The Development of On-Line Language Comprehension Abilities in Children

A great deal of psycholinguistic research has focused on the question of how adults interpret language in real time. This work has revealed a complex and interactive language processing system capable of rapidly coordinating linguistic properties of the message with information from the situation or context [e.g., 1-7]. In the study of language acquisition, however, surprisingly little is known about how children interpret language in real time, in large part because most existing techniques have relied upon the adult skill of reading [8, 9]. We present here results from a new method for studying children’s language processing abilities, in which a head-mounted eyetracking system was used to monitor eye movements as subjects responded to spoken instructions. The results revealed systematic differences in how children and adults process spoken language: Five year olds relied heavily on the linguistic properties of the input, showing less ability or inclination than adults to coordinate these properties with information from the visual context. This finding suggests that an important component of human development is acquiring the capacity to rapidly coordinate information generated from multiple perceptual and cognitive systems. Recent developments in miniaturized eye-tracking technology have permitted us to establish a new method for studying spoken language comprehension in children. Prior research with adults has shown that by monitoring eye movements during listening, much can be inferred about the processes underlying language interpretation [6, 10]. These studies have found that adults’ eye movements are closely time-locked with speech — e.g., within a few hundred milliseconds of hearing a word that uniquely refers to an object in the world, adults can launch an eye movement to the intended referent. Moreover, consistent with the hypothesis that the language processing system is highly interactive, adults are capable of guiding their interpretation of grammatically ambiguous phrases based upon relevant information from the visual context. By using a new light-weight eye-tracking visor specifically designed for children (see Methods, Figure 1), we set out to examine the extent to which children process speech in real time and resolve local ambiguities based on cues from the visual context. Two age groups participated in the study: sixteen Five Year Olds, and twelve Adults. Subjects’ eye movements were monitored as they responded to spoken instructions to move stuffed animals on a tabletop. We compared instructions containing a temporary syntactic ambiguity, such as “Put the frog on the napkin in the box”, with unambiguous versions, such as “Put the frog that’s on the napkin in the box”. In the ambiguous version, the phrase “on the napkin” could be interpreted as a destination of the putting event, indicating where to put the frog, or as a modifier of the preceding noun phrase, indicating which frog. Each target sentence was heard in one of two visual contexts (Figure 1). One context supported the modifier interpretation, consisting of two frogs, one of which was on a napkin, an empty napkin and an empty box (2-Referent Context). In this case, upon hearing “the frog”, a listener would not know which frog is being referred to, and should thus interpret the phrase “on the napkin” as a modifier. The other context supported the destination interpretation and consisted of the same scene except the second frog was replaced with another animal such as a horse (1-Referent context). In this case, modification of “the frog” with “on the napkin” would be unnecessary because there is only one frog [1, 11]. Hence, we would expect listeners to interpret the prepositional phrase as a destination, referring to the empty napkin. In an earlier eye-tracking study, Tanenhaus et al. found that adults can use these referential constraints to inform syntactic commitments. Adults initially misinterpreted the ambiguous phrase as the destination in 1-Referent but not 2-Referent contexts, as shown by substantially more looks to the incorrect destination (the empty napkin) in I-Referent as compared to 2Referent contexts [6]. Before examining how children deal with ambiguous phrases, it is important to note that their eye movements revealed that they were processing speech in real time, recognizing referential expressions with nearly the same speed as adults. When a definite noun phrase, such as “the frog”, uniquely referred to a single object in the scene, subjects’ direction of gaze quickly moved to the referent object — as measured from the onset of the noun, an average of 667 ms for Five Year Olds and 564 ms for Adults. This finding is well within the range of other eye movement studies [6, 10] and indicates that children’s word recognition processes proceed at approximately the same speed as adults. An examination of how subjects parsed the ambiguous prepositional phrase “on the napkin” revealed systematic differences between ages. As opposed to Adults, who showed sensitivity to context, the findings from Five Year Olds indicated they had consistently interpreted the ambiguous phrase as a destination and were unable to revise their initial commitment. This finding is of considerable interest because a preference for the destination interpretation over the modifier interpretation is precisely what is expected from a processing system that is relying on the linguistic properties of the message and not the context — i.e., the verb “put” has a strong syntactic expectation for a second argument in the form of a prepositional phrase [3, 5]. This preference was most apparent in Five Year Olds’ responses to ambiguous instructions, in which 58% of the trials resulted in an incorrect action (Figure 2). An action was considered correct if the intended object (e.g., the frog on the napkin) was moved directly to the intended destination (e.g., the empty box). An action was also considered correct if both the object and the modifying object were moved to the intended destination (e.g., both the frog and the napkin underneath the frog were placed in the box), consisting of less than 3.3% of responses overall.