Proprioceptive And Somatosensory Influences On Visual And Auditory Localization

Proprioception and somatosensory cues can also be used to create errors in sensory localization. When the hand is in contact with a target light in an otherwise dark room, if apparent motion of the arm is induced by muscle vibration, then apparent visual motion of the physically stationary target will be perceived. The subject will see the target change its apparent position even though recordings of eye position will indicate that stable fixation is maintained.16 Subjects often report that it feels as if their eyes are moving in their orbits, tracking the visual target's "motion." Vibration of an arm muscle activates its muscle-spindle receptors, eliciting a reflexive contraction. It has been known since the classic observations of Matthews and his colleagues,17 that if the motion of a limb moving under the action of a tonic vibration

FIGURE 5.5 Perceptual remapping of visual angle elicited by vibratory myesthetic illusions. Left: Small target lights (filled circles) are attached to the index fingers of both arms, which are immobilized (restraints not shown, for clarity). Vibrators are positioned over both biceps brachii muscles, and the subject stably fixates one of the targets in the otherwise dark room. Middle: Activation of the vibrators entrains spindle afferent activity, artificially signaling biceps elongation and eliciting illusory extension of both unseen forearms (arrows). The perceived distance between the two lights increases during the felt arm displacement. Right: Cutaneous contact is denied by attaching the target lights to the immobile restraint (not shown) 1 mm away from the fingers, increasing dramatically the vibration-induced felt displacements of the fingers and abolishing the illusory visual separation of the targets.

FIGURE 5.5 Perceptual remapping of visual angle elicited by vibratory myesthetic illusions. Left: Small target lights (filled circles) are attached to the index fingers of both arms, which are immobilized (restraints not shown, for clarity). Vibrators are positioned over both biceps brachii muscles, and the subject stably fixates one of the targets in the otherwise dark room. Middle: Activation of the vibrators entrains spindle afferent activity, artificially signaling biceps elongation and eliciting illusory extension of both unseen forearms (arrows). The perceived distance between the two lights increases during the felt arm displacement. Right: Cutaneous contact is denied by attaching the target lights to the immobile restraint (not shown) 1 mm away from the fingers, increasing dramatically the vibration-induced felt displacements of the fingers and abolishing the illusory visual separation of the targets.

reflex is resisted, apparent motion of that limb will be experienced in the direction that would be associated with lengthening of the vibrated muscle. Thus, biceps brachii activation leads to apparent forearm extension and triceps brachii activation to apparent flexion. The arm movement illusion has two components: a spatial displacement and a velocity one. The limb will seem to move physically to a new spatial location and then to still be moving but no longer displacing. The induced motion of a visual target attached to the hand of the vibrated arm has the same features and shows a peaking of apparent spatial displacement while still being perceived to be in continuous motion.

If a target light is attached to the index finger of each hand and the fingers are separated by a few centimeters, as illustrated in Figure 5.5, then when illusory extension of both arms is induced by vibration of both bicep muscles, the targets will be seen to increase in separation. Bilateral triceps vibration induces apparent flexion of the two arms and the targets will be seen to move closer together. This visual direction remapping occurs for both monocular and binocular viewing and eye position recordings indicate that steady fixation is maintained.18 The illusory visual effect is eliminated if the fingers are not in direct contact with the targets. These results demonstrate that proprioceptive-tactile information about visual target location can influence the apparent direction of gaze, visual direction, and the apparent physical separation of two targets (the latter, without affecting apparent distance, which means that there must be a central remapping of retinal local signs). These findings indicate that somatosensory and proprioceptive information provides, through hand contact, a mechanism for calibrating the direction of gaze and visual spatial localization.

FIGURE 5.6 Illusions experienced in darkness during biceps brachii vibration with a speaker emitting noise bursts attached to the hand. Left: The forearm and the sound source are actually immobilized (restraint not shown for clarity) in a sagittal plane but are experienced as moving down; apparent movement of the sound is consistent with the physical pattern of non-changing auditory arrival time and intensity cues at the ears. Right: The forearm and speaker are actually restrained in a horizontal plane but are experienced as moving rightward relative to the head which feels stationary, a pattern inconsistent with non-changing binaural cues.

FIGURE 5.6 Illusions experienced in darkness during biceps brachii vibration with a speaker emitting noise bursts attached to the hand. Left: The forearm and the sound source are actually immobilized (restraint not shown for clarity) in a sagittal plane but are experienced as moving down; apparent movement of the sound is consistent with the physical pattern of non-changing auditory arrival time and intensity cues at the ears. Right: The forearm and speaker are actually restrained in a horizontal plane but are experienced as moving rightward relative to the head which feels stationary, a pattern inconsistent with non-changing binaural cues.

Auditory localization can be similarly remapped.19 Figure 5.6 shows a subject seated with an auditory speaker emitting noise bursts attached to his hand. Two arm orientations are illustrated — a horizontal and a vertical one. When the biceps brachii is vibrated, illusory extension of the forearm will be experienced horizontally in one case and vertically in the other. Simultaneously, the sound source will be heard to change in position in keeping with the hand's apparent displacement, reaching a peak displacement while still seeming to be moving. The illusions can be 20 or 30 degrees or more, in magnitude. Vertical movement of a sound source in the subject's sagittal plane is consistent with non-changing arrival time and intensity cues at the ears. However, lateral physical movement of a sound source would necessarily change these cues. Since the head is movable, one might expect if lateral displacement of the hand and auditory target was experienced that a change in head orientation would also be experienced. For example, in the dark with a single target light attached to the hand, a change in eye position with respect to the head is experienced during illusory motion of the target. But this does not happen with the sound source. A remapping of auditory localization cues occurs instead, analogous with the two visual targets case, in which the retinal loci associated with particular visual directions are remapped.

5.5 SOMATOSENSORY AND PROPRIOCEPTIVE CONTRIBUTIONS TO SELF-CALIBRATION

The remappings of arm position, eye position, and auditory and visual localization, associated with muscle vibration, are transient. After the vibration is terminated or contact of the hand with the target is broken, the original localizations are re-achieved. Nevertheless, the influence of hand position on sensory and postural localization provides a mechanism by which calibration of auditory and visual maps may be achieved normally. It is clear from recent physiological work that the cortical maps devoted to vision, somatosensation, and audition are highly plastic and subject to reorganization.20-21'22'23-24 The mobility of the hand and the possibility of gaining veridical information about the external environment and objects within it by exploration with the hand underscore its flexible use as a tool for self-recalibration.

The hand can also be used for calibration of the body schema. This concept refers to the knowledge the CNS has of the dimensions and spatial characteristics of the body that allows an individual to touch accurately without visual guidance different parts of the body and that allows appropriate guidance of the body relative to the surroundings. The "Pinocchio illusion" illustrates how the hand may be involved. Figure 5.7 shows an individual, with eyes closed, grasping his nose while the biceps brachii of his arm is vibrated to create an illusion of arm extension. The subject perceives his nose to grow longer and longer in keeping with the apparent position of his hand which seems to move farther and farther from his face. With appropriate positioning of the hands on the body, broad-ranging perceptual remap-pings of body configuration, body dimensions, and body orientation can be gener-ated.25 Figure 5.8 shows an individual with arms akimbo. Bilateral vibration of the biceps brachii leads to apparent expansion of the waist as the hands seem to move farther apart. With bilateral triceps vibration, the pattern most often experienced by subjects is the waist diminishing in size. Although these effects are transient and the correct body size is experienced with cessation of vibration, they nevertheless demonstrate how veridical mappings could be generated and refreshed by hand contact under conditions of accurate hand movement control. In other words, these studies provide a fast-forward view of how the normal calibration process may be achieved.

5.6 SOMATOSENSORY CONTRIBUTIONS TO ARM MOVEMENT CONTROL

The use of the hand in calibration of the body schema raises the issue of how the calibration of the arm itself is achieved. The hand and arm change size during development and arm length affects the physical amplitude of hand displacement through space. For example, if the arm grows an additional 10 cm in length, then a 1° change in joint angle at the shoulder will produce a larger linear displacement of the hand than prior to the growth change. Normally, when the hand is moved over a stationary surface, that surface is perceived to be stationary. This means that the changing tactile input at the fingers is attributed to the motion of the hand. Katz26 recognized the significance of this fact in his classic studies of touch. Gibson27 later

FIGURE 5.7 The Pinocchio illusion. Left: The person being tested holds his nose in a pincer grip while a vibrator is positioned over the biceps brachii muscle of his right arm. Right: When the vibrator is turned on, illusory extension of the forearm is felt and the spatially fixed point of contact is also experienced as moving, resulting in apparent extension of the nose (right).

FIGURE 5.7 The Pinocchio illusion. Left: The person being tested holds his nose in a pincer grip while a vibrator is positioned over the biceps brachii muscle of his right arm. Right: When the vibrator is turned on, illusory extension of the forearm is felt and the spatially fixed point of contact is also experienced as moving, resulting in apparent extension of the nose (right).

FIGURE 5.8 Perceptual remapping of body dimensions in the vicinity of contact with an appendage in which a vibratory myesthetic illusion is induced. In the test situation, the subject holds his arms akimbo in darkness (center) during bilateral biceps (left) or triceps (right) brachii muscle vibration. Biceps vibration elicits illusory forearm extension and body expansion at the point of hand contact. Triceps vibration induces illusory forearm flexion and pinching of the waist.

FIGURE 5.8 Perceptual remapping of body dimensions in the vicinity of contact with an appendage in which a vibratory myesthetic illusion is induced. In the test situation, the subject holds his arms akimbo in darkness (center) during bilateral biceps (left) or triceps (right) brachii muscle vibration. Biceps vibration elicits illusory forearm extension and body expansion at the point of hand contact. Triceps vibration induces illusory forearm flexion and pinching of the waist.

measured the threshold for detecting the motion of a surface moving under the fingertips. He found that the stationary, restrained fingertips resting on a surface could detect motion of 1 mm/s or even less. He also had subjects move their hands back and forth laterally with their fingertips contacting a yardstick that also could be in motion. He found that the direction and approximate speed of yardstick motion could be detected although he did not provide details about the frequencies, velocities, and amplitudes of arm and yardstick movements studied. The apparatus illus-

FIGURE 5.9 Apparatus for manipulating the coupling between voluntary movements and tactile slip cues. In the situation illustrated, the surface on which the finger tips are dragging moves rightward an equal amount to the hand movement thereby eliminating slip cues. Exposure to novel couplings of slip and voluntary movement (without vision) causes immediate alterations in detection of surface motion and estimation of voluntary arm movement magnitude. Longer term exposure leads to adaptive remappings.

FIGURE 5.9 Apparatus for manipulating the coupling between voluntary movements and tactile slip cues. In the situation illustrated, the surface on which the finger tips are dragging moves rightward an equal amount to the hand movement thereby eliminating slip cues. Exposure to novel couplings of slip and voluntary movement (without vision) causes immediate alterations in detection of surface motion and estimation of voluntary arm movement magnitude. Longer term exposure leads to adaptive remappings.

trated in Figure 5.9 permits related questions to be asked. It couples motion of the contact surface with motion of the hand in the same or opposite direction by a fraction thereof. Consequently, the tactile feedback associated with voluntary movements of the hand over a surface can be systematically remapped. This allows measurement of the detection thresholds for identifying motion of the surface during conjoint hand movements.

Subjects are better able to detect surface motion when it is in the same direction as arm motion. The displacement detection threshold is about 7% for "with" displacement of the surface but 40% for "against" displacements. Thus, subjects are much more able to detect displacements of the contact surface during voluntary motion of the hand when the surface moves so as to reduce the magnitude of slip displacement across the fingertips. Surface displacement against the direction of hand motion is considerably more difficult to detect reliably. An interesting additional feature is that subjects also tend to overestimate the magnitude of their arm movements in experimental conditions involving counter-displacements of the contact surface and to underestimate them when the surface moves in the same direction as their arm movements. This pattern suggests that the slip cues at the fingertips as the hand sweeps over a surface contribute to the appreciation of the magnitude of the arm movement.

A contribution of tactile input at the fingertip to the appreciation of arm position is consistent with recent observations demonstrating that when pointing movements are made to targets on a surface, the pattern of shear force stimulation of the fingertip provides a spatial coding of hand position with respect to the body.1 The region of the fingertip stimulated also indicates far vs. near and left vs. right. The shear reaction force vectors generated on the fingertip during the first 30 msec after impact are oriented to a locus near the shoulder of the reaching arm and code the spatial position of the hand with respect to the body. Thus, when the finger touches the surface, the pattern of contact cues on the fingertip literally specifies where the finger is in relation to the body. They code hand location in an egocentric reference frame. The shear forces abate within 100 msec of the finger contacting the surface after which only a small normal force remains as the arm is actively supported with the finger in contact with the surface.

The importance of the finger impact forces for calibration of reaching movements is emphasized by two additional experimental paradigms. When a head-mounted visual display is used to present virtual targets, a subject can point to them but even if his or her finger is in spatial register with the virtual target there will be no somatosensory feedback related to the target position because there is no physical contact. The head-mounted display obstructs sight of the hand so visual feedback about hand position is also absent. In this circumstance, if the subject reaches repeatedly to the virtual target, a remarkable thing will happen. The subject's reaching movements to the target will become more and more inaccurate, showing greater and greater dispersion.28 Importantly, if a solid surface is placed at the spatial plane of the virtual targets so they coincide with its surface, then, even after one single contact with the surface, subsequent reaches will be much more accurate. If the surface is removed, movement accuracy again will rapidly degrade. This pattern means that absence of contact at the end of movements coupled with absence of sight of the hand leads to degradation of limb position sense. Contact is necessary to preserve calibration; proprioceptive information alone is not adequate.

The importance of terminal contact cues for calibration is also apparent in experiments involving Coriolis perturbations of reaching movements.29 If a reaching movement is made in a room rotating at constant velocity, an inertial force known as a Coriolis force will be generated on the arm. Its magnitude is proportional to the velocity of room rotation and the velocity of the moving arm relative to the room; consequently, it is only present during a reaching movement itself. The transient Coriolis force deflects the path of the arm in the direction opposite rotation of the room. When the room is turning at 60°/s, subjects will miss visual targets by many centimeters (see Figure 5.10 on left).

Subjects rapidly adapt to the Coriolis perturbations when allowed visual feedback of their movements. Adaptation takes less than 10 reaches, when repeated reaches are made to a single target. If subjects point to the location of a visual target that is extinguished at the onset of their reach (in the otherwise dark rotating room), they still are able to adapt within 10-15 reaches, if their hand makes contact with the surface in which the target is embedded. Terminal contact allows them to adapt fully to the Coriolis perturbations even though the surface provides no textural cues to the target's location (see Figure 5.10 on right, top). By contrast, if subjects point in the air just above the location of the just-extinguished visual target, they do not regain accurate movement endpoints and will continue to show endpoint deviations in the direction of the Coriolis forces generated by their movements (see Figure 5.10 on right, bottom). The absence of terminal contact cues prevents the sensorimotor control mechanism from registering or detecting the discrepancy between desired

FIGURE 5.10 Role of fingertip contact in adaptation of reaching movements to Coriolis force perturbations in a rotating room. Left: Forward reaching movements (varm) during counterclockwise rotation (wsrr) generate rightward Coriolis forces on the arm (FCor). Right, top: When reaches end on a smooth surface encasing the target, they are initially deviated by Coriolis forces but return to baseline endpoint and curvature patterns within 40 movements. The initial post-rotation reaches show mirror image deviations to the initial per-rotation reaches. Right, bottom: Reaches ending in the air above the target per-rotation show no adaptation to Coriolis deviations of their endpoint but return to normal curvature. Post-rotation, there is little endpoint deviation but curvature is mirror image to initial per-rotation reaches.

FIGURE 5.10 Role of fingertip contact in adaptation of reaching movements to Coriolis force perturbations in a rotating room. Left: Forward reaching movements (varm) during counterclockwise rotation (wsrr) generate rightward Coriolis forces on the arm (FCor). Right, top: When reaches end on a smooth surface encasing the target, they are initially deviated by Coriolis forces but return to baseline endpoint and curvature patterns within 40 movements. The initial post-rotation reaches show mirror image deviations to the initial per-rotation reaches. Right, bottom: Reaches ending in the air above the target per-rotation show no adaptation to Coriolis deviations of their endpoint but return to normal curvature. Post-rotation, there is little endpoint deviation but curvature is mirror image to initial per-rotation reaches.

hand position and actual hand position. Thus, full endpoint adaptation cannot be achieved.

In this experimental situation, the adaptation takes place "automatically" if terminal contact is allowed. With repeated reaches, the subject becomes progressively more accurate without awareness of what he or she is doing. Moreover, after adaptation is complete subjects no longer perceive the presence of the Coriolis forces generated by their reaching movements. Their movements seem completely natural and normal, and the Coriolis forces, although still generated during movements, seem absent. Space does not permit pursuit of this important fact and its implications for perception of the forces encountered during self-generated movements but this issue is discussed further elsewhere by Lackner and DiZio.1

These studies demonstrate the importance of somatosensory cues from the hand in the calibration and maintenance of limb position sense and in the adaptive recalibration of arm movement control. A final series of studies will be described to illustrate the contribution of haptic cues from the hand to balance.

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