Stimulus-specific adaptation beyond pure tones.

Detecting rare and surprising events is a useful strategy for sensory -systems. In the human auditory system, deviance detection is indexed by an important component of the auditory event-related potentials, the mismatch negativity (MMN). Responses of single neurons in the inferior colliculus, medial geniculate body, and auditory cortex of mammals (cats, rats, and mice) show responses that share some properties with MMN: they are evoked by rare events, are preattentive (in as much as they occur in anesthetized animals), and, at least at the level of primary auditory cortex, cannot be accounted for by simple fatigue of the incoming sensory information. Here we extend these results to deviations beyond tone frequency. Recording in rat primary auditory cortex and using oddball sequences consisting of two frozen tokens of broadband noise samples, we found differences between the responses to the same token when used as the common and when used as the deviant, showing an exquisite sensitivity to the small differences between two spectro-temporally similar sounds. Similarly, differential adaptation can be demonstrated when using two word-like stimuli that have been derived from human speech but adapted to the rat auditory system. Thus, differential adaptation to common and rare sounds is present also with sounds whose complexity mirrors that of natural environments.