Neuromagnetic responses were recorded to frequent "standard tones of l000 Hz and to infrequent 1100-Hz "deviant" tones with a 24-channel planar SQUID gradiometer. Stimuli were presented at constant interstimulus intervals (ISIs) ranging from 0.75 to 12 sec. The standards evoked a prominent 100-msec response, N100m, which increased in amplitude with increasing ISI. N100m could be dissociated into two subcomponents with different source areas. The posterior component, N100m2, increased when the ISI grew up to 6 sec, whereas the more anterior component, N100m2, probably continued its growth beyond the 12-sec ISI. At ISIs from 0.75 to 9 sec, the deviants elicited additionally a mismatch field (MMF). The equivalent sources of both N100m and MMF were at the supra-temporal auditory cortex. We assume that auditory stimuli leave in the auditory system a trace that affects the processing of the subsequent stimuli. The decrement of the N100m amplitude as well as elicitation of MMF can be considered as indirect evidence of active traces. A behavioral estimate of the persistence of the sensory auditory memory was obtained in a separate experiment in which the subject compared, without attending to the stimuli, tones presented at the daerent ISIs. The subjects discriminated the stimuli better than merely by chance at ISIs of 0.75-9 sec. The ISI dependence of the behavioral estimate as well as of N100m2 and MMF are similar enough to suggest a common underlying mechanism that retains information for a period of about 10 sec.
- magnetoencephalography (MEG)