High-frequency action-potential bursts in the thalamocortical system directly drive associative learning and track reward rules in mice, according to a study published in Nature Communications.
Researchers tracked neural activity across four brain areas in freely moving male mice trained on a whisker-dependent discrimination task. Animals learned to associate aperture widths with condensed milk rewards or white-noise punishments across 13.5 sessions, or roughly 388 trials, to achieve expert performance. Removing whiskers reduced discrimination scores from 2.36 to 0.79, confirming the sensory mechanism.
Tetrode recordings in the barrel cortex, ventral posteromedial nucleus, posterior medial nucleus of the thalamus, and zona incerta ventralis identified a subset of cells termed burst-coding neurons. As learning progressed, the proportion of whisker touch-responsive units increased from 12.5% to 52.2%, while locomotion-encoding units remained steady at around 10%. Burst-coding neurons accounted for the largest aperture-coding group in expert mice, with 92.11% showing selective burst enhancement for rewarded apertures.
These neuronal firing patterns adapted directly to rule changes rather than fixed physical widths. When researchers inverted task rules to reward narrow openings instead, burst-coding neurons inverted their response profiles accordingly. Introducing an intermediate neutral aperture produced intermediate burst and lick rates, while randomizing stimulus-outcome associations eliminated burst coding entirely.
Pharmacological and focal genetic suppression of thalamocortical bursting disrupted learning and task performance. Logistic regression showed that burst bias in the barrel cortex significantly predicted licking behavior, exhibiting an inflection point near zero bias.
