Proprioceptive Biases in Different Experimental Designs

Looking Precisely at Your Fingertip Requires Visual Guidance of Gaze

People often look at objects that they are holding in their hands. It is therefore reasonable to expect them to be able to direct their gaze precisely with respect to their fingers. However, we know that people make reproducible idiosyncratic errors of up to a few centimetres when they try to align a visible cursor to their own finger hidden below a surface. To find out whether they also make idiosyncratic errors when they try to look at their finger, we asked participants to hold their finger in front of their head in the dark, and look at it. Participants made idiosyncratic errors of a similar magnitude to those previously found when matching a visual cursor to their hidden finger. This shows that proprioceptive position sense of finger and gaze are not aligned, suggesting that people rely on vision to guide their gaze to their own finger.

Proprioceptive Localization of the Fingers: Coarse, Biased, and Context-Sensitive

The proprioceptive sense provides somatosensory information about positions of parts of the body, information that is essential for guiding behavior and monitoring the body. Few studies have investigated the perceptual localization of individual fingers, despite their importance for tactile exploration and fine manipulation. We present two experiments assessing the performance of proprioceptive localization of multiple fingers, either alone or in combination with visual cues. In the first experiment, we used a virtual reality paradigm to assess localization of multiple fingers. Surprisingly, the errors averaged 3.7 cm per digit, which represents a significant fraction of the range of motion of any finger. Both random and systematic errors were large. The latter included participant-specific biases and participant-independent distortions that evoked similar observations from prior studies of perceptual representations of hand shape. In a second experiment, we introduced visual cues about positions of nearby fingers, and observed that this contextual information could greatly decrease localization errors. The results suggest that only coarse proprioceptive information is available through somatosensation, and that finer information may not be necessary for fine motor behavior. These findings may help elucidate human hand function, and inform new applications to the design of human-computer interfaces or interactions in virtual reality.

Visuo-Proprioceptive Matching Errors Are Consistent with Biases in Distance Judgments

People make systematic errors when matching the location of an unseen index finger with that of a visual target. These errors are consistent over time, but idiosyncratic and surprisingly task-specific. The errors that are made when moving the unseen index finger to a visual target are not consistent with the errors when moving a visual target to the unseen index finger. To test whether such inconsistencies arise because a large part of the matching errors originate during movement execution, we compared errors in moving the unseen finger to a target with biases in deciding which of two visual targets was closer to the index finger before the movement. We found that the judgment as to which is the closest target was consistent with the matching errors. This means that inconsistencies in visuo-proprioceptive matching errors are not caused by systematic errors in movement execution, but are likely to be related to biases in sensory transformations.

Spatial Representation of the Workspace in Blind, Low Vision, and Sighted Human Participants

It has been proposed that haptic spatial perception depends on one's visual abilities. We tested spatial perception in the workspace using a combination of haptic matching and line drawing tasks. There were 132 participants with varying degrees of visual ability ranging from congenitally blind to normally sighted. Each participant was blindfolded and asked to match a haptic target position felt under a table with their nondominant hand using a pen in their dominant hand. Once the pen was in position on the tabletop, they had to draw a line of equal length to a previously felt reference object by moving the pen laterally. We used targets at three different locations to evaluate whether different starting positions relative to the body give rise to different matching errors, drawn line lengths, or drawn line angles. We found no influence of visual ability on matching error, drawn line length, or line angle, but we found that early-blind participants are slightly less consistent in their matching errors across space. We conclude that the elementary haptic abilities tested in these tasks do not depend on visual experience.

Matching locations is not just matching sensory representations

People make systematic errors when matching locations of an unseen index finger with the index finger of the other hand, or with a visual target. In this study, we present two experiments that test the consistency of such matching errors across different combinations of matching methods. In the first experiment, subjects had to move their unseen index fingers to visually presented targets. We examined the consistency between matching errors for the two hands and for different postures (hand above a board or below it). We found very little consistency: The matching error depends on the posture and differs between the hands. In the second experiment, we designed sets of tasks that involved the same matching configurations. For example, we compared matching errors when moving with the unseen index finger to a visual target, with errors when moving a visual target to the unseen index finger. We found that matching errors are not invertible. Furthermore, moving both index fingers to the same visual target results in a different mismatch between the hands than directly matching the two index fingers. We conclude that the errors that we make when matching locations cannot only arise from systematic mismatches between sensory representations of the positions of the fingers and of visually perceived space. We discuss how these results can be interpreted in terms of sensory transformations that depend on the movement that needs to be made.

Proprioceptive Localization of the Hand Changes When Skin Stretch around the Elbow Is Manipulated

Cutaneous information has been shown to influence proprioceptive position sense when subjects had to judge or match the posture of their limbs. In the present study, we tested whether cutaneous information also affects proprioceptive localization of the hand when moving it to a target. In an explorative study, we manipulated the skin stretch around the elbow by attaching elastic sports tape to one side of the arm. Subjects were asked to move the unseen manipulated arm to visually presented targets. We found that the tape induced a significant shift of the end-points of these hand movements. Surprisingly, this shift corresponded with an increase in elbow extension, irrespective of the side of the arm that was taped. A control experiment showed that this cannot be explained by how the skin stretches, because the skin near the elbow stretches to a similar extent on the inside and outside of the arm when the elbow angle increases and decreases, respectively. A second control experiment reproduced and extended the results of the main experiment for tape on the inside of the arm, and showed that the asymmetry was not just a consequence of the tape originally being applied slightly differently to the outside of the arm. However, the way in which the tape was applied does appear to matter, because applying the tape in the same way to the outside of the arm as to the inside of the arm influenced different subjects quite differently, suggesting that the relationship between skin stretch and sensed limb posture is quite complex. We conclude that the way the skin is stretched during a goal-directed movement provides information that helps guide the hand toward the target.