Do infants reach for perceived objects? A reply to Stiles-Davis

Stiles-Davis proposes that the infants in our experiments (Hofsten & Spelke, 1985) did not reach for perceived objects in order to manipulate them, but rather touched perceived surfaces in order to explore their boundaries. Her commentary raises questions about infants' perception of the boundaries, the unity, and the manipulability of objects. More deeply, it raises the question of what an object is for an infant. We consider each of these questions in turn, in light of our own findings and those of other studies of object-directed reaching, object perception, and the object concept. We suggest that young infants organize the visual world into entities that are bounded, unitary, and manipulable and that infants endow those entities with the core properties of physical objects.     

Parts, cavities, and object representation in infancy

Part representation is not only critical to object perception but also plays a key role in a number of basic visual cognition functions, such as figure-ground segregation, allocation of attention, and memory for shapes. Yet, virtually nothing is known about the development of part representation. If parts are fundamental components of object shape representation early in life, then the infant visual system should give priority to parts over other aspects of objects. We tested this hypothesis by examining whether part shapes are more salient than cavity shapes to infants. Five-month-olds were habituated to a stimulus that contained a part and a cavity. In a subsequent novelty preference test, 5-month-olds exhibited a preference for the cavity shape, indicating that part shapes were more salient than cavity shapes during habituation. The differential processing of part versus cavity contours in infancy is consistent with theory and empirical findings in the literature on adult figure-ground perception and indicates that basic aspects of part-based object processing are evident early in life.     

The time course of perceptual grouping: the role of segregation and shape formation

The time course of perceptual grouping was examined in two experiments, using a primed matching task. In different conditions, elements were grouped into columns/rows by common lightness, into a shape (triangle/ arrow or square/cross) by common lightness, and into a shape without segregation of elements. The results showed an early and rapid grouping into columns/rows by common lightness and into a shape when no segregation from other elements was involved. Goodness of shape (i.e., triangle/arrow vs. square/cross) had no influence on how early grouping was evident, but the relatively poorer shapes appeared to consolidate with time. In contrast, grouping into a shape that involved segregation and required resolving figure-ground relations between segregated units, as grouping into a shape by common lightness, consumed time, regardless of shape goodness. These results suggest that the time course of grouping varies as a function of the processes involved in it (e.g., segregation and shape formation) and the conditions prevailing for each process.     

Smelling objects

Objects are central to perception and our interactions with the world. We perceive the world as parsed into discrete entities that instantiate particular properties, and these items capture our attention and shape how we interact with the environment. Recently there has been some debate about whether the sense of smell allows us to perceive odours as discrete objects, with some suggesting that olfaction is aspatial and doesn’t allow for object-individuation. This paper offers two empirically tractable criteria for assessing whether particular objects are exhibited in perceptual experience—(1) susceptibility to figure-ground segregation and (2) perceptual constancies—and argues that these criteria are fulfilled by olfactory perception, and thus there are olfactory objects. I argue that there are, in fact, two different ways that olfaction allows for figure-ground segregation. First, I look at various Gestalt grouping principles, which are thought to govern when features are perceived as grouped into structured wholes, segregated from everything around them. I argue that these principles apply to olfactory experience, providing evidence of non-spatial figure-ground segregation. Second, I defend the contentious idea that a spatial variety of figure-ground segregation can also occur in olfaction. To see this, however, we need to look to empirical evidence showing that tactile stimulation and bodily movements play a crucial role in olfactory phenomenology. Finally, I draw on empirical evidence and olfactory phenomenology to argue that there are perceptual constancies in olfactory experience, allowing us to perceive odours as coherent objects that survive shifts in our perspectives on the world.

     

Perceptual grouping in space and time: Evidence from the Ternus display

We report three experiments investigating the effect of perceptual grouping on the appearance of a bistable apparent-motion (Ternus) display. Subjects viewed a Ternus display embedded in an array of context elements that could potentially group with the Ternus elements. In contrast to several previous findings, we found that grouping influenced apparent motion perception. In Experiment 1, apparent motion perception was significantly affected via grouping by shape similarity, even when the visible persistence of the elements was controlled. In Experiment 2, elements perceived as moving without context were perceived as stationary when grouped with stationary context elements. In Experiment 3, elements perceived as stationary without context were perceived as moving when grouped with moving context elements. We argue that grouping in the spatial and temporal domains interact to yield perceptual experience of apparent-motion displays.     

Objects, parts, and categories

Concepts may be organized into taxonomies varying in inclusiveness or abstraction, such as furniture, table, card table or animal, bird, robin. For taxonomies of common objects and organisms, the basic level, the level of table and bird, has been determined to be most informative (Rosch, Mervis, Gray, Johnson, & Boyes-Braem, 1976). Psychology, linguistics, and anthropology have produced a variety of measures of perception, behavior, and communication that converge on the basic level. Here, we present data showing that the basic level differs qualitatively from other levels in taxonomies of objects and of living things and present an explanation for why so many measures converge at that level. We have found that part terms proliferate in subjects' listings of attributes characterizing category members at the basic level, but are rarely listed at a general level. At a more specific level, fewer parts are listed, though more are judged to be true. Basic level objects are distinguished from one another by parts, but members of subordinate categories share parts and differ from one another on other attributes. Informants agree on the parts of objects, and also on relative "goodness" of the various parts. Perceptual salience and functional significance both appear to contribute to perceived part goodness. Names of parts frequently enjoy a duality not evident in names of other attributes; they refer at once to a particular appearance and to a particular function. We propose that part configuration underlies the various empirical operations of perception, behavior, and communication that converge at the basic level. Part configuration underlies the perceptual measures because it determines the shapes of objects to a large degree. Parts underlie the behavioral tasks because most of our behaviors is indirect toward parts of objects. Labeling appears to follow the natural breaks of perception and behavior; consequently, part configuration also underlies communication measures. Because elements of more abstract taxonomies, such as scenes and events, can also be decomposed into parts, this analysis provides a bridge to organization in other domains of knowledge. Knowledge organization by parts (partonomy) is contrasted to organization by kinds (taxonomy). Taxonomies serve to organize numerous classes of entities and to allow inference from larger sets to sets included in them. Partonomies serve to separate entities into their structural components and to organize knowledge of function by components of structure. The informativeness of the basic level may originate from the availability of inference from structure to function at that level.     

Emergent features, attention, and perceptual glue in visual form perception

We examined the grouping of line segments into unitary shapes and attempted to identify procedures to diagnose when such grouping is taking place. Previous research has indicated that attentional measures may diagnose grouping: With grouped parts, selective attention to individual parts is difficult and divided attention across parts is easy, whereas with ungrouped parts selective attention is easy and divided attention is difficult. This result suggests that grouping operates via a perceptual glue binding parts into wholes that are difficult or impossible to divide. Other studies have suggested in addition that grouped parts produce emergent features, possibly including symmetry and closure, that make possible configural superiority effects, where whole shapes are more discriminable than are their distinguishing contours shown in isolation. The 13 experiments reported here indicate that perceptual glue is not needed to explain known findings about grouping, a claim that agrees with conclusions by other investigators using other criteria. Rather, emergent features alone may suffice to explain grouping, provided that reliable and accurate diagnostic criteria can be identified. It is shown that the diagnostics now available are not fully adequate for this purpose. Surprisingly, it appears that some prime candidates for emergent features--namely, closure and line terminators--may not be of central importance to form perception.     

Time course of perceptual grouping by color

Does perceptual grouping operate early or late in visual processing? One position is that the elements in perceptual layouts are grouped early in vision, by properties of the retinal image, before perceptual constancies have been determined. A second position is that perceptual grouping operates on a postconstancy representation, one that is available only after stereoscopic depth perception, lightness constancy, and amodal completion have occurred. The present experiments indicate that grouping can operate on both a preconstancy representation and a postconstancy representation. Perceptual grouping was based on retinal color similarity at short exposure durations and based on surface color similarity at long durations. These results permit an integration of the preconstancy and postconstancy positions with regard to grouping by color.     

Perceptual grouping and attention: Not all groupings are equal

We examined grouping under inattention using Driver, Davis, Russell, Turatto, & Freeman’s (2001) method. On each trial, two successive displays were briefly presented, each comprising a central target square surrounded by elements. The task was to judge whether the two targets were the same or different. The organization of the background elements stayed the same or changed, independently of the targets. In different conditions, background elements grouped into columns/rows by color similarity, a shape (a triangle/arrow, a square/cross, or a vertical/horizontal line) by color similarity, and a shape with no other elements in the background. We measured the influence of the background on the target same-different judgments. The results imply that background elements grouped into columns/rows by color similarity and into a shape when no segregation from other elements was involved and the shape was relatively “good.” In contrast, no background grouping was observed when resolving figure-ground relations for segregated units was required, as in grouping into a shape by color similarity. These results suggest that grouping is a multiplicity of processes that vary in their attentional demands. Regardless of attentional demands, the products of grouping are not available to awareness without attention.     

Grouping based on phenomenal similarity of achromatic color.

It is widely acknowledged that a precondition for the perception of the world of objects and events is an early process of organization, and it has generally been assumed that such organization is based on the Gestalt laws of grouping. However, the stage at which such grouping occurs, whether early or late, is an empirical question. It is demonstrated in two experiments that grouping by similarity of neutral color is based not on similarity of absolute luminance at the level of the proximal stimulus, but on phenomenal similarity of lightness resulting from the achievement of lightness constancy. An alternative explanation of such grouping based on the equivalence of luminance ratios between elements and background is ruled out by appropriate control conditions.     

Late influences on perceptual grouping: illusory figures

Two experiments demonstrate that grouping can be strongly influenced by the presence of figures defined by illusory contours. Rectangular arrays were constructed in which a central column of figures could group either with those on one side, on the basis of perception of figures defined by illusory contours, or with those on the other side, on the basis of physically present inducing elements. In all displays, subjects grouped according to the illusory figures significantly more often than for control displays that contained the same inducing elements, but rearranged so that illusory contours were degraded or eliminated. A second experiment showed that in objectively defined grouping tasks, subjects grouped faster by illusory figures than by inducing elements. These results indicate that grouping can occur after illusory contours have been perceived.     

Incremental grouping of image elements in vision

One important task for the visual system is to group image elements that belong to an object and to segregate them from other objects and the background. We here present an incremental grouping theory (IGT) that addresses the role of object-based attention in perceptual grouping at a psychological level and, at the same time, outlines the mechanisms for grouping at the neurophysiological level. The IGT proposes that there are two processes for perceptual grouping. The first process is base grouping and relies on neurons that are tuned to feature conjunctions. Base grouping is fast and occurs in parallel across the visual scene, but not all possible feature conjunctions can be coded as base groupings. If there are no neurons tuned to the relevant feature conjunctions, a second process called incremental grouping comes into play. Incremental grouping is a time-consuming and capacity-limited process that requires the gradual spread of enhanced neuronal activity across the representation of an object in the visual cortex. The spread of enhanced neuronal activity corresponds to the labeling of image elements with object-based attention.     

Multielement visual tracking: attention and perceptual organization.

Two types of theories have been advanced to account for how attention is allocated in performing goal-directed visual tasks. According to location-based theories, visual attention is allocated to spatial locations in the image; according to object-based theories, attention is allocated to perceptual objects. Evidence for the latter view comes from experiments demonstrating the importance of perceptual grouping in selective-attention tasks. This article provides further evidence concerning the importance of perceptual organization in attending to objects. In seven experiments, observers tracked multiple randomly moving visual elements under a variety of conditions. Ten elements moved continuously about the display for several seconds; one to five of them were designated as targets before movement initiation. At the end of movement, one element was highlighted, and subjects indicated whether or not it was a target. The ease with which the elements in the target set could be perceptually grouped was systematically manipulated. In Experiments 1-3, factors that influenced the initial formation of a perceptual group were manipulated; this affected performance, but only early in practice. In Experiments 4-7, factors that influenced the maintenance of a perceptual group during motion were manipulated; this affected performance throughout practice. The results suggest that observers spontaneously grouped the target elements and directed attention toward this coherent but nonrigid virtual object. This supports object-based theories of attention and demonstrates that perceptual grouping, which is usually conceived of as a purely stimulus-driven process, can also be governed by goal-directed mechanisms.     

Perception of rhythmic grouping: testing the iambic/trochaic law

This study was designed to test the iambic/trochaic law, which claims that elements contrasting in duration naturally form rhythmic groupings with final prominence, whereas elements contrasting in intensity form groupings with initial prominence. It was also designed to evaluate whether the iambic/trochaic law describes general auditory biases, or whether rhythmic grouping is speech or language specific. In two experiments, listeners were presented with sequences of alternating /ga/ syllables or square wave segments that varied in either duration or intensity and were asked to indicate whether they heard a trochaic (i.e., strong-weak) or an iambic (i.e., weak-strong) rhythmic pattern. Experiment 1 provided a validation of the iambic/trochaic law in English-speaking listeners; for both speech and nonspeech stimuli, variations in duration resulted in iambic grouping, whereas variations in intensity resulted in trochaic grouping. In Experiment 2, no significant differences were found between the rhythmic-grouping performances of English- and French-speaking listeners. The speech/ nonspeech and cross-language parallels suggest that the perception of linguistic rhythm relies largely on general auditory mechanisms. The applicability of the iambic/trochaic law to speech segmentation is discussed.     

How do humans group non-rigid objects in multiple object tracking?: Evidence from grouping by self-rotation

Previous studies on perceptual grouping found that people can use spatiotemporal and featural information to group spatially separated rigid objects into a unit while tracking moving objects. However, few studies have tested the role of objects’ self-motion information in perceptual grouping, although it is of great significance to the motion perception in the three-dimensional space. In natural environments, objects always move in translation and rotation at the same time. The self-rotation of the objects seriously destroys objects’ rigidity and topology, creates conflicting movement signals and results in crowding effects. Thus, this study sought to examine the specific role played by self-rotation information on grouping spatially separated non-rigid objects through a modified multiple object tracking (MOT) paradigm with self-rotating objects. Experiment 1 found that people could use self-rotation information to group spatially separated non-rigid objects, even though this information was deleterious for attentive tracking and irrelevant to the task requirements, and people seemed to use it strategically rather than automatically. Experiment 2 provided stronger evidence that this grouping advantage did come from the self-rotation per se rather than surface-level cues arising from self-rotation (e.g. similar 2D motion signals and common shapes). Experiment 3 changed the stimuli to more natural 3D cubes to strengthen the impression of self-rotation and again found that self-rotation improved grouping. Finally, Experiment 4 demonstrated that grouping by self-rotation and grouping by changing shape were statistically comparable but additive, suggesting that they were two different sources of the object information. Thus, grouping by self-rotation mainly benefited from the perceptual differences in motion flow fields rather than in deformation. Overall, this study is the first attempt to identify self-motion as a new feature that people can use to group objects in dynamic scenes and shed light on debates about what entities/units we group and what kinds of information about a target we process while tracking objects.     

Perceptual organization based on illusory regions in infancy

Prior research indicates that, like adults, infants use enclosed regions to group elements. It is not clear whether infants or adults can use regions that have to be inferred from illusory contours to group elements. We examined whether 3- to 4-month-olds use illusory regions to group elements and generalize this organization to novel regions. Infants habituated to pairs of shapes in illusory vertical or horizontal regions subsequently discriminated, in novel regions, pairs of elements that had previously shared a region from pairs of elements that had been in different regions. A control group of infants, who had experienced the same stimuli except for the presence of illusory regions, failed to discriminate between within-region and between-region pairs of stimuli. These results reveal that (1) illusory regions can be used to group elements, (2) perceptual organization is sufficiently developed early in life for 3- to 4-month-olds to group on the basis of ecologically relevant illusory contours, and (3) such grouping in infancy generalizes to novel regions.     

The role of functional information for infant categorization

This report examines whether knowledge about function influences the formation of artifact categories in 11-12- month old infants. Using an object-examination task, a set of artificial stimuli was presented that could either be grouped according to overall similarity or according to similarity in one functionally relevant part. Experiment 1 revealed that infants categorized the objects according to overall similarity but not part similarity under control conditions. Experiment 2 showed that after having seen the experimenter demonstrating the functional use of the critical part, infants later categorized the stimuli according to part similarity. When the same actions were performed without producing any effect, infants failed to categorize according to the critical part. This set of findings suggests that 11-12-month old infants use functional information as a cue to categorization.     

The differentiation of rhythmic structure.

Listeners judged whether two five-tone nonmetric rhythms were the same or different. Each rhythm was presented one, two, or four times to study the process of perceptual differentiation. The results indicated that the listeners perceived these rhythms in terms of the grouping of the tones, and not in terms of the timing between the groups. Two rhythms that had the same perceptual grouping were judged as being identical, even if the timing between the groups was different. The perception of the groupings of tones developed gradually. If each rhythm was presented only once, then the listeners had only a global percept, focused on groups (runs) of three elements, and often judged two different rhythms as being identical. If the rhythms were presented two or four times, then the grouping of the tones became more differentiated and the listeners were less likely to judge different patterns as being identical. Thus, perception of auditory rhythmic structure appears to follow the same developmental process as the perception of visual spatial structure.     

Induced perceptual grouping

ABSTRACT— The term perceptual grouping is associated with classical principles such as similarity and proximity. This article reports induced perceptual grouping, a phenomenon that occurs when placement of a uniform set of items near a structured set induces grouping within the otherwise uniform set. For example, when items grouped pair-wise by similarity are placed near another set of unstructured items, an analogous pair-wise grouping links elements of the second set. Induced grouping affected reaction times in two different visual search tasks, with reaction times depending on whether the target properties were contained within a group or crossed group boundaries as defined by induced grouping due to similarity, proximity, or common fate. Induced grouping was reduced when grouping between the structured and unstructured sets was weakened by means of a common-region cue or decreased similarity. Induced grouping appears to reflect the computation of hierarchical structure in visual images.