Psychosomatic Medicine 6:40-55 (1944)
© 1944 American Psychosomatic Society

Effects of Injury to the Cerebral Cortex upon Sexually-Receptive Behavior in the Female Rat

¹ Department of Animal Behavior, American Museum of Natural History, New York, N. Y

Methods

Twenty virgin female rats raised in segregation were ovariectomized at 3 to 4 months of age. Estrous was induced in these animals by the injection of estradiol benzoate and progesterone. Each animal was observed in mating tests with a sexually vigorous male during 3 successive periods of induced estrous. Records taken during every test, included the number of times the female was mounted by the male, the number of times the female exhibited lordosis (concave arching of the back facilitating intromission), the number of hopping responses and ear-vibration responses executed by the female, and the number of times the female kicked backward in an attempt to resist the male's sexual mounts.

After the third preoperative mating test part or all of the neocortex was removed from one hemisphere. Following postoperative recovery estrous was induced by hormone administration and a second series of 3 sex tests was administered to each female. At the conclusion of these tests 9 rats were subjected to a second operation in which part or all of the remaining cortex was destroyed. Only the records of 7 females deprived of 97 to 100 per cent of the cortex were subjected to extensive analysis. Following recovery these animals were observed in 3 additional sex tests during induced estrous.

Results

Unilateral removal of 29 to 50 per cent of the neocortex in 20 females. The tendency to exhibit lordosis (as measured by the Copulatory Quotient) showed no consistent change. Ear-wiggling was eliminated in 3 individuals, decreased in 6, increased in 10, and remained the same in one animal. Increases in the frequency of ear-wiggling were of greater magnitude than were decreases. Hopping was eliminated in one female, decreased in 8 cases, and increased in 11. Increases were more extensive than decreases. Back-kicking was eliminated in 4 rats and reduced markedly in the remaining 16 individuals.

Destruction of 97 to 100 per cent of the neocortex in 7 females. Average CQ (Copulatory Quotient) scores were relatively unaffected, but the female's tendency to show lordosis became more variable. In the same test a female would display lordosis when the male investigated without mounting; and 2 minutes later the same animal might fail to show lordosis in response to several successive copulatory attempts by the male. Six of the 7 decorticates displayed lordosis in response to stimuli which had not elicited this reaction prior to operation (pipette in the vagina, grasp of experimenter's hand, male's preliminary investigations, etc.). Two females frequently maintained the lordosis position long after the male had dismounted, and one animal often permitted several successive copulations during the same lordosis. Five of 7 rats experienced periods of hyperactivity, repeatedly circling the perimeter of the cage at a fast run. During such periods males were rarely able to force a lordosis. This fact contributed to the increased variability of the CQ scores.

Ear-wiggling increased in 4 of the 7 females, decreased in 2 and remained the same in one rat. Increases were of greater magnitude than were decreases, and the average frequency of this response for the group rose from 6.6 per positive test before operation to 11.2 after decortication. Two rats displayed ear-wiggling in response to stimuli which had not elicited the behavior preoperatively (pipette in the vagina, and grasp of the experimenter's hand). In 2 individuals although the ear-wiggle reaction increased in frequency after decortication, it lacked normal integration with remaining elements of the receptive pattern, appearing only after the male dismounted instead of before and during the copulation.

Hopping was eliminated in 2 cases, decreased in 2 and increased in 3. In 2 females postoperative hopping usually occurred immediately following the male's dismount and rarely appeared before the mount as it does in the pattern of the normal female.

Back-kicking was eliminated in 6 of 7 females and the frequency of its occurrence was markedly reduced in the seventh individual.

Incidental observation suggested that completely receptive decorticate females were sexually less stimulating to the male than were intact females.

Records of changes in the bodily weight of the operated females indicate that after operation the animals were in good health.

Careful study of incidental invasions to non-cortical structures appeared to support the conclusion that postoperative changes in behavior were primarily if not exclusively the result of neocortical damage.

Note:
The experiment herein reported was supported by a grant from the Committee for Research in Problems of Sex, National Research Council

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