Neuronal mechanisms of sensory binding

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When the electroencephalogram (EEG) has been discovered, one early observations was that the brain activity follows rhythmic changes, termed oscillations. Slow and fast oscillations, occur whether we are awake or in sleep, our eyes are open or closed, or whether we are moving or at rest. Recently, it has been found that brain oscillations are important for neural communication between distant brain areas.***We recorded fast gamma oscillations (40 Hz) with magnetoencephalography (MEG) in human volunteers, who were listening to 40-Hz amplitude modulated sound. 40-Hz oscillations informed us about hearing and how auditory perception is organized in the brain, such as when we localize a sound in space or understand speech in noise. Previous animal research helped us interpreting our MEG results and to develop a model of central auditory processing. However, the previous studies were rather different from our MEG work and did not completely explain the neural mechanisms underlying the non-invasively recorded MEG. Therefore, in the current project we will perform a set of studies that will help us interpreting MEG and EEG findings. ***First, we will test our hypothesis, that one role of fast oscillations is, to determine at which exact time point a neuron fires. This is an important mechanism, because the brain oscillations would determine which neurons are firing at the same time and thus would be connected and work together. We will record neural firing with electrodes in the auditory cortex in chinchilla while stimulating with 40-Hz sound. ***Second, we will measure the concentration of the neurotransmitter GABA using magnetic resonance spectroscopy and compare the GABA level with the amplitude of gamma oscillations recorded in MEG. The results will inform us about how the gamma oscillations are generated, specifically how inhibitory interneurons in the cortex are involved in generating gamma oscillations.***Third, we will investigate how 40-Hz oscillations influence auditory perception. We will perform a gap detection task while listening to 40-Hz AM sounds and test our hypothesis, that gap detection will be better at a certain phase of the 40-Hz sound than on others, according to the hypothesis, that gamma oscillations determine the time point of neural firing. Moreover we will present parts of speech stimuli separately to the left and right ear and test, whether common 40-Hz modulation helps to fuse the parts into understandable speech.***We will use the results of this research for interpreting our MEG findings about changes in auditory processing in elderly people and their problem of understanding speech in noise. Moreover, it has been found that gamma oscillations are different in brain conditions and disorders such as in autism and in schizophrenia. If we know more about the mechanisms and functional meaning of gamma oscillations, we can use EEG or MEG recording of gamma oscillations as sensitive diagnostic tool. **