When planning target-directed reaching movements, human subjects combine
visual and proprioceptive feedback to form two estimates of the arm's
position: one to plan the reach direction, and another to convert
that direction into a motor command. These position estimates are
based on the same sensory signals but rely on different combinations
of visual and proprioceptive input, suggesting that the brain weights
sensory inputs differently depending on the computation being performed.
Here we show that the relative weighting of vision and proprioception
depends both on the sensory modality of the target and on the information
content of the visual feedback, and that these factors affect the
two stages of planning independently. The observed diversity of weightings
demonstrates the flexibility of sensory integration and suggests
a unifying principle by which the brain chooses sensory inputs so
as to minimize errors arising from the transformation of sensory
signals between coordinate frames.