Abstract
Neuronal oscillations are observed in many brain areas in various frequency bands. Each of the frequency bands is associated with a particular functional role. Gamma oscillations (30-80 Hz) are thought to be related to cognitive tasks like memory and attention and possibly also involved in the information transfer between brain areas. This thesis explores a theoretical framework to understand the concept of neuronal coherence and its functional role in neuronal communication. Coherence is used as a measure to quantify the synchronisation i) between pre-synaptic and post-synaptic signals and ii) between neuronal signals. Coherence estimates are in general larger for multi-unit recordings than for single-unit recordings. This can be understood from the fact that the correlations between single-unit activities in a multi-unit recording contribute to a larger signal-to-noise ratio of the neural signal. Further, an increased synchrony of pre-synaptic spike activity results in an increase in coherence between pre-synaptic and post-synaptic activity. This property of neurons, that they are coincidence detectors and that they respond preferably to synchronized input, explains why an increased coherence between pre-synaptic signals can serve as a mechanism to select this input and ignore uncorrelated input signals. In this thesis a model with two pulse-coupled phase oscillators shows that the exact phase relation between the response signals of two interacting oscillators depends on the conduction delay between the two oscillators, the coupling strengths, and coupling type (excitatory or inhibitory). Stability analysis revealed that for symmetric coupling strengths more than one stable state exists. For asymmetric coupling the region of bistability is reduced, suggesting a beneficial role of asymmetric coupling for reliable neuronal information transfer. The cortico-spinal model in this thesis shows that the effectiveness of the neuronal communication between motor cortex and spinal cord is modulated by the phase of the receiving population.
