Contribution du cortex pariétal postérieur au contrôle de la locomotion sous guidage visuel chez le chat

During locomotion, vision provides crucial information about the type of terrain one walks on, as well as the size, shape and location of possible obstacles in the path. In the case of a predator, vision also provides information about the speed of a potential prey. The neuronal mechanisms involved in the execution of visually guided gait modifications are now well studied, but those related to their planning remain poorly understood. One likely candidate involved in movement planning is the posterior parietal cortex (PPC). Based on a consideration of the properties of the PPC in visually-guided reaching, the present study was designed to determine the contribution of the PPC to the control of visually-guided locomotion.  In the first study presented in this thesis, we hypothesize that the cat PPC is involved in the planning of precise foot placement during voluntary gait modifications. We trained animals to step over moving obstacles attached on a treadmill belt before and after a lesion to area 5. To increase the need for visual and proprioceptive integration, we dissociated the speed of the obstacles from that of the treadmill. We noticed that deficits were largest when the importance of vision was the greatest. Our analysis showed that the deficits were due to inappropriate paw placement prior to the step over the obstacle and thus suggests that the PPC is involved in the planning of precise paw placement during visually-guided locomotion.   Direct visual information is available to guide forelimb gait modifications, but is lost when the obstacle passes under the body. Therefore, hindlimb gait modifications must rely on remembered information and must be coordinated with those of the forelimbs. Our second study is designed to determine the neuronal mechanisms responsible for this coordination. We propose that the PPC is responsible for coordinating the activity of the forelimbs and the hindlimbs during obstacle negotiation. To test this hypothesis, we recorded the activity of area 5 neurones in the same task and found two cell populations: one that discharged in relation to the passage of the obstacle between the fore and hindlimbs, and the other between the two hindlimbs. In the visual dissociation task, the discharge was modified to account for the increased time taken by the obstacle to pass under the body and the change in interlimb coupling when cats modified their stepping strategy during the step over the obstacle. We also found that these same cells maintained their discharge when the cat straddled an obstacle either between the fore and hindlimbs or between the two hindlimbs. The discharge was sustained up to the limit tested (~ 1-2min). These neurones could be responsible for the retention of obstacles characteristics to guide future hindlimb movements.   Our results provide support for the hypothesis that the PPC is involved in the integration of visual and proprioceptive information for the planning of precise paw placement in front of obstacles. The discharge of our neuronal populations suggests it also encodes temporal and spatial information regarding obstacle’s location and speed to coordinate all four limbs during the task. Finally, we propose that one function of the PPC is to estimate the position of the limbs with respect to the advancing obstacle.

Keyphrases: gait modifications, lesions, posterior parietal cortex, movement planning, interlimb coordination, working memory