AUDITORY CORTICAL RESPONSES IN HEARING SUBJECTS AND UNILATERAL DEAF PATIENTS AS DETECTED BY FUNCTIONAL MAGNETIC-RESONANCE-IMAGING

Functional magnetic resonance imaging is a non-invasive method for the detection of focal brain activity at high spatial resolution. Acousti c stimulation leads to a blood oxygenation level dependent signal chan ge in the plane of the superior temporal gyrus. The dependence of this response in the auditory cortex on binaural, monaural left and monaur al right acoustic stimulation for 10 healthy subjects and five monaura l deaf patients is described. Acoustic stimulation consists of 1000 Hz pulsed sine tones at a pulse rate of 6 Hz and a sound pressure level of 95 dB. For monaural stimulation, normal-hearing subjects revealed a strong lateralization of cortical response towards the contralateral hemisphere. The lateralization ratios between left and right hemispher ic response areas were 3.4-5.2 for monaural stimulation and nearly bal anced for binaural stimulation. Additionally. the sum of cortical acti vation volumes induced by monaural left and right stimulation was simi lar to 30% smaller than for binaural stimulation, indicating either in hibitory mechanisms or neuronal facilitation within the auditory pathw ays. For monaural deaf subjects the lateralization ratio between left to right response was just 1.3 towards the contralateral hemisphere of the healthy ear. which is comparable to binaural responses of normal- hearing subjects. This observation seems to indicate a plasticity or a reorganization of auditory pathways of monaural deaf patients.