Muscle Vibration-Induced Illusions: Review of Contributing Factors, Taxonomy of Illusions and User’s Guide

in Multisensory Research
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Limb muscle vibration creates an illusory limb movement in the direction corresponding to lengthening of the vibrated muscle. Neck muscle vibration results in illusory motion of visual and auditory stimuli. Attributed to the activation of muscle spindles, these and related effects are of great interest as a tool in research on proprioception, for rehabilitation of sensorimotor function and for multisensory immersive virtual environments. However, these illusions are not easy to elicit in a consistent manner. We review factors that influence them, propose their classification in a scheme that links this area of research to perception theory, and provide practical suggestions to researchers. Local factors that determine the illusory effect of vibration include properties of the vibration stimulus such as its frequency, amplitude and duration, and properties of the vibrated muscle, such as contraction and fatigue. Contextual (gestalt) factors concern the relationship of the vibrated body part to the rest of the body and the environment. Tactile and visual cues play an important role, and so does movement, imagined or real. The best-known vibration illusions concern one’s own body and can be classified as ‘first-order’ due to a direct link between activity in muscle spindles and the percept. More complex illusions involve other sensory modalities and external objects, and provide important clues regarding the hidden role of proprioception, our ‘silent’ sense. Our taxonomy makes explicit this and other distinctions between different illusory effects. We include User’s Guide with tips for anyone wishing to conduct a vibration study.

Muscle Vibration-Induced Illusions: Review of Contributing Factors, Taxonomy of Illusions and User’s Guide

in Multisensory Research



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  • View in gallery

    An easy way to demonstrate and experience vibration-induced movement illusion (similar to the method used by Goodwin et al., 1972a). The demonstrator actively supports volunteer’s right arm by holding it at the wrist to ensure the arm is stationary. A hand-held vibrator purchased in a department store (vibration frequency approx. 90 Hz) is firmly pressed into the biceps tendon of the blindfolded volunteer. The volunteer uses her other arm to indicate the extent of illusory movement and displacement of the vibrated arm, which usually begin a few seconds following the onset of vibration. Here the volunteer indicates an illusory forearm extension of approximately 30 deg.

  • View in gallery

    Vibration of right dorsal neck results in the false sensory input that indicates contralateral head movement (consistent with the lengthening of right dorsal neck muscles). If during such stimulation the observer fixates a small stationary object in an otherwise dark visual field, the object will appear to move to the left, consistent with the misleading signal regarding head movement. Consistent with the oft-silent contribution of proprioceptive messages to sensory processing, the head movement itself may not be consciously perceived even if the visual motion is (Taylor and McCloskey, 1991; see Section 4.2 for details).

  • View in gallery

    Muscle spindle and vibration-induced sensations. Muscle spindles are elongated sensory organelles embedded in the muscle and lying in parallel with muscle fibres (see image). Muscle spindles have sensory endings that respond to stretch. Primary endings respond most to dynamic stretch and are served by Ia afferent nerve fibres. Secondary endings respond most to static stretch and are served by type II afferent fibres. Thus natural muscle stretch during limb movement results in the stretch of the muscle spindle and activation of sensory neurons.

    A vibrating object pressed firmly into the muscle causes rapid repetitive muscle stretches to which muscle spindles respond very well. Both types of sensory endings respond to vibration, but they are most sensitive to different frequencies. Dissociation between illusory movement and illusory displacement (Section 4.1) is attributed to their different response properties.

    The process is complicated by the fact that muscle spindle sensory endings need to remain taut at different muscle lengths in order to stretch when the muscle stretches. This is achieved through the activity of intrafusal muscle fibres on which the sensory endings are located. Because these tiny muscle fibres within the spindle capsule only contract at their ends, their activation stretches their central portions where the sensory endings lie. When the muscle contracts (due to the commands sent through the alpha motor system), so do the intrafusal fibres (due to the activity of the gamma motor system, see image), keeping the spindle endings taut although the muscle has shortened — a process known as alpha–gamma co-activation (Vallbo, 1970). Information about commands sent to muscles is also available to the brain (known as efference copy or corollary discharge) and may itself contribute to the movement sensation. Thus rather than being a simple consequence of muscle stretch, sensory input and the sensation will vary depending on the overall muscle length, movement preceding it or any concurrent movement, fatigue and other factors (see Section 2). A number of physiological studies provided detailed description of response to vibration in primary afferents (Burke et al., 1976a, b; Cordo et al., 1993; Roll et al., 1989). Central response to muscle vibration involves a network of areas, including both sensory and motor cortices (Naito et al., 2005).

  • Mechanical properties of vibrators. Sourcing of vibrators that can be used for purposes described in this manuscript is varied — from pneumatic drills, physiotherapy massagers and sex toys to custom-made devices. Mechanical properties of vibrators need to match their intended use in muscle vibration studies, and basic understanding of those properties is therefore desirable. We present some of the relevant information below, relying on sources that describe vibrators use in haptic research and applications (see Precision Microdrives Ltd., 2016: “AB-004: Understanding ERM vibration motor characteristics”, 1238; “AB-029: Vibration motors  — voltage vs frequency vs amplitude”, 1238; “VAB-02: How do vibration motors work?”, 1238; Yao and Hayward, 2010). A typical vibrator used for our purposes is larger than vibrators used in haptics but the same general principles of operations apply.

    Vibrators are devices that create fast and repetitive displacement of mass. Vibration frequency (in Herz) is a number of cycles of repetitive motion per second, and is the vibration property most commonly reported in the literature on vibration illusions. Vibration also needs to have sufficient amplitude of displacement with which a normal force is applied to the muscle tendon or belly in order to stretch muscle spindle. The force increases with the mass of vibrator’s moving part, so a very small vibrator will not do for a large muscle. The force actually delivered will also vary with leanness, i.e., bulkiness of the muscle (force delivered is rarely specified, let alone controlled; for one exception, see Cordo et al., 1993). Below we describe two basic vibrator types based on the direction of displacement of mass, i.e., direction in which they exert force.

    1. Eccentric rotating mass (ERM) vibration motor (also known as ‘rumble motor’) has an off-centre spinning load, the rotation of which results in a centrifugal force and motor displacement, i.e., vibration. The speed of rotation, i.e., vibration frequency is positively related to acceleration and the two parameters cannot be independently controlled. For any given vibrator, increasing the input voltage (DC current) increases both frequency and acceleration. Amplitude of displacement decreases with increasing frequency, other factors being constant (see Fig. 1a, Naito et al., 1999). Numerous vibration studies (including Goodwin et al., 1972a) used ERM vibrators and thus necessarily confounded changes in frequency with changes in amplitude of displacement. When applied to the muscle, orientation of the shaft around which the mass rotates should be parallel to it. That way a part of the centrifugal force will act as a normal force on the muscle.

    2. Linear vibration motors have a mass displaced by a force created by magnetic fields and electrical currents. Some of those vibrators operate at a single resonant frequency (linear resonant actuators, LRA) but their amplitude of displacement can be manipulated. In other types, frequency and amplitude can vary independently of each other, which has clear advantages in terms of stimulus control in vibration studies.

    Note: Terminology may confuse: in technical papers, ‘amplitude’ often means rate of acceleration and is measured in G or ms−2, but in the literature on vibration illusions, ‘amplitude’ usually refers to the extent of peak-to-peak vibrator displacement measured in µm or mm (e.g., Goodwin et al., 1972a; Naito et al., 1999; Schofield et al., 2015). We refer to the former as acceleration, and to the latter, as amplitude of displacement.


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