Hippocampal pyramidal cell activity encodes conditioned stimulus predictive value during classical conditioning in alert cats

We have recorded the firing activities of hippocampal pyramidal cells throu ghout the classical conditioning of eyelid responses in alert cats. Pyramid al cells (n = 220) were identified by their antidromic activation from the ipsilateral fornix and according to their spike properties. Upper eyelid mo vements were recorded with the search coil in a magnetic field technique. L atencies and firing profiles of recorded pyramidal cells following the pair ed presentation of conditioned (CS) and unconditioned (US) stimuli were sim ilar, regardless of the different sensory modalities used as CS (tones, air puffs), the different conditioning paradigms (trace, delay), or the differ ent latency and topography of the evoked eyelid conditioned responses. Howe ver, for the three paradigms used here, evoked neuronal firing to CS presen tation increased across conditioning, but remained unchanged for US present ation. Contrarily, pyramidal cell firing was not modified when the same sti muli used here as CS and US were presented unpaired, during pseudoconditing sessions. Pyramidal cell firing did not seem to encode eyelid position, ve locity, or acceleration for either reflex or conditioned eyelid responses. Evoked pyramidal cell responses were always in coincidence with a beta osci llatory activity in hippocampal extracellular field potentials. In this reg ard, the beta rhythm represents a facilitation, or permissive time window, for timed pyramidal cell firing. It is concluded that pyramidal cells encod e CS-US associative strength or CS predictive value.