Auditory cortical responses evoked by pure tones in healthy and sensorineural hearing loss subjects: functional MRI and magnetoencephalography.

BACKGROUND: Blood oxygen level dependent functional magnetic resonance imaging (fMRI) and magnetoencephalography are new techniques of brain functional imaging which can provide the information of excitation of neurons by measure the changes of hemodynamics and electrophysiological data of local brain tissue. The purpose of this study was to study functional brain areas evoked by pure tones in healthy and sensorineural hearing loss subjects with these techniques and to compare the differences between the two groups.

METHODS: Thirty healthy and 30 sensorineural hearing loss subjects were included in this study. In fMRI, block-design paradigm was used. During the active epoch the participants listened to 1000 Hz, sound pressure level 140 dB pure tones at duration 500 ms, interstimulus interval 1000 ms, which presented continuously via a magnetic resonance-compatible audio system. None stimulus was executed in control epoch. In magnetoencephalography study, every subject received stimuli of 1000 Hz tone bursts delivered to the bilateral ear at duration 8 ms, interstimulus intervals 1000 ms. Sound pressure level in healthy subjects was 30 dB; in sensorineural hearing loss subjects was 20 dB above everyone's hearing threshold respectively. All subjects were examined with 306-channel whole-scalp neuromagnetometer.

RESULTS: In fMRI, all subjects showed significant activations in bilateral Heschl's gyri, anterior pole of planum temporale, planum temporale, precentral gyri, postcentral gyri, supramarginal gyri, superior temporal gyri, inferior frontal gyri, occipital lobes and cerebellums. The healthy subjects had more intensive activation in bilateral Heschl's gyri, anterior pole of planum temporale, inferior frontal gyri, left superior temporal gyri and right planum temporale than the hearing loss subjects. But in precentral gyri, postcentral gyri and occipital lobes, the activation is more intensive in the hearing loss subjects. In magnetoencephalography study, both in the hearing loss and the healthy subjects, the most evident audio evoked fields activated by pure tone were N100m, which located precisely on the Heschl's gyrus. Compared with the hearing loss subjects, N100m of the healthy subjects was stronger and had longer latencies in right hemisphere.

CONCLUSIONS: Under proper pure tone stimulus the activation of auditory cortex can be elicited both in the healthy and the sensorineural hearing loss subjects. Either at objective equivalent stimuli or at subjectively perceived equivalent stimuli, the auditory responses were more intensive in healthy subjects than hearing loss subjects. The tone stimuli were processed in a network in human brain and there was an intrinsic relation between the auditory and visual cortex. Blood oxygen level dependent fMRI and magnetoencephalography could reinforce each other.