(49/868) Abnormal interaction between vestibular and voluntary head control in patients with spasmodic torticollis.
The functional status of vestibulo-collic reflexes in the sternocleidomastoid (SCM) muscles was investigated in 24 patients with spasmodic torticollis using small, abrupt 'drops' of the head. None had been treated with botulinum toxin injections during at least 4 months preceding the study. Eight of the patients, four of whom had been studied before surgery, were also studied after selective peripheral denervation of neck muscles. The reflex was of normal latency and duration in the 'passive drop' condition, in which subjects were instructed not to oppose the fall of the head. To study voluntary interaction with the reflex response, subjects were then asked to flex the neck as quickly as possible after onset of the head drop ('active drop'). In this condition, voluntary responses in patients were delayed, smaller and less effective in counteracting the head fall than in normal subjects. The same abnormalities were also found in patients after surgery when the head posture was improved. Somatosensory/auditory voluntary reaction times in SCM were normal, as was the latency of the startle reflex. We conclude that voluntary interaction with the vestibulo-collic reflex is disrupted in patients with spasmodic torticollis, a finding which corroborates the patients' aggravation of their symptoms by head or body perturbations. Lack of effective interaction between two major systems controlling head position may contribute to torticollis. (+info)
(50/868) Adaptive changes of saccadic eye-head coordination resulting from altered head posture in torticollis spasmodicus.
We asked whether and how the abnormal head posture in torticollis patients affects saccadic gaze shifts and impairs the associated head movements. We wanted to learn to what extent observed changes directly result from the disease or reflect compensatory mechanisms, secondary to the altered head posture. We compared the results of patients with those of normal subjects. When patients viewed a centric target, their heads were a priori deviated in the direction of the torticollis, with orbital eye position showing a compensatory offset in the opposite direction. These abnormal eye and head positions were re-established when patients returned from an eccentric gaze position by means of a centripetal gaze shift, independently of its direction and magnitude, unlike in normal subjects who always recentred eyes and head. In normal subjects the share of the head in the total gaze shift amounted to about 70%, whereas in patients it contributed only 30%, necessitating correspondingly larger orbital eye displacements and eccentricities. Moreover, patients' head movements were asymmetric; they were larger when gaze was shifted into, or returned from the hemifield contralateral to the torticollis direction compared with gaze shifts in the ipsilateral hemifield. The eyes displayed a reversed asymmetry. Patients showed a significant increase in gaze latency and head versus eye delay as well as in the number of corrective saccades. However, head velocity was normal in four out of seven patients. Moreover, all patients made normal eye saccades (peak velocity, duration, gaze error), except for the increase in latency, which also occurred when gaze was shifted without head movements. Thus, patients' saccadic eye-head coordination showed abnormalities which mainly concerned the involved head movements. We suggest that the observed changes do not reflect a direct involvement of the disease upon the gaze shift mechanism, but can be interpreted as adaptive changes that compensate for the altered head posture. We formalized this view in the form of a dynamic model. (+info)
(51/868) Eye position influences auditory responses in primate inferior colliculus.
We examined the frame of reference of auditory responses in the inferior colliculus in monkeys fixating visual stimuli at different locations. Eye position modulated the level of auditory responses in 33% of the neurons we encountered, but it did not appear to shift their spatial tuning. The effect of eye position on auditory responses was substantial-comparable in magnitude to that of sound location. The eye position signal appeared to interact with the auditory responses in at least a partly multiplicative fashion. We conclude that the representation of sound location in primate IC is distributed and that the frame of reference is intermediate between head- and eye-centered coordinates. The information contained in these neurons appears to be sufficient for later neural stages to calculate the positions of sounds with respect to the eyes. (+info)
(52/868) Selective processing of vestibular reafference during self-generated head motion.
The vestibular sensory apparatus and associated vestibular nuclei are generally thought to encode head-in-space motion. Angular head-in-space velocity is detected by vestibular hair cells that are located within the semicircular canals of the inner ear. In turn, the afferent fibers of the vestibular nerve project to neurons in the vestibular nuclei, which, in head-restrained animals, similarly encode head-in-space velocity during passive whole-body rotation. However, during the active head-on-body movements made to generate orienting gaze shifts, neurons in the vestibular nuclei do not reliably encode head-in-space motion. The mechanism that underlies this differential processing of vestibular information is not known. To address this issue, we studied vestibular nuclei neural responses during passive head rotations and during a variety of tasks in which alert rhesus monkeys voluntarily moved their heads relative to space. Neurons similarly encoded head-in-space velocity during passive rotations of the head relative to the body and during passive rotations of the head and body together in space. During all movements that were generated by activation of the neck musculature (voluntary head-on-body movements), neurons were poorly modulated. In contrast, during a task in which each monkey actively "drove" its head and body together in space by rotating a steering wheel with its arm, neurons reliably encoded head-in-space motion. Our results suggest that, during active head-on-body motion, an efferent copy of the neck motor command, rather than the monkey's knowledge of its self-generated head-in-space motion or neck proprioceptive information, gates the differential processing of vestibular information at the level of the vestibular nuclei. (+info)
(53/868) Visual influences on the development and recovery of the vestibuloocular reflex in the chicken.
Whenever the head turns, the vestibuloocular reflex (VOR) produces compensatory eye movements to help stabilize the image of the visual world on the retina. Uncompensated slip of the visual world across the retina results in a gradual change in VOR gain to minimize the image motion. VOR gain changes naturally during normal development and during recovery from neuronal damage. We ask here whether visual slip is necessary for the development of the chicken VOR (as in other species) and whether it is required for the recovery of the VOR after hair cell loss and regeneration. In the first experiment, chickens were reared under stroboscopic illumination, which eliminated visual slip. The horizontal and vertical VORs (h- and vVORs) were measured at different ages and compared with those of chickens reared in normal light. Strobe-rearing prevented the normal development of both h- and vVORs. After 8 wk of strobe-rearing, 3 days of exposure to normal light caused the VORs to recover partially but not to normal values. In the second experiment, 1-wk-old chicks were treated with streptomycin, which destroys most vestibular hair cells and reduces hVOR gain to zero. In birds, vestibular hair cells regenerate so that after 8 wk in normal illumination they appear normal and hVOR gain returns to values that are normal for birds of that age. The treated birds in this study recovered in either normal or stroboscopic illumination. Their hVOR and vVOR and vestibulocollic reflexes (VCR) were measured and compared with those of untreated, age-matched controls at 8 wk posthatch, when hair cell regeneration is known to be complete. As in previous studies, the gain of the VOR decreased immediately to zero after streptomycin treatment. After 8 wk of recovery under normal light, the hVOR was normal, but vVOR gain was less than normal. After 8 wk of recovery under stroboscopic illumination, hVOR gain was less than normal at all frequencies. VCR recovery was not affected by the strobe environment. When streptomycin-treated, strobe-recovered birds were then placed in normal light for 2 days, hVOR gain returned to normal. Taken together, the results of these experiments suggest that continuous visual feedback can adjust VOR gain. In the absence of appropriate visual stimuli, however, there is a default VOR gain and phase to which birds recover or revert, regardless of age. Thus an 8-wk-old chicken raised in a strobe environment from hatch would have the same gain as a streptomycin-treated chicken that recovers in a strobe environment. (+info)
(54/868) Interference of propylene glycol with the hole-board test.
Experimental drugs and/or plant extracts are often dissolved in solvents, including propylene glycol. Nevertheless, there is evidence for psychoactive properties of this alcohol. In this study we found that in the hole-board test 10% propylene glycol did not modify the head-dipping behavior. However, 30% propylene glycol induced an increase in the number of head-dips (46.92 +/- 2.37 compared to 33.83 +/- 4.39, P<0.05, ANOVA/Student-Newman-Keuls), an effect comparable to that obtained with 0.5 mg/kg diazepam (from 33.83 +/- 4.39 to 54 +/- 3.8, P<0.01, ANOVA/Student-Newman-Keuls). These results demonstrate that 30% propylene glycol has significant anxiolytic effects in this model and therefore cannot be used as an innocuous solvent. (+info)
(55/868) A spatial hearing deficit in early-blind humans.
An important issue in neuroscience is the effect of visual loss on the remaining senses. Two opposing views have been advanced. On the one hand, visual loss may lead to compensatory plasticity and sharpening of the remaining senses. On the other hand, early blindness may also prevent remaining sensory modalities from a full development. In the case of sound localization, it has been reported recently that, under certain conditions, early-blind humans can localize sounds better than sighted controls. However, these studies were confined to a single sound source in the horizontal plane. This study compares sound localization of early-blind and sighted subjects in both the horizontal and vertical domain, whereas background noise was added to test more complex hearing conditions. The data show that for high signal-to-noise (S/N) ratios, localization by blind and sighted subjects is similar for both azimuth and elevation. At decreasing S/N ratios, the accuracy of the elevation response components deteriorated earlier than the accuracy of the azimuth component in both subject groups. However, although azimuth performance was identical for the two groups, elevation accuracy deteriorated much earlier in the blind subject group. These results indicate that auditory hypercompensation in early-blind humans does not extend to the frontal target domain, where the potential benefit of vision is maximal. Moreover, the results demonstrate for the first time that in this domain the human auditory system may require vision to optimally calibrate the elevation-related spectral pinna cues. Sensitivity to azimuth-encoding binaural difference cues, however, may be adequately calibrated in the absence of vision. (+info)
(56/868) Variability in the control of head movements in seated humans: a link with whiplash injuries?
The aim of this study was to determine how context and on-line sensory information are combined to control posture in seated subjects submitted to high-jerk, passive linear accelerations. Subjects were seated with eyes closed on a servo-controlled linear sled. They were asked to relax and received brief accelerations either sideways or in the fore-aft direction. The stimuli had an abrupt onset, comparable to the jerk experienced during a minor car collision. Rotation and translation of the head and body were measured using an Optotrak system. In some of the subjects, surface electromyographic (EMG) responses of selected neck and/or back muscles were recorded simultaneously. For each subject, responses were highly stereotyped from the first trial, and showed little sign of habituation or sensitisation. Comparable results were obtained with sideways and fore-aft accelerations. During each impulse, the head lagged behind the trunk for several tens of milliseconds. The subjects' head movement responses were distributed as a continuum in between two extreme categories. The 'stiff' subjects showed little rotation or translation of the head relative to the trunk for the whole duration of the impulse. In contrast, the 'floppy' subjects showed a large roll or pitch of the head relative to the trunk in the direction opposite to the sled movement. This response appeared as an exaggerated 'inertial' response to the impulse. Surface EMG recordings showed that most of the stiff subjects were not contracting their superficial neck or back muscles. We think they relied on bilateral contractions of their deep, axial musculature to keep the head-neck ensemble in line with the trunk during the movement. About half of the floppy subjects displayed reflex activation of the neck muscles on the side opposite to the direction of acceleration, which occurred before or during the head movement and tended to exaggerate it. The other floppy subjects seemed to rely on only the passive biomechanical properties of their head-neck ensemble to compensate for the perturbation. In our study, proprioception was the sole source of sensory information as long as the head did not move. We therefore presume that the EMG responses and head movements we observed were mainly triggered by the activation of stretch receptors in the hips, trunk and/or neck. The visualisation of an imaginary reference in space during sideways impulses significantly reduced the head roll exhibited by floppy subjects. This suggests that the adoption by the central nervous system of an extrinsic, 'allocentric' frame of reference instead of an intrinsic, 'egocentric' one may be instrumental for the selection of the stiff strategy. The response of floppy subjects appeared to be maladaptive and likely to increase the risk of whiplash injury during motor vehicle accidents. Evolution of postural control may not have taken into account the implications of passive, high-acceleration perturbations affecting seated subjects. (+info)