Reference frame and haptic discrimination of orientations in infants

Body-tilt effect on the haptic discrimination of orientations and the 'oblique effect' (better discrimination of the vertical orientation than of an oblique orientation) were examined in 5-month-old infants. Body tilt leads to a mismatch between egocentric and gravitational reference frames and indicates in which reference frame orientations and oblique effect are defined. A familiarization/reaction to novelty procedure was used in upright body and tilted body conditions. Results revealed the occurrence of a haptic oblique effect in the upright body position, which disappeared when the body was tilted. The results suggest that spatial orientations and the oblique effect depend on a mixed reference frame that integrates not only gravitational information but also egocentric information.     

The awareness of body orientation modulates the perception of visual vertical

It is established that the body position influences verticality perception. In contrast, the possible influence of the awareness of the body orientation on verticality perception has never been investigated. This hypothesis, explored in the present study, is supported by the role played by the parietal cortex and the insula in both body position awareness and verticality perception. Nine subjects were asked to estimate the direction of the visual vertical (VV) by 12 adjustments of a luminous line in three conditions: (1) a control condition (subjects were upright and aware of their position), (2) a condition of congruence between the lateral body tilt and the awareness of this tilt, and (3) a condition of dissociation of subjective and objective orientations (tilted subjects who felt upright). The dissociation between objective and subjective orientations was obtained by inducing experimentally a postural vertical (PV) bias through 5min of lateral body tilt at 30° in darkness in a motorized flight simulator (mean 8.8°±4; min 6.2°; max 17.4°). VV orientation and variability were measured (expressed below in this order). As compared to the upright condition (0.3°±0.2; 0.8°±0.5), subjects showed similar VV orientation (0.1°±0.6; p=0.82) but an increased variability (1.4°±0.5; p<0.001) when tilted and aware of their tilt. In contrast, when they were tilted but felt upright, VV was biased in the direction of body tilt (2°±0.5; p<0.005) without increase of variability (0.9°±0.5; p=0.7). Our study reveals that the awareness of body orientation modulates verticality representation, which means that in addition to sensory integration, mental processes play also a role in the sense of verticality. We propose a novel model of verticality representation, based both on bottom-up and top-down processes.     

Spinal and supraspinal postural networks

Different species maintain a particular body orientation in space (upright in humans, dorsal-side-up in quadrupeds, fish and lamprey) due to the activity of a closed-loop postural control system. We will discuss operation of spinal and supraspinal postural networks studied in a lower vertebrate (lamprey) and in two mammals (rabbit and cat). In the lamprey, the postural control system is driven by vestibular input. The key role in the postural network belongs to the reticulospinal (RS) neurons. Due to vestibular input, deviation from the stabilized body orientation in any (roll, pitch, yaw) plane leads to generation of RS commands, which are sent to the spinal cord and cause postural correction. For each of the planes, there are two groups of RS neurons responding to rotation in the opposite directions; they cause a turn opposite to the initial one. The command transmitted by an individual RS neuron causes the motor response, which contributes to the correction of posture. In each plane, the postural system stabilizes the orientation at which the antagonistic vestibular reflexes compensate for each other. Thus, in lamprey the supraspinal networks play a crucial role in stabilization of body orientation, and the function of the spinal networks is transformation of supraspinal commands into the motor pattern of postural corrections. In terrestrial quadrupeds, the postural system stabilizing the trunk orientation in the transversal plane was analyzed. It consists of two relatively independent sub-systems stabilizing orientation of the anterior and posterior parts of the trunk. They are driven by somatosensory input from limb mechanoreceptors. Each sub-system consists of two closed-loop mechanisms - spinal and spino-supraspinal. Operation of the supraspinal networks was studied by recording the posture-related activity of corticospinal neurons. The postural capacity of spinal networks was evaluated in animals with lesions to the spinal cord. Relative contribution of spinal and supraspinal mechanisms to the stabilization of trunk orientation is discussed.     

Ocular vestibular evoked myogenic potentials: The effect of head and body tilt in the roll plane

ObjectiveTo explore effects of whole-head/body tilt in the roll plane on ocular-vestibular evoked myogenic potentials (oVEMP).MethodsTwenty healthy subjects were randomly tilted in an Eply Omniax rotator across a series of eight angles from 0° to 360° (at 45° separations) in the roll plane. At each position, oVEMPs to air-conducted (AC) and bone-conducted (BC) stimulation were recorded from unrectified infra-orbital surface electromyography during upward gaze. oVEMP amplitudes, latencies and amplitude asymmetry were compared across each angle of orientation.ResultsHead orientation had a significant effect on oVEMP reflex amplitudes for both AC and BC stimulation (p < 0.001). For both stimuli there was a trend for lower amplitudes with increasing angular departure from the upright position. Mean amplitudes decreased by 42.6–56.8% (AC) and 23.2–25.5% (BC) when tilted 180°. Roll-plane tilt had a significant effect on amplitude asymmetry ratios recorded in response to AC stimuli (p < 0.001), indicating a trend for lower amplitudes from the dependent (down) ear. Amplitude asymmetry ratios for BC stimuli were unaffected by head and body orientation.ConclusionsThe results confirm an effect of head and body orientation on oVEMP reflexes recorded in response to air- and bone-conducted stimuli.SignificanceThe upright position yields an optimal oVEMP response.     

The origin of contraversive pushing: evidence for a second graviceptive system in humans

BACKGROUND: Stroke patients may exhibit the peculiar behavior of actively pushing away from the nonhemiparetic side, leading to lateral postural imbalance and a tendency to fall toward the paralyzed side. This phenomenon has been called the "pusher syndrome." OBJECTIVE: The current study analyzes the mechanism leading to contraversive pushing. METHODS: The subjective postural vertical (SPV) and subjective visual vertical (SVV) were determined in five consecutively admitted patients with severe contraversive pushing and in controls. Whereas adjustment of the SPV reflects the perceived upright orientation of the body, the SVV provides a sensitive and direction-specific measurement of peripheral and central vestibular dysfunction. RESULTS: The deficit leading to contraversive pushing is an altered perception of the body's orientation in relation to gravity. Pusher patients experience their body as oriented "upright" when it is tilted 18 degrees to the nonhemiparetic, ipsilesional side. In contrast, perception of the SVV was undisturbed. CONCLUSIONS: A separate pathway seems to be present in humans for sensing the orientation of gravity apart from the one for orientation perception of the visual world. This second graviceptive system decisively contributes to humans' control of upright body posture. Contraversive pushing occurring after stroke lesions may represent the behavioral correlate of a disturbed neural representation of this system.     

Organization of stabilizing reflex responses in tibialis anterior muscles following ankle flexion perturbations of standing man

Reflex activity in human ankle muscles in response to 36 deg./s dorsi-flexion rotations of the feet was investigated in subjects standing upright and when leaning back so as to preactivate ankle flexor muscles. Short latency stretch reflex activity in soleus and inhibition in tibialis anterior muscles occured at 50 ms from ankle rotation onset. Two prominent bursts of tibialis activity followed at 83 and 131 ms, and preceded large stabilizing ankle torques. Head movements commenced 20 ms after foot rotations and acquired accelerations exceeding 100 deg./s² within 60 ms. It is suggested that the tibialis anterior activity is either a vestibulospinal reflex resulting rom the head movement, or a strethc reflex only present standing, since this activity was not observed when seated subjects received identical foot rotations.     

Pusher Syndrome – a frequent but little-known disturbance of body orientation perception

Abstract Disturbances of body orientation perception after brain lesions may specifically relate to only one dimension of space. Stroke patients with “pusher syndrome” suffer from a severe misperception of their body's orientation in the coronal (roll) plane. They experience their body as oriented ‘upright’ when it is in fact markedly tilted to one side. The patients use the unaffected arm or leg to actively push away from the unparalyzed side and resist any attempt to passively correct their tilted body posture. Although pusher patients are unable to correctly determine when their own body is oriented in an upright, vertical position, they seem to have no significant difficulty in determining the orientation of the surrounding visual world in relation to their own body. Pusher syndrome is a distinctive clinical disorder occurring characteristically after unilateral left or right brain lesions in the posterior thalamus and –less frequently– in the insula and postcentral gyrus. These structures thus seem to constitute crucial neural substrates controlling human (upright) body orientation in the coronal (roll) plane. A further disturbance of body orientation that predominantly affects a single dimension of space, namely the transverse (yaw) plane, is observed in stroke patients with spatial neglect. Apparently, our brain has evolved separate neural subsystems for perceiving and controlling body orientation in different dimensions of space.     

Effect of Body Orientation with Respect to Gravity on Directional Accuracy of Human Pointing Movements

Errors in pointing to remembered target locations were measured in normal subjects in the upright posture and in a supine or prone lying position, three conditions that differed in the orientation of the moving arm with respect to gravity. Vertical errors, or, more strictly, errors along the body axis, were significantly larger in both supine and prone lying positions as compared with the vertical posture. Subjects' orientation with respect to gravity produced a consistent error pattern, i.e. subjects pointed below the remembered target locations in the vertical posture and above them under the two lying conditions, in the body-related co-ordinates. On the contrary, variable error (the confidence ellipse) was differentially affected by the orientation of the movement trajectory with respect to gravity. In the supine body position the variable error was larger, and in the prone position it was smaller than in the upright posture. The finding of gravity-related bias in pointing errors suggests that the motor programme generated in the lying position is not modified to adapt to an altered orientation with respect to gravity.     

Neuronal representation of object orientation

The dissociation between object identity and object orientation observed in six patients with brain damage, has been taken as evidence for a view-invariant model of object recognition. However, there was also some indication that these patients were not generally agnosic for object orientation but were able to gain access to at least some information about objects' canonical upright. We studied a new case (KB) with spared knowledge of object identity and impaired perception of object orientation using a forced choice paradigm to contrast directly the patient's ability to perceive objects' canonical upright vs non-upright orientations. We presented 2D-pictures of objects with unambiguous canonical upright orientations in four different orientations (0 degrees, -90 degrees, +90 degrees, 180 degrees ). KB showed no impairment in identifying letters, objects, animals, or faces irrespective of their given orientation. Also, her knowledge of upright orientation of stimuli was perfectly preserved. In sharp contrast, KB was not able to judge the orientation when the stimuli were presented in a non-upright orientation. The findings give further support for a distributed view-based representation of objects in which neurons become tuned to the features present in certain views of an object. Since we see more upright than inverted animals and familiar objects, the statistics of these images leads to a larger number of neurons tuned for objects in an upright orientation. We suppose that probably for this reason KB's knowledge of upright orientation was found to be more robust against neuronal damage than knowledge of other orientations.     

Activation of selected frontal trunk and extremities muscles during rolling from supine to side lying in healthy subjects and in post-stroke hemiparetic patients

The purpose of this work was to get insight into the role of frontal trunk and proximal extremity muscles in rolling from supine to side lying. Participants were seventeen hemiparetic patients and 14 healthy subjects. Electromyographic (EMG) activity of the Sternocleidomasoid (ScM), Pectoralis Major (PM), External Oblique (ExO) and Rectus Femoris (RF) muscle pairs was recorded during rolling performance. Analysis included the establishment of EMG response times and magnitudes. For all muscles, initiation of EMG activity was delayed in the patients as compared with the controls. Among the healthy subjects, EMG activity level of the SCM was more enhanced on the mobile than on the stable body side, while activation level of the PM muscle was larger on the stable body side. In the hemiparetic group, the SCM demonstrated similar results as the controls yet, PM activity on the paretic side was lower than on the non-paretic side regardless of rolling direction. The difference in activation level between the corresponding ExO and RF muscles showed inconsistency among the healthy subjects, whereas in the hemipareic group the muscles on the paretic side never displayed higher activation levels than on the non-paretic side. In conclusion, rolling sideways in hemiparetic subjects is characterized by a normal relationship between activation levels of the SCM muscles while the relationship between the corresponding PM, ExO and RF muscles is hampered due to reduced activation level of the muscles on the paretic body side.     

"Pusher syndrome" following cortical lesions that spare the thalamus

Stroke patients with "pusher syndrome" show severe misperception of their own upright body orientation although visualvestibular processing is almost intact. This dissociation argues for a second graviceptive system in humans for the perception of body orientation. Recent studies revealed that the posterior thalamus is an important part of this system. The present investigation aimed to study the cortical representation of this system beyond the thalamus. We evaluated 45 acute patients with and without contraversive pushing following left–or right–sided cortical lesions sparing the thalamus. In both hemispheres, the simple lesion overlap associated with contraversive pushing typically centered on the insular cortex and parts of the postcentral gyrus. The comparison between pusher patients and controls who were matched with respect to age, lesion size, and the frequency of spatial neglect, aphasia and visual field defects revealed only very small regions that were specific for the pusher patients with cortical damage sparing the thalamus. Obviously, the cortical structures representing our control of upright body orientation are in close anatomical proximity to those areas that induce aphasia in the left hemisphere and spatial neglect in the right hemisphere when lesioned. We conclude that in addition to the subcortical area previously identified in the posterior thalamus, parts of the insula and postcentral gyrus appear to contribute at cortical level to the processing of the afferent signals mediating the graviceptive information about upright body orientation.     

Klinik, Ursache und Therapie der Pusher-Symptomatik

Stroke patients may exhibit the peculiar behavior of actively pushing away from the nonhemiparetic side, leading to lateral postural imbalance and a tendency to fall towards the paralyzed side. These patients use the nonparetic extremities to stem actively against attempts of passive correction towards upright orientation. This phenomenon has been called the "pusher syndrome". Recent findings disclose that the deficit leading to contraversive pushing is an altered perception of the body's orientation in relation to gravity. Pusher patients experience their body as upright when they are actually tilted to the nonhemiparetic side. In contrast, processing of visual and vestibular inputs for the determination of visual vertical was undisturbed. The results argue for a separate pathway in humans for sensing gravity apart from that for perception of the visual world. This second graviceptive system decisively contributes to our control of upright body posture. The present article describes this still largely unknown neurological disease. The clinical examination of contraversive pushing, its underlying disturbance, lesion location, and approaches for therapy are considered.     

Task-specific effects of orientation information: neuropsychological evidence

The deficits underlying orientation agnosia in a patient (MB) with a right fronto-temporo-parietal lesion were examined. Like similar patients in the literature, MB was impaired at discriminating whether objects were upright or not and, in copying, she tended to re-represent stimuli as upright. In addition, MB failed to show the normal effects of rotation on object identification; her naming of objects rotated 45 degrees from upright was no slower than her naming of upright items. Effects of the degree of rotation did emerge, however, when she had to perform a matching task that required mental rotation. The evidence suggests that orientation may be coded in several ways (e.g. separately between objects and relative to the viewer), and that brain-damage can selectively affect the use of some but not all types of orientation information.     

Efficacy of tilt-suppression in postrotatory nystagmus in cats

The phenomenon of tilting the head away from an upright position immediately after a horizontal head-rotation, thus reducing the duration of postrotatory nystagmus (PRN), has more than once been called "tilt-suppression." It represents an example of the semicircular canal-otolith interaction in the central vestibular system. In the present study we investigated how head roll-tilt influences the time constants of PRN in the horizontal and vertical planes in cats. The head/body was roll-tilted by 30 degrees toward the upright or the side down from initial roll positions immediately after termination of earth-vertical axis (EVA) rotation. Changes in head orientation either towards or away from the EVA reduced horizontal PRN. The reducing effect was small when the head was roll-tilted toward the EVA. Vertical nystagmus decreased only when the head orientation moved toward alignment with the EVA. Otolithic "tilt-suppression" may be a central neuronal mechanism that is activated to minimize the tumbling sensation of turning about a tilted axis and postural instability, but our results indicate that tilt-suppression of PRN depends on a change in head orientation with respect to the EVA.     

Three-dimensional orientation of the eye rotation axis during the Purkinje sensation

The otolith-semicircular canal interaction during postrotatory nystagmus was studied in six normal human subjects by applying fast, short-lasting, passive head and body tilts (90° in the roll of pitch plane) 2 s after sudden stop from a constant velocity rotation (100°/s) about the earth-vertical axis in yaw. Eye movements were measured with 3-D magnetic search coils. Following the head tilt, activity in the semicircular canal primary afferents continues to reflect the postrotatory angular velocity vector in head-centered coordinates, whereas otolith primary afferents signal a different orientation of the head relative to gravity. Pitch (roll) tilts away from upright during postrotatory nystagmus after yaw rotation elicited a transient vertical (torsional) VOR. Despite the change in head orientation relative to gravity, postrotatory eye velocity decayed closely along the axis of semicircular canal stimulation (horizontal in head coordinates). These results suggest that postrotary nystagmus is largely organized in head-centered rather than gravity-centered coordinates in humans as suggested by the Purkinje-sensation.     

A review on assessment and treatment of the trunk in stroke A need or luxury

Trunk function has been identified as an important early predictor of functional outcome after stroke and the same deteriorates on both contralateral and ipsilateral sides of the body following stroke.The primary contribution of the trunk muscles is to allow the body to remain upright,adjust weight shifts,and control movements against constant pull of gravity and is considered central key point of the body.Proximal stability of the trunk is a pre-requisite for distal limb mobility,balance,gait and functional activities and its positive correlation in hemiplegia has been demonstrated in a cross-sectional study.Both isokinetic and handheld dynamometer muscle strength testing demonstrated the weakness of bilateral trunk flexors,extensors and rotator muscles in both acute and chronic hemiplegic patients.This was confirmed by electromyography analysis which identified poor bilateral trunk muscles activity in patients with stroke.Trunk impairment scale is sensitive to evaluate the selective muscle control of upper and lower trunk,and it has been reported that lateral flexion of the trunk is easier than rotation of the trunk and the clinical observation concurs to the difficulty in lower trunk rotation of stroke patients.However,trunk exercises given early after stroke could produce enhanced balance performance post-stroke.This review attempts to report the evidence supporting the involvement of the trunk and its influence on balance and functional performance in post-stroke hemiplegia.     

Leg muscle activity during tandem stance and the control of body balance in the frontal plane

ObjectivesWe investigated the pattern of activity of the tibialis anterior (TA), soleus (SOL) and peroneus longus (PER) muscles of both legs during tandem stance, in order to highlight their respective role in maintaining balance.MethodsTwelve young healthy subjects were asked to stand with feet in line for successive 15 s-epochs, on a dynamometric platform with (EO) and without (EC) vision. EMG was recorded from the six muscles simultaneously. Collected signals were displacement of the centre of feet pressure (CoP) and EMG. Variables calculated for each recorded epoch were mean level, variability and distribution between legs of EMG, and cross-correlation between EMG and CoP traces and between EMG of homonymous muscles.ResultsCoP motion was larger along the medio–lateral (M–L) than antero–posterior (A–P) axis, and larger with EC than EO particularly in the M–L axis. Muscle activity was larger in the rear than in the front leg, as expected, except for PER. Activity increased with the increase in M–L CoP oscillations, except for SOL, which was tonically active, both legs, regardless of the amplitude of the oscillations. Manipulating vision had no effect on the variability of the EMG for equal mean levels of activity, for any muscle. Cross-correlation between EMG of rear leg muscles and M–L CoP sway gave higher coefficients for TA and PER than SOL, and appropriate time-delays between TA or PER and CoP motion, indicating a role of these muscles in the control of M–L sway. Except for the tonically active SOL, the homonymous muscles of the two legs were active out-of-phase, indicating a mutual push–pull action of the pairs. This was confirmed by the reciprocal activation of TA and PER of the same leg.ConclusionsOverall, in spite of a large inter-trial and inter-subject variability, the neural command to the leg muscles during tandem stance implies a task-sharing rule, whereby SOL keeps the body upright while the reciprocal PER and TA activities produce the alternate impulses necessary for body stabilization in the frontal plane.SignificanceKnowledge of the normal mode of control of balance in frontal plane can foster new investigation in both posture and gait control, in addition to offering tools for understanding balance problems of elderly persons and patients at risk of fall.     

One good turn deserves another: an event-related brain potential study of rotated mirror-normal letter discriminations

The time to decide if a letter is normal or backwards (mirror-reversed) increases as the letter is rotated away from the upright. It is widely accepted that this increase in time reflects the mental rotation of the stimulus to the upright orientation in order to determine the mirror-normal status of the stimulus. Although response times tend to be longer for mirrored stimuli than for normal stimuli, the difference is constant across orientation. Little work has been focused on why mirror-image stimuli produce longer response times than normal stimuli. This study examines the question of whether or not mirrored stimuli are rotated in the picture plane at the same time as normal stimuli, and if so, why response times to mirrored stimuli are longer than that for normal stimuli. Both the behavioural and electrophysiological findings suggest that the mirrored stimuli are not only rotated in the picture plane, but that they are subsequently rotated to the normal view. It is this additional rotation that produces, at least in part, the delayed response times for mirror-image stimuli.     

Effects of personal familiarity on early neuromagnetic correlates of face perception

This study investigated effects of familiarity and orientation on face processing by means of magnetoencephalography. Participants were presented with photographs of personally familiar, famous and unfamiliar faces in both upright and inverted orientation. They had to decide whether faces were familiar by means of manual yes/no responses. Independent of orientation, we observed a clear modulation of the M170 by familiarity, with personally familiar faces evoking larger amplitudes than unknown faces. The M170 was also sensitive to orientation, with larger amplitudes for inverted than upright faces. Moreover, the M170 exhibited larger amplitudes over the right than over the left hemisphere, but this asymmetry was present for upright faces only. The present data suggest that at least for personally familiar faces, neural correlates of identification start no later than approximately 170 ms, and underline a special role of the right hemisphere for faces in their typical upright orientation.     

Metabolic and contractile changes in fast and slow muscles of the cat after glucocorticoid-induced atrophy.

The susceptibility of fast- and slow-twitch hind limb muscles to glucocorticoid-induced atrophy was investigated in adult male cats treated for 10 to 14 days with triamcinolone (4 mg/kg/day), using several histochemical, biochemical, and functional indices. After treatment, muscle weight loss in the fast-twitch muscles (medial gastrocnemius and vastus medialis) occurred to a greater extent than in the slow-twitch muscles (soleus and vastus intermedius), with the latter muscles decreasing in weight proportional to the body weight. Fast-twitch glycolytic (FG) fibers responded with similar degrees of atrophy in the muscles examined; however, slow-twitch oxidative (SO) and fast-twitch oxidative glycolytic (FOG) fibers atrophied more in the fast-twitch compared to the slow-twitch muscles. Phosphofructokinase and NADP⁺-linked isocitrate dehydrogenase specific activities decreased similarly in the fast-twitch muscles, while no change occurred in the slow-twitch muscles. Functionally, the soleus and medial gastrocnemius remained unchanged in abilities to generate tension tetanically, when this was expressed per unit muscle mass or per unit contractile protein. As a result of the treatment, however, the medial gastrocnemius fatigued faster in response to repetitive stimulation in the glucocorticoid-treated animals. The results suggest that the response of muscle to glucocorticoid-induced atrophy is not regulated by the primary metabolic pathways used for energy production. The differences in response of the SO and FOG fiber types in fast- versus slow-twitch muscles suggest basic differences in metabolic and activity profiles of the same fiber types in different muscles, which may influence susceptibility to atrophy.