 minutes after insertion of the coils. All observed effects seemed to be transient and dissipated after coils were removed. CONCLUSIONS: Scleral coils may not be appropriate for all subjects. The findings suggest that there is a need for thorough subject prescreening and that clinicians should consider the risk/benefit ratio. Acute reduction in visual acuity may confound search coil findings. More research is needed to determine the maximum wearing time for properly screened subjects.

Adult↗