Biomedical subjects

B G Cumming

Publications and source records attributed to B G Cumming.

28 records · Page 2Linked to original sources

PubMed1995

The relationship between stereoacuity and stereomotion thresholds.

There are in principle at least two binocular sources of information that could be used to determine the motion of an object towards or away from an observer; such motion produces changes in binocular disparities over time and also generates different image velocities in the two eyes. It has been argued in the past that stereomotion is detected by a mechanism that is independent of that which detects static disparities. More recently this conclusion has been questioned. If stereomotion detection in fact depends upon detecting disparities, there should be a clear correlation between static stereo-detection thresholds and stereomotion thresholds. If the systems are separate, there need be no such correlation. Four types of threshold measurement were performed by means of random-dot stereograms: (1) static stereo detection/discrimination; (2) stereomotion detection in random-dot stereograms (temporally uncorrelated); (3) stereomotion detection in temporally correlated random-dot stereograms; and (4) binocular detection of frontoparallel motion. Three normal subjects and five subjects with unusually high stereoacuities were studied. In addition, two manipulations were performed that altered stereomotion thresholds: changes in mean disparity, and image defocus produced by positive spectacle lenses. Across subjects and conditions, stereomotion thresholds were well correlated with stereo-discrimination thresholds. Stereomotion was poorly correlated with binocular frontoparallel-motion thresholds. These results suggest that stereomotion is detected by means of registering changes in the output of the same disparity detectors that are used to detect static disparities.

Cues↗

PubMed1994

Integration of stereopsis and motion shape cues.

A global shape judgement task was used to investigate the combination of stereopsis and kinetic depth. With both cues present, there were no distortions of shape perception, even under conditions where either cue alone did show such distortions. We suggest that the addition of motion information overcomes the stereo distance scaling problem. However, when incongruent combinations of disparity and motion were used, the results did not match predictions of a number of combination theories. These data could be described by a model which used weighted linear combination after correctly scaling disparities for viewing distance. When the motion cue was weakened by presenting only two frames of each motion sequence, stereo was weighted more heavily.

Cues↗

PubMed1994

Binocular mechanisms for detecting motion-in-depth.

There are in principle at least two binocular sources of information that could be used to determine the motion of an object towards or away from an observer: such motion produces changes in binocular disparities over time and also generates different image velocities in the two eyes. Existing psychophysical and physiological evidence is reviewed. It is concluded that these data are inconclusive concerning whether one or both of these sources of information are used in primate vision. Thresholds were measured for disparity modulations in dynamic (temporally uncorrelated) random dot stereograms (RDS), and for RDS in which the same random dot pattern was used throughout (temporally correlated). Although the first stimulus contains no consistent inter-ocular velocity differences, thresholds were generally slightly lower for this stimulus than for temporally correlated stimuli. Sensitivity to the temporal derivative of disparity is therefore adequate to account for human stereomotion detection. A stimulus was devised in which monocular motion was clearly visible to each eye (with opposite velocities) but in which all disparity changes were beyond the temporal resolution of stereopsis. This produced no sensation of motion-in-depth. Similarly, stimuli beyond the spatial resolution of stereopsis did not support stereomotion detection. These data strongly suggest that stereomotion is primarily detected by means of temporal changes in binocular disparity. We argue that there is no experimental evidence that supports the existence of a mechanism sensitive to inter-ocular velocity differences.

Depth Perception↗

PubMed1991

Vertical disparities and perception of three-dimensional shape.

The information about depth and three-dimensional shape available from the horizontal component of the stereo disparity field requires interpretation in conjunction with information about egocentric viewing distance (D). A novel computational approach for estimating D was proposed by Mayhew and Longuet-Higgins, who demonstrated that the horizontal gradient of vertical disparities uniquely specifies the viewing distance. We have now used random dot stereograms in a shape judgement task to show that changes in vertical disparities have no effect on perceived three-dimensional shape. Changes in ocular convergence do alter perceived shape, suggesting substantial changes in the subjects' scaling of horizontal disparities. We conclude that vertical disparities are not used to scale disparities for viewing distance, and that extraretinal signals must be considered when analysing human three-dimensional shape perception.

Depth Perception↗

PubMed1986

Disparity-induced and blur-induced convergence eye movement and accommodation in the monkey.

The dynamics of vergence eye movement and of ocular accommodation were studied in two monkeys trained to track a haploscopically presented target that appeared to move in depth. The target was presented under four conditions: monocular viewing, normal binocular viewing, accommodation-open-loop binocular viewing, and "conflict" viewing, in which the accommodation and vergence stimuli did not correspond to those produced by any real target in three-dimensional space. The first and third conditions were chosen because in each case only one of the two primary cues that guide accommodation and vergence was operative: blur in the first case and disparity in the third. We usually studied responses to apparent target movement directly toward or away from the right eye, in which accommodation was measured. The latencies of the accommodation responses to steps toward the monkey were approximately 180 and 240 ms in the two monkeys, while in both monkeys the latencies of convergence were approximately 160 ms. Neither the vergence latencies nor the accommodation latencies were greatly different in monocular and binocular viewing. Responses to a sinusoidally moving target (frequencies 0.1-1.2 Hz; peak-to-peak amplitude 0.5-4 diopters or meter-angles) were studied in the first three of the above viewing conditions. In binocular viewing, even with accommodation open-loop, vergence and accommodation showed much smaller phase lags than in monocular viewing. Furthermore, in response to step changes, both vergence and accommodation velocities were higher in binocular viewing than in monocular viewing. Thus the dynamic control of both vergence and accommodation relies predominantly on disparity signals. At low frequencies (0.2 or 0.3 Hz) the monkeys showed only a modest ability to separate their accommodation and vergence responses when presented with conflicting blur and disparity cues. A simple linear calculation based on the data above was used to predict the responses in such situations. The predicted and observed responses were in reasonable agreement.

Accommodation, Ocular↗

PubMed1986

Neurons in the monkey midbrain with activity related to vergence eye movement and accommodation.

We recorded from neurons dorsal and dorsolateral to the third nerve nucleus of the monkey whose discharge rates modulated when the monkey tracked targets moving in depth but not when it tracked targets moving from side to side. The neurons' activity modulated equally well whether the target moved directly toward one eye or the other. For most neurons the amplitude of modulation was similar whether the monkey tracked monocularly (blur cue alone), binocularly with accommodation open-loop (disparity cue alone), or in normal binocular viewing. By comparing the modulation in normal binocular viewing with that when the blur and disparity cues were in conflict we were able to show that 19 neurons discharged in relation to the vergence response alone and not to accommodation. Eight neurons discharged exclusively in relation to accommodation. While the monkeys tracked targets moving in depth so that target vergence varied with a sinusoidal time course (frequency 0.1 or 0.2 Hz) the discharge modulations of identified vergence cells generally showed much more phase lead than expected of motoneurons. We examined the activity of a subset of these vergence cells in response to a range of stimulus frequencies to compare the dynamics of these neurons with motoneurons. The phase leads were larger than those expected of motoneurons over the entire frequency range tested. We speculate that vergence neurons may selectively activate (directly or indirectly) motoneurons with longer time constants than the mean.

Accommodation, Ocular↗

PubMed1977

The in vitro propagation of amaryllis (Hippeastrum spp. hybrids).

A new, rapid technique for the propagation of amaryllis (Hippeastrum spp. hybrids) by means of tissue culture is reported. Leaf bases, scapes, peduncles, inner bulb scales and ovaries were cultured successfully in vitro and plantlets were induced readily at various concentrations of growth regulators. Some plantlets also were produced in the absence of growth regulators. The most productive tissues for propagation were inverted scapes and peduncles, cultured in a modified Murashige and Skoog salt solution with added organic constituents and 1 mg per 1 (4.5 micron) 2,4-dichlorophenoxyacetic acid (2,4-D) and 1 mg per 1 (4.4 micrometer) 6-benzylaminopurine (BAP). Plantlets induced axenically also grew roots on the generalized shoot-inducing medium so that no special rooting medium was required. Although friable callus was obtained from ovary tissue cultured on a medium containing 2 mg per 1 (11 micrometer) naphthaleneacetic acid and 4 mg per 1 (18 micrometer) BAP, it produced shoots after 8 weeks of further subculture on the same medium. An average of 10 rooted plantlets was obtained from each scape or peduncle explant on the shoot-propagating medium. Thus, if 45 explants are obtained from each bulb, 450 rudimentary plantlets could be obtained from each mother bulb in 8 weeks of culture. This is a substantial increase over present propagation methods.

Culture Techniques↗

PubMedDate not supplied

Integration of depth modules: stereopsis and texture.

Global shape judgements were employed to examine the combination of stereopsis and shape-from-texture in the determination of three-dimensional shape. Adding textural variations to stereograms increased perceived depth. Thus, texture was not simply vetoed by the strong stereo cue. In experiments where the depth specified by texture was incongruent with that specified by stereo, the data were well described by a weighted linear combination rule. Although only a small weight was assigned to texture, this weight was somewhat greater at a farther viewing distance. This could be a consequence of the decreased reliability of stereopsis at far viewing distances.

Depth Perception↗

PubMedDate not supplied

Effects of different texture cues on curved surfaces viewed stereoscopically.

Stereoscopic shape judgements can be modified by the addition of texture cues. This paper examines the properties of texture that are responsible for this effect. When a three-dimensional curved surface is projected onto a two-dimensional image, changes in surface orientation result in gradients of texture element size (or area), shape (compression) and density in the image. Manipulating each of these gradients independently we found that 97% of the variance in the results could be accounted for by the compression gradient. When the texture pattern corresponds to a highly anisotropic texture on the object's surface, shape-from-texture becomes ineffective. These results suggest that human shape-from-texture proceeds under the assumption that textures are statistically isotropic, and not that they are homogeneous.

Depth Perception↗

PubMedDate not supplied

Modeling V1 neuronal responses to orientation disparity.

The contribution of interocular orientation differences to depth perception, at either the neuronal or the psychophysical level, is unclear. To understand the responses of binocular neurons to orientation disparity, we extended the energy model of Ohzawa et al. (1990) to incorporate binocular differences in receptive-field orientation. The responses of the model to grating stimuli with interocular orientation differences were examined, along with the responses to random dot stereograms (RDS) depicting slanted surfaces. The responses to combinations of stimulus orientations in the two eyes were left-right separable, which means there was no consistent response to the binocular orientation difference. All existing neuronal data concerning orientation disparity can be well described by this type of model (even a version with no disparity selectivity). The disparity sensitive model is nonetheless sensitive to changes in RDS slant, although it requires narrow orientation bandwidth to produce substantial modulation. The disparity-insensitive model shows no selectivity to slant in this stimulus. Several modifications to the model were attempted to improve its selectivity for orientation disparity and/or slant. A model built by summing several disparity-sensitive models showed left-right inseparable responses, responding maximally to a consistent orientation difference. Despite this property, the selectivity for slant in RDS stimuli was no better than the simple disparity-selective model. The range of models evaluated here demonstrate that interocular orientation differences are neither necessary nor sufficient for signaling slant. In contrast, within the framework of the energy model, positional disparity sensitivity appears to be both necessary and sufficient.

Animals↗