Detection of auditory (cross-spectral) and auditory-visual (cross-modal) synchrony

Detection thresholds for temporal synchrony in auditory and auditory-visual sentence materials were obtained on normal-hearing subjects. For auditory conditions, thresholds were determined using an adaptive-tracking procedure to control the degree of temporal asynchrony of a narrow audio band of speech, both positive and negative in separate tracks, relative to three other narrow audio bands of speech. For auditory-visual conditions, thresholds were determined in a similar manner for each of four narrow audio bands of speech as well as a broadband speech condition, relative to a video image of a female speaker. Four different auditory filter conditions, as well as a broadband auditoryvisual speech condition, were evaluated in order to determine whether detection thresholds were dependent on the spectral content of the acoustic speech signal. Consistent with previous studies of auditory-visual speech recognition which showed a broad, asymmetrical range of temporal synchrony for which intelligibility was basically unaffected (audio delays roughly between -40 ms and +240 ms), auditory-visual synchrony detection thresholds also showed a broad, asymmetrical pattern of similar magnitude (audio delays roughly between -45 ms and +200 ms). No differences in synchrony thresholds were observed for the different filtered bands of speech, or for broadband speech. In contrast, detection thresholds for audio-alone conditions were much smaller (between -17 ms and +23 ms) and symmetrical. These results suggest a fairly tight coupling between a subject's ability to detect cross-spectral (auditory) and cross-modal (auditory-visual) asynchrony and the intelligibility of auditory and auditoryvisual speech materials.