A PDP approach to semantic cognition 1 Running Head : A PDP APPROACH

Over the first year of life, infants gain conceptual skills which allow them to construe semantically related items as similar, even when they have few if any directly-perceived attributes in common. Moreover, this skill first encompasses only broad semantic categories, and only later extends to more subtle distinctions, when conceptual and perceptual similarity relations do not coincide. In this paper we suggest that a new mechanism must be added to the mix of possible bases for this observed developmental change. In agreement with many others, we suggest that infants’ earliest conceptual representations are organised with respect to certain especially useful or salient properties, regardless of whether such properties can be directly observed. However we suggest that in many cases this salience may itself be acquired, through domain-general learning mechanisms that are sensitive to the high-order coherent covariation of directly-observed stimulus properties across a breadth of experience. To support this argument we will describe simulations with a simple PDP model of semantic memory. When trained with backpropagation to complete queries about the properties of different objects, the model’s internal representations differentiate in a coarse-to-fine manner. As a consequence, different sets of properties come to be especially “salient” to the model at different points during development. Such dynamics provide a simple account of the acquisition of conceptual structure from perceptual experience. A PDP approach to semantic cognition 3 A Parallel Distributed Processing approach to semantic cognition: Applications to conceptual development Although conceptual development in infancy is a wide and varying field of study which encompasses many different points of view, there are, broadly speaking, two sets of empirical findings that are relevant to understanding the competencies that emerge over the first year of life. First, infants as young as 3-4 months are sensitive to visual similarities existing across discriminably different stimulus events, and can capitalize on these to guide their behavior in preferential-looking and dishabituation studies. In some cases, this perceptual learning skill is sufficient for very young infants to discriminate natural semantic categories, on the basis of the visual similarity and variability of the stimulus items encountered during habituation and test (e.g. Eimas & Quinn, 1994). However at the youngest testable ages, infant behaviour in this respect seems to be strongly linked to the visual structure of the specific items encountered during habituation and test, and does not appear to reflect stored knowledge about the conceptual relationships that may exist among stimulus items (Mareschal, French, & Quinn, 2000; Quinn & Johnson, 1997). Thus several researchers have suggested that very young infants are capable of forming “perceptual categories” to guide their expectations in looking tasks. The second set of findings suggest that, toward the end of the first year of life, infants are less bound to the immediately-observed perceptual properties of objects, and are sometimes able to respond to objects on the basis of somewhat abstract properties that are not directly apparent in the situation. In a series of studies employing several related methods, Mandler and her collaborators (e.g. Mandler & Bauer, 1988; Mandler, Bauer, & McDonough, 1991; Mandler & McDonough, 1993, 1996; Mandler, 1997, 2000) and other investigators (e.g. Pauen, 2002b, 2002a) have argued that pre-verbal children’s choices of replicas of objects to touch, to play with, and to use in imitations of actions performed by others, suggest that at the earliest testable ages (about seven months) infants treat objects that share certain informative but abstract properties (such as self-initiated movement) as similar to one another, even if the objects happen to differ in many other perceptible respects. Moreover, infants at this age seem to be sensitive predominantly to very general A PDP approach to semantic cognition 4 conceptual distinctions among perceptually-varying items, and only later come to honor finer distinctions when perceptual and conceptual relations do not happen to coincide (e.g. Mandler & Bauer, 1988; Mandler & McDonough, 1993; Mandler, 2002). There is considerable dispute regarding the interpretation of these data—specifically, whether they can be explained solely with reference to perceptual learning mechanisms; whether or not confounding factors have been adequately controlled; and whether or not the infant behaviour really reveals the first emergence of conceptual knowledge structures. But whichever position one takes on these issues, the data raise two important questions about conceptual development in infancy. First, how do infants come to construe semantically related items as similar to one another when they have few (or perhaps no) directly-perceived properties in common? Second, why are coarse-grained conceptual distinctions available before finer-grained distinctions? Other investigators have offered answers to one or both of these questions. These answers come with somewhat different slants on the empirical evidence; and both the evidence itself and the theories are the subject of considerable current discussion, as witnessed by the other articles in this volume. Here, we would like to raise the possibility that a process not considered by any of these investigators needs to be added to the mix of possible bases for the characteristics of children’s earliest conceptual abilities. Specifically, we will suggest that the learning mechanisms that give rise to infants’ earliest concepts are strongly sensitive to patterns of coherent covariation amongst the directly-perceived properties of objects and events. This sensitivity influences the first conceptual distinctions that emerge in infancy, as well as determining which properties of objects infants will weight in particular semantic tasks. Our goal will be first to show that children’s initial conceptual distinctions may reflect the effects of coherent covariation; and second, that the importance infants assign to particular cues may reflect the fact that these cues vary coherently with others, and thereby come to acquire salience. The mechanism that supports these effects derives from our assumptions about the nature of the representations and processes that subserve cognition generally, and semantic cognition in particular: namely, the assumptions inherent in the parallel distributed processing (PDP) framework. Accordingly, we will make our arguments with reference to a simple PDP model of semantic cogntion first described by A PDP approach to semantic cognition 5 Rumelhart (Rumelhart, 1990; Rumelhart & Todd, 1993). This model has strongly influenced our thinking about the acquisition of conceptual knowledge, and we believe that the principles that govern its behaviour can shed light on many different aspects of semantic cognition (Rogers & McClelland, in press). In this paper we will focus on properties of the model that may help to explain how infants are able to represent conceptual similarity relations among perceptually varying stimulus items, and why they are first sensitive to coarse conceptual distinctions. The principles we will emphasize will not and cannot refute the possibility that factors raised by other investigators are also at work. However, if we are able to establish that these principles can play a role in shaping children’s conceptual distinctions and in determining what cues they are sensitive to, our account raises questions about the degree to which it is necessary to invoke some of the proposals that have been offered by others. A Brief Survey of the Landscape Though most theorists may agree that infant abilities in semantic tasks change in some fashion over the course of the first year of life, there is a diversity of opinion on how best to characterise the underlying cause of the observed differences. It is difficult to do justice to the richness of the literature in this regard; however it will be useful for us to briefly survey some of the stances others have taken on this issue, to set the stage for our later consideration of the PDP approach. Painting with broad strokes, we will here outline four positions regarding the nature of the knowledge that permits older infants to treat items from the same conceptual domain as similar, even if they do not share directly-perceived properties. We should note that these positions need not be considered all mutually exclusive of one another—in some cases they are complimentary, and in other cases not. Perceptual enrichment. One possibility offered by several researchers (e.g. Quinn & Eimas, 1997; ? ; Quinn & Johnson, 1997; Mareschal, 2000) is that older infants come to discern conceptual relationships on the basis of learned associations among directly-perceived properties of items across different episodes and situations. For example, older infants, on the basis of their encounters with particular dogs, cats, birds, and other animals, have learned to associate the various perceptual properties that co-occur together in these A PDP approach to semantic cognition 6 encounters—including some properties that may be directly-observable in static photographs and toy replicas (such as eyes, legs, and fur) as well as some that are not (such as patterns of movement and behaviour). Capitalizing on this stored knowledge, older infants in experiments like Mandler’s make inferences about properties of the stimulus objects that cannot be directly observed in the experiment, but which have been directly observed on past occasions. On this view, semantically related items come to be associated with similar sets of properties, and as children gain experience they can come to exploit conceptual similarity relations on this basis. Investigators who adopt this perspective emphasize studies