Novel approaches to characterize neural responses to standard and non-standard sounds in humans: A single-trial MEG/EEG study of the auditory mismatch negativity.

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A cowboy resting next to his fire is alerted immediately when an approaching intruder breaks a tiny twig. Non-invasive brain imaging by means of magneto- or electroencephalography suggests that this frequent scene in Westerns may have a real basis. An infrequent deviant sound, here produced by the twig, presented in a sequence of frequent standard sounds produced by the fire, evokes a so-called mismatch negativity (MMN) from the auditory cortex, even pre-attentively or during sleep, and thus could alert the cowboy. In the prevailing interpretation, the MMN is considered to reflect a memorybased process, in which the incoming sensory information is compared with a transient trace of the auditory memory formed by the standards. Alterations of the MMN are a potential indicator of various neurodegenerative diseases and developmental disorders. Since the signal-to-noise ratio of the MMN is typically small in scientific experiments, the MMN is derived from averages of responses to a large number of standards and deviants. The cowboy, however, should better detect the deviant sound on a single trial, as otherwise he might never enjoy a fire again. We plan to study the MMN in humans by critically assessing the roles attributed to the standards and the deviants not only on the average data, but, specifically, on the single-trial recordings. We expect to reach this aim by means of novel analysis techniques, the adaptive approximation of the signals using the matching pursuit algorithm.