Internal state is at least two dimensional

Normative models of behavior typically assume that the goal of animals is to optimize some measure of reward. It is well known, however that animals don't always conform to this ideal. Especially in tasks involving repeated trials, attention waxes and wanes: sometimes performance is near optimal; other times it's near chance. Such variation in behavior must be driven by changes in the internal state of the animal. To gain insight into this process, we examined three potential measures of internal state in a two-alternative forced choice task: the complexity of physical movements, the degree to which animals make use of the stimulus, and pupil diameter. Pupil diameter was almost independent of behavior, whereas the first two measures were strongly correlated with it -- but in very different ways. This suggests that the internal state is at least two dimensional. In our experiments those dimensions corresponded to how much animals focus on the current stimulus, and how much they use their priors over task variables. These results add insight into internal states in general, and also provide a cautionary tale: internal state is not one-dimensional, and it may take several measurements to infer it accurately.