Figure 1.

Two broad classes of models for a sequence generating circuit. a, Neurons could form a feed-forward synaptically-connected chain within HVC such that activity propagates from one group of neurons to the next. b, Alternatively, sequential activity could occur in the absence of directed connections between neurons, from temporal and spatial gradients of excitability. For example, the network could receive a global and gradual ramping-down of an inhibitory input over time (red synapses), producing a sequential activation. The order of activation would be determined by neuronal excitability. In the example model shown here, neurons receive different levels of constant excitatory input (green synapses). The neuron with the largest excitatory input (neuron 1) would be most depolarized and would be the first to reach spiking threshold. The neuron with the smallest constant excitatory input (neuron 8) would be the last to reach threshold. In the model depicted here, the timescale of the sequence produced corresponds to one song syllable (shown above).