Abstract

This study is to examine whether dopamine is involved in the conditioned blocking. In the blocking paradigm, prior training to a conditioned stimulus(CS-A) blocks the ability to attend to a second conditioned stimulus(CS-B) when the two form a compound stimulus(CS-AB) in a subsequent training. Blocking is an associative process by which animal learn to ignore CS-B because it contains no new information regarding the reinforcing event. The blocking effect was assessed in a standard shuttle box. The onset of the tone signalled the footshock in stage 1. In stage 2, the compound stimulus of the tone and the light signalled the shock. The testing stage consisted of 16 nonreinforced presentations each of CS-A, CS-B and a novel dim light stimulus. The blocking ratio was obtained by dividing the number of conditioned avoidance responses (CAR) emitted to the light by the total number of CARs emitted to both light and tone. Experiment 1 showed that 1mg/kg apomorphine attenuated the blocking effect. In experiment 2, blocking was also disrupted with 0.1mg/kg apomorphine coupled with dopamine receptor supersensitivity produced by prolonged pretreatment with haloperidol. These results suggest that dopamine hyperactivity in the brain disrupted blocking which is interpreted as a selective attention deficit. This attentional deficit bears some resemblance to attentional disorder seen among schizophrenic patients.