Persistent activity in a cortical-to-subcortical circuit: bridging the temporal gap in trace eyelid conditioning.

We have addressed the source and nature of the persistent neural activity that bridges the stimulus-free gap between the conditioned stimulus (CS) and unconditioned stimulus (US) during trace eyelid conditioning. Previous work has demonstrated that this persistent activity is necessary for trace eyelid conditioning: CS-elicited activity in mossy fiber inputs to the cerebellum does not extend into the stimulus-free trace interval, which precludes the cerebellar learning that mediates conditioned response expression. In behaving rabbits we used in vivo recordings from a region of medial prefrontal cortex (mPFC) that is necessary for trace eyelid conditioning to test the hypothesis that neurons there generate activity that persists beyond CS offset. These recordings revealed two patterns of activity during the trace interval that would enable cerebellar learning. Activity in some cells began during the tone CS and persisted to overlap with the US, whereas in other cells, activity began during the stimulus-free trace interval. Injection of anterograde tracers into this same region of mPFC revealed dense labeling in the pontine nuclei, where recordings also revealed tone-evoked persistent activity during trace conditioning. These data suggest a corticopontine pathway that provides an input to the cerebellum during trace conditioning trials that bridges the temporal gap between the CS and US to engage cerebellar learning. As such, trace eyelid conditioning represents a well-characterized and experimentally tractable system that can facilitate mechanistic analyses of cortical persistent activity and how it is used by downstream brain structures to influence behavior.