Human auditory cortical mechanisms of sound lateralisation: III. Monaural and binaural shift responses

Neuromagnetic responses were recorded over the whole head with a 122-channel gradiometer. A pair of 150-ms 1-kHz tones separated by an interval of 150 ms was presented to one ear every 2 s. The other ear received either no input, an identical pair simultaneous to the first, an identical pair alternating with the first or a continuous 600-ms tone. The 'monaural shift' condition in which stimuli alternated between ears produced a clear perception of changing lateralisation, but the evoked response could be explained as merely the sum of simple monaural onset and offset responses; thus we found no evidence for a separate response to interaural intensity difference in this condition. The 'binaural shift' condition, in which intensity changed in one ear while the other received a continuous tone, evoked a transient response (N130m) at a latency of about 130 ms. N130m was larger over the hemisphere contralateral to the direction of shift, and larger than the corresponding monaural response, whether to an onset or an offset. We concluded that N130m also was not a separate directional response, but was analogous to a simple monaural response, the prolonged latency being due to masking and the enhanced amplitude to facilitation by the sustained response to the continuous tone.