Disruption of the conditioned emotional response by caudate nucleus stimulation

The effectiveness of two forms of caudate nuclear stimulation in disrupting the development of a conditioned emotional response (CER) in rats was investigated. When a 5-sec train of 2/sec light flashes was used as a conditioned stimulus (CS), single pulse caudate stimulation delivered 100 msec after each light flash effectively retarded acquisition of the CER. Posttrial caudate stimulation (delivered after the CS--US pairing) was less effective. The differential effectiveness of these two forms of stimulation in disrupting acquisition processes was most clearly seen during extinction of the CER. The group which received caudate stimulation during the CS extinguished faster than the group receiving caudate stimulation after the CS--US pairing. A nonstimulated control group was the fastest to acquire, and the slowest to extinguish the CER.