MULTIPLE-UNIT AND SINGLE-UNIT ACTIVITY IN AREA-32 (PRELIMBIC REGION) OF THE MEDIAL PREFRONTAL CORTEX DURING PAVLOVIAN HEART-RATE CONDITIONING IN RABBITS
B. Maxwell et al., MULTIPLE-UNIT AND SINGLE-UNIT ACTIVITY IN AREA-32 (PRELIMBIC REGION) OF THE MEDIAL PREFRONTAL CORTEX DURING PAVLOVIAN HEART-RATE CONDITIONING IN RABBITS, Cerebral cortex, 4(3), 1994, pp. 230-246
Data from both brain lesion and brain stimulation experiments suggest
that area 32 of the medial prefrontal cortex (prelimbic area) particip
ates in the acquisition and/or expression of conditioned bradycardia.
This report describes experiments designed to determine whether cells
in this area of the brain exhibit changes in neural activity during cl
assical conditioning that can be related to these learned heart rate c
hanges. Thus, multiple- and single-unit activity was recorded from are
a 32 in rabbits during Pavlovian heart rate (HR) conditioning. In the
first experiment, neuronal discharge recorded from chronically implant
ed multiple-unit electrodes in the superficial and deep layers of area
32 increased systematically in response to the presentation of tone c
onditioned stimuli (CS) paired with paraorbital electric shock as the
unconditioned stimulus (US). This tone-evoked increase in multiple-uni
t activity (MUA) closely paralleled the acquisition of the decelerativ
e HR conditioned response in animals that received paired CS/US presen
tations, but was of smaller amplitude in animals given unpaired CS/US
presentations. Tone-evoked increases in MUA also occurred during tone-
alone presentations prior to training, but this activity declined over
trials. During extinction, CS-evoked MUA increases also declined over
trials. These findings suggest that the CS-evoked increase in neurona
l activity in area 32 was associatively produced. A second experiment
examined the CS-evoked response of single units (n = 98) in area 32 du
ring differential Pavlovian conditioning, in which one tone (CS+) was
consistently followed by the paraorbital shock US and a second tone (C
S-) was not. Four types of cells were found: (1) 32% of the cells stud
ied showed no change in response to either CS+ or CS- presentation, (2
) 48% showed CS-evoked increases to either CS+ or CS-, (3) 14% showed
CS-evoked decreases to CS+ or CS-, and (4) 7% showed a biphasic respon
se, in which an increase was followed by a decrease in activity. These
cell types are similar to those previously found in the more dorsal a
nterior cingulate cortex (area 24) and frontal eyefields (area 8). How
ever, a major difference between the present findings and those of our
previous studies of areas 8 and 24 is that in several instances chang
es in CS-evoked activity in area 32 were significantly greater in resp
onse to the CS- than to the CS+, suggesting that some cells in the pre
limbic region may code the absence of aversive stimulation, that is, a
period of relative ''safety.''