An investigation of the sensory properties of electrical brain stimulation is being conducted. Rats are trained in a signal detection paradigm, where reinforcement is contingent upon correctly detecting the presence or the absence of very weak stimuli delivered to loci typically involved in electrical brain stimulation reinforcement. By the use of standard psychophysical techniques, the accuracy of detection of the weak stimuli is assessed as a function of the parameters of the stimulation and the locus of stimulation.