Head roll influences perceived hand position.

Visual and proprioceptive sensory inputs are naturally coded in different reference frames, i.e., eye-centered and body-centered, respectively. To use these signals in conjunction for motor planning or perception ultimately requires converting them into a common frame of reference using estimates of the relative orientation of the eyes, head, and body. Here, we examine whether extraretinal signals-specifically head roll-alter multisensory perception through noisy reference frame transformations. To do so, we examine the accuracy of visual localization relative to proprioceptive hand position for different head roll orientations. Subjects were required to judge whether a visual target was located closer or further and left or right (4-alternative forced-choice task) from their unseen hand. This was done for three different head roll rotations (-30, 0, and 30 deg). We show that eccentric head roll increased the variability in the subjects' ability to discriminate target location relative to the fingertip. We conclude that sensory perception is sensitive to body-geometry-dependent noise affecting the coordinate matching transformations of sensory data.