Stimulus-Specific Adaptation in the Auditory Midbrain

University Of Washington, Seattle WA

Investigators

Abstract

All animals that hear must be able to ignore common sounds that are repetitive and uninteresting and be alerted by novel sounds that may require attention. Based on noninvasive recordings of summed electrical brain activity in humans, it has long been thought that detection of novel sounds is a higher cognitive function performed by the cerebral cortex. However, work in the PI's laboratory has shown that a subcortical auditory center located in the midbrain (the inferior colliculus) contains neurons that are unresponsive to a repeated sound and respond exclusively to novel sounds. These neurons are termed ?novelty detectors?. The process by which they become unresponsive to a commonly occurring sound is termed ?stimulus-specific adaptation?. The mechanism(s) underlying stimulus-specific adaptation are not known, so one goal of the proposed research is to discover the underlying processes that allow neurons to ignore an expected stimulus but respond to an unexpected one. Although novelty detector neurons constitute a specialized population, new evidence from the PI's lab suggests that all neurons in the inferior colliculus exhibit some degree of stimulus-specific adaptation and enhancement of responses to novel stimuli. The proposed work will investigate the hypothesis that novelty detection is built up in stages through connections among neurons that exhibit different degrees of stimulus-specific adaptation, and that neural inhibition may somehow be involved in suppressing responses to an expected sound. The proposed experiments will use a paradigm in which a ?standard? stimulus is presented most of the time and an ?oddball? stimulus is presented on rare occasions. The specific aims are to 1) determine how ubiquitous stimulus-specific adaptation and enhanced novelty responses are in the inferior colliculus; 2) to characterize the conditions under which individual neurons show stimulus-specific adaptation and novelty responses, specifically examining the effect of stimulus repetition rate, probability of the oddball stimulus occurring, and the amount of contrast (i.e., difference between the oddball and standard stimuli along a specific dimension such as pitch); 3) to determine whether there is a map or gradient in the tendency of inferior colliculus neurons to show stimulus-specific adaptation and novelty responses and 4) to determine whether local blocking of the action of inhibitory neurotransmitters on the neuron being studied will reduce stimulus-specific adaptation and allow the neuron to respond to all stimuli regardless of how common or uncommon they are. These experiments will be conducted in the big brown bat, a mammal whose auditory system is highly specialized for processing sound patterns used in echolocation, and the mouse, a mammal with a generalized auditory system. This research will provide new insights into the broad issues of how mammals focus attention on novel sounds, and how this ability has evolved to serve different behaviors in different species. The findings of these studies should also provide convincing evidence that at least some processes that are considered higher cognitive functions occur at a level prior to the cerebral cortex. It is likely that the results of these experiments will be generalizable to auditory processing and attention mechanisms in humans as well as in the specific animals studied, and to novelty detection in sensory systems other than hearing. The research will have a broader impact in providing excellent training opportunities for graduate, undergraduate, and high school students.