Influence of speech stimuli intensity on the activation of auditory cortexinvestigated with functional magnetic resonance imaging

Understanding the impact of variations in the acoustic signal is critical f or the development of auditory and language fMRI as an experimental tool. W e describe the dependence of the BOLD signal and speech intelligibility on the intensity of auditory stimuli. Eighteen subjects were imaged on a 1.5-T MRI scanner. Speech stimuli were English monosyllabic words played at five intensity levels. Intrasubject reproducibility was measured on one subject by presenting the stimulus five times at the same intensity level. Intelli gibility was measured during data acquisition as subjects signaled when hea ring two targets. Each functional trial consisted of four cycles (30 s off- 30 s on). Five oblique slices covering primary and association auditory are as were imaged. Activated voxels were identified by cross-correlation analy sis and their percent signal change (Delta S) was measured. Intersubject di fferences in activation extent, asymmetry, and dependence on intensity were striking. Volume of activation was significantly greater in the left than in the right hemisphere. Intrasubject reproducibility for Delta S was highe r than for volume of activation. Delta S and intelligibility showed a simil ar dependence on intensity suggesting that not only intensity but also inte lligibility affect the fMRI signal. (C) 1999 Acoustical Society of America. [S0001-4966(99)01404-6].