The effects of apomorphine (0.001--32.0 mg/kg) on elicited and operant pecking were studied in pigeons. Elicited pecking was measured in a 1-h observation test. Apomorphine caused dose-related increases in the pecking elicited by the drug in all the subjects, with maximal responding at 3.2 mg/kg. In contrast, operant responding on a multiple, 5 min fixed interval, 30 response fixed-ratio schedule revealed individual differences in sensitivity to the drug. A dose of 0.32 mg/kg eliminated key pecking in fixed-interval and fixed-ratio components in 4 (group 1) of the 15 subjects while 3.2 mg/kg eliminated responding in 9 other subjects (group 2), and 2 of the subjects (group 3) required 32.0 mg/kg to eliminate responding. The 13 birds in groups 1 and 2 showed decreases in operant responding with concomitant increases in elicited pecking. For the 2 remaining birds, increases in operant behavior were highly correlated with increased stereotypy. The effects of apomorphine on operant behavior appeared to depend on induced stereotypy, with rate-decreasing effects resulting from the disruption of ongoing behavior by stereotyped pecking aimed elsewhere in the chamber, and rate increases resulting from the redirection of elicited pecking towards the operant key.
The present study examined the course of extinction of a learned taste aversion using prolonged exposure to the aversive food as the extinction test. A conditioned aversion to sucrose was established in deprived rats. They were then exposed for a 4-h period to sucrose alone where it was (1) response contingent, (2) delivered intermittently, or (3) freely available. The aversion extinguished in the 4-h period under all three test conditions. The results suggest that the extinction rate of a learned taste aversion is based on the duration of the exposure to the aversive food after the poisoning experience, not on the time since poisoning.
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