How Do Brain Circuits Generate Complex Sequential Behaviors?

There is a precise sense of time in the songs of songbirds. The areas of the brain that generate the electrical signals that control the muscles generating song in songbirds is known. In addition, the code itself is well characterized. However, how that neural output is generated remains controversial. The new paper “Support for a synaptic chain model of neuronal sequence generation” (published November 18, 2010 in Nature) looks at detailed electrical activity in neurons controlling song to see how the neural code is generated.

Motor output to muscles responsible for the creation of song project from the RA nucleus in the songbird brain shown in Figure 1. Above RA nucleus is the HVC nucleus that sends input to the RA nucleus and is known to have a central role in controlling the timing of sounds that compose a birdsong. Neurons in the HVC nucleus that project to the RA nucleus produce “only a single highly stereotyped burst of spikes during each repetition of a song.” How is this stereotyped code generated in the HVC neurons projecting to the RA nucleus?

Note: A nucleus is a cluster of neurons in the central nervous system.

The paper presents two possibilities:

• activity could propagate through the HVC network of neurons “like a chain of falling dominoes” forming a kind of multi-geared clock that underlies song timing (they call this a chain network)
• periodic electrical activity or other cell dynamics may modulate the excitability of neurons and therefore control the timing of their activity through oscillations.

The research team recorded from inside the neurons that reside in the HVC nucleus and project to the RA nucleus in freely behaving songbirds. Their findings were consistent with the idea that the control of song temporal structure is produced by the propagation of calcium-mediated bursts through a synaptically connected chain of neurons.

I’m struck by the apparent similarity of this team’s findings with ideas pursued by Eugene Izhikevich. I’d be interested in seeing neuron activity in the HVC nucleus presented in a dynamical systems format.

Other related blog posts:

Memory and the Precise Timing of Signals in the Brain