The nociceptive withdrawal response of the tail in the spinalized rat employs a hybrid categorical-continuous spatial...

Complexity in movement planning, arising from diverse temporal and spatial sources, places a computational burden on the central nervous system. However, the efficacy with which humans can perform natural, highly trained movements suggests that they have evolved effective behavioral strategies that simplify the computational burden. The specific aim of our research was to use three-dimensional high-speed video to determine whether the tail nociceptive withdrawal response (NWR) to noxious heat stimuli delivered at locations that varied both circumferentially and rostral-caudally on the tail depended on the location of the stimulus in spinalized rats. In particular, we sought to determine whether the movement strategy was categorical (limited number of directions) or continuous (any variation in stimulus location results in a variation in response direction). In spinalized rats, localized, noxious heat stimuli were delivered at eight locations circumferentially around the tail and at five rostral-caudal levels. Our results demonstrate that at all rostral-caudal levels, response movement direction was bimodal regardless of circumferential stimulus location-either ~ 64° left or right of ventral. However, in spite of tight clustering, movement direction varied significantly but weakly according to circumferential location, in that responses to stimuli were more lateral for lateral stimulus locations. In contrast, changes in stimulus level strongly affected movement direction, in that a localized bend response closely matched the level of the stimulus. Together, our results demonstrate, based on movement analysis in spinalized rats, that the NWR employs a hybrid categorical-continuous strategy that may minimize the harmful consequences of noxious stimuli.