Do Reward and Drive Neurons Exist?

Abstract

Brain stimulation and lesion studies have shown that a bundle of pathways extending bidirectionally from the medulla to the telencephalon through the lateral hypothalamus may contain the axons of a set of reward neurons. In self-stimulation experiments stimulation of these pathways caused reward behavior; in lesion studies, cutting them suspended it temporarily and modified it permanently. Lesions removing much of the forebrain showed that at least some of the critical neurons did not have their cell bodies in the front end of this system. Other experiments gave strong but not yet compelling evidence that two or three families of catecholamine-containing neurons with cell bodies at the back end of the system and widely broadcast axons might be the neurons in question. “Unit recording” studies pointed to different neurons which monitored the bidirectional bundle in midcourse at the hypothalamic level as being possibly drive neurons. These were active in striving animals, further activated by conditioned stimuli associated with rewards, but silenced by several different kinds of rewards. Indirect evidence suggested that these were excited also by visceral and/or hormonal inputs; and that their axons might become connected during development and learning to cell assemblies in the cortex and basal ganglia, possibly to cell assemblies active at the time of their being “silenced” by rewards. The wide ramifications of the supposed reward axons suggested that besides “inhibiting drives” there might also be other functions. The most likely ones would be to stamp in sensory-motor connections (possibly in the cerebellum), to motivate the “replay” of sequential behavior memories (possibly in the hippocampus), and to “charge” (or connect drives to) cell assemblies active at the time of reward (possibly in the neocortex).