The Motion-Silencing Illusion Depends on Object-Centered Representation.

Motion silencing is a striking and unexplained visual illusion wherein changes that are otherwise salient become difficult to perceive when the changing elements also move. We develop a new method for quantifying illusion strength (Experiments 1a and 1b), and we demonstrate a privileged role for rotational motion on illusion strength compared with highly controlled stimuli that lack rotation (Experiments 2a to 3b). These contrasts make it difficult to explain the illusion in terms of lower-level detection limits. Instead, we explain the illusion as a failure to attribute changes to locations. Rotation exacerbates the illusion because its perception relies upon structured object representations. This aggravates the difficulty of attributing changes by demanding that locations are referenced relative to both an object-internal frame and an external frame. Two final experiments (4a and 4b) add support to this account by employing a synchronously rotating external frame of reference that diminishes otherwise strong motion silencing. All participants were Johns Hopkins University undergraduates.

Properties of visual episodic memory following repeated encounters with objects.

A person sees an object once, and then seconds, minutes, hours, days, or weeks later, she sees it again. How is the person's visual memory for that object changed, improved, or degraded by the second encounter, compared to a situation in which she will have only seen the object once? The answer is unknown, a glaring lacuna in the current understanding of visual episodic memory. The overwhelming majority of research considers recognition following a single exposure to a set of objects, whereas objects reoccur regularly in lived experience. We therefore sought to address some of the more basic and salient questions that are unanswered with respect to how repetition affects visual episodic memory. In particular, we investigated how spacing between repeated encounters affects memory, as well as variable input quality across encounters and changes in viewed orientation. Memory was better when the spacing between encounters was larger, and when a first encounter with an object supplied high quality input (compared to low quality input first, followed later by higher quality input). These experiments lay a foundation for further understanding how memory changes, improves, and degrades over the course of experience.

Visual working memory is more tolerant than visual long-term memory.

Human visual memory is tolerant, meaning that it supports object recognition despite variability across encounters at the image level. Tolerant object recognition remains one capacity in which artificial intelligence trails humans. Typically, tolerance is described as a property of human visual long-term memory (VLTM). In contrast, visual working memory (VWM) is not usually ascribed a role in tolerant recognition, with tests of that system usually demanding discriminatory power-identifying changes, not sameness. There are good reasons to expect that VLTM is more tolerant; functionally, recognition over the long-term must accommodate the fact that objects will not be viewed under identical conditions; and practically, the passive and massive nature of VLTM may impose relatively permissive criteria for thinking that two inputs are the same. But empirically, tolerance has never been compared across working and long-term visual memory. We therefore developed a novel paradigm for equating encoding and test across different memory types. In each experiment trial, participants saw two objects, memory for one tested immediately (VWM) and later for the other (VLTM). VWM performance was better than VLTM and remained robust despite the introduction of image and object variability. In contrast, VLTM performance suffered linearly as more variability was introduced into test stimuli. Additional experiments excluded interference effects as causes for the observed differences. These results suggest the possibility of a previously unidentified role for VWM in the acquisition of tolerant representations for object recognition. (PsycINFO Database Record

Exploiting core knowledge for visual object recognition.

Humans recognize thousands of objects, and with relative tolerance to variable retinal inputs. The acquisition of this ability is not fully understood, and it remains an area in which artificial systems have yet to surpass people. We sought to investigate the memory process that supports object recognition. Specifically, we investigated the association of inputs that co-occur over short periods of time. We tested the hypothesis that human perception exploits expectations about object kinematics to limit the scope of association to inputs that are likely to have the same token as a source. In several experiments we exposed participants to images of objects, and we then tested recognition sensitivity. Using motion, we manipulated whether successive encounters with an image took place through kinematics that implied the same or a different token as the source of those encounters. Images were injected with noise, or shown at varying orientations, and we included 2 manipulations of motion kinematics. Across all experiments, memory performance was better for images that had been previously encountered with kinematics that implied a single token. A model-based analysis similarly showed greater memory strength when images were shown via kinematics that implied a single token. These results suggest that constraints from physics are built into the mechanisms that support memory about objects. Such constraints-often characterized as 'Core Knowledge'-are known to support perception and cognition broadly, even in young infants. But they have never been considered as a mechanism for memory with respect to recognition. (PsycINFO Database Record

A deficit perceiving slow motion after brain damage and a parallel deficit induced by crowding.

Motion perception is known to involve at least 2 kinds of mechanisms-lower level signal detectors and higher level algorithms for comparing object positions over time. When stimulus motion is modal (continuously visible), it is generally assumed that processing via lower level mechanisms is sufficient to make accurate motion judgments. We investigated the possibility that higher level mechanisms may also be involved in the processing of slow motion, even when it is smooth and continuous. This possibility was suggested by results from a brain-damaged patient, JKI, who showed left visual field deficits in both the explicit representation of object position and judgments concerning the direction of slow, but not fast, smooth motion. We investigated the possibility further by using crowding to induce a behaviorally similar motion-perception deficit in healthy observers. Crowding, which is known to impair object-position representation, impaired direction judgments for slow, but not for faster, smooth motion. The results suggest an everyday role for higher level mechanisms in the perception of slow motion, and they reinforce the taxonomy of motion perception in terms of underlying processing mechanisms as opposed to stimulus properties.

Why some colors appear more memorable than others: A model combining categories and particulars in color working memory.

Categorization with basic color terms is an intuitive and universal aspect of color perception. Yet research on visual working memory capacity has largely assumed that only continuous estimates within color space are relevant to memory. As a result, the influence of color categories on working memory remains unknown. We propose a dual content model of color representation in which color matches to objects that are either present (perception) or absent (memory) integrate category representations along with estimates of specific values on a continuous scale ("particulars"). We develop and test the model through 4 experiments. In a first experiment pair, participants reproduce a color target, both with and without a delay, using a recently influential estimation paradigm. In a second experiment pair, we use standard methods in color perception to identify boundary and focal colors in the stimulus set. The main results are that responses drawn from working memory are significantly biased away from category boundaries and toward category centers. Importantly, the same pattern of results is present without a memory delay. The proposed dual content model parsimoniously explains these results, and it should replace prevailing single content models in studies of visual working memory. More broadly, the model and the results demonstrate how the main consequence of visual working memory maintenance is the amplification of category related biases and stimulus-specific variability that originate in perception.

Why do people appear not to extrapolate trajectories during multiple object tracking? A computational investigation.

Intuitively, extrapolating object trajectories should make visual tracking more accurate. This has proven to be true in many contexts that involve tracking a single item. But surprisingly, when tracking multiple identical items in what is known as "multiple object tracking," observers often appear to ignore direction of motion, relying instead on basic spatial memory. We investigated potential reasons for this behavior through probabilistic models that were endowed with perceptual limitations in the range of typical human observers, including noisy spatial perception. When we compared a model that weights its extrapolations relative to other sources of information about object position, and one that does not extrapolate at all, we found no reliable difference in performance, belying the intuition that extrapolation always benefits tracking. In follow-up experiments we found this to be true for a variety of models that weight observations and predictions in different ways; in some cases we even observed worse performance for models that use extrapolations compared to a model that does not at all. Ultimately, the best performing models either did not extrapolate, or extrapolated very conservatively, relying heavily on observations. These results illustrate the difficulty and attendant hazards of using noisy inputs to extrapolate the trajectories of multiple objects simultaneously in situations with targets and featurally confusable nontargets.

Relating color working memory and color perception.

Color is the most frequently studied feature in visual working memory (VWM). Oddly, much of this work de-emphasizes perception, instead making simplifying assumptions about the inputs served to memory. We question these assumptions in light of perception research, and we identify important points of contact between perception and working memory in the case of color. Better characterization of its perceptual inputs will be crucial for elucidating the structure and function of VWM.

Stimulus-specific variability in color working memory with delayed estimation.

Working memory for color has been the central focus in an ongoing debate concerning the structure and limits of visual working memory. Within this area, the delayed estimation task has played a key role. An implicit assumption in color working memory research generally, and delayed estimation in particular, is that the fidelity of memory does not depend on color value (and, relatedly, that experimental colors have been sampled homogeneously with respect to discriminability). This assumption is reflected in the common practice of collapsing across trials with different target colors when estimating memory precision and other model parameters. Here we investigated whether or not this assumption is secure. To do so, we conducted delayed estimation experiments following standard practice with a memory load of one. We discovered that different target colors evoked response distributions that differed widely in dispersion and that these stimulus-specific response properties were correlated across observers. Subsequent experiments demonstrated that stimulus-specific responses persist under higher memory loads and that at least part of the specificity arises in perception and is eventually propagated to working memory. Posthoc stimulus measurement revealed that rendered stimuli differed from nominal stimuli in both chromaticity and luminance. We discuss the implications of these deviations for both our results and those from other working memory studies.

How undistorted spatial memories can produce distorted responses.

Reproducing the location of an object from the contents of spatial working memory requires the translation of a noisy representation into an action at a single location-for instance, a mouse click or a mark with a writing utensil. In many studies, these kinds of actions result in biased responses that suggest distortions in spatial working memory. We sought to investigate the possibility of one mechanism by which distortions could arise, involving an interaction between undistorted memories and nonuniformities in attention. Specifically, the resolution of attention is finer below than above fixation, which led us to predict that bias could arise if participants tend to respond in locations below as opposed to above fixation. In Experiment 1 we found such a bias to respond below the true position of an object. Experiment 2 demonstrated with eye-tracking that fixations during response were unbiased and centered on the remembered object's true position. Experiment 3 further evidenced a dependency on attention relative to fixation, by shifting the effect horizontally when participants were required to tilt their heads. Together, these results highlight the complex pathway involved in translating probabilistic memories into discrete actions, and they present a new attentional mechanism by which undistorted spatial memories can lead to distorted reproduction responses.

Two items remembered as precisely as one: how integral features can improve visual working memory.

In the ongoing debate about the efficacy of visual working memory for more than three items, a consensus has emerged that memory precision declines as memory load increases from one to three. Many studies have reported that memory precision seems to be worse for two items than for one. We argue that memory for two items appears less precise than that for one only because two items present observers with a correspondence challenge that does not arise when only one item is stored--the need to relate observations to their corresponding memory representations. In three experiments, we prevented correspondence errors in two-item trials by varying sample items along task-irrelevant but integral (as opposed to separable) dimensions. (Initial experiments with a classic sorting paradigm identified integral feature relationships.) In three memory experiments, our manipulation produced equally precise representations of two items and of one item.

Off to a bad start: uncertainty about the number of targets at the onset of multiple object tracking.

Visual tracking abilities are limited to only a few objects at a time. When do errors arise? We hypothesized that some errors arise prior to tracking; specifically, during the first moments of a trial because of an inability to correctly perceive the number of targets in a display. To test this hypothesis, we modified a basic multiple object tracking (MOT) task in two ways: (1) we distilled the first moments of MOT into a static working memory task, requiring participants to remember and then identify targets among nontargets in displays without motion; (2) we unconstrained the number of responses a participant could make, asking them to terminate each trial when they felt that they had made an adequate number of responses. In Experiment 1, participants made the wrong number of responses in a considerable number of trials, and they tendered the wrong number of responses more frequently with larger loads. Comparisons across different delay durations demonstrated that these results were not caused by temporal decay. Follow-up experiments produced similar results when participants stated the cardinal number of targets perceived in a static trial (Experiment 2), and when they reported whether or not a test display included the same number of targets as a memory display (Experiment 3). Finally, with a typical tracking duration, participants also produced the wrong number of responses frequently (Experiment 4). Thus, some of the difficulty associated with MOT originates from uncertainty about the number of targets at the start of an episode.

Close encounters of the distracting kind: identifying the cause of visual tracking errors.

Why can we track only so many objects? We addressed this question by asking when and how tracking errors emerge. To test the hypothesis that many tracking errors are target/nontarget confusions emerging from close encounters, we compared standard multiple-object tracking trials with trials on which a nontarget turned a random color whenever it approached within 4° of a target. This manipulation significantly improved performance by alleviating the correspondence challenge of a close encounter. Two control experiments showed that color change benefits were not merely due to target recovery. Follow-up experiments demonstrated that color change benefits did not accrue monotonically with distance but, instead, seemed to obey a step function; and an additional experiment demonstrated that, without color changes, the frequency of close encounters predicts tracking performance. Taken together, these experiments suggest that uncertainty about target location imposes the primary constraint on tracking, at times causing errors by leading to confusions between targets and nontargets.

Amodal causal capture in the tunnel effect.

In addition to identifying individual objects in the world, the visual system must also characterize the relationships between objects, for instance when objects occlude one another or cause one another to move. Here we explored the relationship between perceived causality and occlusion. Can one perceive causality in an occluded location? In several experiments, observers judged whether a centrally presented event involved a single object passing behind an occluder, or one object causally launching another (out of view and behind the occluder). With no additional context, the centrally presented event was typically judged as a non-causal pass, even when the occluding and disoccluding objects were different colors--an illusion known as the 'tunnel effect' that results from spatiotemporal continuity. However, when a synchronized context event involved an unambiguous causal launch, participants perceived a causal launch behind the occluder. This percept of an occluded causal interaction could also be driven by grouping and synchrony cues in the absence of any explicitly causal interaction. These results reinforce the hypothesis that causality is an aspect of perception. It is among the interpretations of the world that are independently available to vision when resolving ambiguity, and that the visual system can 'fill in' amodally.

Spatiotemporal object continuity in human ventral visual cortex.

Coherent visual experience requires that objects be represented as the same persisting individuals over time and motion. Cognitive science research has identified a powerful principle that guides such processing: Objects must trace continuous paths through space and time. Little is known, however, about how neural representations of objects, typically defined by visual features, are influenced by spatiotemporal continuity. Here, we report the consequences of spatiotemporally continuous vs. discontinuous motion on perceptual representations in human ventral visual cortex. In experiments using both dynamic occlusion and apparent motion, face-selective cortical regions exhibited significantly less activation when faces were repeated in continuous vs. discontinuous trajectories, suggesting that discontinuity caused featurally identical objects to be represented as different individuals. These results indicate that spatiotemporal continuity modulates neural representations of object identity, influencing judgments of object persistence even in the most staunchly "featural" areas of ventral visual cortex.

Attentional resources in visual tracking through occlusion: the high-beams effect.

A considerable amount of research has uncovered heuristics that the visual system employs to keep track of objects through periods of occlusion. Relatively little work, by comparison, has investigated the online resources that support this processing. We explored how attention is distributed when featurally identical objects become occluded during multiple object tracking. During tracking, observers had to detect small probes that appeared sporadically on targets, distracters, occluders, or empty space. Probe detection rates for these categories were taken as indexes of the distribution of attention throughout the display and revealed two novel effects. First, probe detection on an occluder's surface was better when either a target or distractor was currently occluded in that location, compared to when no object was behind that occluder. Thus even occluded (and therefore invisible) objects recruit object-based attention. Second, and more surprising, probe detection for both targets and distractors was always better when they were occluded, compared to when they were visible. This new attentional high-beams effect indicates that the ability to track through occlusion, though seemingly effortless, in fact requires the active allocation of special attentional resources.

A temporal same-object advantage in the tunnel effect: facilitated change detection for persisting objects.

Meaningful visual experience requires computations that identify objects as the same persisting individuals over time, motion, occlusion, and featural change. This article explores these computations in the tunnel effect: When an object moves behind an occluder, and then an object later emerges following a consistent trajectory, observers irresistibly perceive a persisting object, even when the pre- and postocclusion views contrast featurally. This article introduces a new change detection method for quantifying percepts of the tunnel effect. Observers had to detect color changes in displays where several objects oscillated behind occluders and occasionally changed color. Across comparisons with several types of spatiotemporal gaps, as well as manipulations of occlusion versus implosion, performance was better when objects' kinematics gave the impression of a persisting individual. The results reveal a temporal same-object advantage: better change detection across temporal scene fragments bound into the same persisting object representations. This suggests that persisting objects are the underlying units of visual memory.

Rhesus monkeys (Macaca mulatta) spontaneously compute addition operations over large numbers.

Mathematics is a uniquely human capacity. Studies of animals and human infants reveal, however, that this capacity builds on language-independent mechanisms for quantifying small numbers (<4) precisely and large numbers approximately. It is unclear whether animals and human infants can spontaneously tap mechanisms for quantifying large numbers to compute mathematical operations. Moreover, all available work on addition operations in non-human animals has confounded number with continuous perceptual properties (e.g. volume, contour length) that correlate with number. This study shows that rhesus monkeys spontaneously compute addition operations over large numbers, as opposed to continuous extents, and that the limit on this ability is set by the ratio difference between two numbers as opposed to their absolute difference.

The activation of attentional networks.

Alerting, orienting, and executive control are widely thought to be relatively independent aspects of attention that are linked to separable brain regions. However, neuroimaging studies have yet to examine evidence for the anatomical separability of these three aspects of attention in the same subjects performing the same task. The attention network test (ANT) examines the effects of cues and targets within a single reaction time task to provide a means of exploring the efficiency of the alerting, orienting, and executive control networks involved in attention. It also provides an opportunity to examine the brain activity of these three networks as they operate in a single integrated task. We used event-related functional magnetic resonance imaging (fMRI) to explore the brain areas involved in the three attention systems targeted by the ANT. The alerting contrast showed strong thalamic involvement and activation of anterior and posterior cortical sites. As expected, the orienting contrast activated parietal sites and frontal eye fields. The executive control network contrast showed activation of the anterior cingulate along with several other brain areas. With some exceptions, activation patterns of these three networks within this single task are consistent with previous fMRI studies that have been studied in separate tasks. Overall, the fMRI results suggest that the functional contrasts within this single task differentially activate three separable anatomical networks related to the components of attention.

Rhesus monkeys attribute perceptions to others.

Paramount among human cognitive abilities is the capacity to reason about what others think, want, and see--a capacity referred to as a theory of mind (ToM). Despite its importance in human cognition, the extent to which other primates share human ToM capacities has for decades remained a mystery. To date, primates [1, 2] have performed poorly in behavioral tasks that require ToM abilities, despite the fact that some macaques are known to encode social stimuli at the level of single neurons [3-5]. Here, we presented rhesus macaques with a more ecologically relevant ToM task in which subjects could "steal" a contested grape from one of two human competitors. In six experiments, monkeys selectively retrieved the grape from an experimenter who was incapable of seeing the grape rather than an experimenter who was visually aware. These results suggest that rhesus macaques possess an essential component of ToM: the ability to deduce what others perceive on the basis of where they are looking. These results converge with new findings illustrating the importance of competitive paradigms in apes [6]. Moreover, they raise the possibility that, in primates, cortical cells thought to encode where others are looking [7] may encode what those individuals see as well.