Neuronal firing in anterior cingulate neurons changes modes across trials in single states of multitrial reward schedules

The recorded responses of single neurons often vary considerably in the numbers of spikes emitted across repeats of a single experimental condition. Because of this irregularity and for theoretical convenience the responses are often approximated using a Poisson process. However, it has been frequently pointed out that many details of the responses, including the distribution of spike counts across similar trials, are not consistent with a Poisson process, even an inhomogeneous one. Wiener and Richmond (2003, J Neurosci 23:2394–2406) showed that the spike count distributions could usually be fitted nicely by mixtures of a few (1–3) Poisson distributions, a step they regarded as a computational convenience. Now, we find that a substantial proportion (47%) of the neuronal responses from anterior cingulate cortex, which we conceptualize as part of a system related to the balance between work and reward, have responses with multimodal firing rate distributions. When these distributions are modeled as mixtures of Poisson distributions, the proportions of the different Poisson distributions are related to behavioral state, and might be related to cognitive factors. This suggests that the neurons undergo behaviorally-related mode changes.