Lexical stress and lexical access: effects in read and spontaneous speech

This thesis examines three issues which are of importance in the study of auditory word recognition: the phonological unit which is used to access representations in the mental lexicon; the extent to which hearers can rely on words being identified before their acoustic offsets; and the role of context in auditory word recognition. Three hypotheses which are based on the predictions of the Cohort Model (Marslen-Wilson and Tyler 1980) are tested experimentally using the gating paradigm. First, the phonological access hypothesis claims that word onsets, rather than any other part of the word, are used to access representations in the mental lexicon. An alternative candidate which has been proposed as the initiator of lexical access is the stressed syllable. Second, the early recognition hypothesis states that polysyllabic words, and the majority of words heard in context, will be recognised before their acoustic offsets. Finally, the context-free hypothesis predicts that during the initial stages of the processing of words, no effects of context will be discernible.Experiment 1 tests all three predictions by manipulating aspects of carefully articulated, read speech. First, examination of the gating responses from three context conditions offers no support for the context-free hypothesis. Second, the high number of words which are identified before their acoustic offsets is consistent with the early recognition hypothesis. Finally, the phonological access hypothesis is tested by manipulation of the stress patterns of stimuli. The dependent variables which are examined relate to the processes of lexical access and lexical retrieval; stress differences are found on access measures but not on those relating to retrieval. When the experiment is replicated with a group of subjects whose level of literacy is lower than that of the undergraduates who took part in the original experiment, differences are found in measures relating to contextual processing.Experiment 2 continues to examine the phonological access hypothesis, by manipulating speech style (read versus conversational) as well as stress pattern. Gated words, excised from the speech of six speakers, are presented in isolation. Words excised from read speech and words stressed on the first syllable elicit a greater number of responses which match the stimuli than conversational tokens and words with unstressed initial syllables. Intelligibility differences among the four conditions are also reported.Experiment 3 aims to investigate the processing of read and spontaneous tokens heard in context, while maintaining the manipulation of stress pattern. A subset of the words from Experiment 2 are presented in their original sentence contexts: the test words themselves, plus up to three subsequent words, are gated. Although the presence of preceding context generally enhances intelligibility, some words remain unrecognised by the end of the third subsequent word. An interaction between stress and speech style may be explained in terms of the unintelligibility of the preceding context.Several issues arising from Experiments 1, 2 and 3 are considered further. The characteristics of words which fail to be recognised before their offsets are examined using the statistical technique of regression; the contributions of phonetic and phonological aspects of stressed syllables are assessed; and a further experiment is reported which explores top-down processing in spontaneous speech, and which offers support for the interpretation of the results of Experiment 3 offered earlier