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.
BackgroundAdaptation to constant stimulation has often been used to investigate the mechanisms of perceptual coding, but the adaptive processes within the proprioceptive channels that encode body movement have not been well described. We investigated them using vibration as a stimulus because vibration of muscle tendons results in a powerful illusion of movement.Methodology/Principal FindingsWe applied sustained 90 Hz vibratory stimulation to biceps brachii, an elbow flexor and induced the expected illusion of elbow extension (in 12 participants). There was clear evidence of adaptation to the movement signal both during the 6-min long vibration and on its cessation. During vibration, the strong initial illusion of extension waxed and waned, with diminishing duration of periods of illusory movement and occasional reversals in the direction of the illusion. After vibration there was an aftereffect in which the stationary elbow seemed to move into flexion. Muscle activity shows no consistent relationship with the variations in perceived movement.ConclusionWe interpret the observed effects as adaptive changes in the central mechanisms that code movement in direction-selective opponent channels.
While viewing an unambiguously rotating circular array of bars for an extended period, most perceive the array to occasionally move in the direction opposite to its true motion. We find that this alternation in perception has similar dynamics to rivalry, including little correlation among the durations of successive percepts. We also describe analogous reversals in touch and in proprioception. In the proprioceptive case, biceps vibration induces illusory forearm extension. Occasionally, although the same stimulation continues, reversals occur-flexion is perceived rather than extension. Temporal sampling is often invoked to explain the visual reversals but it cannot explain these proprioceptive reversals. Instead, after initial adaptation to the stimulus, rivalry between signals indicating the opposing directions could potentially explain reversals in all three modalities.
IntroductionOrganization of tactile input into somatotopic maps enables us to localize stimuli on the skin. Temporal relationships between stimuli are important in maintaining the maps and influence perceived locations of discrete stimuli. This points to the spatiotemporal stimulation sequences experienced as motion as a potential powerful organizing principle for spatial maps. We ask whether continuity of the motion determines perceived location of areas in the motion path using a novel tactile stimulus designed to ‘convince’ the brain that a patch of skin does not exist by rapidly skipping over it.MethodTwo brushes, fixed 9 cm apart, moved back and forth along the forearm (at 14.5 cm s−1), crossing a 10-cm long ‘occluder’, which prevented skin stimulation in the middle of the motion path. Crucially, only one brush contacted the skin at any one time, and the occluder was traversed almost instantaneously. Participants pointed with the other arm towards the felt location of the brush when it was briefly halted during repetitive motion, and also reported where they felt they had been brushed.ResultsParticipants did not report the 10-cm gap in stimulation – the motion path was perceptually completed. Pointing results showed that brush path was ‘abridged’: locations immediately on either side of the occluder, as well as location at the ends of the brush path, were perceived to be >3 cm closer to each other than in the control condition (F(1,9) = 7.19; p = .025 and F(1,9) = 6.02, p = .037 respectively). This bias increased with prolonged stimulation.ConclusionsAn illusion of completion induced by our Abridging stimulus is accompanied by gross mislocalization, suggesting that motion determines perceived locations. The effect reveals the operation of Gestalt principles in touch and suggests the existence of dynamic maps that quickly adjust to the current input pattern.
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