Attention: Spike or Burst

How can a neuron hear anything over the cacophony created by ten thousand inputs? A logic gate's handful of inputs pales in comparison. The massive feedback projection from cortex to the thalamus, which relays sensory information, may solve this problem.

Photo of a brain
Bursting in a silicon thalamocortical cell Retinal input (blue) triggers single spikes (red) or bursts (green). Bursts require priming by inhibition (from a reticular neuron) evoked by vigorous responses; otherwise the cell switches to single-spike mode. [Kai Hynna 2002]

Our calculations suggest that fifty spikes arriving within 1ms or seven bursts arriving within 100ms will fire a cortical cell reliably.

Setting a threshold so high that it is rarely exceeded by background activity makes reliable transmission possible. Assume a neuron's 10,000 inputs fire 10 times per second, on average. Then 100±10 spikes (mean ± sigma) occur in 1ms. Hence, the chance that more than 150 spikes (five sigma) are received is just 1%. For a 100ms interval, thresholds of 7 spikes (seven sigma) at each dendritic terminal and 7 active terminals at the soma yield the same probability.

We are investigating how bursting and synchrony can reduce cacophony and boost saliency.

All the neurons in the thalamocortical circuit, including reticular interneurons, which inhibit relay neurons, can burst. But they require a strong input to do so, as a large leak must be overcome. When this leak is turned off, possibly by cortical feedback, the neurons switch their firing mode to single spikes, allowing reciprocal inhibition to synchronize the interneurons, which in turn synchronize the relays.

Students
Kai Hynna's thalamus chip has two firing-modes.
Kareem Zaghloul's retina chip can drive the thalamus chip.
Paul Merolla's cortex chip can send/receive spikes to/from the thalamus chip.

Collaborators
Tirin Moore

Funding
NSF's KDI Program
NSF's CAREER Program