Cats could be dogs, but dogs could not be cats: what if they bark and mew? A Connectionist Account of Early Infant Memory and Categorization

Cats could be dogs, but dogs could not be cats: what if they bark and mew? A Connectionist Account of Early Infant Memory and Categorization Robert A.P. Reuter (rreuter@ulb.ac.be) Cognitive Science Research Unit, CP 191, Av. F.D. Roosevelt, 50 B-1050 Brussels, Belgium Abstract The goal of this paper is to replicate and extend the connectionist model presented by Mareschal and French (1997) as an account of 'the particularities of […] infant memory and categorization'. With infants, the sequential presentation of cats followed by dogs yields an expected increase in infants' looking time, whereas the reversed presentation order does not. This intriguing asymmetry of infants' category formation, first reported by Quinn, Eimas, and Rosenkrantz (1993), was simulated by Mareschal et al.'s simple connectionist network. In addition, the authors proposed that this asymmetric categorization is a natural byproduct of the 'asymmetric overlaps of the visual feature distributions' of cats and dogs. Using a simple feedforward backpropagation network, we successfully replicated this asymmetric categorization effect, as well as a reported asymmetric exclusivity effect in the two categories, and an asymmetric interference effect of learning dogs on the memory for cats, but not of learning cats on the memory for dogs. We furthermore investigated the authors' explanation of the asymmetric effects, firstly, by systematically varying the overall similarity between learned items and interfering items, and secondly, by adding a binary feature to the input set, namely the animal cry (barking vs. mewing). The results of the present modeling underscore the authors' explanation of the observed effects in infants' memory and categorization, but also suggest lines of further experimental research susceptible to undermine the proposed connectionist account. Introduction In this paper, we report on a replication and two extensions of Mareschal and French's (1997) simple connectionist model that accounts fairly well for unexpected findings observed in spontaneous category formation in young infants (e.g., Quinn, Eimas, & Rosenkrantz, 1993) and in infant memory. Indeed, Mareschal et al. focused their modeling efforts on three target behaviors of very young infants concerning their categorization and memory, namely: (a) the ability to categorize complex visual stimuli, (b) the asymmetric effect in early categorization, and (c) interference effects in early memory. The empirical data Mareschal et al. modeled show that infants, aged 3 to 4 months, are able to accurately categorize cats and dogs, and that they form an exclusive category of cats (thus excluding novel dogs) but an inclusive category of dogs (thus novel cats may well fall into the category of dogs) (Quinn et al., 1993). Furthermore, infants are known to present catastrophic forgetting of previously learned stimuli when shown certain other intervening material (e.g., Cohen & Gelber, 1975). This interference effect decreases precisely when infants' categorization abilities increase (Quinn & Eimas, 1996). Infant memory and categorization can thus be thought to be closely linked to each other and to depend on the same basic mechanisms. Indeed, the connectionist account given for the observed asymmetric effects in categorization and for the interference effects in memory is based on the same reasoning. The asymmetry in category formation arises from the unequal overlap of the visual features distribution and not just the variance of the distribution itself. In other words, the values of the cat features fall within those of the dog values. Hence, based merely on the statistical structure of the input features, infants (and neural networks) form a category of dogs including cats, whereas they form a category of cats excluding (some) dogs 1 . The correlational structure extracting mechanism is precisely what accounts for the observed asymmetric effects of unequally exclusive (or inclusive) cat and dog categorizations. Catastrophic forgetting, on the other hand, can be understood as the deleterious effect of representing, within the same connections, stimuli whose features are very differently distributed. This, consequently, washes out the relevant knowledge previously stored in the network. However learning items whose features lie within the same range as those learned previously should not create very different internal representations - and may hence even consist in learning more of the same . Based on this connectionist account of the relationship between infant categorization and memory, Mareschal et al. successfully produced the conjectured asymmetry in catastrophic interference consistent with the asymmetry observed in category elaboration (i.e., dogs interfered with previously learned cats, whereas cats did not interfere with learned dogs). Indeed, some dogs look like cats, since that they fall into the range of the cat distribution for some features. We will come back to this later.