Where are my fingers? Assessing multi-digit proprioceptive localization

The proprioceptive sense is an important component in the sensory system, providing position estimates for the parts of our body that are needed in order to guide behavior. While prior work has characterized proprioceptive localization of the upper limb as a whole, few studies have investigated the proprioceptive localization of individual digits, despite their central role in tactile exploration and fine manipulation. In this study, we asked subjects to report the perceived positions of the endpoints of each finger by means of a virtual reality and multi-finger tracking system, and we investigated the magnitude and variability of the proprioceptive localization errors that ensued. We found the errors to be large, averaging 4.3 cm per digit, even when the location of the wrist was disclosed to participants. This error magnitude is comparable to those that have been reported for whole limb localization, and represents a significant fraction of the length of an entire digit. Localization accuracy is greatest for the thumb and smallest for the little finger, reflecting in all cases, significant biases in perceived locations. The precision of localization was similar for each finger, with uncertainties on the order of several centimeters. The posture of the hand and specific location of the finger both affected errors. The specific error patterns for each subject were idiosyncratic, and the results suggest an underlying proprioceptive hand representation in which the fingers are distorted from the true shape of the hand. These results may have implications for understanding the role of position sensing in tactile exploration and fine manipulation, and suggest that an improved understanding of sensorimotor integration of proprioceptive information from the fingers is needed.