Biomedical subjects

S B Stevenson

Publications and source records attributed to S B Stevenson.

At least 19 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↗

PubMed1999

The effect of target size and eccentricity on reflex disparity vergence.

This study examined the effects of stimulus size and eccentricity on reflex disparity vergence: the small, involuntary corrections of eye alignment which serve to minimize the binocular disparity of fixated targets. Subjects were instructed to fixate steadily on a small, stationary mark superimposed on the center of a dynamic random dot stereogram. The stereogram was binocularly uncorrelated except for a fully correlated patch whose size and eccentricity were varied systematically across trials. The disparity of the patch was varied sinusoidally over time to stimulate vergence following movements. The overall purpose was to determine the relative contributions of various field loci in controlling binocular fixation by finding the smallest patch which would reliably drive vergence against the effort to fixate steadily. Psychophysical thresholds for detection of the correlated patch stimuli were also measured for comparison to the oculomotor results. Results showed that the smallest effective patch increased with eccentricity similarly for both vergence responses and psychophysical detection, suggesting they depend on a common, presumably cortical matching process. The dependence of response on eccentricity is roughly consistent with changes in the cortical magnification factor, suggesting that the area of cortex stimulated may be the determining factor in vergence responses to this class of stimulus.

Convergence, Ocular↗

PubMed1999

Post-retinal processing of background luminance.

It is generally thought that mean luminance and low spatial frequency information in a visual image are sharply attenuated at the retina, due to processes of light adaptation and the spatial filtering effects of lateral inhibition. Our results from interocular luminance masking suggest, however, that cortical masking effects play a primary role in the attenuation of low frequency sensitivity. Results also revealed that interocular luminance masking saturates and that semisaturation occurs where left and right eye luminances are equal, implying that the test luminance limits the effectiveness of the mask through interocular gating.

Adaptation, Ocular↗

PubMed1998

Vergence eye movements elicited by stimuli without corresponding features.

We have observed quantitative depth perception with a dichoptic stimulus which possessed no contrast-defined binocular corresponding features (phantom stereogram). The depth perception can be the result of appreciation of a partial-occlusion situation depicted by the stimulus, or the result of activities of low-level disparity detectors which are capable of combining dissimilar local features in the stimulus. Although both mechanisms predict similar depth perception, they predict different vergence eye-movement outputs, especially in the vertical dimension. To identify the underlying mechanisms of the phantom stereopsis, we recorded vergence tracking eye movements to four types of dichoptic stimuli: (a) conventional stereogram with horizontal disparity (HD); (b) horizontal phantom stereogram (HP); (c) conventional stereogram with vertical disparity (VD); and (d) vertical phantom stereogram (VP). We found that HD, HP, and VD stimuli could elicit robust vergence tracking eye movements but VP stimulus could not. While the success of HP stimulus in eliciting vergence tracking may be explained by proximal vergence, the failure of VP stimulus in eliciting vergence tracking clearly indicates that phantom stereogram could not elicit coherent responses among low-level disparity detectors. Partial occlusion, therefore, has to play an important role in the depth perception from the phantom stereogram.

Data Interpretation, Statistical↗

PubMed1998

Effect of background components on spatial-frequency masking.

Previous studies of spatial-frequency masking and adaptation have shown that the contrast-detection threshold elevates maximally when the test spatial frequency is the same as the masking (or adapting) frequency but changes only slightly when they are separated by two or more octaves. At low spatial frequencies, however, the peak of the threshold-elevation function does not obey this rule: there is a well-established peak shift in the threshold-elevation functions toward higher spatial frequencies. We investigated whether this shift might be due to the masking effects caused by the background field, which contributes energy at the very low end of the spectrum. We first measured the effect of a 3-cycles/deg (c/deg) mask on detection of a range of test frequencies, compared with unmasked detection thresholds. We then measured the combined effect of a 2-c/deg and a 3-c/deg mask on detection, compared with detection with just the 2-c/deg mask. The comparison in the second case still tests the effect of the 3-c/deg mask, but the presence of the hidden 2-c/deg mask causes the peak masking effect to shift toward higher frequencies. This result provides a proof of concept for the hypothesis that the peak shift at low spatial frequencies is caused by the low-frequency energy in the background field, which is present in both masked and unmasked conditions. A five-parameter quantitative model of frequency masking is presented that describes the pure contrast-detection function, the frequency-masking functions at mask frequencies of 0.25, 0.5, 2, and 3 c/deg, and the peak-shift phenomenon.

Contrast Sensitivity↗

PubMed1997

Binocular matching of dissimilar features in phantom stereopsis.

Previously we have demonstrated that quantitative depth perception can be elicited from a stereogram that lacks contrast defined binocular corresponding elements (phantom stereopsis). In this report, we use computer simulation to demonstrate that it is biologically plausible for some known binocular cortical cell types to combine non-conventional matching features. Therefore, binocular matching processes based on the responses of these cells could be a conventional one, namely, looking for similar response patterns in the two eyes. While at cell types we simulated gave identical disparity outputs to the conventional stereogram, they responded differently to the phantom stereogram. Processes other than low-level disparity detectors may have to be invoked in order to achieve a unique depth solution.

Computer Simulation↗

PubMed1997

Human stereo matching is not restricted to epipolar lines.

Computational approaches to stereo matching have often taken advantage of a geometric constraint which states that matching elements in the left and right eye images will always fall on "epipolar lines". The use of this epipolar constraint reduces the search space from two dimensions to one, producing a tremendous saving in the computation time required to find the matching solution. Use of this constraint requires a precise knowledge of the relative horizontal, vertical and torsional positions of the two eyes, however, and this information may be unavailable in many situations. Experiments with dynamic random element stereograms reveal that human stereopsis can detect and identify the depth of matches over a range of both vertical and horizontal disparity. Observers were able to make accurate near/far depth discriminations when vertical disparity was as large as 45 arcmin, and were able to detect the presence of correlation over a slightly larger range. Thus, human binocular matching sensitivity is not strictly constrained to epipolar lines.

Depth Perception↗

PubMed1997

The influence of subject instruction on horizontal and vertical vergence tracking.

Previously it has been reported that horizontal disparity vergence is strongly influenced by subject instructions to vary attention or tracking effort. This paper describes experiments which compared these instruction effects on horizontal and vertical disparity vergence. Within-trial comparisons were made possible by use of oblique (combined horizontal and vertical) disparity modulation. Subjects viewed a flat, fully correlated, dynamic random noise stereogram pattern through stationary circular apertures, with a small stationary fixation cross superimposed in the center. The disparity of the noise pattern was either modulated sinusoidally or changed abruptly. Subjects were instructed either to (1) hold fixation on the cross and ignore the disparity modulation of the noise pattern; or (2) follow the movement of the noise pattern as accurately as possible. Subjects showed clear effects of instruction on the horizontal component of tracking, but showed little or no effect on the vertical component. Horizontal and vertical components of oblique vergence tracking appear to be largely independent, and vertical vergence is affected minimally, if at all, by an effort to track.

Attention↗

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

Effects of spatial frequency, duration, and contrast on discriminating motion directions.

The minimum speed required for discriminating the direction of drifting gratings was measured at a variety of spatial frequencies, display durations, and contrasts. As was reported previously, speed thresholds were relatively constant for middle and high spatial frequencies, but speed threshold was found to be almost inversely proportional to spatial frequency in the range of 0.25 to 1.0 c/deg. Speed threshold was also found to be inversely proportional to duration between 73 and 40 ms. These results at low frequencies and short durations are shown to be consistent with limits set by the spread of energy in the stimuli, producing velocity uncertainty. A quantitative model of temporal filtering is presented that largely accounts for results at all spatial frequencies and durations by the inclusion of constant positional noise. A discussion includes the possible roles of magnocellular and parvocellular mechanisms in mediating speed thresholds.

Contrast Sensitivity↗

PubMed1994

Quantitative stereoscopic depth without binocular correspondence.

What features in a stereogram define the disparities that lead to stereoscopic depth? The usual answer is that luminance-defined edges from the two eyes are matched and produce depth perception. But parts of an object may be occluded by other objects and absent from one eye's view. It was suggested that unpaired monocular elements might signal occlusion in depth, and the qualitative perception of depth associated with unmatched elements has been shown to be consistent with the geometry of occlusion. We designed a stereogram that simulates a particular occlusion situation: an opaque white rectangle is stereoscopically in front of a large black rectangle pasted on a white background. The position of the occluder is adjusted so that its left edge obscures the left-hand edge of the black rectangle in the right eye view and its right edge obscures the right-hand edge of the black rectangle in the left eye view. We report here that quantitative stereopsis can be seen from this stereogram, even though there are no binocular corresponding luminance edges to match.

Depth Perception↗

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↗

PubMed1994

A polar coordinate system for describing binocular disparity.

When a meridional magnifier is introduced in front of one eye, a planar surface is perceived as slanted about a vertical axis. If the horizontal meridian is magnified, the perceived slant is away from the eye with the magnifier (geometric effect). If the vertical meridian is magnified, the slant is towards the eye with the magnifier (induced effect). While the geometric effect can be explained by the binocular horizontal disparities introduced by the horizontal magnifier, the induced effect has to be explained differently. Various models have been developed and the induced effect has generally been explained as a reinterpretation of horizontal disparity under a new reference of stereoscopic localization which is resultant from the vertical positional disparity introduced by the vertical magnifier. In this paper we describe binocular disparity in a polar coordinate system. Under this system, horizontal and vertical disparities are combined into a single stimulus variable, polar angle disparity. We show that the spatial distribution of polar angle disparity can faithfully describe the three-dimensional slant and inclination of a planar surface relative to the gaze normal plane. Both geometric and induced effects can be explained as direct responses to the polar angle disparity map distorted by the magnifier. Theoretical predictions based on the polar angle disparity are compared with experimental findings.

Depth Perception↗

PubMed1994

Isovergence surfaces: the conjugacy of vertical eye movements in tertiary positions of gaze.

Conjugate gaze is often defined as the equal angle rotation of the two eyes. For fixation at far distances, the optical axes are parallel and conjugacy is defined irrespective of the coordinate system. For nearby or finite fixation distances, the evaluation of conjugacy for many gaze postures depends on the coordinate system used to measure it. For example, if the eye is elevated or depressed and the eye is rotated about a vertical axis, the intersections of lines of sight with a tangent screen will describe either straight lines or arcs depending on whether the vertical axis is fixed with respect to the head or to the eye. Because of the horizontal separation of the two eyes, the binocular fixation of near targets at tertiary positions of gaze will require a vertical vergence component for head-referenced but not eye-referenced measurements. The vertical gaze alignment of three human subjects was measured as they viewed targets placed at secondary and tertiary eye positions at two different distances. Vertical vergence was either held open or closed-loop. The lines of sight were found to intersect (i.e. vertical gaze was aligned) regardless of target position or viewing condition.

Convergence, Ocular↗

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↗