Unit responses in the medial forebrain bundle to rewarding stimulation in the hypothalamus

The effects of different intensities of stimulation were studied on self-stimulation behavior and on the neural responses in the medial forebrain bundle correlated with it. Of the two neural responses observed, an excitatory type was more diffusely distributed throughout this pathway than an inhibitory type, and its properties varied more as a function of the rate of self-stimulation at the various intensities tested than the properties of the inhibitory responses did. The latter were more localized to the caudal portion of the medial forebrain bundle studied, and their threshold was generally lower than the threshold of self-stimulation behavior. Also, the inhibitory responses appeared to be more sensitive indicators of stimulation applied to the medial forebrain bundle, whereas the excitatory responses were more sensitive indicators of variations in the rate of responding for brain reward. The neural responses in the MFB to rewarding stimulation are evaluated in terms of the possibility that they are specific to the positive reinforcement mechanism in the diencephalon.