Middle-latency auditory-evoked magnetic fields in patients with auditory cortex lesions

Objective-To demonstrate the influence of auditory cortex lesions on auditory middle-latency responses (AMLRs) and middle-latency auditory-evoked magnetic fields (MLAEFs) in humans. Material and Methods-A total of 15 normal subjects, 9 patients with left auditory cortex lesions and I patient with a right auditory cortex lesion were studied. MLAEFs were recorded from each hemisphere of the brain in a magnetically shielded room using a 37-channel SQUID gradiometer. Simultaneously, AMLRs were recorded from the scalp at the vertex, C3 and C4. Tone bursts were used as auditory stimuli. Results-Pam responses of the MLAEF, which are typically evoked in the latency range of the Pa of the AMLR, and are localized at the auditory cortex as dipoles, were impaired or abolished over the left auditory cortex lesion in the patients with left-hemisphere lesions, but the Pa of the AMLR. persisted. Conclusion-The main generator of the Pam in MLAEF was demonstrated to be the auditory cortex. The results also show that the Pa of the AMLR is evoked only partly from the auditory cortex.