A proper understanding of neurological studies of animal pain begins with the distinction between nociception and pain. Nociception — the capacity to sense noxious stimuli — is one of the most primitive sensory capacities. Neurons functionally specialized for nociception have been described in invertebrates such as the medical leech and the marine snail Aplysia californica (Walters 1996). Because nociceptors are found in a very wide range of species, and are functionally effective even in decerebrate or spinally transected animals, their presence and activity in a species provides little or no direct evidence for phenomenally conscious pain experiences. The gate control theory of Melzack and Wall (1965) describes a mechanism by which “top-down” signals from the brain modulate “bottom-up” nociception, providing space for the distinction between felt pain and nociception.