The effects of amygdalectomy on acquisition performance in a two phase discrimination task



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The ability to discriminate visual stimuli in both a simple discrimination task and a complex discrimination task was examined for one group of four cats with bilateral ablation of the amygdaloid nuclei and compared with the ability of another group of four normal control animals. In the simple discrimination task (Phase I), the animals had to learn to bar press for milk during a 10 second reinforcement period indicated by a 7 cps flashing light (SD period) and inhibit bar pressing during the 30 second variable interval intertrial interval. Each animal received 50 trials per day and the following measures were obtained: (1) number of trials to criterion, (2) number of SD responses, and (3) number of intertrial responses. In the complex discrimination task (Phase II), the animals received 60 trials per day, one half of which were the same as in Phase I (SD trials) and the other half of which were indicated by a 3 cps flickering light (S∆ trial). No response was reinforced during an trial and the animals had to inhibit bar pressing during this period. The following measures were obtained: (1) trials to criterion, (2) SD responses, (3) responses, and (4) intertrial responses. The obtained results were as follows: (1) the operated animals averaged significantly more SD responses in Phase I than did the control animals; (2) the amygdalectomized animals reached criterion in significantly fewer trials than did the control animals in Phase II; and (3) there were no differences in the two groups on any of the other performance measures. These results were compared with Grandstaff's (1965) findings for olfactorybulbectomized animals and it was found that the two experimental groups were essentially equivalent on all measures. Two explanations were put forth to account for the faster learning exhibited by animals with amygdala ablations and animals with olfactory bulb ablations as compared with normal controls animals. The first explanation points to a differential response tendency between the experimental and control groups at the outset of Phase II as exemplified by a higher SP response rate in Phase I. This may be accounted for by the fact that amygdalectomized animals are more attentive and responsive to visual stimuli than normal animals. The second explanation pointed to a disruption of excitatory and inhibitory stimulus generalization gradients as perhaps being the relevant variable. The operated animals may generalize responding from SD to So less than controls and the experimental animals may also show a steeper inhibitory gradient from S∆ to SD.



Amygdaloid body, Learning in animals