Effects of hypothalamic stimulation on unit responses recorded from neurons of sensorimotor cortex of awake cats during conditioning.

1. The activity of single units of the coronal pericruciate (CPC) cortex was studied in 11 awake cats during sessions in which a click conditioned stimulus (CS) was repeatedly paired with glabella tap unconditioned stimulus (US) and hypothalamic stimulation (HS). Effects of HS on the activity of cortical units were also studied during sessions in which HS alone was delivered repeatedly every 10 s. 2. HS evoked an increase in spike activity of less than 60 ms latency in 89 of 116 units tested. 3. Repeated presentation of HS that was effective in producing rapid behavioral conditioning resulted in a characteristic reduction in the latency of discharge evoked by HS in cortical units. 4. Short-latency activation (less than 20 ms) of units of the sensorimotor cortex appeared to be characteristic of HS that led to enhanced rates of conditioned response (CR) acquisition. One of the cells responding in this way was identified as a pyramidal cell of layer V by intracellular injection of horseradish peroxidase (HRP). 5. Further analyses of activity were performed on 16 units of the CPC cortex that were followed through conditioning (or reconditioning) and extinction of the CR. After less than 10 CS-US-HS pairings, there was a selective augmentation of unit response to the CS but not of response to an explicitly unpaired discriminative stimulus (DS). Responses to the CS were not similarly augmented when presentations of HS preceded rather than followed the presentations of the CS and US. The rapid development of CS-evoked unit activity coincided with the rapid acquisition of discriminative CRs behaviorally. 6. During conditioning, the most conspicuous increases in CS-evoked unit response occurred at latencies 100 ms or more after onset of the click CS. This corresponded with the behavioral observation that the majority of eye blink CRs occurred with onset latencies longer than 100 ms.