Human cortical responses evoked by dichotically presented tones of different frequencies.

Behavioral and patient studies have suggested that during dichotic listening the ipsilateral auditory pathways are strongly inhibited, so that each hemisphere is treats the sound coming to the contralateral ear. We analysed the auditory N100m neuromagnetic evoked response following passive listening of dichotic tones of different frequencies. We found that the N100m in each hemisphere depended on both ipsilateral and contralateral stimuli, revealing no strong inhibition of ipsilateral pathways. The N100m increased with the interaural frequency disparity and was reduced as both ears received identical stimuli. The results can be explained by the existence of a frequency-dependent excitatory/inhibitory organization of the auditory cortex, as has been described in the cat. We suggest that the N100m might also reflect automatic processes involved in multiple-stream perception.