The effect of brain state on rapid auditory processing in the rat:

The correct interpretation of natural sounds, such as language, depends on an individual's ability to perform rapid auditory processing (RAP) - processing of auditory stimuli that occurs on a time scale of tens of milliseconds. Indeed, individuals who exhibit deficits in RAP also demonstrate impairments in the acquisition of normal language skills. Interestingly, auditory training that is designed to engage the attention of the subject can ameliorate these deficits. Even though the physiological basis of the improvement in language skills in these individuals is currently unknown, one possible correlate could be changes in brain state that occur through training. Indeed, changes in brain state have been shown to influence neuronal responsiveness to sensory stimuli. In addition, changes in the level in alertness, or attention, are associated with changes in the degree of cortical activation. In this thesis, I explored the relationship between brain state and RAP by recording the simultaneous responses of large neuronal populations in the rat auditory cortex to temporally structured auditory stimuli. I systematically quantified the magnitude of evoked responses across different brain states, both under anesthesia and during wakefulness, and assessed the efficiency of RAP by estimating how well the type of sensory stimulus could be predicted from the population activity.First, RAP was assessed during the inactivated state. Surprisingly, even though response amplitude varies systematically with the phase of the slow oscillation, the efficiency of RAP does not. Second, RAP was examined across the different global activated and inactivated states observed under urethane anesthesia. This showed that RAP is overall more efficient in the activated state because of the ability of auditory populations to strongly respond to temporally structured stimuli. Finally, the effect of changes in the instantaneous level of activation during wakefulness on RAP was studied in chronically implanted rats, again showing that the efficiency in the processing of temporally structured stimuli increases with the level of cortical activation.Together, these results establish a significant link between cortical activation and RAP performance. This suggests that improvement in language ability after training in humans might reflect an increased ability to produce cortical activation when required.Ph.D.Includes bibliographical references (p. 100-132)by Liad Hollende