A general deficit of the 'automatic pilot' with posterior parietal cortex lesions?

Lesions of the parieto-occipital junction (POJ) in humans cause gross deviations of reaching movements and impaired grip formation if the targets are located in the subjects' peripheral visual field. Movements to central targets are typically less impaired. This disorder has been termed "optic ataxia". It has been suggested that a general deficit of online corrections under central as well as peripheral viewing conditions might be sufficient to explain this discrepancy. According to this hypothesis, patients with optic ataxia should demonstrate an impaired online correction of grip aperture under central viewing conditions if the target object changes its size during the grasping movement. We investigated this prediction in a patient with optic ataxia (I.G.) in a virtual visuo-haptic grasping task. We imposed an isolated need for online corrections of the hand aperture independently of positional changes of the target object. While we found some general inaccuracies of her grasping movements, the patient did not show a specific impairment of online adjustment of grip aperture. On the contrary, I.G. smoothly adjusted her grip aperture comparable to healthy subjects. A general deficit of fast movement correction affecting targets in peripheral as well as central visual fields thus does not appear to account for the overt visuomotor deficits in optic ataxia. Rather, it seems more likely that an anatomical dissociation between visuomotor pathways related to actions in the central and in the peripheral visual field underlies the dissociation of visuomotor performance depending on the retinotopic target position in optic ataxia.

Do patients with neglect show abnormal hand velocity profiles during tactile exploration of peripersonal space?

It has been suggested that the movement impairments experienced by patients with neglect are not restricted to spatial disorders, but also affect higher-order kinematics (velocity and acceleration) to the extent that movements towards the neglected side are slower than movements away from it. In a recent study, we could not confirm this hypothesis, but found that patients with unilateral neglect exhibited no distinct direction-specific deficits in hand velocity when performing goal-directed reaching movements. Here we investigated whether neglect patients might reveal direction-specific deficits during exploratory hand movements. Six patients with left-sided neglect and six age-matched healthy control subjects scanned with their right hands the surface of a large table searching for a (non-existent) tactile target. Movements were performed in darkness. Time-position data of the hand were recorded with an optoelectronic camera system. Median activity of the patients' exploratory hand movements was shifted to the right (Karnath and Perenin 1998). Hand trajectories were partitioned into sections of leftward/rightward or, along the sagittal plane, into sections of near/far movements. For each movement section average and peak velocities were computed. The patients' hand movements were bradykinetic when compared with the control group. However, we found no evidence that average or peak velocities of leftward intervals were systematically lower than during rightward motion. Direction-specific deficits in velocity were also not observed for movements to and away from the body (sagittal plane). In conclusion, we found evidence for general bradykinesia in neglect patients but not for a direction-specific deficit in the control of hand velocity during exploratory hand movements.

Scanning direction and line bisection: a study of normal subjects and unilateral neglect patients with opposite reading habits.

Sixty normal dextrals (30 left-to-right and 30 right-to-left readers) and two left unilateral neglect patient with opposite reading habits performed a passive line bisection task. In order to study the effect of scanning direction on performance, subjects had to stop a mark moving on the to-be-bisected line either from the left to the right or in the opposite direction. Results showed that the position of the subjective middle was dependent upon the scanning direction of the line for all the subjects. A leftward deviation appeared for left to right scanning, whereas a rightward shift occurred when the mark moved from the right to the left. These results emphasize the role of scanning direction in space organization and are discussed with respect to the explanatory hypotheses of unilateral neglect.

Prism adaptation to a rightward optical deviation rehabilitates left hemispatial neglect.

A large proportion of right-hemisphere stroke patients show hemispatial neglect-a neurological deficit of perception, attention, representation, and/or performing actions within their left-sided space, inducing many functional debilitating effects on everyday life, and responsible for poor functional recovery and ability to benefit from treatment. The frequent parietal locus of the lesion producing neglect reflects the impairment of coordinate transformation used by the nervous system to represent extrapersonal space. Given that adaptation to a visual distortion can provide an efficient way to stimulate neural structures responsible for the transformation of sensorimotor coordinates, the aim of our study was to investigate the effect of prism adaptation on various neglect symptoms, including the pathological shift of the subjective midline to the right. All patients exposed to the optical shift of the visual field to the right were improved on their manual body-midline demonstration and on classical neuropsychological tests. Unlike other physiological manipulations used to improve neglect, this improvement lasted for at least two hours after prism removal and thus could be useful in rehabilitation programmes. The positive effect found for both sensorimotor and more cognitive spatial functions suggests that they share or depend on a common level of space representation linked to multisensory integration.

Tactile exploration of peripersonal space in patients with neglect.

Recent studies investigated the pattern of visual exploration along the horizontal axis in patients with neglect. They found a deviated distribution of activity with the centre shifted -15 degrees to the right of the body's mid-sagittal plane. The present study reports comparable observations concerning tactile exploration of space in neglect. We found the whole distribution of patients' exploratory hand movements shifted towards the right. The median activity lay 17.7 cm right of the body's mid-sagittal plane. The frequency of tactile exploration decreased towards the periphery of peripersonal space on the right and the left side. The findings are in clear contrast to the predictions of the attentional gradient model of neglect. They rather favour a deviation of the whole frame for exploratory behaviour towards a new equilibrium on the right. The character of this frame appears to be supramodal in that it determines the distribution of exploratory movements irrespective of whether the subject explores the surround visually or tactually.

Improvement of the motor deficit of neglect patients through vestibular stimulation: evidence for a motor neglect component.

The effect of vestibular stimulation on motor performance has been studied comparatively in 2 groups of hemiplegic patients, one including 9 right brain-damaged patients (RBD) with neglect, the other 9 left brain-damaged patients (LBD) without neglect. In the RBD group, a transient but significant improvement of motor performance was observed following stimulation, although motor scores remained unchanged in two cases. Moreover, a temporary remission of personal neglect and anosognosia was obtained in 8 out of 9 patients. In contrast to the RBD group, the motor performance of the LBD group was not improved through vestibular stimulation, although a moderate improvement of force was noticed in one ambidextrous patient who had shown transient signs of neglect at the acute stage. These results suggest the participation of a motor neglect component in the motor deficit of neglect patients. The motor neglect component may be considered as one of the many manifestations of the neglect syndrome and, as such, can be improved by the sensory manipulations which presumably restore a conscious representation of the left side of space.

Grasping without form discrimination in a hemianopic field.

Although patients with blindsight are usually unable to discriminate forms, recent neuropsychological data have suggested that they could still use some form attributes in object-oriented actions. One patient with a complete right hemianopia due to a medial occipital lesion has been tested for his capacities to process orientation and size of visual objects. He was presented with either a slot of variable orientation or with rectangular objects of the same surface but variable length. His performance was studied in three types of tasks: motor, in which he had to insert a card in the slot or to grasp the rectangle between thumb and indexfinger; verbal, in which forced-choice verbal guesses were required; and matching, which required matching orientation or size with wrist or fingers. Although responses were at chance level in the two latter conditions, motor responses were systematically influenced by both orientation and size of the stimulus. These data provide further evidence for two dissociable modes of visual information processing dealing respectively with 'what"' the object is vs 'how' to grasp it. They also indicate that the neural pathway controlling visuomotor transformation in humans is much less dependent on V1 input than the pathway involved in visual discrimination and identification.

[Neglect of the representational space: demonstration by mental evocation of the map of France]. / Négligence de l'espace représenté: mise en évidence par l'évocation mentale de la carte de France.

Neglect of the representational space has been shown in 8 patients asked to evoke mentally the map of France and to name as many towns as possible successively on each side of a vertical axis splitting the map in two equivalent halves. They had first to imagine the map with an axis in the south-north direction and then with an axis in the reverse direction (north-south). The results were similar to those reported by Bisiach and Luzzatti (1979). They reflect a disturbance of the mechanisms involved in conscious representation of space. In addition we report on the effects of vestibular stimulation on the reversibility of representational neglect in 3 patients. These preliminary results provide support for the idea that vestibular stimulation would play centrally by restoring the symmetry of the internal representation of egocentric space.

Temporary remission of representational hemineglect through vestibular stimulation.

This study was aimed at finding whether vestibular stimulation could reduce neglect of representational space, in the same way it reduces other manifestations of the neglect syndrome. Representational neglect was shown in 8 patients asked to evoke mentally the map of France and to name as many towns as possible successively on each side of a vertical axis splitting the map in two equivalent halves. Performances appeared much poorer on the left than on the right side. Immediately after a cold left ear irrigation, the left side scores improved in all patients, although the right scores remained unchanged. The results are discussed within the framework of the representational theory of neglect. It is proposed that mental representation of space would result from dynamic and distributed neural mechanisms.

Partial remission of hemiplegia and somatoparaphrenia through vestibular stimulation in a case of unilateral neglect.

In a case of long lasting severe neglect resulting from a large right parieto-temporo-occipital infarct, vestibular stimulation produced a temporary reduction of the motor deficit and disappearance of the somatoparaphrenic delusion, in addition to the already reported improvement of extrapersonal and personal neglect and anosognosia. These data open new perspectives in the understanding of the neglect syndrome and of functional involvement of the parietal lobe in space representation.

Discrimination of motion direction in perimetrically blind fields.

Three patients with bilateral and two with unilateral striate cortex lesions have been tested for their ability to discriminate motion direction within their perimetrically blind field. No optokinetic nystagmus could be recorded in any of the three cases of cortical blindness, but all the patients could discriminate with a high level of success between two opposite directions of movement (at least along the horizontal plane) in a forced-choice indication task. In contrast, the two hemispherectomized patients remained at chance level in the same task. This indicates that the residual capacities observed in the patients with occipital lesions depend upon cortical structures. According to neurophysiological findings, the most likely substrate would be the homologue of area MT of the monkey.

Optic ataxia: a specific disruption in visuomotor mechanisms. I. Different aspects of the deficit in reaching for objects.

Visually directed arm movements have been studied by film recordings in 10 patients with optic ataxia resulting from unilateral lesions of the parietal region, in 3 cases on the right and in 7 on the left. Half of the patients also underwent visuospatial perceptive tests. The results indicate the following. (1) Optic ataxia is a specific visuomotor disorder, independent of visual space misperception. (2) The proximal and the distal components of the movements are equally affected as shown in reaching and hand orientation tasks. (3) The percentages of spatial and orientation errors quantified, respectively, in these two situations show a different distribution across the different hand-field combinations according to the side of the lesion: whereas the right-damaged patients show a deficit essentially related to a field effect, the left-damaged patients show in addition to the latter an impairment related to a hand effect. These findings suggest that the 2 types of visuomotor mechanisms responsible for the proximal and distal components of visually-directed arm movements are controlled by the parietal cortex and that there should exist a hemisphere asymmetry in the functional organization of these mechanisms. (4) Reconstruction of the lesions drawn from CT scans in 8 of the patients shows a salient and constant involvement of the posterior parietal cortex, always including the intraparietal sulcus and either the superior part of the inferior parietal lobule or more often various parts of the superior parietal lobule. The weak co-occurrence of optic ataxia and hemispatial neglect, and their different lesion sites, indicate a double dissociation between these two symptoms.

Central and peripheral residual vision in humans with bilateral deprivation amblyopia.

Residual visual capacities were investigated in the central and peripheral visual fields of 13 patients with bilateral deprivation amblyopia secondary to congenital cataracts removed at an early age. In comparison with a control group of normal subjects, spatial modulation sensitivity function of the amblyopes was markedly impaired in each experimental condition, i.e. both when the stimuli were stationary or drifting (8 Hz) gratings and both in central or peripheral visual field, at 10 and 20 degrees eccentricity. The sensitivity drop was observed over the whole spatial frequency range, although it was much more severe at high frequencies. Threshold elevation, with respect to controls, was very similar in conditions using stationary or drifting gratings, suggesting that both sustained and transient mechanisms are affected by stimulus deprivation amblyopia. Temporal modulation sensitivity function was uniformly impaired over the whole temporal frequency range. When compared with other types of amblyopia, this pattern of spatiotemporal sensitivity loss appeared characteristic of deprivation amblyopia. The peripheral deficit was particularly striking by its severity and extent, as ascertained by static perimetry and visual acuity measurements up to 50 degrees eccentricity. This finding emphasizes the susceptibility of peripheral as well as central human vision to early deprivation and suggests that peripheral visual functions are still immature in newborns. Finally, the overall deficit varied with the severity of the deprivation, in that patients with complete neonatal cataract performed much worse than those with incomplete cataract.

[Prosopagnosia without hemianopsia after unilateral right occipitotemporal lesion]. / Prosopagnosie sans hémianopsie après lésion unilatérale occipito-temporale droite.

Three atypical features were noted in a patient with prosopagnosia due to a right occipital hematoma: prosopagnosia was unassociated with achromatopsia, spatial disorientation or left visual field alexia; the right visual field was normal and the left field only slightly impaired; the lesion was apparently unilateral.

[Bilateral parietal syndrome approximating a Balint syndrome]. / Syndrome pariétal bilatéral proche d'un syndrome de Balint.

A patient presented with disorders of saccadic and pursuit movements a wandering gaze, the impossibility to maintain fixation, a concentric shortening of visual attention mainly of the right side, an incomplete right homonymous hemianopia (respecting 10 to 20 degrees), and optical and auditory ataxia predominantly affecting the right hand. Pathology showed a large left parietal glioma in the white matter in the superior and inferior parietal lobes, with microscopic evidence of parietal cortex invasion. A hematoma on the right side was situated entirely parasagitally in the precuneus, approaching the cortex at this level. The two lesions were independent and the splenium was intact. The frontal lobes were normal. A review of nearly 30 cases of bilateral syndrome of the parieto-occipital junction showed that anomalies of visually guided ocular movements were always present. These cases require differentiation from those, such as the present case, which involve mainly a visuomotor coordination disorder and from those with predominant visual disorientation.

[The function of contrast sensitivity in deprivation amblyopia]. / La fonction de sensibilité au contraste dans l'amblyopie de privation.

Visual deprivation during the first few months of life is known to result in a severe and irreversible visual deficit. Animal models of amblyopia developed during the past 20 years has considerably increased our knowledge of the "sensitive period" during which deprivation can lead to irreversible effects, and of the anatomical and physiological changes underlying this visual deficit. However precise data regarding either the duration of the sensitive period or the nature of the amblyopic deficit are still mostly lacking in humans. Preliminary results on the latter topic are reported in the present paper. Contrast sensitivity function (CSF) has been studied using stationary sinusoidal gratings in three patients with deprivation amblyopia resulting from bilateral congenital cataracts and in nine normal emmetropic subjects. Compared with these control subjects, the three amblyopic patients showed a dramatic decrease of their CSF with a maximal drop in the high and medium spatial frequency range and a relative sparing at low spatial frequencies. Within the framework of current theories of parallel processing of visual information through the geniculostriate pathway, these results indicate that early visual deprivation in man severely affects development of "sustained" channels involved in form perception. Further studies are underway to explore "transcient" channels in similar cases.

Macular sparing investigated by means of Haidinger brushes.

Haidinger brushes, an entoptic phenomenon perceived only through the most central macular retina, have been used in addition to kinetic and static perimetry with the Tubingen perimeter for determining macular sparing and splitting in hemianopic patients. Seven patients were examined: 2 with a bitemporal hemianopia resulting from traumatic damage to the optic chiasma, and 5 with a homonymous hemianopia resulting from extensive lesions of the optic radiations and/or the occipital cortex (including 2 cases of hemidecortication). On perimetric examination some exceptions could be observed in the correspondence of macular splitting and sparing respectively with pre- and postgeniculate lesions, but this was not the case when Haidinger brushes were used. Half of the figure was then perceived by the patients with lesions of the chiasma and the entire figure by all the patients with geniculostriate lesions. We consider that results obtained by this simple method are more reliable than those obtained by perimetry and that Haidinger brushes should be used for macular field examination in neuro-ophthalmic practice.

[Optic ataxia: analysis of eye and hand responses in pointing at visual targets (author's transl)]. / Ataxie optique: analyse des réponses oculaires et manuelles dans une tâche de pointage vers une cible visuelle.

A visuomotor task was used in three cases of isolated optic ataxia, in patients with an unilateral parietal lesion (one right and two left). Eye and hand movements were recorded in pointing at visual targets in a closed loop condition. Saccades directed to the contralateral side were abnormal in two patients. Hand pointing was always accurate in foveal vision conditions. Errors observed in peripheral vision condition were different according to the side of the lesion. The contribution of oculomotor signals in optic ataxia and the influence of hemispheric specialization are discussed.