THE ROLE OF BRACHIAL MUSCLE-SPINDLE SIGNALS IN ASSIGNMENT OF VISUAL DIRECTION

1. In the oculobrachial illusion, a target light attached to the unsee n stationary hand is perceived as moving and changing spatial position when illusory motion of the forearm is elicited by brachial muscle vi bration. Our goal was to see whether we could induce apparent motion a nd displacement of two retinally fixed targets in opposite directions by the use of oculobrachial illusions. 2. We vibrated both biceps brac hii, generating illusory movements of the two forearms in opposite dir ections, and measured any associated changes in perceived distance bet ween target lights on the unseen stationary hands. The stability of vi sual fixation of one of the targets was also measured. 3. The seen dis tance between the stationary targets increased significantly when vibr ation induced an illusory increase in felt distance between the hands, both with binocular and monocular viewing. 4. Subjects maintained fix ation accuracy equally well during vibration-induced illusory increase s in visual target separation and in a no-vibration control condition. Fixation errors were not correlated with the extent or direction of i llusory visual separation. 5. These findings indicate that brachial mu scle spindle signals can contribute to an independent representation o f felt target location in head-centric coordinates that can be interre lated with a visual representation of target location generated by ret inal and oculomotor signals. 6. A model of how these representations a re interrelated is proposed, and its relation to other intersensory in teractions is discussed.