Changes in Visual Object Recognition Between 18 and 24 Months

Visual object recognition is foundational to processes of categorization, tool use, and real-world problem solving. Despite considerable effort across many disciplines and many specific advances, there is no comprehensive or well-accepted account of this ability. Moreover, none of the extant approaches consider how human object recognition develops. New evidence indicates a period of rapid change in toddlers’ visual object recognition between 18 and 24 months that is related to the learning of object names and to goal-directed action. Children appear to shift from recognition based on piecemeal fragments to recognition based on geometric representations of threedimensional shape. These findings may lead to a more unified understanding of the processes that make human object recognition as impressive as it is. KEYWORDS—visual object recognition; object name learning; development; visuomotor development Human visual object recognition is fast, robust, and successful in the service of a variety of different tasks. For example, people routinely recognize the dog whose nose is sticking out from the blanket; they recognize deck chairs and kitchen chairs as chairs; and they recognize their favorite cup as their own. The range of these abilities suggests that visual object recognition depends not on a single process but on several distinct processes (Peissig & Tarr, 2007). This article is primarily concerned with the visual representations that support recognition at the level of basic categories—for example, the processes that enable us to recognize easy chairs, lawn chairs, and rocking chairs as chairs. New developmental evidence indicates a significant shift in the nature of these representations in children between the ages of 18 and 24 months. Two classes of theories—so-called object-based and viewbased theories of adult object recognition—are relevant to the developmental findings. The best-known theory on the objectbased side, Biederman’s (1987) recognition-by-components (RBC) account proposes that humans form internal representations that are geometric models of objects’ shapes and that are internally manipulated via processes analogous to mental rotation (Marr & Nishihara, 1978). These representations, built from a primitive set of geometric volumes (Fig. 1a) capture the whole object’s geometric structure independent of one’s viewing perspective. The alternative class of theories explains object recognition not in terms of the geometry of three-dimensional shapes but rather in terms of picture-like (and therefore viewdependent) images (see Peissig & Tarr, 2007, for review). One of these, Ullman’s (2007) ‘‘fragment’’ account (Fig. 1b), specifically explains recognition at the basic-category level in terms of classspecific fragments. In this account, horses, for example, are recognized via piecemeal and category-specific local fragments such as the the ears, legs, and head shape. Both theories have been widely tested in studies of object recognition in adults and each captures important phenomena. Neither approach has seriously considered the developmental origins of visual object recognition, however. The findings reviewed below suggest an early shift from more fragment-based object recognition to recognition based on geometric shape. CHANGE BETWEEN 18 AND 24 MONTHS The development of visual object recognition is relatively unstudied, and there are many open questions (see Kellman, 2001, for a review). The representations and processes that underlie the visual recognition of three-dimensional real-world objects have been particularly neglected. There are, however, reasons to Address correspondence to Linda B. Smith, Department of Psychological and Brain Sciences, 1101 East 10 Street, Indiana University, Bloomington, IN 47405; e-mail: [email protected]. CURRENT DIRECTIONS IN PSYCHOLOGICAL SCIENCE 290 Volume 18—Number 5 Copyright r 2009 Association for Psychological Science expect that these processes will undergo significant change during development: First, many models of high-level vision recognition, as well as behavioral and neuroscience studies, indicate a formative role for category learning (see Peissig & Tarr, 2007). Second, evidence from a different domain of visual recognition, that of face recognition, indicates a protracted course of development and learning that stretches from infancy into adolescence (e.g., Maurer, Le Grand, & Mondloch, 2002). Object recognition might well show a similarly long developmental trajectory. Growth in Geometric Representations The period between 18 and 24 months is an interesting one with respect to the development of object recognition, because this is when children acquire a substantial number of object names, names that refer to categories of things that are principally alike in their in shape (Gershkoff-Stowe & Smith, 2004). Moreover, there is a well-documented increased attention to object shape over material properties such as color and texture during this same period (Gershkoff-Stowe & Smith, 2004). Motivated by these findings, I (Smith, 2003) wanted to know if 18to 24month-old children could recognize common objects given minimal information about their geometric shape, the same kind of information posited by Biederman’s RBC model to account for adult recognition. Smith compared children’s recognition of two kinds of holdable and manipulable three-dimensional objects: geometric ‘‘caricatures’’ (Fig. 2a) constructed from two to four volumes arranged to represent overall shape but without any fine-grained detail, color, or textural information; and richly detailed typical examples (Fig. 2b). There were two measures of object recognition. In the nonlinguistic play task, children were presented with caricatures or detailed examples and their play actions were scored as indicating recognition. For example, pretending to brush hair with a brush, eat the toy slice of pizza, or take a picture with the camera were scored as indicating recognition, whereas banging, stacking, or rolling were not. In the name-comprehension task, children were shown three objects and asked to indicate one (e.g., ‘‘show me the camera’’). Both tasks yielded the same result: Older children recognized the shape caricatures as well as they did the detailed instances. Younger children did not. They recognized only the detailed examples but not the caricatures. These results provide two new insights: First, representations of global geometric shape—of the kind posited in some theories of adult object recognition— are sufficient, in and of themselves, for object recognition in 2-year-olds, just as they are in adults. The fact that the older children in this sample—who are, after all, very young—recognized the caricatures just as well as they did the detailed examples shows that these children have abstracted the geometric structure of the shapes of common objects. Second, the additional fact that younger children recognized the detailed examples in both the nonlinguistic and linguistic tasks but failed to recognize the shape caricatures suggests a change in the representations that support visual object recognition. In particular, geometric representations of object shape appear to first emerge between 18 and 24 months, a result that has been replicated in additional studies (Jones & Smith, 2005; Pereira & Smith, 2009; Son, Smith, & Goldstone, 2008). Why might this developmental period be crucial for developing whole-object representations of geometric shape? One possibility is that object-name learning itself plays a role. I (Smith, 2003; see also Pereira & Smith, 2009) specifically examined the relation between children’s recognition of the shape caricatures and the number of object names in children’s vocabularies and found that known object names were a better predictor of children’s shape-caricature recognition than was age. Jones and Smith (2005) provided further evidence by showing delays in visual object recognition in children with delayed vocabulary development. Fig. 2. Stimuli used in experiments on shape caricature recognition: (a) sparse three-dimensional caricatures of the geometry of common categories, (b) richly detailed and lifelike instances, (c) caricatures with localized category specific features, (d) caricatures with no added features, (e) scrambled caricatures with localized category specific features, and (f) scrambled caricatures with no features. Fig. 1. Two representations of a horse: (a) a sparse geometric representation and (b) a category-specific-fragment representation. Volume 18—Number 5 291 Linda B. Smith