[Auditory cortical response to monaural stimulation as detected by functional magnetic resonance imaging]

In order to confirm the crossed-innervation between auditory cortex and the ear that receives monosyllabic sound, the auditory cortical response to monaural monosyllabic stimulation as detected by functional magnetic resonance imaging (fMRI) was investigated in six normal hearing subjects. Stimulus amplitude averaged 95 dBSPL at the distal end of the audio system. A series of 440 echo planar images was acquired during the acoustic stimulation within the four OFF-ON cycle paradigm. Five image series with 10 slices were collected within each OFF or On period. Each scanning session began with four baseline images before the OFF-ON paradigm. Monosyllabic sounds were presented monaurally during the ON period at a rate of one monosyllable/sec. Functional MRI data were analyzed with SPM99b software (Statistical Parametric Mapping). The background scanner noise averaged 97dBSPL. The selicon ear plug and headphone as acoustic shields attenuated the noise as much as 17 dB. A broad and intense auditory cortical response was observed bilaterally in response to monaural monosyllable stimulation. Sound presentation to the right ear was followed by a larger response in the left auditory cortex than in the right, and left ear stimulation evoked a larger response in the right auditory cortex than in the left. This pattern was consistent in all subjects examined. The primary auditory cortex responded to monosyllabic words presented to the contralateral ear. The results confirmed the crossed-innervation between the auditory cortex and ear for listening to monosyllables. Functional MRI is a useful tool for investigating auditory cortex function, if the scanner noise is adequated controlled.