The Relative Contribution of Top-down and Bottom-up Information During Lexical Access
The research reported in this thesis examines the relative contributions of top-down and bottom-up information during lexical access. I evaluate the Cohort Model of lexical access (Marslen-Wilson and Welsh, 1978; Marslen-Wilson and Tyler, 1980; and Marslen-Wilson, 1987) which specifies that the first stage in lexical access is fully autonomous and that during this stage all processing proceeds in terms of analysis of the acoustic-phonetic input. Implicit in this model is the assumption that bottom-up processing is immune to any effects of contextual or top-down information. I examine the extent to which listeners ever rely exclusively on bottom-up information during lexical access and investigate this issue empirically, by measuring effects of context on both the production and the perception of words in various contexts. I test the hypothesis that a word uttered in a constraining context will be acoustically indistinguishable from its competitors by, first, measuring one acoustic parameter (VOT) across constraining and non-constraining contexts and, then, examining the intelligibility of tokens of that parameter taken from the varyingly constraining contexts. The data from these experiments suggest that the realization of VOT is not an aspect of bottom-up information which would create problems for a bottom-up processor in terms of providing ambiguous acoustic-phonetic information. I then investigate whether bottom-up processing during lexical access is immune to effects of context. Following Grosjean (1980) and Tyler (1984), I utilize the Gating Paradigm. Using incongruous contexts, I argue that direct assessment of the contributions made by different information sources during lexical access can be made. By presenting bottom-up information which is inappropriate to the contextual (topdown) information, I evaluate the extent to which one information source is given priority over the other. I vary both the contextual constraints available to the listener and the acoustic clarity of bottom-up information. The observed pattern of listeners' identifications of the words suggested that whilst bottomup information was given priority, top-down information was available and was utilized during lexical access. I present data which support the working structure of the Cohort Model of lexical access. I conclude, however, that the model places disproportionate emphasis on initial bottom-up processing. It appears that top-down information is not prohibited from contributing to processing during the initial stage of lexical access.