Biomedical subjects

L K Cormack

Publications and source records attributed to L K Cormack.

12 recordsLinked to original sources

PubMed2000

A contrast paradox in stereopsis, motion detection, and vernier acuity.

Stereoacuity improves with increasing contrast, unless the increase is monocular. In this case performance paradoxically suffers. This study examined whether this contrast paradox occurs for two other classes of visual judgment: two-frame motion and vernier acuity. We constructed three homologous tasks in which the two components of a gabor stimulus (stereo half-images, motion frames, vernier components) were either both high contrast, both low contrast, or mismatched. The contrast paradox was evident in all three tasks and showed a similar spatial frequency dependence. We suggest the contrast paradox results from the combination of mismatched signals by a single filter.

Contrast Sensitivity↗

PubMed1997

Interactions of spatial frequency and unequal monocular contrasts in stereopsis.

Increasing the contrast of just one eye's image degrades stereothresholds; this phenomenon is referred to as the stereo contrast paradox. In experiment one, this paradox was found to be absent in dynamic random-element stereograms; thresholds were simply limited by the lower of the two eyes' contrasts. In experiment two, in which narrowband Gabor targets were used, the paradox was found to be strongest at relatively low spatial frequencies (1 cycle deg-1). As spatial frequency was increased, the paradox gradually disappeared. At relatively high spatial frequencies (5 cycles deg-1), thresholds were generally limited by the lower of the two eyes' contrasts, as was found for the dynamic noise targets. These results demonstrate the interactions of spatial frequency and contrast in binocular image combination and yield clues as to the different roles which high and low spatial frequencies may play in stereopsis.

Contrast Sensitivity↗

PubMed1997

Element density and the efficiency of binocular matching.

Constraints on binocular matching were investigated by comparing the thresholds for interocular correlation in random element displays for human and model observers, with element density manipulated as a parameter. The models consisted of ideal decision rules operating on the entire stimulus, only on the edges in the stimulus, or only on the sparse minority elements in the stimulus. The results indicate that the human visual system selectively attends to the stimulus edges or to the sparse elements under most circumstances. Efficiencies (human or model) were highest at very low element densities (approximately 20%) and decreased with increasing element density with a log-log slope of -0.5, indicating that dynamic random element stereograms at the traditional 50% element density are vastly undersampled.

Computer Simulation↗

PubMed1997

Asymmetries and errors in perception of depth from disparity suggest a multicomponent model of disparity processing.

In three experiments, asymmetries between the processing of crossed and uncrossed disparities were investigated. The target was a luminance-defined circle concentric to a fixation mark, viewed stereoscopically on a computer monitor for 105 msec. Fifteen disparities were presented according to the method of constant stimuli. Observers indicated the apparent direction of target depth relative to fixation. All experiments measured both the accuracy and latency of this response. Experiment 1 showed fewer errors and shorter reaction times for identifying crossed disparities. Experiments 2 and 3 replicated Experiment 1 and also showed that observers may often perceive a target in the direction opposite that prescribed by the disparity information. We propose that the asymmetries and reversals result from differences in computation of sign, not of magnitude. This notion is consistent with a scheme of continuous disparity tuning and accounts for such asymmetries and errors without positing disparity pooling mechanisms.

Adult↗

PubMed1996

Binocular correlation detection with oriented dynamic random-line stereograms.

Stereopsis relies principally on the extraction of horizontal retinal disparities. As such, we assume that the vertical contours (i.e., horizontal contrast energy) are of principle import for stereopsis. Yet there are theoretical reasons for believing that horizontal contours should be involved in binocular matching (if not stereopsis proper) as well. First, they would facilitate the computation of vertical disparities, which are necessary for the control of disjunctive eye movements and perhaps the computation of absolute depth. Second, the process of binocular matching is a two-dimensional one; its solution requires information along both principle orientations. In this study, we have measured the efficacy with which horizontal or vertical contours can be binocularly matched by measuring thresholds for the detection of interocular correlation for oriented dynamic random-line stereograms. We find that the slopes of the psychometric functions are almost a factor of two steeper when matching vertical contours, indicating a narrower noise distribution along the decision axis associated with these stimuli.

Fixation, Ocular↗

PubMed1994

The effect of stimulus contrast and interocular correlation on disparity vergence.

In previous reports, we developed a metric for describing the signal strength of a dynamic random-dot stereogram (DRDS) stimulus at binocular (cyclopean) levels of the human visual system, which takes both contrast and interocular correlation into account. In this study we tested the generality of that metric in relation to the control of horizontal vergence eye movements. Signal strength was assessed by measuring the extent to which a DRDS stimulus could elicit involuntary vergence responses from a subject who was attempting to fixate steadily. Results for both step and sinusoidal disparity modulation paradigms showed that vergence velocity increased when either interocular correlation (IOC) or contrast was increased. Furthermore, IOC and contrast were found to contribute to signal strength for vergence in the same proportion as was found psychophysically. In general, the results indicate that the signals that drive this passive form of vergence are derived according to the same binocular combination rules as the signals that give rise to the perception of surfaces in DRDS stimuli.

Contrast Sensitivity↗

PubMed1994

An upper limit to the binocular combination of stimuli.

It is important to know the spatial extent over which the binocular visual system searches for "matches" or image correspondence. Most models of stereopsis define fixed neighbourhoods in one monocular image in which a search is conducted for a match to some element in the other image. We were unable to experimentally determine fixed values for the extent of these neighbourhoods. We were, however, able to derive a simple rule that predicts performance on a binocular matching task over a large spatio-temporal region, and from which we can calculate the efficiency of our observers. We conclude that the human visual system does not use neighbourhoods of a single, fixed extent to perform binocular matching in a single region of the visual field, but uses more flexible scheme that allows it to function well under a broad range of stimulus conditions.

Depth Perception↗

PubMed1992

Disparity tuning in mechanisms of human stereopsis.

The change in sensitivity across some stimulus dimension which follows adaptation to a particular stimulus can reveal a great deal about the tuning characteristics of underlying sensory/perceptual mechanisms. In this study, a psychophysical adaptation paradigm was employed to characterize the disparity tuning of perceptual mechanisms involved in stereopsis. The stimulus was a dynamic random-dot stereogram (DRDS) portraying a surface which varied in interocular correlation (IOC) and retinal disparity. Adaptation to a fully correlated DRDS surface produced an elevation in IOC threshold over a relatively narrow range of disparities, with maximum effect at the disparity of the adapting stimulus. The width of these disparity tuning functions varied from 5 arc min for adaptation at the horopter to 20 arc min for adaptation at 20 arc min disparity. Frequently, IOC sensitivity was enhanced for disparities on either side of the adapted disparity, suggesting that an opponent center-surround organization operates at an early level of disparity processing. A model of underlying channel structure consistent with these data is presented.

Adaptation, Ocular↗

PubMed1991

Depth attraction and repulsion in random dot stereograms.

Previous studies of perceived attraction or repulsion of adjacent visual targets have used local targets whose positions were varied in both depth and direction. We have measured these effects in three subjects using dynamic random-dot stereograms to isolate depth-axis effects. Results show that both attraction and repulsion effects can occur for overlapping, positively correlated, random-dot surfaces. The results were quantitatively similar to those reported previously for local targets. Manipulation of interocular correlation confirmed that the effects are produced by binocular interactions. Results are explained as accurate judgments based on the stimulus at the cyclopean level.

Depth Perception↗

PubMed1991

Interocular correlation, luminance contrast and cyclopean processing.

We have investigated the nature and viability of interocular correlation as a measure of signal strength in the cyclopean domain. Thresholds for the detection of interocular correlation in dynamic random element stereograms were measured as a function of luminance contrast, a more traditional measure of stimulus strength. At high contrasts, correlation thresholds were independent of contrast. At low contrasts, correlation thresholds were inversely proportional to the square of contrast. Stereothresholds were also measured as a function of both contrast and interocular correlation. At low contrasts, stereoacuity was inversely proportional to both interocular correlation and the square of contrast. These results are consistent with an inherently multiplicative mechanism of binocular combination, such as a cross-correlation of the two eye's inputs.

Contrast Sensitivity↗

PubMed1989

Hyperacuity, superresolution and gap resolution in human stereopsis.

Different types of stereoscopic acuity were studied with tasks adapted from studies of visual direction acuity. Dynamic, random-element stereograms portraying multiple surfaces in depth and a temporal 2AFC procedure were used for all measurements. The three tasks required detection of a depth offset (Hyperacuity task), a depth-axis thickening (Superresolution task), and a depth-axis gap between surfaces (Gap Resolution task). Thresholds for the three tasks were on the order of 3 sec arc, 30 sec arc and 200 sec arc of retinal disparity, respectively. These results are comparable to those for the analogous visual direction tasks on which they were patterned, suggesting that the underlying judgments involved are similar. Results are used to estimate the intrinsic noise of horizontal disparity processing.

Depth Perception↗

PubMedDate not supplied

Disparity-tuned channels of the human visual system.

Traditionally, it has been thought that the processing of binocular disparity for the perception of stereoscopic depth is accomplished via three types of disparity-selective channels--"near," "far," and "tuned." More recent evidence challenges this notion. We have derived disparity-tuning functions psychophysically using a subthreshold summation (i.e. low-level masking) technique. We measured correlation-detection thresholds for dynamic random-element stereograms containing either one or two surfaces in depth. The resulting disparity-tuning functions show an opponent-type profile, indicating the presence of inhibition between disparity-tuned units in the visual system. Moreover, there is clear inhibition between disparities of the same sign, obviating a strict adherence to near-far opponency. These results compare favorably with tuning functions derived psychophysically using an adaptation technique, and with the tuning profiles from published single-unit recordings. Our results suggests a continuum of overlapping disparity-tuned channels, which is consistent with recent physiological evidence as well as models based on other psychophysical data.

Depth Perception↗