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|Title:||RESPONSE OF POSTERIOR PARIETAL CORTEX CELLS TO MANIPULATIONS OF VISUAL INFORMATION DURING OBSTACLE AVOIDANCE IN THE CAT|
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|Authors/Affiliations:||1 Daniel Marigold*; 1 Trevor Drew; |
1 Universite de Montreal, QC, Canada
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|Content:||Objectives: Vision plays an essential role in goal-directed locomotion, particularly when planning anticipatory gait modifications to avoid objects in the travel path. Recent work suggests that the posterior parietal cortex (PPC) may contribute to the planning of these modifications (Lajoie & Drew, J Neurophysiol 97:2339-2354, 2007). Of particular interest is our finding that some neurons increase their discharge frequency prior to the step over an obstacle (Drew et al. Brain Res Rev 57:199-211, 2008). Such cells may provide information about the characteristics (e.g. size, distance, velocity, and/or time to contact) of the approaching obstacle. In this study, we asked (1) whether continuous visual input from the advancing obstacle is responsible for the increase in discharge activity observed in PPC cells prior to the step over the obstacle and (2) whether acceleration of the obstacle towards the cat alters this cell discharge activity.|
Materials and Methods: We addressed the first question by occluding vision while recording neurons in the PPC of 1 cat trained to step over obstacles attached to a moving treadmill belt. The obstacle speed either matched the speed of the treadmill belt on which the cat was walking or was decreased with respect to it. The experiments were performed in a room lit only by several LED lights. During the visual occlusion experiments, these lights were extinguished for 900 ms (approximate duration of 1 step cycle) 1-3 steps prior to the step over the obstacle. We addressed the second question by accelerating the obstacle towards the cat for a period of 900 ms 1-3 steps prior to the step over the obstacle.
Results: The effects of visual occlusion were tested on 20 cells and the effects of obstacle acceleration were tested on 23 cells that showed increased activity prior to the step over the obstacle. In 8/20 cells, visual occlusion resulted in a significant decrease in discharge frequency when applied 1-2 steps before the obstacle. In the other 12/20 cells, however, there was no significant change in activity during the occlusion. In 16/23 cells, obstacle acceleration increased the discharge frequency when initiated 1-2 steps before the obstacle. In contrast, 7/23 cells showed no effect of obstacle acceleration. In both the visually occluded trials and acceleration trials, the cat occasionally hit the obstacle.
Conclusion: We suggest that the cells that decreased activity with visual occlusion have primarily a sensory function, signaling direct visual information on the characteristics of the obstacle. Conversely, the population of cells with no change in activity to visual occlusion demonstrates short-term memory and might be more involved in planning motor activity on the basis of the prior visual information. This latter population of cells would form the neuronal substrate that allows a precise control over locomotion even when only intermittent visual information is available (see Patla et al., Exp Brain Res 112:513-522, 1996). Cells responding to acceleration would provide the substrate for on-line corrections of gait.
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