Thalamic relay cells fire bursts of action potentials. Once a long hyperpolarization primes (de-inactivates) the T-type calcium channel, a depolarizing input will trigger a calcium spike with a burst of action potentials. During sleep, bursts are frequent, rhythmic, and non-visual. Bursts have been observed in alert animals, and burst timing is known to carry visual information under light anesthesia. We extend this finding by showing that bursts without visual triggers are rare. Nevertheless, if the channel were primed at random with respect to the stimulus, then bursts would have the same visual significance as single spikes. We find, however, that visual signals influence when the channel is primed. First, natural time-varying stimuli evoke more bursts than white noise. Second, specific visual stimuli reproducibly elicit bursts whereas others reliably elicit single spikes. Therefore visual information is encoded by the selective tagging of some responses as bursts. The visual information attributable to visual priming (as distinct from the information attributable to bursts’ visual triggering) was 2 bits/burst on average. Although bursts are reportedly rare in alert animals, this must be investigated as a function of visual stimulus. Moreover, we propose methods to measure the extent of both visual triggering and visual priming of bursts. Whether or not bursts are rare, our methods could help determine whether bursts in alert animals carry a distinct visual signal.

A Raster plot of responses of an LGN neuron to 128 repeats of the same full-field white noise stimulus. Red points indicate the action potentials that were classified as being part of a burst. Diamonds indicate the times at which we classified the response as a visually evoked firing event (cell responded in >10% of trials). Filled diamonds indicate the subset of firing events that were highly reliable (cell responded during the event in >80% of trials). B Responses of the same neuron to 128 repeats of a full-field stimulus with a time course recorded in nature. C Percent of isolated spikes (x axis) vs. percent of bursts (y axis) that occurred during a visually evoked firing event (open diamonds in panels A and B). Each symbol represents results for a single cell’s response to either the natural stimulus (open circles) or the white noise stimulus (open triangles). D Percent of isolated spikes (x axis) vs. bursts (y axis) that occurred in a highly reliable visual event (closed diamonds in panels A and B).