Propagating waves and variable neural dynamics
Adam Keane · The Sydney eScholarship Repository (The University of Sydney) · 2016
In cortical circuits, neural responses are highly variable during both spontaneous and evoked activity. Nonetheless, coherent structures, such as propagating waves, can form at the population level. In this thesis, we provide a unified account of these seemingly contrasting dynamics by investigating spiking neural circuits that incorporate two essential features of cortical circuits: distance-dependent connectivity and the balance of excitation and inhibition. We show that propagating waves with complex dynamics only emerge when the neural circuits are balanced. These waves sweep past neurons, to which they provide highly synchronized synaptic inputs. We thus reconcile two major views of irregular neurodynamics, namely, the balanced state view and the synchronized input view. The propagating waves also provide a mechanism for double stochasticity of firing activity, and non-Gaussian dynamics of membrane potential. By applying a localized input to our balanced networks, we show that, as observed in experimental studies, a weak stimulus evokes a wave pattern propagating along lateral connections, whereas a strong stimulus triggers a localized pattern. We further identify the mechanisms underlying such response patterns, and show that their collective dynamics account for a range of recent experimental observations regarding cortical response properties. Such observations include the stimulus-evoked shift of cortical states from synchrony to asynchrony, and a decline in neural variability at stimulus onset. Furthermore, we extend previous theoretical studies of temporal chaos in balanced networks by showing that spatiotemporal chaos occurs in our network. This spatiotemporal chaos is characterized by the Lyapunov spectrum, and indicates that there are great fluctuations across space and time. By calculating finite-time Lyapunov vectors, we show that the spiking fronts of propagating wave patterns provide a mechanism for such spatiotemporal chaos.