Humans perform vertical and horizontal arm motions with different temporal patterns. The specific velocity profiles are chosen by the central nervous system by integrating the gravitational force field to minimize energy expenditure. However, what happens when a visuo-motor rotation is applied, so that a motion performed in the horizontal plane is perceived as vertical? We investigated the dynamic of the adaptation of the spatial and temporal properties of a pointing motion during a prolonged exposure to a 90° visuo-motor rotation, where a horizontal movement was associated to a vertical visual feedback. We found that participants immediately adapted the spatial parameters of motion to the conflicting visual scene in order to maintain their arm trajectory straight. On the contrary, the initial symmetric velocity profiles specific of a horizontal motion were progressively modified during the conflict exposure, becoming more asymmetric and similar to those appropriate for a vertical motion. Importantly, this visual effect that increased with repetitions was not followed by a consistent after effect when the conflicting visual feedback was absent (catch and washout trials). In a control experiment we demonstrated that an intrinsic representation of the temporal structure of perceived vertical motions could provide the error signal allowing for this progressive adaptation of motion timing. These finding suggest that gravity strongly constrains motor learning and the reweighting process between visual and proprioceptive sensory inputs, leading to the selection of a motor plan which is sub-optimal in terms of energy expenditure.
- Sensori-motor adaptation
- visual vertical
- motor control
- internal model of gravity
- Copyright © 2014, Journal of Neurophysiology