Flexibility and individual differences in visuo-proprioceptive integration: evidence from the analysis of a morphokinetic control task.

Conflicting theories of visuo-proprioceptive integration in movement control suggest that each modality can be weighted according to either its statistical reliability or the computations in which the integrated estimates will be used. However, the psychophysical experiments on which most studies are based use sensory conflicts and are therefore likely to reflect particular rather than normal behavior. In this paper, we: (1) propose a method avoiding the use of sensory conflicts (delayed recall task), (2) restrict our interest to spontaneous rather than adapted behavior, and (3) focus on a complex task requiring fine online control in order for hand movements to fit a precise path during execution. Subjects were provided with either visual, proprioceptive or both cues while their right hand was passively moved to fit a precise three segment pathway. As soon as this encoding phase ended, they were instructed to reproduce actively the trajectory, either with vision, proprioception or both cues. Results provide evidence that vision and proprioception may be used very differently, (1) not only according to the relative resolution of the sensory systems in the actual context, (2) not only according to the processes involved in the task, but also (3) according to subjects. They also suggest that visual and proprioceptive cues are not fused to provide a weighted average position, but that the non-dominant cue could simply be ignored when subjects are provided with multiple sensory cues. We conclude that each of these observations illustrates the same fundamental property of flexibility of integrative mechanisms.