Spatial transformations in the withdrawal response of the tail in intact and spinalized rats

Previous studies of spatial transformations between sensory input and motor output in escape responses have suggested two alternative patterns of spatial integration. The continuous pattern corresponds to withdrawal movements directed 180 degrees away from the location of the stimulus, whereas the categorical pattern corresponds to movements that are biased toward a limited number of preferred directions. The goal of these experiments was to determine which pattern best describes the tail withdrawal response in spinalized and intact rats by applying pinpoint heat stimuli at eight points distributed circumferentially around the tail and measuring the direction and speed of the resulting withdrawal response. Our results are consistent with a novel, hybrid continuous-categorical movement strategy. In the spinalized animal, responses were primarily away from the stimulus (the continuous component) but exhibited a pronounced ventral bias (the categorical component). In addition, stimuli delivered to the ventral surface, which would be expected to evoke a dorsally directed response (opposite to the preferred ventral direction), instead evoked responses that were markedly variable and clustered in the left and right directions. Intact rats showed a similar pattern of response, except reversed; the bias was in the dorsal direction, and the response to dorsal stimulation was now highly variable and lateral in direction. These results reveal a complex tail withdrawal strategy that is modulated by descending supraspinal pathways to adapt the response to the biomechanical and environmental constraints imposed on movement of the rat tail.