Internal attention to features in visual short-term memory guides object learning

Attending to objects in the world affects how we perceive and remember them. What are the consequences of attending to an object in mind? In particular, how does reporting the features of a recently seen object guide visual learning? In three experiments, observers were presented with abstract shapes in a particular color, orientation, and location. After viewing each object, observers were cued to report one feature from visual short-term memory (VSTM). In a subsequent test, observers were cued to report features of the same objects from visual long-term memory (VLTM). We tested whether reporting a feature from VSTM: (1) enhances VLTM for just that feature (practice-benefit hypothesis), (2) enhances VLTM for all features (object-based hypothesis), or (3) simultaneously enhances VLTM for that feature and suppresses VLTM for unreported features (feature-competition hypothesis). The results provided support for the feature-competition hypothesis, whereby the representation of an object in VLTM was biased towards features reported from VSTM and away from unreported features (Experiment 1). This bias could not be explained by the amount of sensory exposure or response learning (Experiment 2) and was amplified by the reporting of multiple features (Experiment 3). Taken together, these results suggest that selective internal attention induces competitive dynamics among features during visual learning, flexibly tuning object representations to align with prior mnemonic goals.     

Understanding the object benefit in visual short-term memory: the roles of feature proximity and connectedness

Past research has identified visual objects as the units of information processing in visual short-term memory (VSTM) and has shown that two features from the same object can be remembered in VSTM as well (or almost as well) as one feature of that object and are much better remembered than the same two features from two spatially separated objects. It is not clear, however, what drives this object benefit in VSTM. Is it the shared spatial location (proximity), the connectedness among features of an object, or both? In six change detection experiments, both location/proximity and connectedness were found to be crucial in determining the magnitude of the object benefit in VSTM. Together, these results indicate that location/proximity and connectedness are essential elements in defining a coherent visual object representation in VSTM.     

Encoding color and shape from different parts of an object in visual short-term memory

Can we find an object-based encoding benefit in visual short-term memory (VSTM) when the features to be remembered are from different parts of an object? Using object parts defined by either figure-ground separation or negative minima of curvature, results from five experiments in which the visual change detection paradigm was used showed that the object-based encoding benefit in VSTM is modulated by how features are assigned to parts of an object: Features are best retained when the color and shape features to be remembered belong to the same part of an object. Although less well retained in comparison, features from different parts of an object are still better remembered than features from spatially separated objects. An object-based feature binding therefore exists even when the color and shape features to be remembered are from different parts of an object.     

Lack of color integration in visual short-term memory binding

Bicolored objects are retained in visual short-term memory (VSTM) less efficiently than unicolored objects. This is unlike shape–color combinations, whose retention in VSTM does not differ from that observed for shapes only. It is debated whether this is due to a lack of color integration and whether this may reflect the function of separate memory mechanisms. Participants judged whether the colors of bicolored objects (each with an external and an internalcolor) were the same or different across two consecutive screens. Colors had to be remembered either individually or in combination. In Experiment 1, external colors in the combined colors condition were remembered better than the internal colors, and performance for both was worse than that in the individual colors condition. The lack of color integration observed in Experiment 1 was further supported by a reduced capacity of VSTM to retain color combinations, relative to individual colors (Experiment 2). An additional account was found in Experiment 3, which showed spared color–color binding in the presence of impaired shape–color binding in a brain-damaged patient, thus suggesting that these two memory mechanisms are different.     

Does visual short-term memory have a high-capacity stage?

Visual short-term memory (VSTM) has long been considered a durable, limited-capacity system for the brief retention of visual information. However, a recent work by Sligte et al. (Plos One 3:e1699, 2008) reported that, relatively early after the removal of a memory array, a cue allowed participants to access a fragile, high-capacity stage of VSTM that is distinct from iconic memory. In the present study, we examined whether this stage division is warranted by attempting to corroborate the existence of an early, high-capacity form of VSTM. The results of four experiments did not support Sligte et al.’s claim, since we did not obtain evidence for VSTM retention that exceeded traditional estimates of capacity. However, performance approaching that observed in Sligte et al. can be achieved through extensive practice, providing a clear explanation for their findings. Our evidence favors the standard view of VSTM as a limited-capacity system that maintains a few object representations in a relatively durable form.     

Visual short-term memory benefit for objects on different 3-D surfaces

Visual short-term memory (VSTM) plays an important role in visual cognition. Although objects are located on different 3-dimensional (3-D) surfaces in the real world, how VSTM capacity may be influenced by the presence of multiple 3-D surfaces has never been examined. By manipulating binocular disparities of visual displays, the authors found that more colored objects could be held in VSTM when they were placed on 2 rather than on 1 planar 3-D surfaces. This between-surface benefit in VSTM was present only when binding of objects' colors to their 3-D locations was required (i.e., when observers needed to remember which color appeared where). When binding was not required, no between-surface benefit in VSTM was observed. This benefit in VSTM could not be attributed to the number of spatial locations attended within a given surface. It was not due to a general perceptual grouping effect either, because grouping by motion and grouping by different regions of the same surface did not yield the same benefit. This increment in capacity indicates that VSTM benefits from the placement of objects in a 3-D scene.     

Feature bindings endure without attention: Evidence from an explicit recall task

Are integrated objects the unit of capacity of visual working memory, or is continued attention needed to maintain bindings between independently stored features? In a delayed recall task, participants reported the color and shape of a probed item from a memory array. During the delay, attention was manipulated with an exogenous cue. Recall was elevated at validly cued positions, indicating that the cue affected item memory. On invalid trials, participants most frequently recalled either both features (perfect object memory) or neither of the two features (no object memory); the frequency with which only one feature was recalled was significantly lower than predicted by feature independence as determined in a single-feature recall task. These data do not support the view that features are remembered independently when attention is withdrawn. Instead, integrated objects are stored in visual working memory without need for continued attention.     

The development of visual short-term memory for multifeature items during middle childhood

Visual short-term memory (VSTM) research suggests that the adult capacity is limited to three or four multifeature object representations. Despite evidence supporting a developmental increase in capacity, it remains unclear what the unit of capacity is in children. The current study employed the change detection paradigm to investigate both the capacity and nature of the representations of VSTM in 7-year-olds, 10-year-olds, and adults. Two experimental conditions were used to compare memory performance for single-feature objects with that for multifeature objects. Results provided further and more convincing evidence for a developmental increase in VSTM. Furthermore, the “loading up” of features in the multifeature condition was found to incur no significant performance deficit in all age groups. These findings are explained in terms of a developmental increase in the number of integrated multifeature object representations in VSTM.     

Object-based representations govern both the storage of information in visual short-term memory and the retrieval of information from it

Interest is growing in how information is retained in visual short-term memory (VSTM). We describe an experiment that assesses VSTM within the context of multidimensional signal detection theory. On every trial, participants were presented with a 250-ms display containing four colored shapes. They were then probed 900 ms later with a colored shape and made separate old/new judgments about the color and the shape. In any particular trial, one, both, or neither of the probed features had been presented. Performance differed according to whether both probed features belonged to a single object or to two different objects. When both probed features belonged to the same object, featural retrieval was better than would be predicted by independent feature storage. When both probed features belonged to two different objects, featural retrieval was worse than would be predicted by independent feature storage. We conclude that storage in and retrieval from VSTM operate on the basis of object-based representations.     

What are the units of visual short-term memory, objects or spatial locations?

We investigated whether the capacity of visual short-term memory (VSTM) is defined by number of objects or number of spatial locations. Previous work is consistent with either alternative. To distinguish these factors, we used overlapping stimuli that allowed us to independently manipulate the number of spatial locations while holding constant the number of objects and features to be encoded (Duncan, 1984; Vecera & Farah, 1994). In Experiment 1, the number of spatial locations had no effect on VSTM, suggesting that VSTM is object based. Experiments 2 and 3 ruled out alternative explanations based on perceptual segregation difficulty or decision noise factors. Our results provide additional support to Luck and Vogel's (1997) demonstration that integrated objects form the units of VSTM capacity.     

Representation of dynamic spatial configurations in visual short-term memory

Locations of multiple stationary objects are represented on the basis of their global spatial configuration in visual short-term memory (VSTM). Once objects move individually, they form a global spatial configuration with varying spatial inter-object relations over time. The representation of such dynamic spatial configurations in VSTM was investigated in six experiments. Participants memorized a scene with six moving and/or stationary objects and performed a location change detection task for one object specified during the probing phase. The spatial configuration of the objects was manipulated between memory phase and probing phase. Full spatial configurations showing all objects caused higher change detection performance than did no or partial spatial configurations for static and dynamic scenes. The representation of dynamic scenes in VSTM is therefore also based on their global spatial configuration. The variation of the spatiotemporal features of the objects demonstrated that spatiotemporal features of dynamic spatial configurations are represented in VSTM. The presentation of conflicting spatiotemporal cues interfered with memory retrieval. However, missing or conforming spatiotemporal cues triggered memory retrieval of dynamic spatial configurations. The configurational representation of stationary and moving objects was based on a single spatial configuration, indicating that static spatial configurations are a special case of dynamic spatial configurations.     

Improving visual short-term memory by sequencing the stimulus array

When multiple objects are presented briefly and simultaneously in a visual array, visual short-term memory (VSTM) can maintain only a limited number of these items. The present research report reveals that splitting the to-be-remembered items into two sequential arrays significantly increases VSTM performance relative to the simultaneous presentation of the same items. A memory benefit also emerges when the full object array is flashed twice (repeated) rather than being presented continuously for the same duration. Moreover, the sequential and repetition benefits are specifically pronounced for individuals with low performance for simultaneously presented items. Our results suggest that the conventional, simultaneous presentation mode may underestimate VSTM performance due to attentional limitations and/or competition between stimulus representations. In contrast, temporal segregation of the stimulus input may help participants maximize their performance and utilize their full VSTM capacity.     

Setting and changing feature priorities in visual short-term memory

Many everyday tasks require prioritizing some visual features over competing ones, both during the selection from the rich sensory input and while maintaining information in visual short-term memory (VSTM). Here, we show that observers can change priorities in VSTM when, initially, they attended to a different feature. Observers reported from memory the orientation of one of two spatially interspersed groups of black and white gratings. Using colored pre-cues (presented before stimulus onset) and retro-cues (presented after stimulus offset) predicting the to-be-reported group, we manipulated observers’ feature priorities independently during stimulus encoding and maintenance, respectively. Valid pre-cues reliably increased observers’ performance (reduced guessing, increased report precision) as compared to neutral ones; invalid pre-cues had the opposite effect. Valid retro-cues also consistently improved performance (by reducing random guesses), even if the unexpected group suddenly became relevant (invalid-valid condition). Thus, feature-based attention can reshape priorities in VSTM protecting information that would otherwise be forgotten.     

Visual short-term memory during smooth pursuit eye movements

Visual short-term memory (VSTM) was probed while observers performed smooth pursuit eye movements. Smooth pursuit keeps a moving object stabilized in the fovea. VSTM capacity for position was reduced during smooth pursuit compared with a condition with eye fixation. There was no difference between a condition in which the items were approximately stabilized on the retina because they moved with the pursuit target and a condition in which the items moved across the retina because they were stationary in space. The reduction of capacity for position was eliminated when miniature items were presented on the pursuit target. Similarly, VSTM capacity for color did not differ between smooth pursuit and fixation. The results suggest that visuospatial attention is tied to the target during smooth pursuit, which impairs VSTM for the position of peripheral objects. Sensory memory during smooth pursuit was only slightly impaired.     

The capacity of visual short-term memory is not a fixed number of objects

Luck and Vogel (1997) have reported several striking results in support of the view that visual short-term memory (VSTM) has a fixed capacity of four objects, irrespective of how many relevant features those objects comprise. However, more recent studies have challenged this account, indicating only a weak effect of the number of objects once other factors are more evenly equated across conditions. Here, we employed a symmetry manipulation to verify object segmentation in our displays, to demonstrate that when spatial and masking factors are held constant, the number of objects per se has no effect on VSTM. Instead, VSTM capacity may reflect the number of object "parts" or feature conjunctions in a given display.     

The role of temporal synchrony in perceptual object formation and updating

The appearance and disappearance of an object in the visual field is accompanied by changes to multiple visual features at the object's location. When features at a location change asynchronously, the cue of common onset and offset becomes unreliable, with observers tending to report the most recent pairing of features. Here, we use these last feature reports to study the conditions that lead to a new object representation rather than an update to an existing representation. Experiments 1 and 2 establish that last feature reports predominate in asynchronous displays when feature durations are brief. Experiments 3 and 4 demonstrate that these reports also are critically influenced by whether features can be grouped using nontemporal cues such as common shape or location. The results are interpreted within the object-updating framework (Enns, Lleras, & Moore, 2010), which proposes that human vision is biased to represent a rapid image sequence as one or more objects changing over time.     

Informative cues can slow search: The cost of matching a specific template

During visual search, observers hold in mind a search template, which they match against the stimulus. To characterize the content of this template, we trained observers to discriminate a set of artificial objects at an individual level and at a category level. The observers then searched for the objects on backgrounds that camouflaged the features that defined either the object’s identity or the object’s category. Each search stimulus was preceded by the target’s individual name, its category name, or an uninformative cue. The observers’ task was to locate the target, which was always present and always the only figure in the stimulus. The results showed that name cues slowed search when the features associated with the name were camouflaged. Apparently, the observers required a match between their mental representation of the target and the stimulus, even though this was unnecessary for the task. Moreover, this match involved all distinctive features of the target, not just the features necessary for a definitive identification. We conclude that visual search for a specific target involves a verification process that is performed automatically on all of the target’s distinctive features.     

Visual cognition influences early vision: the role of visual short-term memory in amodal completion

A partly occluded visual object is perceptually filled in behind the occluding surface, a process known as amodal completion or visual interpolation. Previous research focused on the image-based properties that lead to amodal completion. In the present experiments, we examined the role of a higher-level visual process-visual short-term memory (VSTM)-in amodal completion. We measured the degree of amodal completion by asking participants to perform an object-based attention task on occluded objects while maintaining either zero or four items in visual working memory. When no items were stored in VSTM, participants completed the occluded objects; when four items were stored in VSTM, amodal completion was halted (Experiment 1). These results were not caused by the influence of VSTM on object-based attention per se (Experiment 2) or by the specific location of to-be-remembered items (Experiment 3). Items held in VSTM interfere with amodal completion, which suggests that amodal completion may not be an informationally encapsulated process, but rather can be affected by high-level visual processes.     

Binding feature dimensions in visual short-term memory

We explored several possible influences on binding in visual short-term memory (VSTM) performance. The task was to report whether a test object was the same (“old” trials) or different (“new” trials) from any of the sample objects seen a second ago. The objects were composed of two features that varied from continuous to discrete shapes and colors. In “old” trials the test object appeared either in the same or different position. In “new” trials the test object differed along both features, requiring storage of only one feature per object; along one feature, requiring no binding but storage of all features; or it was created by recombining features from the sample, which requires binding. Existing storage hypotheses are unable to explain the similar sensitivity (d′) obtained in the two last conditions when position remained the same and may suggest that links are created between positions and features. Highest sensitivity occurred when the test object remained at the same position, required no binding, and discrete features were used. Object-type × position, and feature combination × position interactions occurred, suggesting different storage modes depending on whether objects change position during retention.     

Is visual short-term memory object based? Rejection of the “strong-object” hypothesis

Is the capacity of visual short-term memory (VSTM) limited by the number of objects or by the number of features? VSTM for objects with either one feature or two color features was tested. Results show that capacity was limited primarily by the number of colors to be memorized, not by the number of objects. This result held up with variations in color saturation, blocked or mixed conditions, duration of memory image, and absence or presence of verbal load. However, conjoining features into objects improved VSTM capacity when size-orientation and color-orientation conjunctions were tested. Nevertheless, the number of features still mattered. When feature heterogeneity was controlled, VSTM for conjoined objects was worse than VSTM for objects made of single features. Our results support a weak-object hypothesis of VSTM capacity that suggests that VSTM is limited by both the number of objects and the feature composition of those objects.