In Search of the Neural Underpinnings of Temporal Binding: Linking Synaptic Depression with Neural Adaptation in Auditory Cortex

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The perception of sound is intimately bound with time evolution: most sounds, speech sounds in particular, make sense only when perceived against the backdrop of what came immediately before. This requires memory whereby incoming sounds are represented in the context of a continuously updated representation of the immediate past. We term this process temporal binding. While the neural underpinnings of temporal binding are an open question, recent research suggests that synaptic depression might be one of its key mechanisms. Synaptic depression could be considered a local form of memory, and it is difficult to observe directly. However, one of its observable effects is thought to be adaptation, a form of sensory memory which expresses itself as the attenuation of neural responses with repeated stimulation. Both synaptic depression and adaptation can be characterized by the time constants describing the speed with which recovery from these effects occur. We intend to investigate the link between synaptic depression, adaptation, and temporal binding by combining the complementary expertise of the applicants in computational modelling of auditory cortex, recordings of single-neuron activity in monkey auditory cortex, and magnetoencephalography (MEG) and electroencephalography (EEG) measurements in humans. This use of monkey and human subjects will allow us to observe adaptation and temporal binding in the activity of small and large neuronal populations of auditory cortex. These observations combined with computational modelling will form a powerful tool for uncovering the neural underpinnings of temporal binding. The first part of the project will address the relationship between synaptic depression and adaptation by means of simulations and experiments. Our preliminary results suggest that this relationship is non-trivial and complex, but the use of multiple, complementary experimental paradigms for measuring adaptation will allow us to estimate values of the recovery time constant of synaptic depression individually for each subject. In the second part, we will test predictions of the modelling work, utilizing the experimentally gained estimates of the recovery time constant for synaptic depression in humans and monkey auditory cortex. This will allow us to address how auditory cortex represents sound sequences and how it discriminates between sequences differing in the timing of their elements.The project is timely, and its realization requires the amalgamation of expertise in monkey electrophysiology, human MEG/EEG, and computational neuroscience. The Leibniz Institute for Neurobiology in Magdeburg is one of the few research institutes in the world which brings together, to a single site, the required expertise and methodology.