Temporal features of human tendon vibration illusions.

Muscle spindles provide information about the position and movement of our bodies. One method for investigating spindle signals is tendon vibration. Vibration of flexor tendons can produce illusions of extension, and vibration of extensor tendons can produce illusions of flexion. Here we estimate the temporal resolution and persistence of these illusions. In Experiments 1 and 2, sequences of alternating vibration of wrist flexor and extensor tendons produced position illusions that varied with alternation period. When vibrations alternated at 1 Hz or slower, perceived position at the end of the sequence depended on the last vibration. When vibrations alternated every 0.3 s, perceived position was independent of the last vibration. Experiment 2 verified and extended these results using more trials and concurrent electromyographic recording. Although tendon vibrations sometimes induce reflexive muscle activity, we found no evidence that such activity contributed to these effects. Experiment 3 investigated how long position sense is retained when not updated by current information from spindles. Our first experiments suggested that vibrating antagonistic tendons simultaneously could produce conflicting inputs, leaving position sense reliant on memory of position prior to vibration onset. We compared variability in position sense after different durations of such double vibration. After 12 s of double vibration, variability across trials exceeded levels predicted from vibrations of flexor or extensor tendons alone. This suggests that position sense memory had decayed too much to substitute for the current conflicting sensory information. Together, our results provide novel, quantitative insight into the temporal properties of tendon vibration illusions.