Introduction This motor response is congruent with the illusory sensa-

Brain Research 881 2000 128–138 www.elsevier.com locate bres Research report Relations between the directions of vibration-induced kinesthetic illusions and the pattern of activation of antagonist muscles ` ` Sarah Calvin-Figuiere , Patricia Romaiguere, Jean-Pierre Roll ´ Laboratoire de Neurobiologie Humaine , UMR 6562, CNRS-Universite de Provence, Marseille, France Accepted 20 June 2000 Abstract In humans, tendon vibration evokes illusory sensations of movement that are usually associated with an excitatory tonic response in muscles antagonistic to those vibrated antagonist vibratory response, AVR, i.e., in the muscle groups normally contracted if the illusory movement had been performed. The aim of the present study was to investigate the relation between the parameters of the illusory sensation of movement and those of the AVR and to determine whether vectorial models could account for the integration of proprioceptive inputs from several muscles, as well as for the organization of the elementary motor commands leading to one unified motor response. For that purpose, we analyzed the relations between the anatomical site of the tendon vibration, the direction of the illusory movement, the muscles in which the AVR develops, and the characteristics of the AVR surface EMG, motor unit types, firing rates, and activation latencies. This study confirmed the close relationship between the parameters of an AVR and those of the kinesthetic illusion. It showed that, during illusions of movements in different directions, motor units are activated according to a specific pattern correlated with their type, with the direction of the illusory movement and with the biomechanical properties of their bearing muscles. Finally, kinesthetic illusions and AVRs can be effectively represented using similar vectorial computations. These strong relations between the perceptual and motor effects of tendon vibration once again suggest that the AVR may result from a perceptual-to-motor transformation of proprioceptive information, rather than from spinal reflex mechanisms.  2000 Elsevier Science B.V. All rights reserved. Theme : Motor systems and sensorimotor integration Topic : Control of posture and movement Keywords : Vibration-induced kinesthetic illusion; Antagonist vibratory response; Motor unit; Wrist extensor muscle; Human; Microelectromyography

1. Introduction This motor response is congruent with the illusory sensa-

tion of movement because it develops in the muscle groups Kinesthetic illusions can be elicited by artificially man- normally contracted if the illusory movement had been ipulating proprioceptive pathways through tendon vibration performed. In a previous work, we showed that an AVR [4,11,12,24]. The direction of such a kinesthetic illusion can be recorded only when a kinesthetic illusion is elicited, depends on the anatomical site of the vibrated muscles and and that its intensive parameters correlate with the corresponds to their lengthening. cinematic characteristics of the illusory movement per- In most cases, kinesthetic illusions induced by tendon ceived [3]. This correlation suggests that this response vibration are associated with excitatory tonic motor re- could originate from high level processes resulting from a sponses called antagonist vibratory responses AVRs [24] sensori-to-motor transformation of proprioceptive infor- or inverted TVRs Tonic Vibration Reflex [6], which mation [3]. To further examine this hypothesis, we ana- develop in muscle groups antagonistic to those vibrated. lyzed the relations between the direction of the illusory sensation of movement, and the spatial organization of the AVR. We thus generated sensations of illusory movements Corresponding author. Laboratoire de Neurobiologie Humaine, UMR in two orthogonal directions by single vibration of the ´ ´ ˆ 6562, Case 362, 52 Faculte St Jerome, 13397 Marseille cedex 20, France. flexor and adductor muscles of the wrist, and in an Tel.: 133-4-9128-8898; fax: 133-4-9128-8669. ` E-mail address : rcnewsup.univ-mrs.fr S. Calvin-Figuiere. intermediate direction by combined vibration of both 0006-8993 00 – see front matter  2000 Elsevier Science B.V. All rights reserved. P I I : S 0 0 0 6 - 8 9 9 3 0 0 0 2 6 0 4 - 4 S . Calvin-Figuiere et al. Brain Research 881 2000 128 –138 129 muscle groups. The AVRs elicited in all three conditions since these express the whole motor activity, whereas were recorded in the Extensor carpi radialis ECR and motor unit parameters each account for only a part of Abductor pollicis longus APL muscles. These muscles muscle activity. were chosen because they have different biomechanical properties. In isometric contractions, the APL is mainly involved in thumb and wrist abduction, while the ECR are

2. Material and methods