Sensory coupling in tool use

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Sensory information on the position of our hand is typically multimodal, visual and proprioceptive in particular. Sensory integration of the different modalities results in the perception of a single hand in a single position. In tool use, visual and proprioceptive information typically relate no longer to a single object, the hand, but to different objects, the effective part of the tool and the hand, respectively. Movements of the effective part of the tool are related to hand movements by the so-called kinematic transformation. In the most popular experimental paradigm for the study of adaptation to novel kinematic transformations, the direction of the motion of a cursor on a computer monitor is rotated relative to the direction of hand movement. Preliminary studies with a modification of this paradigm revealed sensory coupling of cursor and hand direction, that is, mutual biases of the perceived directions of hand and cursor movements, in spite of different objects in different locations that move in different planes. The purpose of the proposed project is to advance knowledge of this particular type of sensory coupling.A first series of experiments serves to test extensions of widely accepted models of near-optimal sensory integration. These models have been developed for situations in which different sensory modalities refer to a single property of a single object. They shall be generalized to situations, in which the different sensory modalities refer to a property of two different objects, the movements of which are related by a kinematic transformation. Further series of experiments serve to identify factors which determine the strength and the asymmetry of sensory coupling in tool use. The first of these series is designed to compare different kinematic transformations, the second series to assess the effects of learning the kinematic transformation which might differ for different practice regimen. In the third series additional factors are tested that are likely to affect coupling strength. Among these factors are temporal delays between proprioceptive information on hand movements and visual information on cursor motions, and the correlations between the velocity profiles of hand movements and accompanying cursor motions. Finally, as a by-product of several of the planned experiments, estimates of sensory coupling based on explicit psychophysical judgements will be compared with estimates based on a particular kind of indirect measure of the sensed hand position.