Multimodal behavior in a four neuron ring circuit: mode switching.

We study a four-neuron ring circuit comprised of oscillating burst-type neurons unidirectionally coupled via inhibitory synapses. Simple circuits of this type have been used previously to study gait patterns. The ring circuit itself is a variant of the basic reciprocal inhibition network, and it exhibits the property of multistability (multiple stable modes of behavior). That is, different gait modes can be achieved via appropriate initialization of and parameterization of this self-excited oscillatory network. We demonstrate three common gait modes with this circuit: the walk, the bound, and a slightly rotated trot mode. Attention is focused mainly on the mechanisms of rapidly and effectively switching between these modes. Our simulations suggest that neuron membrane dynamics, as well as synaptic junctional properties, strongly influence phase sensitivity in the network; each synapse is a combination of both and can be characterized by a transient phase response curve (PRC). We use the same bursting neuron model to characterize all network neurons, and shape different transient PRCs by using different synaptic properties. The characteristics of these PRCs determine the gait modes sustained in any network configuration, as well as, the ability to switch between modes. The mechanisms explored in this simple circuit, may find application in the switching of more complicated gait pattern networks, as well as, in the design of neuromorphic gait pattern circuits.