A Study on Mismatch Response to a Change in Tone Duration by EEG and MEG

Short-term memory (STM) function has an important role for flexible processing of temporal sequence in human auditory system. In this article, we report the results of basic experiments on mismatch response to a change in tone duration. These experiments were executed to obtain findings for elucidating the mechanism for encoding information of time domain and its short-term memory system. In this study, we measured both mismatch negativity (MMN) by electroencephalogram (EEG) and mismatch field (MMF) by magnetoencephalogram (MEG), and investigated properties of duration mismatch response and its relation to the human cognitive processes. Four subjects were measured MMN on the oddball-paradigm experiments, in which successive tones (durations 100, 200, 300 and 400 ms) as standard stimuli, and the tones with different durations (60, 70, 80, 90% of the standard ones, other attributes were same as standard ones) as deviant stimuli. We observed the relationship between the amplitude and latency of MMN, and the ratio of the durations of standard and deviant stimuli. It was suggested from these results that the duration mismatch response reflected the properties of discrimination and short-term memory of tone duration. One subject was also measured MMF in the same paradigm by the 64 channel first-order SQUID gradiometer, in which the duration of standard stimuli were fixed to 100 ms. As a result, it was confirmed that the properties of the amplitude and latency of duration MMF was almost same as those of duration MMN, and that the estimated current sources of MMF distributed in the auditory cortex and their locations were anterior to those of N100m. The result of MMF recording by the 256 channel whole-head SQUID magnetometer system on one condition is also shown in this article. From this experiment, it was suggested that duration mismatch response is a unique measure for elucidating higher-order brain function, e.g., short-term memory. Such a technique to observe brain activity in a macroscopic way is thought to provide valuable findings to approach to the mechanism of human brain from the systematic sight of view.