Impaired perception of self-motion (heading) in abstinent ecstasy and marijuana users.

RATIONALE: Illicit drug use can increase driver crash risk due to loss of control over vehicle trajectory. This study asks, does recreational use of +/-3,4-Methylenedioxymethamphetamine (MDMA; ecstasy) and tetrahydrocannabinol (THC; marijuana) impair cognitive processes that help direct our safe movement through the world? OBJECTIVE: This study assesses the residual effects of combined MDMA/THC use, and of THC use alone, upon perceived trajectory of travel. METHODS: Perception of self-motion, or heading, from optical flow patterns was assessed using stimuli comprising random dot ground planes presented at three different densities and eight heading angles (1, 2, 4 and 8 degrees to the left or right). On each trial, subjects reported if direction of travel was to the left or the right. RESULTS: Results showed impairments in both drug groups, with the MDMA/THC group performing the worst. CONCLUSIONS: The finding that these psychoactive agents adversely affect heading perception, even in recently abstinent users, raises potential concerns about MDMA use and driving ability.

Perceptual information and attentional constraints in visual search of collision events.

This study examined the detection of collision events when multiple moving objects were present in the scene. Observers were presented with displays simulating a 3-D environment with multiple moving objects. The authors examined the ability of observers to detect collisions using a signal-detection paradigm and a visual search paradigm. The results indicated that, overall, observers were quite accurate at detecting collisions. Observers used both expansion information and static position to detect collisions, with expansion information being the more important source. Singleton search conditions were not processed in parallel, and conjunction search conditions had poorer performance than singleton search conditions. In addition, reaction times were greater for target-present trials as compared with target-absent trials. The results are interpreted in terms of 4 visual search hypotheses for collision detection when multiple moving objects are present.

Perceived depth of 3-D objects in 3-D scenes.

Effects of information specifying the position of an object in a 3-D scene were investigated in two experiments with twelve observers. To separate the effects of the change in scene position from the changes in the projection that occur with increased distance from the observer, the same projections were produced by simulating (a) a constant object at different scene positions and (b) different objects at the same scene position. The simulated scene consisted of a ground plane, a ceiling plane, and a cylinder on a pole attached to both planes. Motion-parallax scenes were studied in one experiment; texture-gradient scenes were studied in the other. Observers adjusted a line to match the perceived internal depth of the cylinder. Judged depth for objects matched in simulated size decreased as simulated distance from the observer increased. Judged depth decreased at a faster rate for the same projections shown at a constant scene position. Adding object-centered depth information (object rotation) increased judged depth for the motion-parallax displays. These results demonstrate that the judged internal depth of an object is reduced by the change in projection that occurs with increased distance, but this effect is diminished if information for change in scene position accompanies the change in projection.

Age-related differences in collision detection during deceleration.

Observers were presented with displays simulating a 3-D environment with obstacles in the path of motion. During the trial, observer motion decelerated at a constant rate and was followed by a blackout prior to the end of the display. On some trials the rate of deceleration resulted in stopping before the collision, whereas on other trials the rate of deceleration resulted in a collision with the obstacles. The observer's task was to detect which trials simulated an impending collision. Proportion of collision judgments was greater for older as compared with younger observers when a collision was not simulated. Older observers showed less sensitivity to detect collisions than younger observers did, particularly at high speeds. The age-dependent results are discussed in terms of analyses based on tau and constant deceleration. The results suggest that increased accident rates for older drivers may be due to an inability to detect collisions at high speeds.

The discrimination of heading from optic flow is not retinally invariant.

Three experiments were conducted to determine whether the discrimination of heading from optic flow is retinally invariant and to determine the importance of acuity in accounting for heading eccentricity effects. In the first experiment, observers were presented with radial flow fields simulating forward translation through a three-dimensional volume of dots. The flow fields subtended 10 degrees of visual angle and were presented at 0 degree, 10 degrees, 20 degrees, and 40 degrees of retinal eccentricity. The observers were asked to indicate whether the simulated movement was to the right or the left of a target that appeared at the end of the display sequence. Eye movements were monitored with an electrooculogram apparatus. In a second experiment, static acuity thresholds were derived for each of the observers at the same retinal eccentricities. There was a significant increase in heading detection thresholds with retinal eccentricity (from 0.92 degree at 0 degree retinal eccentricity to 3.47 degrees at 40 degrees). An analysis of covariance indicated that the variation in sensitivity to radial flow, as a function of retinal eccentricity, is independent of acuity. Similar results were obtained when the Vernier acuity of observers was measured. These results suggest that the discrimination of heading from radial flow is not retinally invariant.

Speed, size, and edge-rate information for the detection of collision events.

In the present study an alternative analysis to tau was considered that was based on perceived speed and size and that assumed constant deceleration for the detection of collision events. Observers were presented with displays simulating a 3-D environment with obstacles in the path of observer motion. During the trial, observer motion decelerated at a constant rate and was followed by a blackout prior to the end of the display. Observers had to detect which trials resulted in a collision. The results indicate that collision detection varied as a function of the size of the obstacles, observer speed, and edge rate--findings not predicted by an analysis of tau. The results suggest that observers use an analysis based on speed and size information. A model that assumes constant deceleration is proposed for braking control.

Cooperativity, priming, and 3-D surface detection from optic flow.

Three experiments were conducted to determine whether the mechanisms responsible for the detection of three-dimensional (3-D) surfaces from optic flow operate in a cooperative manner. The first experiment was conducted to determine whether a hysteresis effect occurs for 3-D surface detection from optic flow. The results of the first experiment demonstrated a hysteresis effect with lower thresholds occurring for decreasing texture density than for increasing texture density. The second experiment used a priming methodology to determine whether this form of cooperativity was based on preactivation of shear detectors or preactivation of 2-D motion detectors. The results suggest that only shear detectors were primed. The third experiment utilized a similar methodology to determine whether a surface representation would produce a priming effect. We found no evidence that the priming effect found in the second experiment was the result of preactivation of a generic representation of the test stimuli. The results of the experiments, considered together, suggest priming of the mechanisms responsible for recovering shear.

The effect of age, retinal eccentricity, and speed on the detection of optic flow components.

Forty observers participated in a study examining the effect of age on the detection of motion in central and peripheral vision. Detection of lamellar (Experiment 1) and radial flow (Experiment 2) was measured for 20 younger observers and 20 older observers (10 men and 10 women in each group). Motion thresholds were measured for angles of 0 degree, 10 degrees, 20 degrees, and 40 degrees off fovea. The results indicated significant differences between older and younger adults for both motion types. The effect of age was mediated by the gender of the observer as well as the retinal eccentricity of the display. Older women showed higher thresholds for lamellar flow at fovea, consistent with previous findings. The findings suggest that age-related changes in visual information processing are affected by changes in the temporal characteristics of the motion processing system. A model is proposed in which 2 different streams of processing are used for the recovery and use of motion information.

The perception of depth and slant from texture in three-dimensional scenes.

The perception of depth and slant in three-dimensional scenes specified by texture was investigated in five experiments. Subjects were presented with computer-generated scenes of a ground and ceiling plane receding in depth. Compression, convergence, and grid textures were examined. The effect of the presence or absence of a gap in the center of the display was also assessed. Under some conditions perceived slant and depth from compression were greater than those found with convergence. The relative effectiveness of compression in specifying surface slant was greater for surfaces closer to ground planes (80 degrees slant) than for surfaces closer to frontal parallel planes (40 degrees slant). The usefulness of compression was also observed with single-plane displays and with displays with surfaces oriented to reduce information regarding the horizon.

Effects of collimation on perceived layout in 3-D scenes.

The effect of varying information for overall depth in a simulated 3-D scene on the perceived layout of objects in the scene was investigated in two experiments. Subjects were presented with displays simulating textured surfaces receded in depth. Pairs of markers were positioned at equal intervals within the scenes. The subject's task was to judge the depth between the intervals. Overall scene depth was varied by viewing through either a collimating lens or a glass disk. Judged depth for equal depth intervals decreased with increasing distance of the interval from the front of the scene. Judged depth was greater for collimated than for non-collimated viewing. Interestingly, collimated viewing resulted in a uniform rescaling of the perceived depth intervals.

Smoothness of the velocity field and three-dimensional surface detection from optic flow.

Five experiments were conducted to determine the importance of smoothness of the velocity field in detecting 3-D surfaces from optic flow. Subjects were presented with optic flow displays simulating either points positioned on a corrugated 3-D surface or points randomly positioned within a 3-D volume. The subject's task was to indicate whether or not the display appeared to be a 3-D surface. Smoothness of the velocity field was examined by systematically varying the speed of individual velocities in the flow field according to a Gaussian distribution with M = 0 and SD = sigma. Variations in frequency, amplitude, density, and surface complexity were also examined. Detection of the corrugated surfaces systematically declined with an increase in sigma. An increase in frequency of the corrugation for simple (single-frequency corrugation) surfaces resulted in a decrease in surface detection accuracy. Accuracy increased with an increase in density and amplitude for both simple and complex (multiple-frequency corrugation) surfaces. An analysis of the deformation of the displays predicted performance on the basis of human observers, providing further support for the importance of deformation for 3-D surface detection.

Relationship between binocular disparity and motion parallax in surface detection.

The ability to detect surfaces was studied in a multiple-cue condition in which binocular disparity and motion parallax could specify independent depth configurations. On trials on which binocular disparity and motion parallax were presented together, either binocular disparity or motion parallax could indicate a surface in one of two intervals; in the other interval, both sources indicated a volume of random points. Surface detection when the two sources of information were present and compatible was not better than detection in baseline conditions, in which only one source of information was present. When binocular disparity and motion specified incompatible depths, observers' ability to detect a surface was severely impaired if motion indicated a surface but binocular disparity did not. Performance was not as severely degraded when binocular disparity indicated a surface and motion did not. This dominance of binocular disparity persisted in the presence of foreknowledge about which source of information would be relevant.

Image velocity magnitudes and perception of heading.

Most theoretical approaches to perception of heading rely on the directions of image velocity vectors as the primary source of heading information. The research described in this article examined an additional source of information for determining heading: distributions of image velocity magnitudes. Displays simulated observer motion relative to rigid three-dimensional environments. Depth was distributed nonuniformly such that image velocity magnitudes provided, for some display conditions, conflicting heading information relative to the radial directions of the flow field. Results indicated that image velocity magnitudes influenced heading performance, suggesting that the perception of heading is not based solely on the radial structure of the directions of image flow.

Detection of smooth three-dimensional surfaces from optic flow.

An assumption central to the study of 3-dimensional (3-D) shape perception is that sufficient information must be present to detect a 3-D surface. Three experiments were conducted to determine the variables important for the detection of 3-D surfaces from optic flow. Observers were presented with optic-flow displays simulating either points positioned on a corrugated 3-D surface or points randomly positioned within a 3-D volume. The task of the observer was to indicate whether the display appeared to be a 3-D surface. An increase in frequency of the corrugation for simple (single-frequency corrugation) surfaces resulted in a decrease in surface detection. Detection performance increased with an increase in density and amplitude for both simple and complex (multiple-frequency corrugation) surfaces. An analysis of the deformation of the displays suggests that 3-D surface detection may be based on the summed absolute value of the 2 shear components of deformation.

Aging and filtering by movement in visual search.

We examined the ability of younger and older adults to selectively process moving items and ignore stationary items in a task that required the search for a target defined by a conjunction of movement and form (i.e., search for a moving X among moving Os and stationary Xs) in displays of 5, 9, 17, and 25 stimuli (Experiment 1) and displays of 5, 10, and 20 stimuli (Experiment 2). We also investigated subjects' performance in two feature search tasks, the search for a target defined by movement or form. Finally, we examined the influence of practice on feature and conjunction search. Younger and older adults searched the displays at similar rates in the feature and conjunction search tasks. Older and younger adults also benefited equivalently from practice. These data suggest age-equivalence in the processes which underlie feature search in dynamic environments as well as those processes responsible for the segregation of moving and stationary objects in the visual field.

Age-related differences in the detection of three-dimensional surfaces from optic flow.

The present study examined age-related differences in sensitivity to optic flow for the detection of 3-D surfaces. Observers were presented with optic flow displays simulating either a 3-D corrugated surface or a random velocity pattern and were asked to detect the 3-D corrugated surface display. Performance decrements were found for older (mean age = 71) compared with younger (mean age = 21) observers across variations in density and corrugation frequency. Older observers also showed performance decrements for a motion coherence task, but performance on this task was not significantly correlated with performance on the 3-D surface detection task. These results suggest that performance on 2-D motion tasks is not necessarily predictive of performance on complex 3-D motion tasks. In addition, the results suggest that different processes underlie the analysis of 2-D and 3-D motion.

Discrimination of speed distributions: sensitivity to statistical properties.

Two experiments examining the ability of human observers to detect differences in the statistical properties underlying velocity distributions were conducted. A four-alternative forced-choice methodology, using four simultaneous velocity distributions, was used in both experiments. In the first experiment the value of one statistical moment (mean, variance, skewness, or kurtosis) was manipulated while the others were held constant. The subjects task was to determine which of four velocity distributions contained the dissimilar value. In the second experiment only the latter three moments were examined. A similar procedure was used, however feedback was given after each trial to maximize observer performance. The results from both experiments indicate that human observers can reliably detect differences in both mean and variance information underlying velocity distributions. The results of this research has important implications for image segmentation and the detection of heading from optic flow.

Detection of surfaces in structure from motion.

In 3 experiments the authors examined the effects of surface shape on the detection of surfaces in structure from motion displays. In experiment 1 they found the number of points necessary to distinguish simple quadratic surfaces from points randomly placed in a volume to be low, in the range of 4-6 points. In experiment 2 they found that a hyperbolic shape is harder to detect than an arch or parabolic shape and that more curved shapes are easier to detect than less curved shapes. In experiment 3 they found that detection of sinusoidal shapes increases with increasing amplitude and decreasing frequency, confirming previous results with motion parallax displays. This pattern of results is consistent with a model in which triangular surface patches are fit through adjacent triples of points and displays with smaller mean orientation change between neighboring triangles are judged to be surface displays.

Active control in interrupted dynamic spatial orientation: the detection of orientation change.

Gibson (1966, 1979) suggested that an important property of perception is that the observer is active. Two experiments were conducted to examine the benefits of active observation in determining dynamic spatial orientation. Subjects were presented with displays simulating locomotion through a three-dimensional environment. Active observers continuously controlled locomotion, whereas passive observers viewed the display. During the trial, the display was blacked out for a brief period, followed by a static image that was at either the correct or the incorrect orientation following the blackout. Subjects were required to indicate whether they were positioned at the correct extrapolated orientation. The presence or absence of orientation change, the type of change (changes in rotation about the depth axis [roll], horizontal axis [pitch], or forward translation), the duration of the blackout, and the consistency of change were varied. In addition, the experiments used either a compensatory or a pursuit tracking task. Active observers had greater sensitivity than did passive observers in detecting a change for both tracking tasks. Subjects in both experiments exhibited greater sensitivity in detecting inconsistent changes (relative to consistent changes), suggesting that the dynamics specified by optical flow were incorporated in extrapolated orientation. In addition, sensitivity decreased with an increase in blackout duration. The results are discussed in terms of an extrapolation model of perception that incorporates the responses executed by active observers.