Rats with ventromedial hypothalamic lesions were trained preoperatively to respond for food reward on a FR 64 schedule of reinforcement. Postoperative comparisons with similarly trained unoperated control rats showed no differences when both groups were maintained at 8070 or 90% of their preoperative baseline weights. Lesioned rats responded at a much higher level. however. at 100~ body weight and at a prefeeding condition run at lOW body weight. These results suggest that rats sustaining such iesions overeat and become obese because these lesions increase their hunger motivation. Miller. Bailey, and Stevenson (I950) and Teitelbaum (I 957) rep orted that hyperphagic ventromedial hypothalamic (VMH) lesioned rats were "less hungry" than nonlesioned rats. More recent data. however, suggest that this interpretation originated, in part, from inappropriate testing procedures and between-group comparisons. Kent and Peters (1973) found that VMH-lesioned rats outperformed control rats on both operant and straight alley tasks when the groups were compared at identical moderate deprivation conditions defined in terms of preoperative free-feeding weights. Operant performance was assessed using a variable-interval (VI) reinforcement schedule rather than a progressively increasing fixed-ratio (FR) schedUle. The latter procedure was used by Teitelbaum (1957), and subsequently by others (Sclafani, 1971; Singh, 1970), to obtain the data that, in part, define the VMH paradox: increased food intake and decreased hunger motivation. The purpose of the present experiment was to compare the performance of VMH-les:oned and control rats on a FR schedule of reinforcement. The discrepancies between the operant data reported by Kent and Peters (1973) and that obtained using progressively increasing FR schedules may partially reflect an interaction with either the schedule of reinforcement or the deprivation conditions and comparison procedures used in the different experiments. On a progressively increasing FR schedule, rats are typically trained to barpress initially on a continuous reinforcement schedule. The number of barpresses required to obtain food reward is subsequently increased to 4, 16, 64, and 256, in ascending order. The rats are usually tested for 2 or 3 days at each ratio, and each rat's entire food ration is obtained during daily 12-h sessions. Since VMH-Iesioned rats often perform at higher levels than control rats on *Requests for reprints should be sent to

The ventromedial hypothalamus was electrolytically destroyed in 18 conscious, sated rats. Operant responding for food reward on an FR·5 reinforcement schedule increased 2 h after the lesions were produced. Performance during the hour immediately following the lesions was not affected.

No abstract

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