Thalamic action potential bursts associated with low threshold spikes (LTS ) occur both during wakefulness, and during sensory and motor activity. We now test the hypothesis that different firing and LTS burst characteristics occur during quiet wakefulness (spontaneous condition) versus mental arithmetic (counting condition). This hypothesis was tested by thalamic recordings during the surgical treatment of essential tremor. Across all neurons and epochs, preburst interspike intervals (PBISIs) were bimodal at median values consistent with the duration of GABAa and GABAb IPSPs. Neuronal spike trains (117 neurons) were categorized by joint interspike interval distributions into those firing as LTS bursts (G, grouped), firing as single spikes (NG, non-grouped), or firing as single spikes with sporadic LTS bursting (I, intermediate). During the the spontaneous condition (46 neurons) only I spike trains changed category. Overall, burst rates (BR) were lower and firing rates (FR) were higher during the counting versus the spontaneous condition. Spike trains in the G category sometimes changed to I and NG categories at the transition to the counting condition, while those in the I category often changed to NG. Among spike trains which did not change category by condition, G spike trains had lower BRs during counting, while NG spike trains had higher FRs. Therefore, many discrete changes in the properties of neuronal spike trains occur between the spontaneous versus counting conditions. These changes are most pronounced for the I category which may be a transitional firing pattern between the bursting (G) and relay modes of thalamic firing (NG).