The active role of partial knowledge in cross-situational word learning

The Active Role of Partial Knowledge in Cross-Situational Word Learning Daniel Yurovsky, Damian Fricker, Chen Yu, and Linda B. Smith {dyurovsk, dfricker, chenyu, smith4} @indiana.edu Department of Psychological and Brain Science, and Cognitive Science Program 1101 East 10 th Street Bloomington, IN 47405 USA Abstract A number of modern word learning theories posit statistical processes in which knowledge is accumulated across many exposures to a word and its potential referents. Accordingly, words do not go directly from unknown to known, but rather pass through intermediate stages of partial knowledge. This work presents empirical evidence for the existence of such partial knowledge, and further demonstrates its active driving role in cross-situational word learning. Subsequently, an incremental model which leverages its partial knowledge of word-object mappings from trial to trial is shown to account well for the data. In contrast, models which do not do so cannot explain the data. These results confirm crucial assumptions made by statistical word learning models and shed light on the representations underlying the acquisition of word meanings. Keywords: word learning; language computational modeling; statistical learning acquisition; Introduction We have a tendency to characterize word learning as an all-or-none process: either a child knows a given word, or she has not yet learned it. This is apparent in our methodology (e.g. forced-choice tests, preferential looking), and assessment of vocabulary size via MCDI (Fenson, Dale, Reznick, Bates, Thal, & Pethick, 1994), as well as some theoretical claims. But this implicit all-or-none characterization may stymie our thinking about potential word-meaning representations. For almost a century we have known that human learning and memory are not binary phenomena (Ebbinghaus, 1913). In learning lists of paired associates, for instance, a failure to recall the correct pair for a prompt does not imply no knowledge of the mapping. Evidence of this knowledge can be recovered using a different test paradigm (e.g. recognition or savings). The knowledge is not absent, but rather partial or sub-threshold. The central idea motivating this work is that such sub-threshold knowledge may play a profound in the course of language acquisition. Several recent theoretical and computational approaches to word learning have made explicit use of partial knowledge. For instance, McMurray (2007) modeled the learning of a word’s meaning as the acquisition of partial meaning tokens. Yu and Smith (2007) argued that early word learning can be thought of as the accumulation of co- occurrence statistics between words and objects across multiple situations. These theories suggest that a word can be learned in bits rather than in a single perfect moment. Other models make an even stronger claim: not only can one build lexical knowledge by accumulating parts; this partial knowledge is an active driver of the learning system (Blythe, Smith, & Smith, in press, McMurray, Horst, Toscano, & Samuelson, in press, Fazly, Alishahi, & Stevenson, in press, Yu, 2008). These models have been tested predominantly on large corpora, reproducing qualitative patterns found in children’s word learning. If they are correct about the presence and role of partial knowledge, however, then we should be able to find empirical evidence for the role of partial knowledge in human word learners. Yurovsky and Yu (2008) presented indirect evidence of the active role of partial knowledge in cross-situational learning. They exposed participants to a series of individually ambiguous learning trials consisting of multiple words and multiple objects. At the end of each trial, participants were asked to indicate how sure they were (1- 10) that they knew the correct label for each object. Yurovsky and Yu showed that a given object’s rating could be predicted from the ratings given to the other objects on the same trial, even after the object’s rating on its previous exposure was taken into account. Thus, participants seemed to be using partial knowledge of word-object mappings to reduce the set of candidates for other labels. This analysis, while promising, was performed on participants’ subjective knowledge ratings. In the present work, we propose to offer stronger and more direct evidence that partial knowledge plays an active role in word learning. To this end, we expose participants to two consecutive blocks of cross-situational learning. Crucially, half of the words and objects in the second block are those which participants failed to learn in the first block. Comparing the results of block 2 to those of several control conditions, we can determine the role of partial knowledge in cross- situational learning. First, we can ask whether partial knowledge exists in the system, whether learners are really accumulating bits of sub-threshold knowledge. At a deeper level, we pursue a more interesting question: does partial knowledge of individual word-referent pairs – interacting in a system with partial knowledge of other word-referent pairs – facilitate the acquisition of new words. To answer this question, a set of computational models are fit to the data to understand the underlying learning mechanisms which give rise to the empirical results. We compare a simple associative model, a biased associative model which increments associations in proportion to their current strength, and a competitive associative model which adds within-trial competition. In the simple associative model, partial knowledge is not used in learning. In the biased associative model, partial knowledge of a word-