Psychophysics of Sensations Evoked by Stimulation of the Human Central Nervous System

The forebrain neuronal system signaling pain has been poorly characterized. We now show that stimulation of thalamic pain-related nuclei in awake humans produces two different responses. The first is a binary response in which a constant painful sensation is evoked regardless of the pattern of stimulation. The second is an analog response in which nonpainful and painful sensations are graded with intensity of the stimulus, and are better defined than the first, both qualitatively and spatially. These results are consistent with those of functional imaging studies that have identified brain regions activated in a binary fashion by a particular stimulus, while further increases in stimulus intensity do not produce increased activation. Binary response sites may participate in all or none attentional or cognitive aspects of pain. Stimulation of these thalamic regions may also mediate memories of the emotional aspect of intense pain and of chronic pain that sometimes results from intense pains. These pain memories are not evoked by stimulation of the spinothalamic tract, which suggests that they involve conditioning of forebrain structures, rather than simple activation of these structures by a painful stimulus.