Srimant P. Tripathy

The effect of similarity and duration on spatial interaction in peripheral vision

Spatial interactions are extensive in the peripheral visual field, extending up to about half the retinal eccentricity of the target (Toet and Levi, Vision Res. 32, 1349-1357, 1992). In the present study it is shown that the degree and extent of peripheral spatial interaction depends in large measure on the similarity between test and flanking stimuli. The stimulus consisted of a test T surrounded by four distracting flanking Ts, each randomly oriented. The task was to determine the orientation of the test T. The test and flanking Ts differed in contrast polarity, shape, depth, color, eye of origin, or contrast. When the target and flanks differed in contrast polarity, depth, or shape, performance improved markedly for all observers. A color difference enhanced the performance of most but not all observers. Eye-of-origin had no effect, that is, spatial interaction was identical when the target and flanks were presented to the same eye, or to opposite eyes. The role of stimulus duration in spatial interaction was examined in two additional experiments. In the first, the stimulus viewing duration was increased in order to allow the observer time to serially search for the test T. In the second experiment, a postmask was presented at the location of the test T. The results of these experiments showed that the influence of similarity was independent of stimulus duration and the postmask, and suggest that serial search does not play an important role in the spatial interaction effects reported here. The extent of spatial interaction is correlated with the ability to do parallel search.

The extent of crowding in peripheral vision does not scale with target size.

Identifying a target is more difficult when distracters are present within a zone of interaction around the target. We investigated whether the spatial extent of the zone of interaction scales with the size of the target. Our target was a letter T in one-of-four orientations. Our distracters were four squared-thetas in one-of-two orientations, presented one in each of the four cardinal directions, equidistant from the target. Target-distracter separation was varied and the proportion of correct responses at each separation was determined. From these the extent of interaction was estimated. This procedure was repeated for different target sizes spread over a 5-fold range. In each case, the contrast of the target was adjusted so that its visibility was constant across target sizes. The experiment was performed in the luminance domain (grey targets on grey background) and in the chromatic domain (green target on equiluminant grey background). In the luminance domain, target size had only a small effect on the extent of interaction; these interactions did not scale with target size. The extents of interaction for chromatic stimuli were similar to those for luminance stimuli. For a fixed target visibility, decreasing the duration of the stimulus resulted in an increase in the extent of interaction. The relevance of our findings is discussed with regard to a variety of proposed explanations for crowding. Our results are consistent with an attention-based explanation for crowding.

On the filling in of the visual blind spot: some rules of thumb.

In monocular viewing there is a region in the peripheral visual field that is blind owing to the absence of photoreceptors at the site where the optic nerve exits the eye. This region, like certain other blind spots, nonetheless appears filled in. Several novel demonstrations of filling in at the blind spot have recently been reported. Here the implications of many of these effects are critically reevaluated. Specifically, it is argued that many blind-spot phenomena taken to support early filling in (eg pop out and alteration in apparent motion) are actually consistent with the thesis that the visual blind spot is treated by early perceptual processing as a region of reduced or absent information. In support of this, it is shown that many perceptual effects observed in blind-spot completion are similar in detail to the amodally perceived completion of partly occluded objects viewed somewhat peripherally. The goals were to point out striking similarities between blind-spot completion and the amodal completion of occluded parts of surfaces, and to provide a common theoretical framework for understanding these phenomena in the context of surface segregation and perceptual interpolation.

Severe loss of positional information when detecting deviations in multiple trajectories

Human observers can simultaneously track up to five targets in motion (Z. W. Pylyshyn & R. W. Storm, 1988). We examined the precision for detecting deviations in linear trajectories by measuring deviation thresholds as a function of the number of trajectories (T ). When all trajectories in the stimulus undergo the same deviation, thresholds are uninfluenced by T for T <or= 10. When only one of the trajectories undergoes a deviation, thresholds rise steeply as T is increased [e.g., 3.3 degrees (T = 1), 12.3 degrees (T = 2), 32.9 degrees (T = 4) for one observer]; observers are unable to simultaneously process more than one trajectory in our threshold-measuring paradigm. When the deviating trajectory is cued (e.g., using a different color), varying T has little influence on deviation threshold. The use of a different color for each trajectory does not facilitate deviation detection. Our current data suggest that for deviations that have low discriminability (i.e., close to threshold) the number of trajectories that can be monitored effectively is close to one. In contrast, when the stimuli containing highly discriminable (i.e., substantially suprathreshold) deviations are used, as many as three or four trajectories can be simultaneously monitored (S. P. Tripathy, 2003). Our results highlight a severe loss of positional information when attempting to track multiple objects, particularly in a threshold paradigm.

On the effective number of tracked trajectories in amblyopic human vision.

We estimated the effective number of trajectories that amblyopic observers could track with their amblyopic eyes and their non-amblyopic eyes using stimuli and methods described in S. P. Tripathy, S. Narasimhan, and B. T. Barrett (2007). The stimuli consisted of dots moving along straight-line trajectories. In Experiment 1, one of the T trajectories (the target) deviated clockwise or counterclockwise by +/-19 degrees , +/-38 degrees , or +/-76 degrees , halfway through the trajectory. In Experiment 2, D of the T trajectories deviated, all in the same direction and with the same magnitude of direction change. In both experiments, we varied T and the angle of deviation. In Experiment 2, we also varied D. Amblyopic observers reported the direction of deviation of the target trajectories and, for each eye, the effective number of tracked trajectories was estimated. This number increased systematically with increasing magnitude of deviation of the targets. On average, the effective numbers of tracked trajectories were approximately 15% smaller for the amblyopic eyes for each of the three magnitudes of deviation. A comparison with data previously published for normal eyes failed to reveal any deficit in the effective number of trajectories tracked by the non-amblyopic eyes of amblyopic observers for the current task.

High-capacity, transient retention of direction-of-motion information for multiple moving objects.

The multiple-object tracking paradigm (MOT) has been used extensively for studying dynamic visual attention, but the basic mechanisms which subserve this capability are as yet unknown. Among the unresolved issues surrounding MOT are the relative importance of motion (as opposed to positional) information and the role of various memory mechanisms. We sought to quantify the capacity and dynamics for retention of direction-of-motion information when viewing a multiple-object motion stimulus similar to those used in MOT. Observers viewed three to nine objects in random linear motion and then reported motion direction after motion ended. Using a partial-report paradigm and varying the parameters of set size and time of retention, we found evidence for two complementary memory systems, one transient with high capacity and a second sustained system with low capacity. For the transient high-capacity memory, retention capacity was equally high whether object motion lasted several seconds or a fraction of a second. Also, a graded deterioration in performance with increased set size lends support to a flexible-capacity theory of MOT.

Gross Misperceptions in the Perceived Trajectories of Moving Dots

We report illusions we observed while investigating thresholds for detecting a deviation in one out of several trajectories of dots moving in straight lines (Tripathy and Barrett 2003). The deviating trajectory consisted of two straight-line segments, with the deviation occurring in the middle of the trajectory, at the intersection of the two segments. Robust distortions were noticed in the shape of the deviating trajectory, in spite of observers knowing exactly when and where the deviation occurred, and the actual shape of the trajectory. The stimulus consisted of several dots undergoing apparent motion along linear trajectories (figure 1). All dots moved with equal average speed (typically 16 deg sÿ1, though speeds from 1 to 16 deg sÿ1 were tested) and reached the midline of the screen (indicated by vertical markers) at exactly the same instant of time. One of the dots/ trajectories (target dot/trajectory) deviated at the midline, while the others continued along straight lines without deviating (distractor dots/trajectories). Observers reported what the perceived shape of the target trajectory was, and the location on the screen where the deviation was perceived. All observers perceived gross distortions in the shape of the target trajectory.

Early age-related decline in the effective number of trajectories tracked in adult human vision

Human performance in many visual and cognitive tasks declines with age, the rate of decline being task dependent. Here, we used a multiple-object tracking (MOT) task to provide a clear demonstration of a steep cognitive decline that begins relatively early in adult life. Stimuli consisted of 8 dots that moved along linear trajectories from left to right. At the midpoint of their trajectories, a certain number of dots, D (1, 2 or 3), deviated either clockwise or counter-clockwise by a certain magnitude (57 degrees, 38 degrees or 19 degrees); the task for observers was to identify the direction of deviation. Percent correct responses were measured for 22 observers aged 18-62 years and were converted to effective numbers of tracked trajectories (E) (S. P. Tripathy, S. Narasimhan, & B. T. Barrett, 2007). In 5 of the 7 conditions tested, there was a significant negative correlation between age and E, indicating an age-related decline in tracking ability. This decline was found to be equivalent to a mean performance drop of 16% per decade over the four decades of adulthood tested. Further analysis suggests that performance in this task starts to decline at around 30 years of age and falls off at the rate of approximately 20% every subsequent decade.

Large crowding zones in peripheral vision for briefly presented stimuli

When a target is flanked by distractors, it becomes more difficult to identify. In the periphery, this crowding effect extends over a wide range of target-flanker separations, called the spatial extent of interaction (EoI). A recent study showed that the EoI dramatically increases in size for short presentation durations (Chung & Mansfield, 2009). Here we investigate this duration-EoI relation in greater detail and show that (a) it holds even when visibility of the unflanked target is equated for different durations, (b) the function saturates for durations shorter than 30 to 80 ms, and (c) the largest EoIs represent a critical spacing greater than 50% of eccentricity. We also investigated the effect of same or different polarity for targets and flankers across different presentation durations. We found that EoIs for target and flankers having opposite polarity (one white, the other black) show the same temporal pattern as for same polarity stimuli, but are smaller at all durations by 29% to 44%. The observed saturation of the EoI for short-duration stimuli suggests that crowding follows the locus of temporal integration. Overall, the results constrain theories that map crowding zones to fixed spatial extents or to lateral connections of fixed length in the cortex.

Bottlenecks of motion processing during a visual glance: the leaky flask model.

Where do the bottlenecks for information and attention lie when our visual system processes incoming stimuli? The human visual system encodes the incoming stimulus and transfers its contents into three major memory systems with increasing time scales, viz., sensory (or iconic) memory, visual short-term memory (VSTM), and long-term memory (LTM). It is commonly believed that the major bottleneck of information processing resides in VSTM. In contrast to this view, we show major bottlenecks for motion processing prior to VSTM. In the first experiment, we examined bottlenecks at the stimulus encoding stage through a partial-report technique by delivering the cue immediately at the end of the stimulus presentation. In the second experiment, we varied the cue delay to investigate sensory memory and VSTM. Performance decayed exponentially as a function of cue delay and we used the time-constant of the exponential-decay to demarcate sensory memory from VSTM. We then decomposed performance in terms of quality and quantity measures to analyze bottlenecks along these dimensions. In terms of the quality of information, two thirds to three quarters of the motion-processing bottleneck occurs in stimulus encoding rather than memory stages. In terms of the quantity of information, the motion-processing bottleneck is distributed, with the stimulus-encoding stage accounting for one third of the bottleneck. The bottleneck for the stimulus-encoding stage is dominated by the selection compared to the filtering function of attention. We also found that the filtering function of attention is operating mainly at the sensory memory stage in a specific manner, i.e., influencing only quantity and sparing quality. These results provide a novel and more complete understanding of information processing and storage bottlenecks for motion processing.