Confronting functional and neural models of speech comprehension and production

A central goal of research in neurobiology of language is to discover the neural underpinning of concepts such as “phoneme”, “morpheme”, “word”, “lemma” and “phrase”, conditions such as “agreement” and operations such as “wh-movement”, which are defined in Linguistics. However, a large proportion of these concepts and operations originated as devices for meta-level discussions about sentences, ages before scientists started asking questions about the neural and cognitive processes that underlie the production and comprehension of utterances. At least some of the concepts may have no neural substance at all, even if they have been successfully invoked in explaining the results of psycholinguistic experiments. In the theory of speech comprehension it is hotly debated what the basic units of processing and representation are. The majority view still holds that the basic units are abstract phonemes or bundles of distinctive features, but there is increasing support for theories that take episodes or exemplars as the basic units. These antagonistic theories have in common that they remain extremelyvague about the details of the neural representations and the computations that are needed for a person to actually understand or produce a spoken utterance. If positions and claims are supported by computational models, it is virtually always so that those models operate on manually constructed discrete symbolic input representations, and the models make no claims about neurobiological plausibility. In the poster we will present the results of a large-scale behavioral experiment aimed at answering the question whether exemplars play a role in comprehension as well as in production. Participants were asked to shadow nonsense words of the form / CVVVVVV-CV-PV/ (Mitterer & Ernestus, 2008), where the vowel V in the central syllable could have normal or somewhat lengthened duration; also the voiceless plosive P that separates the second and third syllable can have normal duration or be lengthened. Native speakers of Dutch have several routes available for linking their perception to the ensuing articulation. At the perception side they may restrict processing to creating on-thefly exemplars without a representation in the form of discrete units, they might create a representation in the form of discrete phonemic units, or they might access their mental lexicon to find the most similar word (Roelofs, 2004). For each of these routes we construct plausible neural computational procedures that could be used to control the speech production process in the shadowing task. Using end-to-end computational models (i.e., models that take acoustic speech signals as input and produce audible speech as output) we simulate the chronometric data and the accuracy with which the stimuli were shadowed, in an attempt to explain differences between participants in terms of different routes. We will use the result to discuss potential discrepancies between representations and processes implied in functional (psycho)linguistic models of speech comprehension and production on the one hand and a detailed account of what is currently known about the neural processes that support auditory processing of speech signals and the production of spoken utterances. Holger Mitterer and Mirjam Ernestus (2008) The link between speech perception and production is phonological and abstract: Evidence from the shadowing task, Cognition 109, 168–173. Ardi Roelofs (2004) Error Biases in Spoken Word Planning and Monitoring by Aphasic and Nonaphasic Speakers: Comment on Rapp and Goldrick (2000), Psychological Review Vol. 111, No. 2, 561–572