Leslie Welch

Velocity-dependent improvements in single-dot direction discrimination

Thirty-six Brown University students participated in three experiments designed to address perceptual learning. In each experiment, visual discrimination thresholds were tracked over 4,200 trials. Results from Experiment 1 suggest that the pattern of threshold reduction on a single-dot motion-direction discrimination task was stimulus-direction specific and matched (in a velocity-dependent manner) the threshold reduction pattern previously reported for a line-orientation discrimination task. In Experiment 2, it was determined that the stationary-line-orientation—specific practice effects originally reported by Vogels and Orban (1985) could be replicated but were contingent on line length. Similarly, the results from Experiment 3 suggest that practice effects originally reported by Ball and Sekuler (1987) could be replicated but were contingent on stimulus velocity. Implications for the mechanisms underlying direction and orientation discrimination are considered.

Recruitment mechanisms in speed and fine-direction discrimination tasks

The minimum information necessary to specify motion requires a change in position across time. Previous studies have shown that human motion measurements improve with more than two frames of motion. This study clarifies how motion information is integrated to produce the best speed and direction discrimination. Using random-dot kinematograms, fine-direction discrimination thresholds and speed discrimination thresholds are assessed as a function of dot lifetime. Specifically, we ask if performance on both tasks depends on dot lifetime in the same manner. If both speed and direction discrimination performance improve the same way with increasing dot lifetime, this would indicate that both tasks have the same integration limit and both tasks may depend on the same underlying mechanisms. Experiment 1 shows that for both tasks a four-frame dot lifetime is necessary for observers to reach asymptotic threshold levels. The absolute level of performance improves with increasing stimulus duration or signal-to-noise ratio, but the integration limit itself does not vary. Experiment 2 examines whether this integration limit is constrained by the number of frames or by the temporal duration of the dot lifetime. The data in Experiment 2 suggest that both a minimum number of samples and a minimal temporal integration period determine the integration limit for recruitment mechanisms. The results suggest that speed and fine-direction discrimination depend upon the same underlying motion mechanisms. These results are discussed in relation to possible underlying physiological substrates and computational models of motion measurement.

The Effect of Inducer Polarity and Contrast on the Perception of Illusory Figures

A study designed to determine how inducer–surround contrast and inducer polarity affect the contour clarity and the lightness of illusory figures is reported. Using magnitude estimation procedures, ten naive subjects rated both the contour clarity and the lightness of Kanizsa squares. The magnitude of the inducer–surround contrast and the inducer polarity (all-black, all-white, or black-and-white) were varied randomly on each trial. The data indicate that contour clarity increases with contrast at the same rate across polarity conditions but that contour clarity at any given contrast level depends significantly on polarity. Contour clarity judgments were significantly lower when the inducers were all-white than when the inducers were all-black or black-and-white, and significantly greater in the ‘mixed’ polarity case (black-and-white inducers) than in the ‘same’ polarity case (the average of the all-black and all-white inducer conditions). Inducer contrast and polarity significantly affected the lightness of the illusory figure in a manner consistent with simultaneous spatial contrast. Also, for a given increment in contrast, contour clarity altered significantly more than surface lightness, regardless of inducer polarity. The findings suggest that the mechanism which mediates boundary formation is sensitive to the direction of contrast, and that the boundary formation mechanism is more sensitive than the surface lightness mechanism to changes in contrast magnitude. The results are considered within the context of neural network models of form perception.

Left visual field attentional advantage in judging simultaneity and temporal order

Dynamic environments often contain features that vary simultaneously as well as features that vary sequentially. In principle, the correspondingly distinct sensations of simultaneity and temporal order could arise from a single shared neural computation that involves differencing two arrival times. On the other hand, simultaneity judgments (SJs) and temporal order judgments (TOJs) have distinct informational requirements that could be optimized by distinct neural events. To explore overlap in the neural events mediating SJs and TOJs, the present experiments built on recent reports that SJ precision in the left visual field (LVF) exceeds that in the right visual field (RVF). Participants completed divided attention tasks requiring either SJs or TOJs to LVF or RVF targets. SJs exhibited a significant LVF advantage, as expected. TOJs also exhibited a significant LVF advantage. Specifically, simply repositioning targets from the LVF to the RVF generated mean TOJ threshold increases (temporal precision reductions) between 39% and 57%, an effect size equivalent to approximately two LVF detectors for each RVF detector. Control experiments indicated that this LVF advantage reflected the temporal resolution of visual attention, rather than lower-level flicker discrimination or masking. These findings constitute additional evidence for an LVF advantage in time-sensitive attentional tasks and further contradict our subjective experience of homogenous temporal precision across the visual field.

Motion interference: perturbing perceived direction

The minimum stimulus necessary to define motion is a change in position from one location to another in time, but past studies have provided evidence that the human motion system integrates motion over more than two positions. In this study we demonstrate strong sequential interactions affecting perceived direction in apparent-motion sequences; a perturbing dot can bias the perceived direction of motion between two test dots to which it is relatively close in space (up to 100 min arc) and time (up to 300 msec). These sequential interactions suggest a motion mechanism sensitive to the spatial characteristics of motion trajectories; the interactions are greatest for evenly spaced targets positioned along a single axis. The implications for motion-detection models and models based on attention as a mechanism to create apparent motion are discussed.

The Absence of Depth Constancy in Contour Stereograms

Stereoscopic surfaces constructed from Kanizsa-type illusory contours or explicit luminance contours were tested for three-dimensional (3-D) shape constancy. The curvature of the contours and the apparent viewing distance between the surface and the observer were manipulated. Observers judged which of two surfaces appeared more curved. Experiment 1 allowed eye movements and revealed a bias in 3-D shape judgment with changes in apparent viewing distance, such that surfaces presented far from the observer appeared less curved than surfaces presented close to the observer. The lack of depth constancy was approximately the same for illusory-contour surfaces and for explicit-contour surfaces. Experiment 2 showed that depth constancy for explicit-contour surfaces improved slightly when fixation was required and eye movements were restricted. These experiments suggest that curvature in depth is misperceived, and that illusory-contour surfaces are particularly sensitive to this distortion.

Simultaneity and Temporal Order Judgments Exhibit Distinct Reaction Times and Training Effects

A considerable body of sensory research has addressed the rules governing simultaneity judgments (SJs) and temporal order judgments (TOJs). In principle, neural events that register stimulus-arrival-time differences at an early sensory stage could set the limit on SJs and TOJs alike. Alternatively, distinct limits on SJs and TOJs could arise from task-specific neural events occurring after the stimulus-driven stage. To distinguish between these possibilities, we developed a novel reaction-time (RT) measure and tested it in a perceptual-learning procedure. The stimuli comprised dual-stream Rapid Serial Visual Presentation (RSVP) displays. Participants judged either the simultaneity or temporal order of red-letter and black-number targets presented in opposite lateral hemifield streams of black-letter distractors. Despite identical visual stimulation across-tasks, the SJ and TOJ tasks generated distinct RT patterns. SJs exhibited significantly faster RTs to synchronized targets than to subtly asynchronized targets; TOJs exhibited the opposite RT pattern. These task-specific RT patterns cannot be attributed to the early, stimulus-driven stage and instead match what one would predict if the limits on SJs and TOJs arose from task-specific decision spaces. That is, synchronized targets generate strong evidence for simultaneity, which hastens SJ RTs. By contrast, synchronized targets provide no information about temporal order, which slows TOJ RTs. Subtly asynchronizing the targets reverses this information pattern, and the corresponding RT patterns. In addition to investigating RT patterns, we also investigated training-transfer between the tasks. Training to improve SJ precision failed to improve TOJ precision, and vice versa, despite identical visual stimulation across tasks. This, too, argues against early, stimulus-driven limits on SJs and TOJs. Taken together, the present study offers novel evidence that distinct rules set the limits on SJs and TOJs.

Motion Capture Depends on Signal Strength

When flickering dots are superimposed onto a drifting grating, the dots appear to move coherently with the grating. In this study we examine: (i) how the perceived direction of a compound stimulus composed of superimposed grating and dots, moving in opposite directions with equal speeds, is influenced by the relative strength of the motion signals; (ii) how the perceived speed of a compound stimulus composed of superimposed grating and dots, moving in the same direction but at different speeds, is influenced by the relative strength of the motion signals; and (iii) whether this stimulus is discriminable from its metameric speed match. Dot signal strength was manipulated by using different proportions of signal dots in noise and different dot lifetimes. Both the perceived direction and speed of these compound stimuli depended upon the relative motion-signal strengths of the grating and the dots. Those compound stimuli that appeared coherent were not discriminable from the speed-matched metameric compound stimuli. When the signals were completely integrated into a coherent compound stimulus, the local motion signals were no longer perceptually available, though both contributed to the global percept. These data strongly support a weighted-combination model where the relative weights depend on signal strength, instead of a winner-takes-all model.

Double Dissociation in Radial & Rotational Motion-Defined Temporal Order Judgments

 Conclusion – The findings suggest a double dissociation between the neural events that limit how precisely we judge asynchronies defined by these two types of MST-mediated motion. Double Dissociation in Radial & Rotational Motion-Defined Temporal Order Judgments Leslie Welch1, Nestor Matthews2, Elena Festa1, Kendra Schafer2 1Brown University Cognitive, Linguistic & Psychological Sciences; 2Denison University – Psychology

Interdisciplinary Teaching of Visual Perception through Art and Science

ABSTRACT This paper tells the tale of an adventure in teaching an interdisciplinary course about visual perception, combining visual art and vision science, called “Making Visual Illusions.” The authors co-taught a course that brought together the hands-on methods of the art studio and the science laboratory, using visual illusions as a theme to guide student explorations. One unexpected issue that arose was the time needed to discuss basic concepts and the connections between fields in order to communicate the deeper ideas the students needed to learn. This paper explores aspects of the course that worked well and makes suggestions for improvement.