Temporal Contiguity in Cross-Situational Statistical Learning

Temporal Contiguity in Cross-Situational Statistical Learning George Kachergis, Chen Yu, and Richard M. Shiffrin {gkacherg, chenyu, shiffrin}@indiana.edu Department of Psychological & Brain Science / Cognitive Science Program Bloomington, IN 47405 USA Abstract Recent research has demonstrated that participants often learn a surprising number of word-referent pairings solely from their co-occurrence statistics across individually ambiguous trials. To isolate processes, past designs prevented the same pairing from appearing in two consecutive trials. Yet such temporal contiguity often appears in real world settings, and seems likely to improve learning. The present research examines and models the effects of such repetitions. Our results show that allowing word-referent pairs to appear in adjacent trials indeed increases overall learning. Not only are the repeated pairs improved, but other pairs are improved, as well. Repetition seems to allow segregation of pairs that are and are not repeated from the previous trial, thereby allowing differential attention between the subsets. However, attention also seems to shift away from pairs that are repeated many times—to their detriment, but to the benefit of concurrent unrepeated pairs. The findings are explored with an associative learning model to provide a formal account of the underlying learning mechanisms. Keywords: statistical learning; cross-situational learning; language acquisition; ambiguity; attention word Introduction To learn a language, people must learn which words refer to which physical referents. A learner listening to a speaker needs to ascertain which objects in the shared environment are the referents intended by the speaker. Almost all occurrences of words occur in settings where there are multiple possible referents. Particularly early in learning, there will be high ambiguity concerning the correct referent. Learning nonetheless takes place because correct pairings of words and referents tend to reoccur over many situations, a phenomenon termed ‘cross-situational learning’ (Pinker, 1984; Gleitman, 1990). A recent proposal of interest in cognitive development implements this well-established idea using statistical learning, which has been shown to work in many distinct perceptual domains (e.g., Conway & Christiansen, 2005). In a cross-situational word learning experiment, a learner acquires word meanings by tracking the co-occurrences of multiple words and referents across situations, ultimately discovering the most likely word- referent mappings. Such statistical word learning has been observed in both infants (Smith & Yu, 2008) and adults (Yu & Smith, 2007). In typical adult studies, participants are instructed to learn which object each (novel) word denotes. They are presented with a series of study trials, each consisting of an array of several novel objects (e.g., a photograph of a metal sculpture, another of a tool, etc.) and successively spoken pseudowords (e.g., “manu”, “bosa”, etc.). Each pseudoword refers to a single onscreen object, but the correct referent for each pseudoword is not indicated, making referents ambiguous on individual trials. In a typical learning scenario, participants attempt to learn 18 pseudoword-object pairings from 27 12-second trials, with four pseudowords and four objects presented per trial. This configuration allows each stimulus (and hence each correct word-referent pairing) to be presented six times. One way to learn the correct pairings would involve the accumulation of pseudoword-object co-occurrence statistics across the training trials. To assess the learning that has taken place during training, each pseudoword is individually presented, and participants are asked to choose the appropriate object from a subset of the 18 objects. Yu & Smith (2007) found that adults learned an average of 9.5 of the 18 pairings when choosing from four alternative referents at test (i.e., 4AFC). Remarkably, several participants manage to learn all 18 mappings. Thus, humans can use the co-occurrences of multiple words and objects across individually ambiguous trials to learn word-object mappings. In order to isolate processes and better control the first studies of this sort (Yu & Smith, 2007), an artificial constraint was used in previous designs of cross-situational learning studies: Word-referent pairs were not allowed to appear in consecutive trials. However, such repetitions are common in real learning environments, partly because of temporal contiguity inherent in the physical environment. For instance, a visual object that a learner is attending to at one moment is quite unlikely to suddenly disappear at the next; rather, it will gradually move away from the central to the peripheral visual field, remaining in sight for some time. This temporal contiguity in the environment may aid learning in a number of ways if the cognitive system is able to make use of this regularity. For a simple example in the cross-situational paradigm, consider two successive trials on which pseudoword A and object a occurred, but all other stimuli – three other words and objects on each of the trials – differed. Assuming memory for the previous trial, the participant could infer that A-a is a correct pairing (“a” was the only repeated pseudoword, and A was the only repeated object). As a second example, consider two successive trials on which three of the four word-referent pairs are repeated (e.g., D E F and f d e appear on both trials), but the first trial contained B-b and the latter C-c. Given memory for the first trial, the participant could infer that D E F must be associated with f d e, albeit the exact pairings would remain ambiguous. Regardless of this ambiguity, it would be possible to infer that B-b must be correct and C-c must be correct, since these are the only remaining possibilities. These are just two