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1 Departement de Physiologie, Universite de Montreal, United States
* To whom correspondence should be addressed. E-mail: trevor.drew{at}umontreal.ca.
We have developed a novel locomotor task in which cats step over obstacles that move at a different speed from that of the treadmill on which the cat is walking: we refer to this as a visual dissociation locomotion task. Slowing the speed of the obstacle with respect to that of the treadmill sometimes led to a major change in strategy so that cats made two steps with the hindlimbs prior to stepping over the obstacle (double step strategy) instead of the single step (standard strategy) observed when the obstacle was at the same speed as the treadmill. In addition, in the step preceding the step over the obstacle, the paws were placed significantly closer to the obstacle in the visual dissociation task than when the treadmill and the obstacle were at the same speed. Following unilateral lesion of area 5 of the posterior parietal cortex (PPC), the cats frequently hit the obstacle as they stepped over it, especially in the visual dissociation task. This locomotor deficit was linked to significant differences in the location in which the forelimbs were placed in the step preceding that over the obstacle compared to the pre-lesion control. Cats also frequently hit the obstacle with their hindlimbs even when the forelimbs negotiated the obstacle successfully; this suggests an important role for the posterior parietal cortex in the coordination of the forelimbs and hindlimbs. Together, these results suggest an important contribution of the PPC to the planning of visually guided gait modifications.
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