Proprioceptive sensitivity to imposed finger deflections

Abstract

Hand proprioception, the sense of the posture and movements of the wrist and digits, is critical to dexterous manual behavior and to stereognosis, the ability to sense the three-dimensional structure of objects held in the hand. To better understand this sensory modality and its role in hand function, we sought to characterize the acuity with which the postures and movements of finger joints are sensed. To this end, we measured the ability of human subjects to discriminate changes in posture and speed around the three joints of the index finger. In these experiments, we isolated the sensory component by imposing the postures on an otherwise still hand, to complement other studies in which subjects made judgments on actively achieved postures. We found that subjects could reliably sense 12-16% changes in joint angle and 18-32% changes in joint speed. Furthermore, the acuity for posture and speed was comparable across the three joints of the finger. Finally, task performance was unaffected by the presence of a vibratory stimulus, calling into question the role of cutaneous cues in hand proprioception.NEW & NOTEWORTHY Manual dexterity and stereognosis are supported by two exquisite sensory systems, namely touch and proprioception. Here, we measure the sensitivity of hand proprioception and show that humans can sense the posture and movements of the fingers with great accuracy. We also show that application of a skin vibration does not impair sensitivity, suggesting that proprioceptive acuity relies primarily on receptors in the muscles (and possibly tendons) rather than the skin.

Keywords: angular acuity; angular speed; muscle spindles; psychophysics.

Graphical abstract

Figure 1.

Experimental rig for metacarpophalangeal (MCP) joint experiments. The subject’s hand is placed in a clamp, and the index finger is positioned at 0° with a protractor fixed above the hand. The index finger is then splinted to allow movement only at 1 of the 3 index joints (setup for the MCP is shown here). The top of the splint is attached to the motor, which moves the index finger to a specified angle at a specified speed.

Figure 2.

Angle discrimination task. A and B: psychometric functions averaged across all subjects for the 20° reference (A; n = 10, 9, 9 subjects for MCP, PIP, DIP, respectively) and the 40° reference (B; n = 7, 6 subjects for MCP, PIP, respectively) for each joint of the index finger. Dashed line indicates the reference angle. Error bars denote the SE across subjects. DIP, distal interphalangeal; MCP, metacarpophalangeal; PIP, proximal interphalangeal. C: just noticeable differences (JNDs) for each joint (colors follow same convention as in A and B). Each point represents 1 subject; boxplots represent interquartile range; and whiskers represent maximum and minimum) for each reference angle. D: Weber fractions for each joint (color-coded as in A and B) and reference angle. Sample sizes in C and D are the same as for A and B. E: isolated passive joint movements for the MCP, PIP, and DIP joints. Colored image shows the finger at rest, prior to the deflection. The solid finger image denotes a 20° or 40° deflection. To either side of this reference is a ghost image of the mean JND for that joint and angle. Anything beyond this ghosted region was, on average, successfully perceived as larger or smaller than the reference movement.

Figure 3.

Angular speed discrimination. A: psychometric functions averaged across all subjects for the 3 joints. DIP, distal interphalangeal; MCP, metacarpophalangeal; PIP, proximal interphalangeal. B: mean just noticeable differences (JNDs) and Weber fractions, separated by joint. Each point denotes 1 subject; boxplots denote interquartile range; whiskers denote maximum and minimum of all subjects’ JNDs for each joint. n = 11, 7, 6 subjects for MCP, PIP, DIP, respectively.

Figure 4.

Effect of cutaneous interference on proprioceptive sensitivity in presence of cutaneous interference. A: sensitivity to angular speed is unaffected by cutaneous interference. We tested speed just noticeable differences (JNDs) with and without a vibratory distractor for the distal interphalangeal (DIP) and proximal interphalangeal (PIP) joints. Each point denotes 1 subject’s JND with lines connecting paired data; boxplots denote the interquartile range; and whiskers denote the maximum and minimum value across subjects. n = 7 subjects for each joint. B: angular acuity is also unaffected by cutaneous interference. We measured the JND at the metacarpophalangeal (MCP) joints with a 40° reference and with a vibration delivered to 1 of 2 locations: on the dorsal surface of the hand, just beneath the index finger, or on the hypothenar eminence on the palm (locations shown in yellow in the schematic). For the former, vibrations would contaminate cutaneous signals at the hand location where skin stretch and movement-related deformations are most pronounced during joint deflections. The latter was used as a control, as the vibrations on the palm do not propagate to the dorsal surface of the hand. To the extent that a vibratory stimulus is distracting, however, JNDs will be affected similarly in the 2 conditions. Conventions are the same as in A; n = 9 subjects.