Human auditory cortex responses to rising versus falling glides.

A 24-channel SQUID magnetometer was used to record signals from the right auditory cortex to tone glides with 16 different centre frequencies (ranging from 0.5 to 2 kHz), sweeping over one octave. The stimulus sequence, presented with an interstimulus onset interval of 0.6 s, consisted of infrequent 'deviant' rising (falling) glides and frequent 'standard' falling (rising) glides; the rising glides were identical to the falling glides but presented in the reverse direction. Deviant glides elicited significantly larger responses than standards at a latency of about 100 ms. This amplitude difference presumably arises from a mismatch response to the deviants. These results suggest that the auditory cortex extracts the direction of frequency transition even when the stimuli do not contain same frequencies.