The psychophysics of chasing: A case study in the perception of animacy

Psychologists have long been captivated by the perception of animacy – the fact that even simple moving shapes may appear to engage in animate, intentional, and goal-directed movements. Here we report several new types of studies of a particularly salient form of perceived animacy: chasing, in which one shape (the ‘wolf’) pursues another shape (‘the sheep’). We first demonstrate two new cues to perceived chasing – chasing subtlety (the degree to which the wolf deviates from perfectly ‘heat-seeking’ pursuit) and directionality (whether and how the shapes ‘face’ each other). We then use these cues to show how it is possible to assess the objective accuracy of such percepts, and to distinguish the immediate perception of chasing from those more subtle (but nevertheless real) types of ‘stalking’ that cannot be readily perceived. We also report several methodological advances. Previous studies of the perception of animacy have faced two major challenges: (a) it is difficult to measure perceived animacy with quantitative precision; and (b) task demands make it difficult to distinguish perception from higher-level inferences about animacy. We show how these challenges can be met, at least in our case study of perceived chasing, via tasks based on dynamic visual search (the Find-the-Chase task) and a new type of interactive display (the Don’t-Get-Caught! task).     

Can infants' object concepts be trained?

Decades of research and debate on the origins of object permanence in infancy have contrasted various types of learning with possible innate contributions. A recent paper by Johnson et al. adds a new perspective to this debate by reporting that even very brief training periods can dramatically influence infants' persisting object representations. Such training studies have the potential to constrain 'nature versus nurture' debates in novel ways, although important challenges remain.     

Attentive tracking of objects versus substances

Recent research in vision science, infant cognition, and word learning suggests a special role for the processing of discrete objects. But what counts as an object? Answers to this question often depend on contrasting object-based processing with the processing of spatial areas or unbound visual features. In infant cognition and word learning, though, another salient contrast has been between rigid cohesive objects and nonsolid substances. Whereas objects may move from one location to another, a nonsolid substance must pour from one location to another. In the study reported here, we explored whether attentive tracking processes are sensitive to dynamic information of this type. Using a multiple-object tracking task, we found that subjects could easily track four items in a display of eight identical unpredictably moving entities that moved as discrete objects from one location to another, but could not track similar entities that noncohesively "poured" from one location to another-even when the items in both conditions followed the same trajectories at the same speeds. Other conditions revealed that this inability to track multiple "substances" stemmed not from violations of rigidity or cohesiveness per se, because subjects were able to track multiple noncohesive collections and multiple nonrigid deforming objects. Rather, the impairment was due to the dynamic extension and contraction during the substancelike motion, which rendered the location of the entity ambiguous. These results demonstrate a convergence between processes of midlevel adult vision and infant cognition, and in general help to clarify what can count as a persisting dynamic object of attention.     

The role of closure in defining the "objects" of object-based attention

Many recent studies have concluded that the underlying units of visual attention are often discrete objects whose boundaries constrain the allocation of attention. However, relatively few studies have explored the particular stimulus cues that determine what counts as an "object" of attention. We explore this issue in the context of the two-rectangles stimuli previously used by many investigators. We first show, using both spatial-cuing and divided-attention paradigms, that same-object advantages occur even when the ends of the two rectangles are not drawn. This is consistent with previous reports that have emphasized the importance of individual contours in guiding attention, and our study shows that such effects can occur in displays that also contain grouping cues. In our divided-attention experiment, however, this contour-driven same-object advantage was significantly weaker than that obtained with the standard stimulus, with the added cue of closure--demonstrating that contour-based processes are not the whole story. These results confirm and extend the observation that same-object advantages can be observed even without full-fledged objects. At the same time, however, these studies show that boundary closure-one of the most important cues to objecthood per se-can directly influence attention. We conclude that object-based attention is not an all-or-nothing phenomenon; object-based effects can be independently strengthened or weakened by multiple cues to objecthood.     

The automaticity of visual statistical learning

The visual environment contains massive amounts of information involving the relations between objects in space and time, and recent studies of visual statistical learning (VSL) have suggested that this information can be automatically extracted by the visual system. The experiments reported in this article explore the automaticity of VSL in several ways, using both explicit familiarity and implicit response-time measures. The results demonstrate that (a) the input to VSL is gated by selective attention, (b) VSL is nevertheless an implicit process because it operates during a cover task and without awareness of the underlying statistical patterns, and (c) VSL constructs abstracted representations that are then invariant to changes in extraneous surface features. These results fuel the conclusion that VSL both is and is not automatic: It requires attention to select the relevant population of stimuli, but the resulting learning then occurs without intent or awareness.     

Perceptual averaging by eye and ear: Computing summary statistics from multimodal stimuli

Beyond perceiving the features of individual objects, we also have the intriguing ability to efficiently perceive average values of collections of objects across various dimensions. Over what features can perceptual averaging occur? Work to date has been limited to visual properties, but perceptual experience is intrinsically multimodal. In an initial exploration of how this process operates in multimodal environments, we explored statistical summarizing in audition (averaging pitch from a sequence of tones) and vision (averaging size from a sequence of discs), and their interaction. We observed two primary results. First, not only was auditory averaging robust, but if anything, it was more accurate than visual averaging in the present study. Second, when uncorrelated visual and auditory information were simultaneously present, observers showed little cost for averaging in either modality when they did not know until the end of each trial which average they had to report. These results illustrate that perceptual averaging can span different sensory modalities, and they also illustrate how vision and audition can both cooperate and compete for resources.     

The relationship between object files and conscious perception

Object files (OFs) are hypothesized mid-level representations which mediate our conscious perception of persisting objects-e.g. telling us 'which went where'. Despite the appeal of the OF framework, not previous research has directly explored whether OFs do indeed correspond to conscious percepts. Here we present at least one case wherein conscious percepts of 'which went where' in dynamic ambiguous displays diverge from the analogous correspondence computed by the OF system. Observers viewed a 'bouncing/streaming' display in which two identical objects moved such that they could have either bounced off or streamed past each other. We measured two dependent variables: (1) an explicit report of perceived bouncing or streaming; and (2) an implicit 'object-specific preview benefit' (OSPB), wherein a 'preview' of information on a specific object speeds the recognition of that information at a later point when it appears again on the same object (compared to when it reappears on a different object), beyond display-wide priming. When the displays were manipulated such that observers had a strong bias to perceive streaming (on over 95% of the trials), there was nevertheless a strong OSPB in the opposite direction-such that the object files appeared to have 'bounced' even though the percept 'streamed'. Given that OSPBs have been taken as a hallmark of the operation of object files, the five experiments reported here suggest that in at least some specialized (and perhaps ecologically invalid) cases, conscious percepts of 'which went where' in dynamic ambiguous displays can diverge from the mapping computed by the object-file system.