Kenith V. Sobel

Neural Synergy Between Kinetic Vision and Touch

Ambiguous visual information often produces unstable visual perception. In four psychophysical experiments, we found that unambiguous tactile information about the direction of rotation of a globe whose three-dimensional structure is ambiguous significantly influences visual perception of the globe. This disambiguation of vision by touch occurs only when the two modalities are stimulated concurrently, however. Using functional magnetic resonance imaging, we discovered that touching the rotating globe, even when not looking at it, reliably activates the middle temporal visual area (MT+), a brain region commonly thought to be crucially involved in registering structure from motion. Considered together, our results show that the brain draws on somatosensory information to resolve visual conflict.

Visual Motion Retards Alternations between Conflicting Perceptual Interpretations

When the visual system is faced with conflicting or ambiguous stimulus information, visual perception fluctuates over time. We found that perceptual alternations are slowed when inducing stimuli move within the visual field, constantly engaging fresh, unadapted neural tissue. During binocular rivalry, dominance durations were longer when rival figures moved compared to when they were stationary, yielding lower alternation rates. Rate was not reduced, however, when observers tracked the moving targets, keeping the images on approximately the same retinal area. Alternations were reliably triggered when rival targets passed through a local region of the visual field preadapted to one of the rival targets. During viewing of a kinetic globe whose direction of rotation was ambiguous, observers experienced fewer alternations in perceived direction when the globe moved around the visual field or when the globes axis of rotation changed continuously. Evidently, local neural adaptation is a key ingredient in the instability of perception.

How Context Influences Predominance during Binocular Rivalry

Variations in the predominance of an object engaged in binocular rivalry may arise from variations in the durations of dominance phases, suppression phases, or both. Earlier work has shown that the predominance of a binocular rival target is enhanced if that target fits well—via common color, orientation, or motion—with its surrounding objects. In the present experiments, the global context outside of the region of rivalry was changed during rivalry, to learn whether contextual information alters the ability to detect changes in a suppressed target itself. Results indicate that context will maintain the dominance of a rival target, but will not encourage a suppressed target to escape from suppression. Evidently, the fate of the suppressed stimulus is determined by neural events distinct from those responsible for global organization during dominance. To reconcile diverse findings concerning rivalry, it may be important to distinguish between processes responsible for selection of one eyes input for dominance from processes responsible for the implementation and maintenance of suppression.

Individual differences in working memory capacity and visual search: The roles of top-down and bottom-up processing

Individual differences in working memory capacity (WMC) have been implicated in a variety of top-down, attention-control tasks: Higher WMC subjects better ignore irrelevant distractions and withhold habitual responses than do lower WMC subjects. Kane, Poole, Tuholski, and Engle (2006) recently attempted to extend these findings to visual search, but found no relation between WMC and search efficiency, even in difficult tasks yielding steep search slopes. Here we used a visual search task that isolated the contributions of top-down versus bottom-up mechanisms, and induced a habitual response via expectation. Searches that relied primarily on bottom-up mechanisms did not vary with WMC, but searches that relied primarily on top-down mechanisms showed an advantage for higher over lower WMC subjects.

Roles of Salience and Strategy in Conjunction Search

In some cases, the search for a conjunction target proceeds through the smaller group of elements in a display, whereas in others, search is limited to those elements that share a particular feature with the target. In 6 experiments, participants searched for a conjunction target among displays consisting of various proportions of 2 distractor types. Smaller-group search was more prevalent than target-feature search with denser displays and with features that were highly discriminable. Explicit instructions to limit search to a specific feature affected performance only when the discriminability of the guiding feature was much greater than the other target feature. Together, these experiments show that bottom-up factors have more influence in guiding conjunction searches than previously thought.

Subjective contours and binocular rivalry suppression

Binocular rivalry probably involves distributed neural processes, some responsible for dominance, others for suppression and still others for fluctuations in perception. Focusing on the suppression process, the present study asks whether neural events underlying rivalry suppression take place prior to, or subsequent to those underlying the synthesis of subjective contours. Specifically, we examined whether (i) a subjective contour could prematurely return a suppressed target to dominance and (ii) whether suppression of a Kanizsa-type inducer precludes the formation of a subjective contour. Suppression durations were not abbreviated by the subjective contour, but suppression did prevent the formation of a subjective contour. Evidently suppression precedes the synthesis of subjective contours in the visual processing hierarchy.

Visual search for conjunctions of physical and numerical size shows that they are processed independently.

The size congruity effect refers to the interaction between numerical magnitude and physical digit size in a symbolic comparison task. Though this effect is well established in the typical 2-item scenario, the mechanisms at the root of the interference remain unclear. Two competing explanations have emerged in the literature: an early interaction model and a late interaction model. In the present study, we used visual conjunction search to test competing predictions from these 2 models. Participants searched for targets that were defined by a conjunction of physical and numerical size. Some distractors shared the target’s physical size, and the remaining distractors shared the target’s numerical size. We held the total number of search items fixed and manipulated the ratio of the 2 distractor set sizes. The results from 3 experiments converge on the conclusion that numerical magnitude is not a guiding feature for visual search, and that physical and numerical magnitude are processed independently, which supports a late interaction model of the size congruity effect.

Using feature preview to investigate the roles of top–down and bottom–up processing in conjunction search

The venerable conjunction search paradigm is a widely used tool to investigate how we search for items of interest from among visually complex surroundings. Models of visual search have long predicted that standard conjunction search is guided primarily by top-down processing. Prior attempts to test this claim experimentally have done so by altering some aspect of the standard conjunction search, whether by manipulating the distractor ratio or by including a feature singleton. Although suggestive, these manipulations result in a task that differs slightly from standard conjunction search. To leave the standard conjunction search paradigm intact, we used the feature preview task developed by Olds and Fockler [Olds, E. S., & Fockler, K. A. (2004). Does previewing one stimulus feature help conjunction search? Perception, 33, 195-216]. Our results show that in standard conjunction search the effect of bottom-up activation is not necessarily detrimental to search performance as previously suggested by computational models of visual search. Instead, bottom-up activation limits the scope of search, thereby boosting the efficiency of standard conjunction searches. Subjects also showed a bias to group items by color rather than orientation even when color differences were reduced nearly to threshold, indicating that the salience advantage of color is complemented by a general bottom-up preference for color.

Bottom-up and top-down attentional contributions to the size congruity effect

The size congruity effect refers to the interaction between the numerical and physical (i.e., font) sizes of digits in a numerical (or physical) magnitude selection task. Although various accounts of the size congruity effect have attributed this interaction to either an early representational stage or a late decision stage, only Risko, Maloney, and Fugelsang (Attention, Perception, & Psychophysics, 75, 1137–1147, 2013) have asserted a central role for attention. In the present study, we used a visual search paradigm to further study the role of attention in the size congruity effect. In Experiments 1 and 2, we showed that manipulating top-down attention (via the task instructions) had a significant impact on the size congruity effect. The interaction between numerical and physical size was larger for numerical size comparison (Exp. 1) than for physical size comparison (Exp. 2). In the remaining experiments, we boosted the feature salience by using a unique target color (Exp. 3) or by increasing the display density by using three-digit numerals (Exps. 4 and 5). As expected, a color singleton target abolished the size congruity effect. Searching for three-digit targets based on numerical size (Exp. 4) resulted in a large size congruity effect, but search based on physical size (Exp. 5) abolished the effect. Our results reveal a substantial role for top-down attention in the size congruity effect, which we interpreted as support for a shared-decision account.

Target grouping in visual search for multiple digits

In four experiments in which participants searched for multiple target digits we hypothesized that search should be fastest when the targets are arranged closely together on the number line without any intervening distractor digits, i.e., the targets form a contiguous and coherent group. In Experiment 1 search performance was better for targets defined by numerical magnitude than parity (i.e., evenness); this result supports our hypothesis but could also be due to the linear separability of targets from distractors or the numerical distance between them. Experiment 2 controlled for target-distractor linear separability and numerical distance, yielding faster search when targets were surrounded by distractors on the number line than when they surrounded distractors. This result is consistent with target contiguity and coherence but also with grouping by similarity of target shapes. Experiment 3 controlled for all three alternative explanations (linear separability, numerical distance, and shape similarity) and search performance was better for contiguous targets than separated targets. In Experiment 4 search performance was better for a coherent target group than one with intervening distractors. Of the possibilities we considered, only the hypothesis based on the contiguity and coherence of the target group on the number line can account for the results from all four experiments.