Psychophysics of CNS pain-related activity: binary and analog channels and memory encoding.
The forebrain neuronal system signaling pain has been poorly characterized. The pain pathway afferent to the thalamus may be a labeled line consisting of neurons in the pain-signaling pathway to the brain (spinothalamic tract, STT) that respond only to painful stimuli. It has recently been proposed that the STT contains a series of analog-labeled lines, each signaling a different aspect of the internal state of the body (interoception), for example, visceral/cold/itch sensations. In this view, pain is the unpleasant emotion produced by disequilibrium of the internal state. The authors now show that stimulation of an STT receiving zone (thalamic principal somatic sensory nucleus, ventral caudal) in awake humans produces two different exteroceptive responses. The first is a binary response signaling the presence of painful stimuli. The second is an analog response in which nonpainful and painful sensations are graded with intensity of the stimulus. Such stimulation can evoke both the sensory and emotional components of previously experienced pain. These results illustrate the diverse functions of human pain signaling pathways.