Temporal processing capabilities in repetition conduction aphasia

This study investigates the temporal resolution capacities of the central-auditory system in a subject (NP) suffering from repetition conduction aphasia. More specifically, the patient was asked to detect brief gaps between two stretches of broadband noise (gap detection task) and to evaluate the duration of two biphasic (WN-3) continuous noise elements, starting with white noise (WN) followed by 3kHz bandpass-filtered noise (duration discrimination task). During the gap detection task, the two portions of each stimulus were either identical ("intra-channel condition") or differed ("inter-channel condition") in the spectral characteristics of the leading and trailing acoustic segments. NP did not exhibit any deficits in the intra-channel condition of the gap detection task, indicating intact auditory temporal resolution across intervals of 1-3ms. By contrast, the inter-channel condition yielded increased threshold values. Based upon the "multiple-looks" model of central-auditory processing, this profile points at a defective integration window operating across a few tens of milliseconds - a temporal range associated with critical features of the acoustic speech signal such as voice onset time and formant transitions. Additionally, NP was found impaired during a duration discrimination task addressing longer integration windows (ca. 150ms). Concerning speech, this latter time domain approximately corresponds to the duration of stationary segmental units such as fricatives and long vowels. On the basis of our results we suggest, that the patient's auditory timing deficits in non-speech tasks may account, at least partially, for his impairments in speech processing.