Effects of predictive mechanisms on head stability during forward trunk perturbation

While much is known about reflex and mechanical contributions to the control of head stability, little is known about predictive control. The goal of this experiment was to determine the contribution of predictive mechanisms to head stability in space, in the pitch plane, during forward trunk perturbations. Eleven standing healthy subjects had their trunk pulled forward by a load-pulley apparatus. The perturbation was either self-triggered or imposed (triggered by the experimenter). Subjects were exposed to two loads: 2% and 4% of their body weight. The contributions of torques acting on the head-neck system were inferred from head and trunk kinematics, neck muscle EMG, and the torques acting on the head, which were computed using inverse dynamics. The results showed that both the head and trunk moved less during the self-triggered than imposed condition during both loads for most of the participants. There was no evidence of predictive neck countertorque or increased neck muscle co-contraction during the self-triggered condition. These findings suggest that most of the subjects improved head stability in the self-triggered condition by reducing trunk motion and the associated interactive torque that perturbed the head. Electronic Publication     

Transcutaneous electrical stimulation and vibration of neck muscles in neglect

Four neglect patients without visual field defects, one with a lesion of the right basal ganglia and three with a right, predominantly parietal lesion, were examined with a cancellation and a copying task before, during and after neck muscle vibration, during transcutaneous electrical stimulation of neck muscles and during vibration of hand muscles on the left side. In all patients, neck muscle vibration improved task performance, while transcutaneous electrical stimulation and hand vibration had little or no effect. The present results demonstrate that the effect of neck muscle vibration cannot be explained as arousal and activation due to unspecific sensory stimulation on the contralesional side of the body. They rather argue for the assumption that the compensatory effect of neck muscle vibration on neglect is an effect induced by the predominant activation of afferent Ia nerve fibres and their specific contribution to the central representation of egocentric space.     

The coordination of rotations of the eyes,head and trunk in saccadic turns produced in natural situations

In real life situations large gaze saccades may involve rotations of the trunk, as well as the eyes and head. When this happens the rotation of the head-in-space is similar whether or not the trunk is also rotating. However, the rotation of the head on the trunk (i.e. the neck movement) is very different in the two circumstances. For similar head-in-space rotations to occur, the neck and trunk movements cannot simply add independently: they must be coordinated. It is argued that this is achieved via a feedback loop in which the semi-circular canals monitor the rotation of the head-in-space, and the neck is driven by an error signal representing the difference between the intended head-in-space trajectory and the actual trajectory. This mechanism, which is essentially the same as the vestibulo-collic reflex, nulls out disturbances to the head-in-space trajectory, whether these are caused by active or passive trunk rotation.     

Effects of preceding movements and contractions on the tonic vibration reflex of human finger extensor muscles

The vibration sensitivity of feline muscle spindle endings is known to vary as a result of preceding muscle conditioning manoeuvres. If similar after-effects occur in man they should be expected to influence the strength of the tonic vibration reflex (TVR). To study this issue, vibration was applied over the finger extensor tendons of 11 volunteers who actively held their fingers in a semi-extended position. The TVR, measured as electromyographic responses and angular deflections at the metacarpophalangeal joints, was found to be stronger when the extensors prior to the test had been contracted in a shortened position than when they had been contracted in a stretched position. This difference was reduced when the vibration stimulus was preceded by a strong extensor contraction in the test position. The antigravity extensor EMG activity required to keep the fingers in the test position was weaker when the conditioning contraction was performed at a short muscle length than when it was performed at a long muscle length. The variations in magnitude of the TVR can be attributed to ‘thixotropic’ properties of intrafusal muscle fibres, resulting in a slack following conditioning at a long muscle length. The different EMG levels required for position holding can be explained by similar properties of extrafusal muscle fibres. In conclusion, the results demonstrate the importance of taking the ‘history of movement’ into account in the design of all TVR studies.     

Illusory changes in head position induced by neck muscle vibration can alter the perception of elbow position

Acuity for elbow joint position sense (JPS) is reduced when head position is modified. Movement of the head is associated with biomechanical changes in the neck and shoulder musculoskeletal system, which may explain changes in elbow JPS. The present study aimed to determine whether elbow JPS is also influenced by illusory changes in head position. Simultaneous vibration of sternocleidomastoid (SCM) and the contralateral splenius was applied to 14 healthy adult human subjects. Muscle vibration or passive head rotation was introduced between presentation and reproduction of a target elbow position. Ten out of 14 subjects reported illusions consistent with lengthening of the vibrated muscles. In these 10 subjects, absolute error for elbow JPS increased with left SCM/right splenius vibration but not with right SCM/left splenius vibration. Absolute error also increased with right rotation, with a trend for increased error with left rotation. These results demonstrated that both actual and illusory changes in head position are associated with diminished acuity for elbow JPS, suggesting that the influence of head position on upper limb JPS depends, at least partially, on perceived head position.     

Feedforward activity of the cervical flexor muscles during voluntary arm movements is delayed in chronic neck pain

The objective of this study was to compare onset of deep and superficial cervical flexor muscle activity during rapid, unilateral arm movements between ten patients with chronic neck pain and 12 control subjects. Deep cervical flexor (DCF) electromyographic activity (EMG) was recorded with custom electrodes inserted via the nose and fixed by suction to the posterior mucosa of the oropharynx. Surface electrodes were placed over the sternocleidomastoid (SCM) and anterior scalene (AS) muscles. While standing, subjects flexed and extended the right arm in response to a visual stimulus. For the control group, activation of DCF, SCM and AS muscles occurred less than 50 ms after the onset of deltoid activity, which is consistent with feedforward control of the neck during arm flexion and extension. When subjects with a history of neck pain flexed the arm, the onsets of DCF and contralateral SCM and AS muscles were significantly delayed (p<0.05). It is concluded that the delay in neck muscle activity associated with movement of the arm in patients with neck pain indicates a significant deficit in the automatic feedforward control of the cervical spine. As the deep cervical muscles are fundamentally important for support of the cervical lordosis and the cervical joints, change in the feedforward response may leave the cervical spine vulnerable to reactive forces from arm movement.     

Visual and oculomotor responses induced by neck vibration in normal subjects and labyrinthine-defective patients

Three-dimensional scleral search coil eye movement recordings were obtained in five normal subjects and four patients with absent vestibular function, during unilateral vibration of the neck in the supine position. The purpose of the experiments was to investigate any role played by eye movements in the illusion that a small fixation target, viewed in an otherwise dark room, moves when vibration is applied to the neck (propriogyral illusion). Vibration was applied to the right dorsal neck muscles in three visual conditions: total darkness, fixating a light-emitting diode (LED) in an otherwise totally dark room and LED fixation in the normally lit room. Normal subjects reported that during vibration, with LED fixation in an otherwise dark room, the target appeared to move predominantly leftwards and patients reported a predominantly downward movement. Eye movements were consistently elicited in all subjects. In normal subjects there was a slow-phase eye movement predominantly to the right, interrupted by nystagmic quick phases in the opposite direction, whereas in the patients slow phases were predominantly upward with quick phases downward. Eye movements were larger in the dark but the velocity of the initial slow-phase component (<200 ms) did not change with visual conditions. Mean latencies of the eye movements were typically 80 ms but in individual trials could be as short as 40– 60 ms. The eye movements were considerably larger in the patients (e.g. mean cumulative slow-phase displacement in the dark 12° vs 2°; maximum velocity ca. 5°/s vs 1°/s). These results indicate that the propriogyral illusion is secondary to vibration-induced eye movements, presumably mediated by the cervico-ocular reflex (COR). The difference in direction of the illusion and eye movements in the patients may be related to a predominant enhancement of the vertical COR, secondary to the prominent exposure to vertical retinal slippage experienced by these patients during daily activities such as locomotion. Received: 25 November 1998 / Accepted: 14 April 1999     

Is the erect posture in microgravity based on the control of trunk orientation or center of mass position?

In the present experiments carried out in microgravity two questions were addressed. First, when the subject was instructed to adopt a vertical erect posture in microgravity with his feet fixed to the floor of the space cabin, would he control anteroposterior position with respect to the ankle joint axis of the ”vertical projection” of his center of mass (CM) or trunk axis orientation with respect to the ”vertical” (perpendicular to the floor of the space cabin)? Secondly, is CM anteroposterior position regulated during upper trunk movements in microgravity, in the absence of equilibrium constraint? Two subjects were tested in a long-term space flight. Video camera recordings were performed and analyzed off line. The results show that during erect vertical posture in microgravity, the trunk axis with respect to the ”vertical” is inclined some 7° forward. The anteroposterior position of the CM ”vertical” projection is not shifted forward, as might be expected in view of the trunk inclination, but remains close to the ankle joint axis. At the end of the upper trunk forward or backward bending movement, the final position of the vertical CM projection remains close to the ankle joint axis in microgravity. These results are interpreted as indicating that CM anteroposterior position continues to be accurately controlled in microgravity; the forward inclination of the trunk axis observed in microgravity is interpreted as being due to a misevaluation of the ”vertical” axis on the basis of biased information from proprioceptive inputs.     

Increase in corticospinal excitability of limb and trunk muscles according to maintenance of neck flexion

The effect of maintenance of neck flexion on corticospinal excitability of limb and trunk muscles was investigated using transcranial magnetic stimulation (TMS). Nine healthy young subjects participated in this experiment. Every measurement was performed with subjects sitting on a chair. Target muscles were the first dorsal interosseous (FDI), biceps brachii (BB), triceps brachii (TB), rectus abdominis (RA), erector spinae (ES), rectus femoris (RF), biceps femoris (BF), tibialis anterior (TA), and gastrocnemius (GcM) on the right side. TMS was applied to the left primary motor cortex, and motor evoked potential (MEP) was measured from the muscles listed above. Optimal stimulus location and resting motor threshold (RMT) were identified for each target muscle, and stimulus intensity used was 120% of RMT. MEPs of the target muscle were recorded with the chin resting on a chin support (chin-on condition) with neck in 20° of flexion, and with voluntary maintenance of the neck flexion posture (chin-off condition). Amplitude and latency of MEP and background activity of target muscles were analyzed. For FDI, BB, TB, ES, and RF, amplitude of MEP increased and latency shortened in the chin-off compared with the chin-on condition. No significant difference in background activity of each target muscle was found between the two conditions. Corticospinal excitability of limb and trunk muscles was selectively enhanced while neck flexion was maintained.     

双手提放前后重物时躯干肌肌电活动规律

目的观察躯干前屈和后伸运动过程提放体前和体后重物时腰腹臀肌的肌电活动规律和人体运动学特征。方法 10位正常健康的男性受试者直腿站立于试验平台内提放体前后的重物,同时监测双侧腰腹臀10块肌肉的肌电活动、躯干角运动和足底力系变化。分析肌电平均振幅、肌肉做功百分比和足底中心压力(center of pressure,COP)的位移,并进行常规的参数统计分析。结果重物在前时,背肌活动较强,并且负重后背肌做功百分比增大,腹肌活动减小;重物在后时,腹肌活动较强,背肌和腹肌的做功百分比接近相等,负重后腹肌的做功百分比增大,背肌活动减小;臀中肌负重后肌电活动增加,但做功百分比基本一致。结论躯干在前屈后伸的动态动作中,肌肉活动随负载大小、位置的不同而变化。前方负重时背肌主动,腹肌拮抗;后方负重时腹肌主动,背肌协同。臀中肌在动态任务中起的是稳定作用。     

Walking along curved paths of different angles: the relationship between head and trunk turning

Walking along a curved path requires coordinated motor actions of the entire body. Here, we investigate the relationship between head and trunk movements during walking. Previous studies have found that the head systematically turns into turns before the trunk does. This has been found to occur at a constant distance rather than at a constant time before a turn. We tested whether this anticipatory head behavior is spatially invariant for turns of different angles. Head and trunk positions and orientations were measured while participants walked around obstacles in 45°, 90°, 135° or 180° turns. The radius of the turns was either imposed or left free. We found that the head started to turn into the direction of the turn at a constant distance before the obstacle (~1.1 m) for turn angles up to 135°. During turns, the head was consistently oriented more into the direction of the turn than the trunk. This difference increased for larger turning angles and reached its maximum later in the turn for larger turns. Walking speeds decreased monotonically for increasing turn angles. Imposing fixed turn radii only affected the point at which the trunk started to turn into a turn. Our results support the view that anticipatory head movements during turns occur in order to gather advance visual information about the trajectory and potential obstacles.     

Kinematics of the freely moving head and neck in the alert cat

 In this study we examined connections between the moment-generating capacity of the neck muscles and their patterns of activation during voluntary head-tracking movements. Three cats lying prone were trained to produce sinusoidal (0.25 Hz) tracking movements of the head in the sagittal plane, and 22.5o and 45o away from the sagittal plane. Radio-opaque markers were placed in the cervical vertebrae, and intramuscular patch electrodes were implanted in five neck muscles, including biventer cervicis, complexus, splenius capitis, occipitoscapularis, and rectus capitis posterior major. Videofluoroscopic images of cervical vertebral motion and muscle electromyographic responses were simultaneously recorded. A three-dimensional biomechanical model was developed to estimate how muscle moment arms and force-generating capacities change during the head-tracking movement. Experimental results demonstrated that the head and vertebrae moved synchronously, but neither the muscle activation patterns nor vertebral movements were constant across trials. Analysis of the biomechanical model revealed that, in some cases, modification of muscle activation patterns was consistent with changes in muscle moment arms or force-generating potential. In other cases, however, changes in muscle activation patterns were observed without changes in muscle moment arms or force-generating potential. This suggests that the moment-generating potential of muscles is just one of the variables that influences which muscles the central nervous system will select to participate in a movement. Received: 27 August 1996 / Accepted: 18 December 1996     

Perception of horizontal head and trunk rotation: modification of neck input following loss of vestibular function

Chronic loss of vestibular function modifies the role of neck afferents in human perception of self-motion. We characterized this change by comparing the self-motion perception of patients with chronic vestibular loss (Ps) to that of normal subjects (Ns). Stimuli consisted of sinusoidal horizontal rotations (0.025–0.4 Hz) of the trunk relative to the head (neck stimulation) and/or of the head in space (vestibular stimulation). Perception of head rotation relative to the trunk, of trunk rotation in space, or of head rotation in space was assessed in terms of gain and phase (veridical perception, G=1 and =0°) as well as detection threshold using a pointing procedure. (1) Perception of head rotation relative to the trunk (neck proprioception). Ps' detection threshold of head-to-trunk rotation was normal (i.e. similar to that of Ns) across all frequencies tested. Also, with peak angular velocities above 5°/s, the gain of their perception was approximately normal. When peak velocity was decreased below this value, however, either by lowering stimulus frequency with peak displacement kept constant (±8°) or by decreasing peak displacement at constant frequency (0.05 Hz), the gain increased above unity, unlike in Ns. In contrast, the phase remained normal (approximately 0°). (2) Perception of trunk rotation in space. Ps perceived their trunks as stationary during neck stimulation and all vestibular-neck combinations at medium to low frequencies. At 0.4 Hz, however, Ps consistently perceived the trunk rotation, conceivably due to somatosensory selfmotion cues arising from high body acceleration. In contrast, Ns perceive a trunk-in-space rotation with the neck stimulation and most of the stimulus combinations across the whole frequency range tested. Ns perceived their trunks as stationary only during head rotation on the stationary trunk (presumed to reflect a mutual cancellation of neck and vestibular signals). (3) Perception of head rotation in space. In Ps, unlike Ns, this perception always resembled that of head rotation relative to the trunk. (4) When Ps were presented with a visual or somatosensory space reference (not motion cues), their perception of trunk and head rotation in space became approximately normal. (5) We suggest that there are basically two changes in the neck induced self motion perception associated with chronic vestibular loss. First, neck proprioception shows a non linear gain that overemphasizes low stimulus velocities, for unknown reasons. Second, the neck signal which normally is used for the perception of trunk rotation in space is suppressed (Ps in the dark, deprived of any space reference, resort to the notion that their trunks are stationary). The change in Ps' perception of head rotation in space is attributed to the former two changes (assuming that they superimposed their notion of head on trunk rotation on that of a stationary trunk).     

Response of the trunk muscles to training assessed by magnetic resonance imaging and muscle strength

Summary A group of 12 sedentary medical students (1 man and 11 women aged 21–27 years) participated in a strength training programme for the trunk muscles lasting 18 weeks. The maximal isometric flexion and extension forces of the trunk muscles were measured before the training and at 18 weeks by dynamometer. The cross-sectional area of the back muscles, i.e. erector spinae, multifidus and psoas muscles, was measured from magnetic resonance images (spin echo sequence TR/TE 1500/80, slice thickness 10 mm) obtained at the L4–L5 disc level before the training, at 11 and 18 weeks. During training, no significant change in the body mass or body fat content was found. Muscle forces or muscle cross-sectional area were not related to body mass. There was a significant increase in both trunk muscle cross-sectional area (psoas muscle P<0.001 and back muscles P<0.01) and trunk muscle forces (flexion and extension forces P<0.01) during the training but no direct association between the muscle cross-sectional area and strength of the flexors and extensors was detected before or after the training.     

Human perception of horizontal trunk and head rotation in space during vestibular and neck stimulation

Summary The vestibular signal of head motion in space must be complemented by a neck signal of the trunk-to-head excursion in order to provide the individual with information on trunk motion in space. This consideration led us to study psychophysically the role of vestibular-neck interaction for human self-motion perception. Subjects (Ss) were presented with passive horizontal rotations of their trunk and/or head (sinusoidal rotations, f=0.025 –0.4 Hz) in the dark for vestibular and neck stimulation, as well as for combinations of both. Ss' perception was evaluated in terms of gain (veridical perception of stimulus magnitude, G=1), phase, and detection threshold. (1) Perception of trunk rotation in space. During vestibular stimulation (whole-body rotation) and neck stimulation (trunk rotation with the head kept stationary) the frequency-transfer characteristics underlying this perception were very similar. The gain fell short; it was only about 0.7 at 0.4 and 0.2 Hz stimulus frequency and was further attenuated with decreasing frequency. In contrast, the phase was close to that of actual trunk position. The gain attenuation was found to be a function of the peak angular velocity of the stimulus, a fact, which we related to a velocity threshold of the order of 1 deg/s. During the various vestibular-neck combinations used, Ss' perception was again erroneous, reflecting essentially the sum of its two non-ideal constituents. However, there was one noticeable exception; during the combination head rotation on stationary trunk, Ss veridically perceived their trunk as stationary (compatible with the notion that the sum yielded zero). (2) Perception of head rotation in space. During vestibular stimulation, Ss' estimates showed the same non-ideal gain-vs.-frequency characteristics as described above for the trunk. Neck stimulation induced an illusion as if the head had been rotated in space. This neck contribution was such that, when it was combined with its vestibular counterpart during head rotation on stationary trunk, the perception became almost veridical. On closer inspection, however, this neck contribution was found to reflect the sum of two components; one was the non-ideal neck signal contributing to the perception of trunk in space, the other was an almost ideal neck signal of head-on-trunk rotation. (3) The results could be described by a simple model. In this model, the erroneous vestibular signal head in space is primarily used to create an internal representation of trunk in space. To this end, it is combined with the closely matching neck signal of trunk to head. The perception of head rotation in space is achieved by summing this trunk in space signal with the almost ideal head on trunk signal, again of nuchal origin. These seeming complex interactions have two implications: (i) the head is referred to trunk coordinates, whereas the trunk is referred to space coordinates; (ii) there is at least one condition in the dark where orientation is correct (despite an erroneous vestibular signal), i.e., during head rotation on stationary trunk.Supported by Deutsche Forschungsgemeinschaft, SFB 325     

Directionality of anticipatory activation of trunk muscles in a lifting task depends on load knowledge

We investigated to what extent subjects base anticipatory activity patterns of trunk muscles before lifting a load on knowledge of the inertial properties of the load. Eight healthy male subjects performed rapid arm lifts of a load with a varying center of mass position in the frontal plane. In one set of trials subjects were familiar with the center of mass position, in another set of trials they were not. In both cases trunk extensor muscles were active before the onset of lift force applied to the load. In the trials with load knowledge this anticipatory activity was specific with respect to center of mass position. In the absence of load knowledge left and right extensor muscles were equally active before the lift and the rate of lifting was reduced. Thus anticipatory control of trunk muscles appears specifically tuned to counteract the expected perturbation. In the absence of load knowledge trunk stiffness is increased by bilateral activity and the perturbation is attenuated since the rate of lifting is reduced. Received: 25 August 1998 / Accepted: 24 March 1999     

Role of vestibular and neck inputs for the perception of object motion in space

Summary The contribution of vestibular and neck inputs to the perception of visual object motion in space was studied in the absence of a visual background (in the dark) in normal human subjects (Ss). Measures of these contributions were obtained by means of a closed loop nulling procedure; Ss fixed their eyes on a luminous spot (object) and nulled its actual or apparent motion in space during head rotation in space (vestibular stimulus) and/ or trunk rotation relative to the head (neck stimulus) with the help of a joystick. Vestibular and neck contributions were expressed in terms of gain and phase with respect to the visuo-oculomotor/joystick feedback loop which was assumed to have almost ideal transfer characteristics. The stimuli were applied as sinusoidal rotations in the horizontal plane (f= 0.025–0.8 Hz; peak angular displacements, 1–16°). Results: (1) During vestibular stimulation, Ss perceived the object, when kept in fixed alignment with the moving body, as moving in space. However, they underestimated the object motion; the gain was only about 0.7 at 0.2–0.8 Hz and clearly decreased at lower stimulus frequencies, while the phase exhibited a small lead. (2) During pure neck stimulation (trunk rotating relative to the stationary head), the object, when stationary, appeared to move in space counter to the trunk excursion. This neck-contingent object motion illusion was small at 0.2–0.8 Hz, but increased considerably with decreasing frequency, while its phase developed a small lag. (3) Vestibular, neck, and visuo-oculomotor effects summed linearly during combined stimulations. (4) The erroneous vestibular and neck contributions to the object motion perception were complementary to each other, and the perception became about veridical (G1, 0°), when both inputs were combined during head rotation with the trunk stationary. The results are simulated by an extended version of a computer model that previously had been developed to describe vestibular and neck effects on human perception of head motion in space. In the model, the perception of object motion in space is derived from the superposition of three signals, representing object to head, (visuo-oculomotor; head coordinates), head on trunk (neck; trunk coordinates), and trunk in space (vestibular-neck interaction; space coordinates).Supported by Deutsche Forschungsgemeinschaft, SFB 325     

Extranodal lymphomas of the head and neck

Malignant lymphomas represent approximately 5% of all malignant neoplasms of the head and neck and may involve nodal or extranodal sites. Nodal head and neck lymphomas are similar to other nodal sites and are not further reviewed here. The head and neck region is the second most frequent anatomical site of extranodal lymphomas (after the gastrointestinal tract). Most are non-Hodgkin's lymphomas of B-cell lineage, and overall diffuse large B-cell lymphoma is the most common type. Hodgkin's lymphoma rarely occurs in extranodal sites. Other hematologic neoplasms that commonly involve extranodal sites of the head and neck are also discussed. In this review, we begin by discussing lymphomas involving the head and neck according to anatomical site. Then we discuss specifically the pathological findings of extranodal marginal zone lymphoma of mucosa-associated lymphoid tissue, plasmablastic lymphoma, extramedullary plasmacytoma, and extranodal natural killer/T-cell lymphoma of nasal type.     

Association of the GSTP1 and NQO1 polymorphisms and head and neck squamous cell carcinoma risk

The GSTP1 and NQO1 have been reported to be associated with an increased risk for smoking related head and neck squamous cell carcinoma (HNSCC). The purpose of this study was to determine the effect of these metabolic gene polymorphisms on the risk of HNSCC. The study population included 294 histologically confirmed HNSCC cases and 333 controls without cancer. Genotyping analysis of the GSTP1 Ile105Val and NQO1 Trp139Arg genes was performed by polymerase chain reaction-based techniques on DNA prepared from peripheral blood. The Mantel-Haenszel chi2 test was used for statistical analysis. The allele frequencies of the GSTP1 and NQO1 polymorphisms were not statistically significant between cases and controls. In analyzing the association between smoking amounts and genetic polymorphisms, GSTP1 and NQO1 polymorphisms were associated with cigarette smoking amounts in cases. G allele containing genotypes in GSTP1 and T allele containing genotypes in NQO1 were associated with a tobacco dose-dependent increase in risk of HNSCC and these genotype distributions were statistically significant (p<0.05). We found that the GSTP1 105Val allele and NQO1 139Arg allele were associated with tobacco dose-dependent increase in risk of HNSCC. GSTP1 and NQO1 genotype polymorphisms may play an important role in the development of smoking related HNSCC.     

Some sensory and motor factors influencing the control and appreciation of eye and limb position

Summary Some aspects of the manner in which the central nervous system uses sensory information for the guidance of eye and arm movements were investigated. When subjects experience apparent motion of their restrained forearm, induced by vibration of their biceps muscle in the dark, they are able to pursue with their eyes at least part of this motion and to point with their nonvibrated limb to the apparent location of the vibrated arm. The presence of a small target light on the vibrated hand limits the extent of illusory change in limb position and results in illusory motion of the target light in the same direction as the arm motion. When asked to indicate the spatial position of the light or hand, subjects still point with their nonvibrated arm to the apparent locations. Although visual pursuit of the illusory motion of the forearm can still be elicited in the presence of the target light on the hand, the subjects' eyes remain steadily fixating the stationary target light when they are instructed to track its illusory motion. These findings demonstrate that sensory and motor factors affecting the perception of visual direction and the guidance of arm and eye movements can be differentially employed at several levels of central nervous control.