The illusion of changed position and movement from vibrating one arm is altered by vision or movement of the other arm

Abstract

Experiments were carried out on blindfolded human subjects to study the contribution of proprioceptive inputs from both arms in a forearm position matching task. Blindfolded matching accuracy was compared with accuracy when the subject could see their indicator (matching) arm, when they used a dummy arm for matching, and when they looked at a mirror image of the matching arm. The position of the mirror had been arranged so that the image of the indicator arm coincided with the position of the reference arm. None of these conditions significantly altered the matching errors. When reference elbow flexors were vibrated at 70-80 Hz, the illusion of extension of the vibrated arm reported by blindfolded subjects was significantly reduced by vision of the mirror image of the indicator arm or when using the dummy arm. It was concluded that visual information about the position of the indicator arm, or the apparent position of the reference arm, could reduce the size of the kinaesthetic illusion from vibration, but not abolish it. In a second experiment, subjects indicated, by tracking with their vibrated arm, the illusion of forearm extension evoked by elbow flexor vibration. It was found that the perceived speed of extension could be reduced by moving the indicator into extension and increased by moving it into flexion. These experiments demonstrate the importance for the matching process of the input provided by the indicator arm. Such a conclusion may help to explain some apparent discrepancies between observations made on position sense using one-arm and two-arm tasks. More broadly, this paper provides support for the idea that aspects of proprioceptive inputs from both arms are processed conjointly, as part of a strategy for use of the two hands as a single instrument in certain skilled tasks.

Figure 1. The apparatus and protocol

A, subjects sat in front of a two-compartment box. The partition between the compartments was a mirror that showed an image of the left arm. The position of the mirror could be adjusted so that the image of the left arm coincided with the position of the right arm. The mirror could be covered up. Subjects’ arms were taped to paddles which hinged at a point close to the elbow joint. The box had a detachable lid to allow subjects to see one or both arms, as well as permitting the experimenter to hold the arms or place them on supports at the test angle. B, before each trial, both arms were conditioned to put them in a comparable mechanical state. The two arms were placed at an angle of 50 deg to the horizontal and the subject was asked to flex each arm with a contraction of 30–50% of maximum while the arms were held by the experimenter. The right forearm (dashed line) was then placed at the test angle of 30 deg and subjects were asked to match its position with the left arm (continuous line). For the vibration trials the right forearm was placed at 30 deg and vibration of its elbow flexors was commenced. Subjects matched the position of the vibrated arm by placement of their left, indicator arm. When the subjects declared a match, the angle difference between indicator and reference arms was determined. When the indicator arm was placed in a more extended position than the reference arm, the error was expressed as a positive value, if the indicator arm was placed in a more flexed position, the error was expressed as a negative value.

Figure 2. Forearm position matching by a single subject

Each panel represents a single matching trial. The reference arm was placed at the test angle and several seconds later the subject was asked to match its position. During each trial the size of the elbow angle has been plotted against time over the 15–25 s duration of the trial. In each panel the dashed line indicates the position of the reference arm, the continuous line the position of the indicator arm. An angle of 0 deg represents the forearm in the horizontal position, an angle of 90 deg the forearm in the vertical position. In A the subject was blindfolded, in B they could see their indicator arm, in C the indicator arm was replaced with a dummy and in D the subject could see a mirror image of their indicator arm. Left-hand panels, matching trials without vibration; right-hand panels, matching trials during vibration of elbow flexors of the reference arm. The period of vibration is shown by the bar at the top of each panel.

Figure 3. Position matching errors for a single subject and for the group

A, single subject; B, group data. Left-hand panels, matching without vibration, right-hand panels with vibration. Matching conditions were with the subject blindfolded (Blind), with their indicator arm visible (Arm), using a dummy arm (Dummy) and with a mirror image of the indicator arm visible (Mirror). Angle difference was calculated as the position of the reference arm minus the position of the indicator arm. 0 deg represented a perfect match. Positive errors were where the indicator arm had adopted a more extended position relative to the reference; negative errors were where the indicator had adopted a more flexed position. In A are shown the angle differences for each of five trials for a particular condition by the one subject. In B are shown the pooled data for the 15 subjects. Values are means (±s.e.m.). Asterisks indicate significant differences (P < 0.05).

Figure 4. Movement sensations in the vibrated reference arm during movement of the indicator arm

Upper panel, perceived movement of the reference arm with the indicator arm stationary and then moving in the direction of extension. Lower panel, perceived movement of the reference with the indicator stationary and with it moving into flexion. In each panel the dashed line represents the angle of the indicator arm, the continuous line the angle of the reference. The period of vibration is shown at the top. Each trial began when the vibrator on the reference arm was turned on. About 3 s later the illusion of forearm extension set in and the subject began to track it (first vertical line). Up to this point the indicator arm had been kept stationary. Once the subject was tracking successfully, the indicator was moved (second vertical line). Tracking was continued for several more seconds and then stopped (third vertical line). Mean slopes, calculated for movement of the reference arm, have been drawn in.

Figure 5. Pooled data for movement sensations

The pooled data from seven subjects are shown. Bars (means ±s.e.m.) show movement illusion speed (deg s⁻¹) when the indicator arm was held stationary (Control), when it was moved in the direction of extension (Extension) and when it was moved in the direction of flexion (Flexion). Asterisks indicate significant differences (both P < 0.05).