Journal of Comparative and Physiological Psychology 1970, Vol. 70, No. 1, 48-59 SEPARABLE FOREBRAIN SYSTEMS CONTROLLING DIFFERENT MANIFESTATIONS OF SPONTANEOUS ACTIVITY 1 GARY S. LYNCH 2 Princeton University Discrete lesions were placed within the dorsal and ventral aspects of the frontal poles of rats and at various levels of two tracts (the medial fore- brain bundle and the inferior thalamic peduncle) which relate this area with the diencephalon. The animals were then tested for wheel and stabilimeter activity during ad-lib and food-deprivation conditions. The results indicated that: (a) a discrete system running from the ventral frontal cortex into the medial forebrain bundle inhibits wheel running, and (b) the substrates of stabilimeter activity are more diffuse with the inferior thalamic peduncle playing a central role. From this, it appears that there are at least two over- lapping neural systems in frontal cortex of the rat which have very different behavioral functions. Previous work has shown that bilateral removal of the frontal pole area in the rat produces two types of activity change: (a) an immediate and chronic increase in daily wheel running (Richter & Hawkes, 1939), and (b) a potentiation of treatments (e.g., starvation, amphetamine) which nor- mally increase stabilimeter cage activity (Campbell & Lynch, 1969; Lynch, Ballan- tine, & Campbell, 1969). This latter re- sult has been interpreted in terms of a reticular loop hypothesis first advanced by Hugelin and Bonvallet (cf. Hugelin, Bonvallet, & Dell, 1959). According to this explanation, the frontal cortex nor- mally reacts to dampen increases in the level of reticular (and behavioral) arousal caused by a wide range of internal changes. Interruption of the reticulo-cortico-reticu- lar loop would be expected to greatly am- plify the effects of conditions which act primarily through the reticular formation. This type of negative feedback model accounts for the effects of frontal lesions on stabilimeter cage activity, but it does not readily explain the chronic increases in wheel running which occur without ex- perimental treatment other than the le- sions. One suggestion for resolving this discrepancy is that wheel-running and stabilimeter cage activity have different anatomical substrates both of which have frontal cortical components. This is given credence by recent anatomical and neuro- behavioral studies showing that the frontal lobes across a range of species contain heterogeneous subdivisions (cf. Divac, Rosvold, & Szwarcbart, 1967; Nauta, 1964) and the extensive behavioral evi- dence dissociating wheel and stabilimeter activity. The present experiments were de- signed to test the hypothesis that overlap- ping neural systems are present in the rat's frontal poles and that simultaneous destruction of these accounts for the di- This research represents a portion of a doctoral vergent activity results reported above. dissertation done at Princeton University under the supervision of Byron A. Campbell and was supported by National Institute of Mental Health Grants MH-08501 and MH-0156 to Byron Camp- bell. The author was the recipient of a National Institute of Mental Health predoctoral fellowship during the time this work was conducted and pre- pared for publication. Requests for reprints should be sent to Gary S. Lynch, who is now at the Department of Psycho- biology. University of California, Irvine, California EXPERIMENT 1 Based on neuroanatomical and behavioral studies, several authors have proposed that the dorsal and ventral ( orbital ) surfaces of the frontal lobes of primates may be functionally distinct (Akert, 1964; Divac et al., 1967). This dichotomy, while prob- ably not directly applicable to rodents, is