25. Temporal edge detection in human auditory cortex

Maria Chait1 m.chait@ucl.ac.uk

1UCL EAR Institute, 332 Gray’s Inn Road, London WC1X 8EE

Auditory environments are constantly fluctuating. These fluctuations are due in part to the oscillatory nature of many acoustic sources and in part to the dynamics of their appearance and disappearance from the auditory scene. Presumably, one of the first stages of detecting these onset and offset events, that are superimposed on the already changing input entering a listener’s ears, is the extraction of auditory temporal edges. The process of deriving a temporal edge depends on the statistical properties of the stimulus before and after the transition. In some cases, edges are detected as a violation of a previously acquired representation of the scene, for example when an ongoing auditory object, against some background, disappears or changes its properties. This is the kind of processing that is tapped by the much used MMN paradigm. However, the opposite side of the coin—the processes by which auditory cortex detects the emergence of regularity out of ‘disorder’, such as when an auditory source appears out of an ongoing random background, is also an ‘edge detection’ task but has been much less explored. We use magnetoencephalography (MEG) to measure early auditory cortical responses to transitions between constant tones, regularly alternating, and randomly alternating tone-pip sequences. Such transitions embody key characteristics of natural auditory temporal edges. Our data demonstrate that the temporal dynamics and response polarity of the neural temporal-edge-detection processes depend in specific ways on the generalized nature of the edge (the context preceding and following the transition) and suggest that distinct neural substrates in core and non-core auditory cortex are recruited depending on the kind of computation (discovery of a violation of regularity, vs. the detection of a new regularity) required to extract the edge from the ongoing fluctuating input entering a listener’s ears