Detection thresholds for electrical stimulation of forebrain and midbrain loci in the rat.

Current intensity thresholds for the detection of electrical brain stimulation were determined from a variety of forebrain and midbrain sites in rats. In a discrete trial, instrumental paradigm, the detection stimulus delivered via one electrode acted as a cue for the availability of response contingent rewarding brain stimulation delivered via a second electrode in the posterolateral hypothalamus. The current intensity of the detection stimulus (20 Hz) was varied according to a modification of the psychophysical method of constant stimuli, while the intensity of contingent rewarding stimulation (160 Hz) was fixed at a highly reinforcing level. Detection thresholds measured from forebrain sites were significantly lower than thresholds from midbrain sites. This difference in detection thresholds was not related to any variability between subjects in site or parameters of the rewarding stimulus used to maintain behavior. Lower detection thresholds from forebrain loci may be due to greater sensitivity to electrical stimuli at the detection site or of the pathways from those sites to areas mediating perception or conditioning.