M. Gareth Gaskell

Integrating Form and Meaning: A Distributed Model of Speech Perception.

We present a new distributed connectionist model of the perception of spoken words. The model employs a representation of speech that combines lexical information with abstract phonological information, with lexical access modelled as a direct mapping onto this single distributed representation. We first examine the integration of partial cues to phonological identity, showing that the model provides a sound basis for simulating phonetic and lexical decision data from Marslen-Wilson and Warren (1994). We then investigate the time course of lexical access, and argue that the process of competition between word candidates during lexical access can be interpreted in terms of interference between distributed lexical representations. The relation between our model and other models of spoken word recognition is discussed.

Sleep-Associated Changes in the Mental Representation of Spoken Words

The integration of a newly learned spoken word form with existing knowledge in the mental lexicon is characterized by the word forms ability to compete with similar-sounding entries during auditory word recognition. Here we show that although the mere acquisition of a spoken form is swift, its engagement in lexical competition requires an incubation-like period that is crucially associated with sleep. Words learned at 8 p.m. do not induce (inhibitory) competition effects immediately, but do so after a 12-hr interval including a nights sleep, and continue to induce such effects after 24 hr. In contrast, words learned at 8 a.m. do not show such effects immediately or after 12 hr ofwakefulness, but show the effects only after 24 hr, after sleep has occurred. This time-course dissociation is best accommodated by connectionist and neural models of learning in which sleep provides an opportunity for hippocampal information to be fed into long-term neocortical memory.

Sleep Spindle Activity is Associated with the Integration of New Memories and Existing Knowledge

Sleep spindle activity has been associated with improvements in procedural and declarative memory. Here, for the first time, we looked at the role of spindles in the integration of newly learned information with existing knowledge, contrasting this with explicit recall of the new information. Two groups of participants learned novel spoken words (e.g., cathedruke) that overlapped phonologically with familiar words (e.g., cathedral). The sleep group was exposed to the novel words in the evening, followed by an initial test, a polysomnographically monitored night of sleep, and a second test in the morning. The wake group was exposed and initially tested in the morning and spent a retention interval of similar duration awake. Finally, both groups were tested a week later at the same circadian time to control for possible circadian effects. In the sleep group, participants recalled more words and recognized them faster after sleep, whereas in the wake group such changes were not observed until the final test 1 week later. Following acquisition of the novel words, recognition of the familiar words was slowed in both groups, but only after the retention interval, indicating that the novel words had been integrated into the mental lexicon following consolidation. Importantly, spindle activity was associated with overnight lexical integration in the sleep group, but not with gains in recall rate or recognition speed of the novel words themselves. Spindle activity appears to be particularly important for overnight integration of new memories with existing neocortical knowledge.

Lexical competition and the acquisition of novel words.

Three experiments examined the involvement of newly learnt words in lexical competition. Adult participants were familiarized with novel nonsense sequences that overlapped strongly with existing words (e.g. cathedruke, derived from cathedral) through repeated presentation in a phoneme-monitoring task. Experiment 1 looked at the immediate effects of exposure to these sequences, with participants showing familiarity with the form of the novel sequences in a two-alternative forced choice task. The effect of this exposure on lexical competition was examined by presenting the existing words (e.g. cathedral) in a lexical decision task. The immediate effect of the exposure was facilitatory, suggesting that the novel words had activated the representation of the closest real word rather than developing their own lexical representations. In Experiment 2, inhibitory lexical competition effects emerged over the course of 5 days for offset-diverging (e.g. cathedruke-cathedral) but not onset-diverging (e.g. yothedral-cathedral) novel words. Experiment 3 disentangled the roles of time and level-of-exposure in the lexicalization process and assessed the generality of the observed lexical inhibition using pause detection. A single, concentrated exposure session was used, which resulted in good recognition performance soon after. Lexicalization effects were absent immediately after exposure but emerged 1 week later, despite no intervening exposure to the novel items. These results suggest that integrating a novel word into the mental lexicon can be an extended process: phonological information is learnt swiftly, but full integration with existing items develops at a slower rate.

A complementary systems account of word learning: neural and behavioural evidence

In this paper we present a novel theory of the cognitive and neural processes by which adults learn new spoken words. This proposal builds on neurocomputational accounts of lexical processing and spoken word recognition and complementary learning systems (CLS) models of memory. We review evidence from behavioural studies of word learning that, consistent with the CLS account, show two stages of lexical acquisition: rapid initial familiarization followed by slow lexical consolidation. These stages map broadly onto two systems involved in different aspects of word learning: (i) rapid, initial acquisition supported by medial temporal and hippocampal learning, (ii) slower neocortical learning achieved by offline consolidation of previously acquired information. We review behavioural and neuroscientific evidence consistent with this account, including a meta-analysis of PET and functional Magnetic Resonance Imaging (fMRI) studies that contrast responses to spoken words and pseudowords. From this meta-analysis we derive predictions for the location and direction of cortical response changes following familiarization with pseudowords. This allows us to assess evidence for learning-induced changes that convert pseudoword responses into real word responses. Results provide unique support for the CLS account since hippocampal responses change during initial learning, whereas cortical responses to pseudowords only become word-like if overnight consolidation follows initial learning.

Learning and consolidation of novel spoken words

Two experiments explored the neural mechanisms underlying the learning and consolidation of novel spoken words. In Experiment 1, participants learned two sets of novel words on successive days. A subsequent recognition test revealed high levels of familiarity for both sets. However, a lexical decision task showed that only novel words learned on the previous day engaged in lexical competition with similar-sounding existing words. Additionally, only novel words learned on the previous day exhibited faster repetition latencies relative to unfamiliar controls. This overnight consolidation effect was further examined using fMRI to compare neural responses to existing and novel words learned on different days prior to scanning (Experiment 2). This revealed an elevated response for novel compared with existing words in left superior temporal gyrus (STG), inferior frontal and premotor regions, and right cerebellum. Cortical activation was of equivalent magnitude for unfamiliar novel words and items learned on the day of scanning but significantly reduced for novel words learned on the previous day. In contrast, hippocampal responses were elevated for novel words that were entirely unfamiliar, and this elevated response correlated with postscanning behavioral measures of word learning. These findings are consistent with a dual-learning system account in which there is a division of labor between medial-temporal systems that are involved in initial acquisition and neocortical systems in which representations of novel spoken words are subject to overnight consolidation.

Leading Up the Lexical Garden Path: Segmentation and Ambiguity in Spoken Word Recognition

Two gating studies, a forced-choice identification study and 2 series of cross-modal repetition priming experiments, traced the time course of recognition of words with onset embeddings (captain) and short words in contexts that match (cap tucked) or mismatch (cap looking) with longer words. Results suggest that acoustic differences in embedded syllables assist the perceptual system in discriminating short words from the start of longer words. The ambiguity created by embedded words is therefore not as severe as predicted by models of spoken word recognition based on phonemic representations. These additional acoustic cues combine with post-offset information in identifying onset-embedded words in connected speech.

Representation and Competition in the Perception of Spoken Words.

We present data from four experiments using cross-modal priming to examine the effects of competitor environment on lexical activation during the time course of the perception of a spoken word. The research is conducted from the perspective of a distributed model of speech perception and lexical representation, which focuses on activation at the level of lexical content. In this model, the strength of competition between simultaneously active lexical items depends on the degree of coherence between their distributed semantic and phonological representations. Consistent with this model, interference effects are more complete when the purely semantic aspects of these coactive representations are probed (using semantic priming) than when phonological aspects are probed as well (using repetition priming).

A Connectionist Model of Phonological Representation in Speech Perception

Abstract A number of recent studies have examined the effects of phonological variation on the perception of speech. These studies show that both the lexical representations of words and the mechanisms of lexical access are organized so that natural, systematic variation is tolerated by the perceptual system, while a general intolerance of random deviation is maintained. Lexical abstraction distinguishes between phonetic features that form the invariant core of a word and those that are susceptible to variation. Phonological inference relies on the context of surface changes to retrieve the underlying phonological form. In this article we present a model of these processes in speech perception, based on connectionist learning techniques. A simple recurrent network was trained on the mapping from the variant surface form of speech to the underlying form. Once trained, the network exhibited features of both abstraction and inference in its processing of normal speech, and predicted that similar behavior will be found in the perception of nonsense words. This prediction was confirmed in subsequent research (Gaskell & Marslen-Wilson, 1994).

Modelling regressive and progressive effects of assimilation in speech perception

Abstract Models of the perception of phonologically variant speech differ in the extent to which contextual information is used predictively versus regressively. These temporal disparities reflect corresponding differences in the experimental literature on the perception of English place assimilation (in which for example the place of the /n/ in “lean bacon” takes on the bilabial place of articulation of the following consonant /b/). Here I present an updated version of a probabilistic connectionist model (Gaskell et al., 1995, Cognitive Science, 19 , 407), which learns to compensate for assimilatory alternations as a consequence of exposure to natural variation in speech. The effects of incorporating graded assimilation on the behaviour of the model are examined, with a pattern of strong predictive effects found for mild and moderate assimilation, and strong regressive effects for more complete assimilations. This behaviour provides a basis for explaining much of the existing experimental data, and provides an explicit profile of temporal effects for testing in further research.