Dynamics of coding and representation of temporal information in bat auditory cortex and midbrain

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The preceding project focussed on the functional role of cortical neurons that are sensitive for echo-delay time in echolocating bats. One of the main results of the project was the demonstration that the temporal neuronal tuning is sharpened considerably by a cortical suppression mechanism that only acts when natural call-echo sequences are used as stimuli. When isolated and randomly presented call-echo pairs are used as stimuli, as it was the case in many previous studies, the cortical responses are stronger but much less specific. These results are quite novel and emphasize the importance of using not only behaviorally relevant stimuli but stimulus sequences rather than isolated sound events to study cortical function. In this elongation project we aim to investigate the mechanisms of sequence-specific processing by (i) studying midbrain responses to natural sequences. This should help to define the first emergence point of the temporally sharp responses observed in the cortex. The latter is important for understanding the transformations that occur along the ascending auditory pathway and that enable bats to effectively extract information from echolocation streams (ii) Within the cortex, layer-specific contributions to sequence processing will be assessed using a current-source density analysis, and (iii) we will analyse synaptic mechanisms that play a role for a stimulus-enhancing suppression by microiontophoresis of GABAergic and glutaminergic pharmaca. The results of this elongation project study should contribute to a general assessment of the role of cortical suppression for processing and focusing on species-relevant information elements in acoustic sequences.