Fine grain temporal analysis in aphasia: evidence from auditory gap detection

Auditory temporal processing was investigated in individuals with acquired aphasia using a task in which they were asked to detect brief silent gaps inserted between noise segments modeled after formants in speech. To examine within-channel gap detection, gaps of 10, 20, 40, and 80ms duration were inserted between an initial segment (IS) and a trailing segment (TS) centered at the same frequency (1kHz). In a between-channel gap detection condition, gaps of 20, 40, 80, and 100ms duration were inserted between an IS that differed in frequency (4kHz) from the TS (1kHz). The effect of gap onset timing was examined in both conditions by systematically varying the duration of the IS by 10, 20, or 40ms. A combined analysis revealed that for both conditions and all gap and IS durations, individuals with aphasia produced fewer correct responses than age-matched neurologically intact controls. Separate condition analyses revealed that when noise segments were centered at the same frequency, individuals with aphasia demonstrated poorer accuracy in detecting 40 and 80ms gaps relative to normal controls (p<0.001). When gaps were inserted between noise segments differing in frequency, on average, aphasic subjects performed less accurately at durations of 40, 80 and 100ms (p<0.025). Detection in both groups decreased with smaller IS durations. The difficulties with gap detection observed in the aphasic group suggest the existence of fundamental problems in processing the temporal form or microstructure of sounds characterized by rapidly changing onset dynamics.