Alain Berthoz

Head stabilization during various locomotor tasks in humans

SummaryThis experiment, which extends a previous investigation (Pozzo et al. 1990), was undertaken to examine how head position is controlled during natural locomotor tasks in both normal subjects (N) and patients with bilateral vestibular deficits (V). 10 normals and 7 patients were asked to perform 4 locomotor tasks: free walking (W), walking in place (WIP), running in place (R) and hopping (H). Head and body movements were recorded with a video system which allowed a computed 3 dimensional reconstruction of selected points in the sagittal plane. In order to determine the respective contribution of visual and vestibular cues in the control of head angular position, the 2 groups of subjects were tested in the light and in darkness. In darkness, the amplitude and velocity of head rotation decreased for N subjects; these parameters increased for V subjects, especially during R and H. In darkness, compared to the light condition, the mean position of a line placed on the Frankfort plane (about 20–30° below the horizontal semi-circular canal plane) was tilted downward in all conditions of movement, except during H, for N subjects. In contrast, this flexion of the head was not systematic in V subjects: the Frankfort plane could be located above or below earth horizontal. In V subjects, head rotation was not found to be compensatory for head translation and the power spectrum analysis shows that head angular displacements in the sagittal plane contain mainly low frequencies (about 0.3–0.8 Hz). The respective contribution of visual and vestibular cues in the control of the orientation and the stabilization of the head in space is discussed.

Motor processes in mental rotation

Much indirect evidence supports the hypothesis that transformations of mental images are at least in part guided by motor processes, even in the case of images of abstract objects rather than of body parts. For example, rotation may be guided by processes that also prime one to see results of a specific motor action. We directly test the hypothesis by means of a dual-task paradigm in which subjects perform the Cooper-Shepard mental rotation task while executing an unseen motor rotation in a given direction and at a previously-learned speed. Four results support the inference that mental rotation relies on motor processes. First, motor rotation that is compatible with mental rotation results in faster times and fewer errors in the imagery task than when the two rotations are incompatible. Second, the angle through which subjects rotate their mental images, and the angle through which they rotate a joystick handle are correlated, but only if the directions of the two rotations are compatible. Third, motor rotation modifies the classical inverted V-shaped mental rotation response time function, favoring the direction of the motor rotation; indeed, in some cases motor rotation even shifts the location of the minimum of this curve in the direction of the motor rotation. Fourth, the preceding effect is sensitive not only to the direction of the motor rotation, but also to the motor speed. A change in the speed of motor rotation can correspondingly slow down or speed up the mental rotation.

Postural readjustments induced by linear motion of visual scenes

SummaryVisually induced linear motion sensation (linear vection) was obtained by projection of a visual scene moving linearly in a sagittal plane at the periphery of the visual field of healthy subjects standing erect on a force platform. This linear vection induces postural readjustments characterized by an inclination of the subject, in the same direction as the movement of the visual scene, followed by an after-effect. The amplitude of the postural changes was: a) logarithmically proportional to the image velocity and the density (or the spatial frequency) of the image pattern, at highest image velocities there was a saturation of the postural effect due to limits in image motion perception and not to biomechanical constraints; b) dependent appreciably on the size of the moving scene and its relative location (in the visual field).The frequency analysis of postural readjustments showed a) an increase of the amplitude of postural sway, especially at the low frequencies (from 0.02–0.2 Hz); b) sharp peaks in the power spectrum located between 0.15 and 0.5 Hz.From the dynamic relationships between the velocity of the moving visual scene and the amplitude of sway, it was concluded that the postural readjustment is proportional to a low pass filtering of the logarithm of the velocity. The contribution of Tibialis Anterior and Soleus was to oppose the body inclination with a resistive force.These results are analyzed in regard to the current theories concerning visual, vestibular and preceptive interaction in postural mechanisms.

Does the brain model Newton's laws?

How does the nervous system synchronize movements to catch a falling ball? According to one theory, only sensory information is used to estimate time-to-contact (TTC) with an approaching object; alternatively, implicit knowledge about physics may come into play. Here we show that astronauts initiated catching movements earlier in 0 g than in 1 g, which demonstrates that the brain uses an internal model of gravity to supplement sensory information when estimating TTC.

The neural basis of egocentric and allocentric coding of space in humans: a functional magnetic resonance study

Abstract. The spatial location of an object can be represented in the brain with respect to different classes of reference frames, either relative to or independent of the subjects position. We used functional magnetic resonance imaging to identify regions of the healthy human brain subserving mainly egocentric or allocentric (object-based) coordinates by asking subjects to judge the location of a visual stimulus with respect to either their body or an object. A color-judgement task, matched for stimuli, difficulty, motor and oculomotor responses, was used as a control. We identified a bilateral, though mainly right-hemisphere based, fronto-parietal network involved in egocentric processing. A subset of these regions, including a much less extensive unilateral, right fronto-parietal network, was found to be active during object-based processing. The right-hemisphere lateralization and the partial superposition of the egocentric and the object-based networks is discussed in the light of neuropsychological findings in brain-damaged patients with unilateral spatial neglect and of neurophysiological studies in the monkey.

Reference Frames for Spatial Cognition: Different Brain Areas are Involved in Viewer-, Object-, and Landmark-Centered Judgments About Object Location

Functional magnetic resonance imaging was used to compare the neural correlates of three different types of spatial coding, which are implicated in crucial cognitive functions of our everyday life, such as visuomotor coordination and orientation in topographical space. By manipulating the requested spatial reference during a task of relative distance estimation, we directly compared viewer-centered, object-centered, and landmark-centered spatial coding of the same realistic 3-D information. Common activation was found in bilateral parietal, occipital, and right frontal premotor regions. The retrosplenial and ventromedial occipitaltemporal cortex (and parts of the parietal and occipital cortex) were significantly more activated during the landmark-centered condition. The ventrolateral occipitaltemporal cortex was particularly involved in object-centered coding. Results strongly demonstrate that viewer-centered (egocentric) coding is restricted to the dorsal stream and connected frontal regions, whereas a coding centered on external references requires both dorsal and ventral regions, depending on the reference being a movable object or a landmark.

A fronto-parietal system for computing the egocentric spatial frame of reference in humans

Abstract Spatial orientation is based on coordinates referring to the subject’s body. A fundamental principle is the mid-sagittal plane, which divides the body and space into the left and right sides. Its neural bases were investigated by functional magnetic resonance imaging (fMRI). Seven normal subjects pressed a button when a vertical bar, moving horizontally, crossed the subjective mid-sagittal plane. In the control condition, the subjects’ task was to press a button when the direction of the bar movement changed, at the end of each leftward or rightward movement. The task involving the computation of the mid-sagittal plane yielded increased signal in posterior parietal and lateral frontal premotor regions, with a more extensive activation in the right cerebral hemisphere. This direct evidence in normal human subjects that a bilateral, mainly right hemisphere-based, cortical network is active during the computation of the egocentric reference is consistent with neuropsychological studies in patients with unilateral cerebral lesions. Damage to the right hemisphere, more frequently to the posterior-inferior parietal region, may bring about a neglect syndrome of the contralesional, left side of space, including a major rightward displacement of the subjective mid-sagittal plane. The existence of a posterior parietal-lateral premotor frontal network concerned with egocentric spatial reference frames is also in line with neurophysiological studies in the monkey.

Reappraisal of the human vestibular cortex by cortical electrical stimulation study

The cortical areas with vestibular input in humans were assessed by electrical stimulation in 260 patients with partial epilepsy who had undergone stereotactic intracerebral electroencephalogram recordings before surgery. Vestibular symptoms were electrically induced on 44 anatomical sites in 28 patients. The patients experienced illusions of rotation (yaw plane: 18, pitch plane: 6, roll plane: 6), translations (n = 6), or indefinable feelings of body motion (n = 8). Almost all vestibular sites were located in the cortex (41/44): in the temporal (n = 19), parietal (n = 14), frontal (n = 5), occipital (n = 2), and insular (n = 1) lobes. Among these sites, we identified a lateral cortical temporoparietal area we called the temporo–peri‐Sylvian vestibular cortex (TPSVC), from which vestibular symptoms, and above all rotatory sensations, were particularly easily elicited (24/41 cortical sites, 58.5%). This area extended above and below the Sylvian fissure, mainly inside Brodmann areas 40, 21, and 22. It included the parietal operculum (9/24 TPSVC sites) which was particularly sensitive for eliciting pitch plane illusions, and the mid and posterior part of the first and second temporal gyri (15/24 TPSVC sites) which preferentially caused yaw plane illusions. We suggest that the TPSVC could be homologous with the monkeys parietoinsular vestibular cortex.

Mental representations of movements. Brain potentials associated with imagination of eye movements

OBJECTIVE Current research in motor imagery is focused on similarities between actual and imagined movements on a central and a peripheral level of the nervous system. The present study measured slow cortical potentials (DC-potentials) during execution and internal simulation of memorized saccadic eye movements. METHODS In 19 healthy righthanded subjects DC-potentials were recorded from 28 electrodes during execution and during imagination of a sequence of memorized eye movements during a visual imagery condition. RESULTS Both oculomotor conditions showed a similar global level and similar topography of performance related DC-potentials, both strongly differed from the visual imagery condition and were lateralized to the left hemisphere. CONCLUSION This study therefore supports the hypothesis that cortical brain structures responsible for execution and imagination of memorized saccadic eye movements are similar. The observed left hemispheric lateralization is in contrast to a previous study using bimanual movements. This discrepancy is discussed in relation to recent observations in apractic patients with parietal lesions.

Goal-directed linear locomotion in normal and labyrinthine-defective subjects

When a subject is walking blindfolded straight ahead towards a previously seen target, the brain must update an internal representation with respect to the environment. This study examines whether the information given by the vestibular system is necessary for this simple path integration task and gives a quantitative description of locomotor behaviour during the walk by comparing ten normal and seven bilateral labyrinthine-defective (LD) subjects. Each subject performed 20 blindfolded walks (EC) and ten walks with eyes open (EO) towards a target attached to the floor 4 m in front of them; these walks were made at different velocities. The positions of head, trunk and feet were recorded using a 3D motion analysis system. No significant difference was found between normal and LD groups in terms of the distance error of reaching the target, while LD subjects showed a larger lateral error. Path curvature, expressed as the standard deviation of the angle between the direction of one step and straight ahead, was found to be significantly larger for LD subjects in the EC condition, demonstrating their instability when walking without vision. Mean walking velocity was lower for LD subjects than for normal subjects in both EC and EO conditions. Both groups walked faster with eyes open; LD subjects increased their velocity by increasing step length, normal subjects by increasing step frequency. Head stabilisation in the frontal plane during locomotion was not significantly different between LD and normal subjects, whereas both head and trunk rotation were slightly larger in LD subjects during blindfolded walking. The results show that bilateral LD subjects are able to perform linear goal-directed locomotion towards memorised targets. Thus, the vestibular system does not appear to be necessary for active linear path integration.