Contextual Influences on Phonetic Identification in Aphasia: The Effects of Speaking Rate and Semantic Bias

Abstract

Two experiments examined the influence of context on stop-consonant voicing identification in fluent and nonfluent aphasic patients and normal controls. Listeners were required to label the initial stop in a target word varying along a voice onset time (VOT) continuum as either voiced or voiceless ([b]/[p] or [d]/[t]). Target stimuli were presented in sentence contexts in which the rate of speech of the sentence context (Experiment 1) or the semantic bias of the context (Experiment 2) was manipulated. The results revealed that all subject groups were sensitive to the contextual influences, although the extent of the context effects varied somewhat across groups and across experiments. In addition, a number of patients in both the fluent and nonfluent aphasic groups could not consistently identify even endpoint stimuli, confirming phonetic categorization impairments previously shown in such individuals. Results are discussed with respect to the potential reliance by aphasic patients on higher level context to compensate for phonetic perception deficits.

The Effects of Contextual Strength on Phonetic Identification in Younger and Older Listeners

“…With regard to the magnitude of the category boundary shifts computed, shifts on the order of 3-6 ms emerged for the older participant group in both the central and peripheral biasing conditions, in keeping with previous findings (e.g., Baum, 2001). Interestingly, upon inspection of Table 2, it is apparent that boundary shifts were generally quite small for the young adult listeners (on the order of 1-2 ms) with a single exception in the peripheral biasing contexts for the 'goal-coal' continuum (~5 ms).…”

“…For instance, sentence contexts like "She drives the car with the__" were coupled with stimuli from a 'dent-tent' continuum. Normal young adult listeners were influenced by the sentential context, producing more phonetic identification responses consistent with the semantic bias and thus yielding a category boundary shift 1 (Connine & Clifton, 1987; see also Borsky et al, 1998;Baum, 2001;Miller et al, 1984). It is this type of contextual influence that is exploited in the current investigation, with a focus on effects of aging.…”

Localization of sublexical speech perception components

“…Patients with fluent and nonfluent aphasias, who generally have left hemisphere lesions, clearly have deficits in word and sentence comprehension, but their sublexical speech perception deficits are more nuanced and do not predict their comprehension (Basso, Casati, & Vignolo, 1977; Blumstein, Baker, & Goodglass, 1977). Generally, aphasic patients perform poorly on phoneme identification tasks (Baum, 2001; Blumstein, Cooper, Zurif, & Caramazza, 1977; Blumstein, Burton, Baum, Waldstein, & Katz, 1994; Boyczuk & Baum, 1999; Gow & Caplan, 1996; Ravizza, 2003). In contrast, phoneme discrimination performance, although not normal, is relatively preserved (Blumstein et al, 1977; Gow & Caplan, 1996).…”

Speech perception when the motor system is compromised

“…This apparent redundancy limits the extent to which speech perception deficits can be observed when any single region, such as speech motor cortex, is damaged. Secondly, all listeners, including patients with Broca's aphasia[4], make extensive use of contextual information in speech comprehension. For example, an altered phoneme midway between /d/ and /t/ is more often perceived as /d/ in the context _ ash , because dash is a word and tash isn't.…”