Cervico-ocular coordination during neck rotation is distorted in people with whiplash-associated disorders

People with whiplash-associated disorders (WAD) not only suffer from neck/head pain, but commonly report deficits in eye movement control. Recent work has highlighted a strong relationship between eye and neck muscle activation in pain-free subjects. It is possible that WAD may disrupt the intricate coordination between eye and neck movement. Electromyographic activity (EMG) of muscles that rotate the cervical spine to the right (left sternocleidomastoid, right obliquus capitis inferior (OI), right splenius capitis (SC) and right multifidus (MF)) was recorded in nine people with chronic WAD. Cervical rotation was performed with five gaze conditions involving different gaze directions relative to cervical rotation. The relationship between eye position/movement and neck muscle activity was contrasted with previous observations from pain-free controls. Three main differences were observed in WAD. First, the superficial muscle SC was active with both directions of cervical rotation in contrast to activity only with right rotation in pain-free controls. Second, activity of OI and MF varied between directions of cervical rotation, unlike the non-direction-specific activity in controls. Third, the effect of horizontal gaze direction on neck muscle EMG was augmented compared to controls. These observations provide evidence of redistribution of activity between neck muscles during cervical rotation and increased interaction between eye and neck muscle activity in people with WAD. These changes in cervico-ocular coordination may underlie clinical symptoms reported by people with WAD that involve visual deficits and changes in function during cervical rotation such as postural control.     

Neck muscle responses to stimulation of monkey superior colliculus. I. Topography and manipulation of stimulation parameters

The role of the primate superior colliculus (SC) in orienting head movements was studied by recording electromyographic (EMG) activity from multiple neck muscles following electrical stimulation of the SC. Combining SC stimulation with neck EMG recordings provides an objective and sensitive measure of the SC drive onto neck muscle motoneurons, particularly in relation to evoked gaze shifts. In this paper, we address how neck EMG responses to SC stimulation in head-restrained monkeys depend on the rostrocaudal, mediolateral, and dorsoventral location of the stimulating electrode within the SC and vary with manipulations of the eye position prior to stimulation onset and changes in stimulation current and duration. Stimulation predominantly evoked EMG responses on the muscles obliquus capitis inferior, rectus capitis posterior major, and splenius capitis. These responses became larger in magnitude and shorter in onset latency for progressively more caudal stimulation locations, consistent with turning the head. However, evoked responses persisted even for more rostral stimulation locations usually not associated with head movements. Manipulating initial eye position revealed that the magnitude of evoked responses became stronger as the eyes attained positions contralateral to the side of stimulation, consistent with a summation between a generic command evoked by SC stimulation and the influence of eye position on tonic neck EMG. Manipulating stimulation current and duration revealed that the relationship between gaze shifts and evoked EMG responses is not obligatory: short-duration (<20 ms) or low-current stimulation evoked neck EMG responses in the absence of gaze shifts. However, long-duration stimulation (>150 ms) occasionally revealed a transient neck EMG response aligned on the onset of sequential gaze shifts. We conclude that the SC drive to neck muscle motoneurons is far more widespread than traditionally supposed and is relayed through intervening elements which may or may not be activated in association with gaze shifts.     

Modulation by eye position of neck muscle contraction evoked by electrical labyrinthine stimulation in the alert cat

Summary Modulation of vestibulo-spinal reflexes by gaze is a model system for studying interactions between voluntary and reflex motor activity. In the alert cat, the EMG of Splenius and Obliquus capitis muscles increases with ipsilateral gaze eccentricity during spontaneous eye movements. Labyrinth stimulation by current pulses evokes EMGs with latencies consistent with a three neuron vestibulocollic pathway. The amplitude of evoked activity increases with eye position. The directions in which eye movements increase EMG was usually the same for both spontaneous and induced EMG activity, namely, horizontal and ipsilateral. However, sometimes the increase in spontaneous EMG occurred with horizontal eye position, whereas the induced EMG changed with vertical eye position. Spontaneous and evoked EMG are then modulated by different eye position signals. Command signals reflecting eye position probably reach two different types of neurons in the vestibulo-collic pathway, most likely secondary vestibular neurons and neck muscle motoneurons.V.J. Wilson's participation was made possible in part by NIH grant NS02619 and in part by a fellowship from I. N. S. E. R. M.     

Neck muscle synergies during stimulation and inactivation of the interstitial nucleus of Cajal (INC)

The interstitial nucleus of Cajal (INC) is thought to control torsional and vertical head posture. Unilateral microstimulation of the INC evokes torsional head rotation to positions that are maintained until stimulation offset. Unilateral INC inactivation evokes head position-holding deficits with the head tilted in the opposite direction. However, the underlying muscle synergies for these opposite behavioral effects are unknown. Here, we examined neck muscle activity in head-unrestrained monkeys before and during stimulation (50 muA, 200 ms, 300 Hz) and inactivation (injection of 0.3 mul of 0.05% muscimol) of the same INC sites. Three-dimensional eye and head movements were recorded simultaneously with electromyographic (EMG) activity in six bilateral neck muscles: sternocleidomastoid (SCM), splenius capitis (SP), rectus capitis posterior major (RCPmaj.), occipital capitis inferior (OCI), complexus (COM), and biventer cervicis (BC). INC stimulation evoked a phasic, short-latency ( approximately 5-10 ms) facilitation and later ( approximately 100-200 ms) a more tonic facilitation in the activity of ipsi-SCM, ipsi-SP, ipsi-COM, ipsi-BC, contra-RCPmaj., and contra-OCI. Unilateral INC inactivation led to an increase in the activity of contra-SCM, ipsi-SP, ipsi-RCPmaj., and ipsi-OCI and a decrease in the activity of contra-RCPmaj. and contra-OCI. Thus the influence of INC stimulation and inactivation were opposite on some muscles (i.e., contra-OCI and contra-RCPmaj.), but the comparative influences on other neck muscles were more variable. These results show that the relationship between the neck muscle responses during INC stimulation and inactivation is much more complex than the relationship between the overt behaviors.     

Effect of neck posture on patterns of activation of feline neck muscles during horizontal rotation

The electromyographic (EMG) patterns of neck muscles were recorded during whole-body horizontal rotation in head-free, alert cats and head-restrained, decerebrate cats. In some trials the cervical column of the animal was oriented vertically, whereas in others it was oriented more horizontally. In alert cats making head movements that compensated for the motion of the platform, neck muscles with modulated patterns of activity could be divided into a subset whose individual EMG patterns changed significantly when the neck posture was altered (including longissimus capitis, obliquus capitis superior and scalenus anterior) and a subset whose individual EMG patterns were invariant regardless of neck posture (including obliquus capitis inferior, levator scapulae and complexus). In head-restrained, decerebrate cats, electromyograms from all implanted muscles were modulated similarly in phase with the platform position. Changing the orientation of the neck had little effect upon these EMG patterns evoked by the horizontal vestibulocollic reflex. One decerebrate cat with strong extensor tone was tested further under head-free conditions. There was very little compensatory head movement, but individual neck muscles displayed patterns of activity that were more similar to those observed in alert, head-free animals.     

Eye position modulates the electromyographic responses of neck muscles to electrical stimulation of the superior colliculus in the alert cat

Rapid gaze shifts are often accomplished with coordinated movements of the eyes and head, the relative amplitude of which depends on the starting position of the eyes. The size of gaze shifts is determined by the superior colliculus (SC) but additional processing in the lower brain stem is needed to determine the relative contributions of eye and head components. Models of eye–head coordination often assume that the strength of the command sent to the head controllers is modified by a signal indicative of the eye position. Evidence in favor of this hypothesis has been recently obtained in a study of phasic electromyographic (EMG) responses to stimulation of the SC in head-restrained monkeys (Corneil et al. in J Neurophysiol 88:2000–2018, 2002b). Bearing in mind that the patterns of eye–head coordination are not the same in all species and because the eye position sensitivity of phasic EMG responses has not been systematically investigated in cats, in the present study we used cats to address this issue. We stimulated electrically the intermediate and deep layers of the caudal SC in alert cats and recorded the EMG responses of neck muscles with horizontal and vertical pulling directions. Our data demonstrate that phasic, short latency EMG responses can be modulated by the eye position such that they increase as the eye occupies more and more eccentric positions in the pulling direction of the muscle tested. However, the influence of the eye position is rather modest, typically accounting for only 10–50% of the variance of EMG response amplitude. Responses evoked from several SC sites were not modulated by the eye position.     

Widespread presaccadic recruitment of neck muscles by stimulation of the primate frontal eye fields

We studied the role of the primate frontal eye fields (FEFs) in eye-head gaze shifts by recording EMG activity from multiple dorsal neck muscles after electrical stimulation of a broad distribution of sites throughout FEF. We assess our results in light of four mechanisms forwarded to account for why eye and head movements follow FEF stimulation. Two mechanisms propose that movements are generated indirectly by FEF stimulation in response to either a percept or an eccentric orbital position. Two other mechanisms propose that movements are evoked directly through the issuance of either a gaze command or separate eye and head commands. FEF stimulation evoked short-latency ( approximately 20 ms) neck EMG responses from the vast majority (>95%) of stimulation sites. Evoked responses usually preceded the gaze shift by approximately 20 ms, even for small gaze shifts (<10 degrees ) not typically associated with head motion. Evoked responses began earlier and attained a larger magnitude when accompanied by larger gaze shifts and took a form consistent with the recruitment of the appropriately directed head movements to accompany the evoked gaze shift. We also observed robust neck EMG even when stimulation failed to evoke a gaze shift and occasionally observed head-only movements when the head was unrestrained. These results resemble neck EMG evoked from the superior colliculus (SC). Neck EMG response latencies approached the minimal conduction time to the motor periphery and hence are not consistent with either of the indirect mechanisms. The widespread nature of the cephalomotor drive from the FEF, the scaling of neck EMG responses with gaze magnitude, and the consistently earlier generation of the EMG versus gaze response are difficult to reconcile with suggestions that separate FEF channels encode eye and head motion independently. The most parsimonious interpretation is that a gaze command issued by the FEF is decomposed into eye and head commands downstream of the SC. The relative timing of the neck EMG and gaze shift responses, and the presence of neck EMG responses on trials without gaze shifts, implies that head premotor elements are not subjected to the same brain stem control mechanisms governing gaze shifts.     

Three-dimensional spatial tuning of neck muscle activation in humans

The complex structure of the neck musculoskeletal system poses challenges to understanding central nervous system (CNS) control strategies. Examining muscle activation patterns in relation to musculoskeletal geometry and three-dimensional mechanics may reveal organizing principles. We analyzed the spatial tuning of neck muscle electromyographic (EMG) activity while subjects generated moments in three dimensions. EMG tuning curves were characterized by their orientation (mean direction) and focus (spread of activity). For the four muscles that were studied (sternocleidomastoid, splenius capitis, semispinalis capitis and trapezius), EMG tuning curves exhibited directional preference, with consistent orientation and focus among 12 subjects. However, the directional preference (orientation) of three of the four neck muscles did not correspond to the muscle's moment arm, indicating that maximizing a muscle's mechanical advantage is not the only factor in determining muscle activation. The focus of muscle tuning did not change with moment magnitude, demonstrating that co-contraction did not increase with load. Axial rotation was found to have a strong influence on neck muscle spatial tuning. The uniform results among subjects indicate that the CNS has consistent strategies for selecting neck muscle activations to generate moments in specific directions; however, these strategies depend on three-dimensional mechanics in a complex manner. Electronic Publication     

Feedforward contraction of transversus abdominis is not influenced by the direction of arm movement

 Because the structure of the spine is inherently unstable, muscle activation is essential for the maintenance of trunk posture and intervertebral control when the limbs are moved. To investigate how the central nervous system deals with this situation the temporal components of the response of the muscles of the trunk were evaluated during rapid limb movement performed in response to a visual stimulus. Fine-wire electromyography (EMG) electrodes were inserted into transversus abdominis (TrA), obliquus internus abdominis (OI) and obliquus externus abdominis (OE) of 15 subjects under the guidance of real-time ultrasound imaging. Surface electrodes were placed over rectus abdominis (RA), lumbar multifidus (MF) and the three parts of deltoid. In a standing position, ten repetitions of shoulder flexion, abduction and extension were performed by the subjects as fast as possible in response to a visual stimulus. The onset of TrA EMG occurred in advance of deltoid irrespective of the movement direction. The time to onset of EMG activity of OI, OE, RA and MF varied with the movement direction, being activated earliest when the prime action of the muscle opposed the reactive forces associated with the specific limb movement. It is postulated that the non-direction-specific contraction of TrA may be related to the control of trunk stability independent of the requirement for direction-specific control of the centre of gravity in relation to the base of support. Received: 29 September 1995 / Accepted: 30 September 1996     

Neck muscle responses to stimulation of monkey superior colliculus. II. Gaze shift initiation and volitional head movements

We report neck muscle activity and head movements evoked by electrical stimulation of the superior colliculus (SC) in head-unrestrained monkeys. Recording neck electromyography (EMG) circumvents complications arising from the head's inertia and the kinetics of muscle force generation and allows precise assessment of the neuromuscular drive to the head plant. This study served two main purposes. First, we sought to test the predictions made in the companion paper of a parallel drive from the SC onto neck muscles. Low-current, long-duration stimulation evoked both neck EMG responses and head movements either without or prior to gaze shifts, testifying to a SC drive to neck muscles that is independent of gaze-shift initiation. However, gaze-shift initiation was linked to a transient additional EMG response and head acceleration, confirming the presence of a SC drive to neck muscles that is dependent on gaze-shift initiation. We forward a conceptual neural architecture and suggest that this parallel drive provides the oculomotor system with the flexibility to orient the eyes and head independently or together, depending on the behavioral context. Second, we compared the EMG responses evoked by SC stimulation to those that accompanied volitional head movements. We found characteristic features in the underlying pattern of evoked neck EMG that were not observed during volitional head movements in spite of the seemingly natural kinematics of evoked head movements. These features included reciprocal patterning of EMG activity on the agonist and antagonist muscles during stimulation, a poststimulation increase in the activity of antagonist muscles, and synchronously evoked responses on agonist and antagonist muscles regardless of initial horizontal head position. These results demonstrate that the electrically evoked SC drive to the head cannot be considered as a neural replicate of the SC drive during volitional head movements and place important new constraints on the interpretation of electrically evoked head movements.     

Short-term changes in neck muscle and eye movement responses following unilateral vestibular neurectomy in the cat

The purpose of this study was to investigate changes in neck muscle and eye movement responses during the early stages of vestibular compensation (first 3 weeks after unilateral vestibular neurectomy, UVN). Electromyographic (EMG) activity from antagonist neck extensor (splenius capitis) and flexor (longus capitis) muscles and eye movements were recorded during sinusoidal visual and/or otolith vertical linear stimulations in the 0.05–1 Hz frequency range (corresponding acceleration range 0.003–1.16 g) in the head-fixed alert cat. Preoperative EMG activity from the splenius and longus capitis muscles showed a pattern of alternate activation of the antagonist neck muscles in all the cats. After UVN, two motor strategies were observed. For three of the seven cats, the temporal activation of the individual neck muscles was the same as that recorded before UVN. For the other four cats, UVN resulted in a pattern of coactivation of the flexor and extensor neck muscles because of a phase change of the splenius capitis. In both subgroups, the response patterns of the antagonist neck muscles were consistent for each cat independently of the experimental conditions, throughout the 3 weeks of testing. Cats displaying alternate activation of antagonist neck muscles showed an enhanced gain of the visually induced neck responses, particularly in the high range of stimulus frequency, and a gain decrease in the otolith-induced neck responses at the lowest frequency (0.25 Hz) only. By contrast, for cats with neck muscle coactivation, the gain of the visually induced neck responses was basically unaffected relative to preoperative values, whereas otolith-induced neck responses were considerably decreased in the whole range of stimulation. As concerns oculomotor responses, results in the two subgroups of cats were similar. The optokinetic responses were not affected by the vestibular lesion. On the contrary, otolith-induced eye responses showed a gain reduction and a phase lead. Deficits and short-term changes after UVN of otolith- and semicircular canal-evoked collic and ocular responses are compared. Received: 15 April 1997 / Accepted: 29 December 1997     

Eye and neck motor signals in periabducens reticular neurons of the alert cat

The purpose of this work was to search for neurons in the ponto-medullary reticular formation which can carry horizontal eye position signals to dorsal neck muscles of the cat. The recordings were localized in an area of the ponto-medullary reticular formation which contains reticulo-spinal neurons projecting to the neck (Peterson et al. 1980). Experiments were performed on alert, head-fixed cats. Eye movements were measured by the search-coil technique. Neuronal activity was recorded extracellularly with glass microelectrodes. Neurons were localized both by stereotaxic coordinates and by their position with respect to the antidromic field potential profile of the abducens nucleus. EMG of longissimus capitis, obliquus capitis and splenius muscles were recorded. Vestibular stimulation was produced by a turn-table. Reticular cells were found below the abducens nucleus (1 to 3.5 mm below the center of the nucleus, A.P.:5.3 to 7.2 mm;L: 1 to 1.5 mm) showing a firing rate closely related to EMG during spontaneous saccades and, to a variable degree, to the ipsilateral horizontal component of eye position. 'Tonic' and 'burst-tonic' cells were found. During vestibular stimulation, the firing rate kept the same relationship with eye position and neck EMG. It is concluded that the analyzed reticular cells are good candidates to be reticulo-spinal neurons which mediate a signal allowing a synergistic movement of head and eye during orientation in the horizontal plane.     

EMG power spectra of cervical muscles in lateral flexion and comparison with sagittal and oblique plane activities

The purpose of this study was to calculate electromyographic (EMG) power spectra of the sternocleidomastoid (SCM), the splenius capitis (SPL) and the trapezius (TRP) muscles from both sides of the body in bilateral lateral flexion and compare them with the EMG spectral characteristics of the same muscles in the sagittal and oblique planes recorded from the same subjects in the same experimental session but reported elsewhere in the literature. Forty normal and healthy young adults (21 males, 19 females) volunteered for the study. The subjects were prepared and positioned appropriately to exert linearly ramping isometric exertion aided by visual feedback against a rigid and load cell instrumental device. Exertions were carried out in a random order. The maximal voluntary contraction was reached within a 5-s test period. The torque and EMG from the SCM, SPL and TRP muscles were sampled bilaterally at a rate of 1 kHz. The EMG data were subjected to fast Fourier transform analysis. During lateral flexion, the torque generated by females was significantly lower than males (P<0.01) and represented at 75% of male torque. Contrary to flexion/extension and oblique plane activities, the ipsilateral SCM was most active with highest power and up to four dominant frequencies demonstrating four distinct peaks. The bandwidth of this muscle progressively increased with the grade of contraction up to 400 Hz, similar to flexion/extension with median frequency (MF) spread between 21 and 109 Hz for males and 78 and 99 Hz for females. The ipsilateral SPL was the second most active muscle, which maintained a narrow bandwidth (200 Hz) and one dominant frequency, rising in power with increasing grade of contraction. Flexion and extension caused symmetrical activity bilaterally. The results of MANOVA revealed a significant main effect of gender, activity direction, grade of contraction and individual muscles (P<0.001). There was a significant interaction between muscle and activity direction (P<0.001) implying significantly different MF and mean power frequency if the direction of exertion was changed. Male and female subjects responded to direction differently (P<0.001). Electronic Publication     

Patterns of neck muscle activation in cats during reflex and voluntary head movements

Summary When the head rotates, vestibulocollic reflexes counteract the rotation by causing contraction of the neck muscles that pull against the imposed motion. With voluntary head rotations, these same muscles contract and assist the movement of the head. The purpose of this study was to determine if an infinite variety of muscle activation patterns are available to generate a particular head movement, or if the CNS selects a consistent and unique muscle pattern for the same head movement whether performed in a voluntary or reflex mode. The relationship of neck muscle activity to reflex and voluntary head movements was examined by recording intramuscular EMG activity from six neck muscles in three alert cats during sinusoidal head rotations about 24 vertical and horizontal axes. The cats were trained to voluntarily follow a water spout with their heads. Vestibulocollic reflex (VCR) responses were recorded in the same cats by rotating them in an equivalent set of planes with the head stabilized to the trunk so that only the vestibular labyrinths were stimulated. Gain and phase of the EMG responses were calculated, and data analyzed to determine the directions of rotation for which specific muscles produced their greatest EMG output. Each muscle exhibited preferential activation for a unique direction of rotation, and weak responses during rotations orthogonal to that preferred direction. The direction of maximal activation could differ for reflex and voluntary responses. Also, the best excitation of the muscle was not always in the direction that would produce a maximum mechanical advantage for the muscle based on its line of pull. The results of this study suggest that a unique pattern of activity is selected for VCR and tracking responses in any one animal. Patterns for the two behaviors differ, indicating that the CNS can generate movements in the same direction using different muscle patterns.     

Interaction between vestibulo-spinal and corticospinal systems: a combined caloric and transcranial magnetic stimulation study

We investigated the interaction between vestibular and corticospinal stimuli in 8 healthy volunteers. Vestibular stimulation was induced with unilateral ear caloric irrigation (30°C) with subjects supine. Single transcranial magnetic stimulation (TMS) pulses were delivered (double-cone coil, intensities 60–75% maximal output) every 10–20 s during vestibular activation and during baseline. Bilateral surface electromyography (EMG) from splenius capitis, sternocleidomastoid (SCM), obliquus externus abdominis, vastus lateralis, biceps femoris (BF), tibialis anterior and peroneus longus was obtained. During whole-body maximal rotatory voluntary isometric contraction (MRVC), only SCM and BF displayed EMG activation/inhibition patterns indicating axial rotatory action. TMS-induced motor evoked potentials (MEPs) after caloric irrigation revealed that only SCM showed consistent vestibular-mediated excitation/inhibition responses, i.e. an increase in MEP area contralateral to the irrigation and a decrease in MEP area ipsilaterally (+12.7 and −6.3% of the MRVC, respectively). A putative head turn induced by this SCM activity pattern would be in the same direction of the slow-phase eye movement. EMG in the 100 ms preceding TMS showed muscle tone values of approximately 10% of MRVC. After caloric irrigation, these values increased by ca. 2% for all muscles bilaterally and hence cannot explain the direction-specific SCM MEP changes. Thus, SCM MEPs show caloric-induced amplitude modulation indicating that SCM is under both horizontal semicircular canal and corticospinal control. This vestibular modulation of corticospinal SCM control likely occurs at cortical levels. The direction of the MEP modulation indicates a directional coupling between vestibularly induced head and eye movements.     

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.     

Neck muscle activity after unilateral labyrinthectomy in the alert guinea pig

 In the guinea pig, lateral deviation of the head is a cardinal symptom of the vestibular syndrome caused by unilateral labyrinthectomy. In the course of recovery from this syndrome (vestibular compensation), lateral deviation of the head disappears completely in 2–3 days. Because this symptom is known to be due to the lesion of the horizontal semicircular canal system, and since obliquus capitis inferior (OCI) muscle is activated predominantly by yaw rotation (horizontal vestibulocollic reflex), we hypothesized that changes in the activity of this muscle could be at least in part responsible for the lateral head deviation caused by unilateral labyrinthectomy. In order to test this hypothesis, electromyographic (EMG) activities of the right and left OCI muscles, as well as eye movements, were recorded in 12 head-fixed alert guinea pigs at various times after left surgical labyrinthectomy (performed with the animals under halothane anesthesia). After the operation, a decrease in tonic EMG activity was observed in the right (contralateral to the lesion) OCI muscle while an increase in tonic EMG activity was detected in the left (ipsilateral) OCI muscle. In addition, phasic changes in EMG activity associated with ocular nystagmic beats occurred in the OCI muscles. These phasic changes were in the opposite direction to those of the tonic changes. There were bursts of activity in the right OCI and pauses in the left OCI. From measurements of rectified averaged EMG activities which took into account both parts (tonic and phasic) of the phenomenon, it was concluded that the labyrinthectomy-induced asymmetry between the activities of the left and right OCI muscles was high enough and lasted long enough to be an important mechanism in the lateral deviation of the head caused by unilateral labyrinthectomy.     

Neural control of superficial and deep neck muscles in humans

Human neck muscles have a complex multi-layered architecture. The role and neural control of these neck muscles were examined in nine seated subjects performing three series of isometric neck muscle contractions: 50-N contractions in eight fixed horizontal directions, 25-N contractions, and 50-N contractions, both with a continuously changing horizontal force direction. Activity in the left sternocleidomastoid, trapezius, levator scapulae, splenius capitis, semispinalis capitis, semispinalis cervicis, and multifidus muscles was measured with wire electrodes inserted at the C(4)/C(5) level under ultrasound guidance. We hypothesized that deep and superficial neck muscles would function as postural and focal muscles, respectively, and would thus be controlled by different neural signals. To test these hypotheses, electromyographic (EMG) tuning curves and correlations in the temporal and frequency domains were computed. Three main results emerged from these analyses: EMG tuning curves from all muscles exhibited well-defined preferred directions of activation for the 50-N isometric forces, larger contractions (25 vs. 50 N) yielded more focused EMG tuning curves, and agonist neck muscles from all layers received a common neural drive in the range of 10-15 Hz. The current results demonstrate that all neck muscles can exhibit phasic activity during isometric neck muscle contractions. Similar oscillations in the EMG of neck muscles from different layers further suggest that neck motoneurons were activated by common neurons. The reticular formation appears a likely generator of the common drive to the neck motoneurons due to its widespread projections to different groups of neck motoneurons.     

Cross-validated models of the relationships between neck muscle electromyography and three-dimensional head kinematics during gaze behavior

The object of this study was to model the relationship between neck electromyography (EMG) and three-dimensional (3-D) head kinematics during gaze behavior. In two monkeys, we recorded 3-D gaze, head orientation, and bilateral EMG activity in the sternocleidomastoid, splenius capitis, complexus, biventer cervicis, rectus capitis posterior major, and occipital capitis inferior muscles. Head-unrestrained animals fixated and made gaze saccades between targets within a 60° × 60° grid. We performed a stepwise regression in which polynomial model terms were retained/rejected based on their tendency to increase/decrease a cross-validation-based measure of model generalizability. This revealed several results that could not have been predicted from knowledge of musculoskeletal anatomy. During head holding, EMG activity in most muscles was related to horizontal head orientation, whereas fewer muscles correlated to vertical head orientation and none to small random variations in head torsion. A fourth-order polynomial model, with horizontal head orientation as the only independent variable, generalized nearly as well as higher order models. For head movements, we added time-varying linear and nonlinear perturbations in velocity and acceleration to the previously derived static (head holding) models. The static models still explained most of the EMG variance, but the additional motion terms, which included horizontal, vertical, and torsional contributions, significantly improved the results. Several coordinate systems were used for both static and dynamic analyses, with Fick coordinates showing a marginal (nonsignificant) advantage. Thus, during gaze fixations, recruitment within the neck muscles from which we recorded contributed primarily to position-dependent horizontal orientation terms in our data set, with more complex multidimensional contributions emerging during the head movements that accompany gaze shifts. These are crucial components of the late neuromuscular transformations in a complete model of 3-D head-neck system and should help constrain the study of premotor signals for head control during gaze behaviors.     

Stimulation of the superior colliculus in the alert cat

Summary Electrical stimulation of the cat superior colliculus (SC), in conjunction with the accurate measurement of elicited eye movements and histologically verified electrode positions, has revealed a striking antero-posterior variation in collicular organization. Three zones could be defined in the SC on the basis of eye movement patterns and associated neck muscle EMG activity evoked from the deeper layers. The Anterior zone was coextensive with the central 25 ° of the visual retinotopically coded map contained in the superficial layers. Saccades evoked from this zone were also retinotopically coded, and the latency of EMG activity depended on the position of the eye in the orbit. A similar observation applies to the entire monkey SC. The Intermediate zone was coextensive with the 25 °–70 ° of visual projections. Saccades evoked from this region were goal-directed and were associated with invariant, short latency EMG responses. The Posterior zone was found in the extreme caudo-lateral portion of the SC. Eye movements evoked from this zone were centering saccades associated with constant latency EMG activity. The present results in conjunction with previously demonstrated antero-posterior variations in projections to the SC, suggest that the motor strategies controlling gaze shifts toward visual targets vary depending on the location of the target in the visual field.