ROBINSON, BL; (2014) Influence of cortical descending pathways on neuronal adaptation in the auditory midbrain. Doctoral thesis , UCL (University College London).

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Abstract

Adaptation of the spike rate of sensory neurones is associated with alteration in neuronal representation of a wide range of stimuli, including sound level, visual contrast, and whisker vibrissa motion. In the inferior colliculus (IC) of the auditory midbrain, adaptation may allow neurones to adjust their limited representational range to match the current range of sound levels in the environment. Two outstanding questions concern the rapidity of this adaptation in IC, and the mechanisms underlying it. I hypothesise that the descending, corticofugal system, a major pathway of the auditory system, plays a role in neuronal adaptation to sound level statistics in the IC. I recorded single unit responses in guinea pig IC to a broadband stimulus which switched every 5s between two distributions of sound level. I then deactivated auditory cortex bilaterally using cooling cryoloops. During cooling, adapted neuronal rate-vs.-sound level functions shifted rightwards, with thresholds tending to increase. This resulted in attenuation of the improvement in neuronal representation of mean sound levels associated with adaptation. In addition, the population of IC neurones adapted more slowly to the switching stimulus during cooling. The data suggest that cortex is not necessary for the generation of midbrain spike-rate adaptation, but that adaptation appears to be a property intrinsic to IC neurones and/or inherited from more peripheral sites. Nonetheless, corticofugal activity appears to be important in midbrain neural coding of sound level, altering adaptation and improving neuronal representation of sound levels around the most common levels in the stimulus.

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