Parietal and occipital lobe contributions to perception of straight ahead orientation

OBJECTIVES

Several studies have investigated how peripheral stimulation affects the perception of body orientation in healthy subjects. The studies showed that opposing stimulation of two different input modalities can cancel out, leaving perception of body orientation unchanged. It was ascertained whether a comparable phenomenon could be found in brain damaged patients with two distinct disorders which individually lead to opposing shifts of the perceived midline.

METHODS

The visual subjective straight ahead was measured in patients with pure neglect, pure hemianopia, or a combination of neglect and hemianopia.

RESULTS

As in previous studies, patients with pure neglect displayed an ipsilesional displacement of the perceived straight ahead. Patients with pure hemianopia showed a contralesional shift. By contrast, no significant midline shift occurred in the patients with both neglect and hemianopia.

CONCLUSIONS

Neglect and hemianopia interact so that opposing biases in the perception of body orientation neutralise each other. Both parietal and occipital areas seem to contribute to the perception of straight ahead body orientation and seem to have counteracting effects when lesioned in the same hemisphere.

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.

Neck-proprioceptive influence on auditory lateralization

The effect of transcutaneous vibration of the posterior neck muscles on the lateralization of dichotic sound was investigated in human subjects. Two-alternative forced-choice (left/right) judgements were made on acoustic stimuli presented with different interaural level differences via headphones during neck-muscle vibration. A shift of the subjective auditory median plane toward the side contralateral of vibration was found, indicating that the sound was perceived as shifted toward the side of vibration. The mean magnitude of the vibration-induced intracranial shift was 1.5 dB. The results demonstrate a neck-proprioceptive influence on sound lateralization and suggest that this proprioceptive input is used for a central-nervous transformation of auditory spatial coordinates onto a body-centered frame of reference.

Is space representation distorted in neglect?

It has been argued that neglect of contralateral stimuli following brain damage might be associated with either a compressed or an anisometric neural representation of space along the earth-horizontal axis. Two different models have been put forward. One model proposes a uniform compression of subjective space, while the other envisages an expansion on one side of space and a compression on the other. We tested these models by determining neglect patients' perception of spatial distances in the horizontal plane. The models differ concerning the expected degree of under- vs overestimation of distances in the left and right hemispace. In the first experiment, patients were asked to position ten red LEDs equidistantly along a semicircle, which was located horizontally in front of them at eye level. A second experiment compared the patients' subjective perception of short, medium and long distances extending into left and right hemispace. We found no evidence for any compression or expansion, nor for anisometry along the earth-horizontal axis. These findings argue against a distortion of subjective space along the horizontal axis in patients with neglect which could account for their failure to orient towards and to explore the contralesional parts of space.

Space exploration in neglect

The present study investigated the gaze as well as the head and the eye-in-head movements of neglect patients while they were exploring their surroundings. A random configuration of letters was presented on the inner surface of a sphere that surrounded the subject, requiring free exploratory eye and head movements. The subjects were requested to search for a single (non-existent) target letter. The co-ordination of eye and head movements in patients with neglect resembled the pattern usually observed in healthy subjects orienting to eccentric visual targets. They performed hypometric head movements with additional shifts of eye-in-head position. Moreover, like healthy subjects, the patients with neglect explored space with gaze, with head and eye-in-head movements that were symmetrically distributed around preferred orientations in space. However, in contrast to controls, these centres of exploration were shifted towards the right. The average horizontal position of gaze and of head movements lay right of the body's mid-sagittal plane, the average eye-in-head position right of the head midline. The preferred orientations were located far away from the anatomical limits of horizontal gaze, head and eye-in-head movements. The decrease of exploration towards more eccentric locations left and right of these orientations thus could not be explained by anatomical restrictions. The results argue against a model of neglect that proposes a lateral gradient of attentional orienting towards the ipsilesional side. Exploring the surroundings, the patients did not orient gaze, the head or the eyes in the head towards the extreme ipsilesional side, nor even close to it. The results favour a deviation model suggesting a shift of the whole frame for exploratory behaviour towards the ipsilesional side. In addition to this shift, we found a second component of altered visual exploration in neglect. The patients' head and gaze movements exhibited a reduced variability around the deviated centre of exploration. The variability was not generally reduced but rather concerned specifically the horizontal dimension. The latter was found even when the area of exploration was paralleled between the groups, requiring the control subjects to search only in that part of the letter array that the neglect patients had explored spontaneously. Possible mechanisms, such as a disturbed ability to update the spatial representation of visual targets or an altered neural representation of space in the horizontal dimension, are discussed.

Perception of horizontal distances in patients with spatial neglect

Two different models have argued that neglect of contralateral stimuli following brain damage might be associated with either a compressed or an anisometric neural representation of space along the earth-horizontal axis. We tested these models by determining neglect patients' perception of spatial distances in the horizontal plane. We found no evidence for any compression or expansion or for anisometry along the earth-horizontal axis. The findings argue against a distortion of subjective space along the horizontal axis in patients with neglect, which could account for their failure to orient towards and to explore the contralesional parts of space.

Disentangling gravitational, environmental, and egocentric reference frames in spatial neglect

Previous studies in neglect patients using rotation of the body around the roll-axis revealed neglect of visual stimuli not only in the egocentric, body-centered left but also in the environmental left. The latter has been taken as evidence for a gravity-based environment-centered component of neglect occurring independently of the subject's actual body orientation. However, by using visual stimuli in a normally lightened room, the studies confounded the gravitational upright with the visible upright of the surround. Thus, it is possible that the visible upright of the environment may have served the role of the gravitational upright relative to which neglect occurred. The present experiment evaluated the influence of gravity on contralateral neglect when no visual information was presented. In complete darkness, neglect patients' exploratory eye movements were recorded in five experimental conditions: body in normal upright position, body titled 30 degrees to the left and 30 degrees to the right, and body pitched 30 degrees backward and 30 degrees forward. In the upright orientation, the patients with neglect showed a bias of ocular exploration to the ipsilesional right side. In egocentric body coordinates, we found no significant differences between the orientation of the biased search field in the different experimental conditions showing that the search field shifted with the orientation of the body. No significant decrease or enhancement of neglect was observed when body orientation was varied in the different conditions. In conclusion, the present results revealed that the modulation of gravitational forces has no significant influence on the exploratory bias of these patients. When visual information was excluded and only graviceptive information was available, the patients' failure to explore the contralesional part of space appeared purely body-centered. The results argue against a disturbed representation of space in neglect that encodes locations in a gravity-based reference system.

Kinematics of goal-directed arm movements in neglect: control of hand velocity

Do patients with unilateral neglect exhibit direction-specific deficits in the control of movement velocity when performing goal-directed arm movements? Five patients with left-sided neglect performed unrestrained three-dimensional pointing movements to visual targets presented at body midline, the left and right hemispace. A group of healthy adults and a group of patients with right-hemispheric brain damage but no neglect served as controls. Pointing was performed under normal room light or in darkness. Time-position data of the hand were recorded with an opto-electronic camera system. We found that compared to healthy controls, movement times were longer in both patient groups due to prolonged acceleration and deceleration phases. Tangential peak hand velocity was lower in both patient groups, but not significantly different from controls. Single peak, bell-shaped velocity profiles of the hand were preserved in all right hemispheric patients and in three out of five neglect patients. Most important, the velocity profiles of neglect patients to leftward targets did not differ significantly from those to targets in the right hemispace. In summary, we found evidence for general bradykinesia in neglect patients, but not for a direction-specific deficit in the control of hand velocity. We conclude that visual neglect induces characteristic changes in exploratory behavior, but not in the kinematics of goal-directed movements to objects in peripersonal space.

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.

Spatial orientation and the representation of space with parietal lobe lesions

Damage to the human parietal cortex leads to disturbances of spatial perception and of motor behaviour. Within the parietal lobe, lesions of the superior and of the inferior lobule induce quite different, characteristic deficits. Patients with inferior (predominantly right) parietal lobe lesions fail to explore the contralesional part of space by eye or limb movements (spatial neglect). In contrast, superior parietal lobe lesions lead to specific impairments of goal-directed movements (optic ataxia). The observations reported in this paper support the view of dissociated functions represented in the inferior and the superior lobule of the human parietal cortex. They suggest that a spatial reference frame for exploratory behaviour is disturbed in patients with neglect. Data from these patients' visual search argue that their failure to explore the contralesional side is due to a disturbed input transformation leading to a deviation of egocentric space representation to the ipsilesional side. Data further show that this deviation follows a rotation around the earth-vertical body axis to the ipsilesional side rather than a translation towards that side. The results are in clear contrast to explanations that assume a lateral gradient ranging from a minimum of exploration in the extreme contralesional to a maximum in the extreme ipsilesional hemispace. Moreover, the failure to orient towards and to explore the contralesional part of space appears to be distinct from those deficits observed once an object of interest has been located and releases reaching. Although patients with neglect exhibit a severe bias of exploratory movements, their hand trajectories to targets in peripersonal space may follow a straight path. This result suggests that (i) exploratory and (ii) goal-directed behaviour in space do not share the same neural control mechanisms. Neural representation of space in the inferior parietal lobule seems to serve as a matrix for spatial exploration and for orienting in space but not for visuomotor processes involved in reaching for objects. Disturbances of such processes rather appear to be prominent in patients with more superior parietal lobe lesions and optic ataxia.

Kinematics of goal-directed arm movements in neglect: control of hand in space

The present study investigated unrestrained, three-dimensional arm movements during goal-directed pointing in five patients with clinically manifest neglect to targets positioned either in the center or the left and right hemispace. Five patients with unilateral right hemispheric lesions without neglect and six healthy subjects served as controls. All subjects were able to point to these targets. Terminal accuracy of pointing did not differ between the three groups along the horizontal, vertical and anterior-posterior axis. Subjects' hand trajectories did not reveal direction-specific deviations from a straightline hand path. Our data show that deviations in the trajectories toward the ipsilesional side are not characteristic for patients with spatial neglect. We argue that exploratory and goal-directed motor behavior might not share the same egocentric, body-centered reference frame. A spatial reference frame for exploratory behavior is disturbed in patients with neglect resulting in a failure to explore the contralesional part of space by eye or limb movements. Its failure does not induce a spatial bias in hand trajectory formation during goal-directed arm movements in peripersonal space. Such deviations of reaching or pointing rather seem to be characteristic for patients with optic ataxia.

Visual extinction and prior entry: impaired perception of temporal order with intact motion perception after unilateral parietal damage

Two patients with left-sided visual extinction after right parietal damage were each given two 'prior entry' tasks that have recently been used to study attentional biases in normals. The first task presented two unconnected bars, one in each visual field, with the patients asked to judge which appeared sooner. Both patients reported that the right bar preceded the left unless the latter led by over 200 msec, suggesting a severe bias to the right affecting the time-course of visual awareness. The second task presented one continuous line in a scrolling format across the same spatial extent, with the patients asked to judge which direction the line moved in. The patients now performed normally. Thus, the perception of temporal order for separate events was impaired by the lesions, but without disrupting motion perception within single events. The implications are discussed for theories of normal and pathological attention, visual awareness, and motion perception.

Ocular exploration of space as a function of neck proprioceptive and vestibular input--observations in normal subjects and patients with spatial neglect after parietal lesions

We recently argued that the specific compensation of spatial neglect by manipulating neck proprioceptive and vestibular input is due to a central "correction" of the disturbed neural transformation process converting the afferent input coordinates from the peripheral sensory organs into a central representation of egocentric space. Both types of stimulation were proposed to induce a reorientation of the deviated or distorted egocentric spatial reference frame. The aim of the present study was to observe this process of reorientation under a condition in which no visual stimulus can attract the subject's attention and thus influence exploration behaviour from outside. We recorded eye movements of normal subjects and of three patients with spatial neglect after right parietal lesions while searching for a non-existent target in complete darkness. It was assumed that the area of the outer space that subjects spontaneously explore under this condition is a direct function of the subject's representation of egocentric space. Ocular space exploration was biased and confined almost entirely to the right side of the midsagittal plane in patients with neglect. This spatial distribution of exploratory eye movements changed remarkably with left-sided neck muscle vibration as well as with left-sided vestibular stimulation using ice water calorics. The spatial area of exploration was significantly enlarged to the contralesional side and the exploration maximum shifted in the same direction. Whereas with both types of stimulation space exploration of patients with neglect was similar to that of normal subjects when not being stimulated, neck proprioceptive and vestibular stimulation in normal subjects induced a quasi neglect-like exploration pattern, i.e. a bias to one side of the objective midsagittal plane. If ocular space exploration was, however, related to the subjectively perceived position of the midsagittal plane in space, eye movements were symmetrically distributed and carried out to both sides of subjective "straight ahead" in all experimental conditions, in normal subjects as well as in patients with neglect. The present results support the above hypothesis and indicate that neck proprioceptive as well as vestibular input directly contribute to the computation of the subject's central representation of egocentric space used for localizing body orientation and for guiding motor behaviour in space.

Optokinetic stimulation influences the disturbed perception of body orientation in spatial neglect

The effect of optokinetic stimulation on the disturbed perception of body orientation in three patients with right brain damage and spatial neglect was examined. The patients were asked to direct a laser point to the position which they felt lay exactly "straight ahead" of their bodies' orientation. Without stimulation they localised the body's sagittal midplane markedly to the right of the objective orientation. The patients' horizontal displacement of the sagittal midplane was reduced by a movement of the surround to the left and worsened by a movement to the right. The findings are consistent with those found in patients with spatial neglect using vestibular and neck proprioceptive stimulation. They show that visual input, together with vestibular and neck proprioceptive input, is used for computing a central representation of egocentric space. In spatial neglect this coordinate transformation works with a systematic error and deviation of the spatial reference frame to the ipsilesional side. The positive effect of optokinetic stimulation in patients with spatial neglect is interpreted with a "correction" of the neural coordinate transformation process by producing asymmetric input at the sensory organs of the contributing channels.

Transcutaneous electrical stimulation and vibration of neck muscles in neglect

Four neglect patients without visual field defects, one with a lesion of the right basal ganglia and three with a right, predominantly parietal lesion, were examined with a cancellation and a copying task before, during and after neck muscle vibration, during transcutaneous electrical stimulation of neck muscles and during vibration of hand muscles on the left side. In all patients, neck muscle vibration improved task performance, while transcutaneous electrical stimulation and hand vibration had little or no effect. The present results demonstrate that the effect of neck muscle vibration cannot be explained as arousal and activation due to unspecific sensory stimulation on the contralesional side of the body. They rather argue for the assumption that the compensatory effect of neck muscle vibration on neglect is an effect induced by the predominant activation of afferent Ia nerve fibres and their specific contribution to the central representation of egocentric space.

Ocular space exploration in the dark and its relation to subjective and objective body orientation in neglect patients with parietal lesions

Eye movements of neglect patients with right parietal lesions were recorded during ocular searching for a (non-existent) target in complete darkness. With respect to the objective orientation of the sagittal midplane, ocular exploration was biased toward the ipsilesional side. However, in relation to the patients' subjective localization of the sagittal midplane in space, exploratory eye movements were symmetrically distributed to the subjective "left" and "right" as observed in non-brain-damaged controls. The present results further support the hypothesis that the essential aspect leading to spatial neglect is a disturbance of those cortical structures that are crucial for computing egocentric, body-centred coordinates that allow use to determine our body position in space and that are necessary for visuomotor coordination and exploration of space. In neglect patients the central coordinate transformation seems to work with a systematic error resulting in a deviation of the spatial reference frame to the ipsilesional side. Consequences of this deviation are a displacement of subjective localization of body orientation and--to the same degree--of the spatial area in which motor behavior (here exploratory eye movements) is executed.

Noninvasive investigation of pericarotid syndrome: role of MR angiography in the diagnosis of internal carotid dissection

A 52-year-old man presented with unilateral left periorbital and frontotemporal pain associated with a partial ipsilateral Horner's syndrome of the postganglionic type and representing a pericarotid syndrome. MRI demonstrated a perivascular subacute hematoma at the level of the cervical portion of the left internal carotid artery with a markedly reduced flow-void signal. MR angiography confirmed the narrowed lumen of the dissected cervical internal carotid artery. There was also a right-sided precavernous carotid aneurysm. Three months later the left-sided pain had subsided, with complete resolution of the hematoma and incomplete restoration of the left carotid lumen seen on MR angiography. Dissection of the carotid wall may cause the oculosympathetic paralysis by producing a lesion of the superior cervical ganglion, the internal carotid nerve, or the perivascular sympathetic plexus. Whereas in pericarotid syndrome the most common cause is cervical carotid dissection, Raeder's syndrome additionally involving parasellar cranial nerves, may be caused by any paracavernous/cavernous lesion, including neoplasms and intracranial carotid aneurysms. The clinical distinction is useful to determine the appropriate diagnostic investigation, in view of the different pathoanatomical localization and different disease spectrum. As demonstrated in the present case, the combination of MRI and MR angiography is a reliable noninvasive tool to investigate the differential diagnosis of pericarotid syndrome, accurately depicting occlusive, stenotic or aneurysmal lesions of the carotid artery. We suggest that intraarterial angiography is no longer necessary.

Subjective body orientation in neglect and the interactive contribution of neck muscle proprioception and vestibular stimulation

Three patients with a right, predominantly parietal lesion and marked left-sided neglect without visual field defects were asked to direct a laser point to the position which they felt to lie exactly 'straight ahead' of their bodies' orientation. Whereas in both light and darkness, the subjective body orientation was close to the objective body position in the control groups, the three neglect patients localized the body's sagittal midplane approximately 15 degrees to the right of the objective orientation. No relevant differences of 'straight ahead' were found between the neglect patients and controls in the vertical plane. The neglect patients' horizontal displacement of sagittal midplane to the right could be compensated for either by neck muscle vibration or by caloric vestibular stimulation on the left side. When vestibular stimulation was combined with neck muscle vibration, the horizontal deviation linearly combined by adding or neutralizing the effects observed when both types of stimulation were applied exclusively in the control groups as well as in the neglect patients. Moreover, data analysis revealed that the neglect patients' ipsilesionally displaced subjective body orientation does not result from a disturbed primary perception or disturbed transmission of the vestibular or proprioceptive input from the periphery. The present results support the hypothesis that the essential aspect leading to neglect in brain-damaged patients is a disturbance of those cortical structures that are crucial for transforming the sensory input coordinates from the peripheral sensory organs--here the retina, neck muscle spindles and cupulae--into an egocentric, body-centred coordinate frame of reference. In neglect patients the coordinate transformation seems to work with a systematic error and deviation of the spatial reference frame to the ipsilesional side leading to a corresponding displacement of subjective localization of body orientation. It can be concluded further that neck muscle proprioception and vestibular stimulation directly interact in contributing to the subject's mental representation of space. The data suggest that the afferent information from these different input channels is used simultaneously for computing egocentric, body-centred coordinates that allow us to determine our body position in space.

Spatial limitation of eye movements during ocular exploration of simple line drawings in neglect syndrome

The eye movement behaviour of a patient suffering from a right basal ganglia infarction with left-sided neglect but without any visual field defects was investigated during the ocular exploration of simple line drawings. The eye movements were registered by means of an i.r. light technique. Each line drawing consisted of different figural elements located in the centre and both the left and the right half of the picture. In each case, only the contextual connection of the left and right elements in a drawing allowed the spectator to describe the whole scene consistently and to perceive the correct theme of the drawing. The drawings fell into two categories. The cognitive impact of the elements in the drawings' centre was varied by "pointing" to the elements located in the left and right halves with a different strength of connection. When exploring the two types of drawings (weak vs. strong connective elements in the centre) the patient showed a similar eye movement pattern, exploring exclusively the right halves of the drawings. Irrespective of the cognitive impact of the drawing's central elements in "pointing" towards supplementary information in the picture's left half, exploration of this half took place in neither case. A striking difference between the two categories was, however, found concerning the patient's ability to recognize the general context, i.e. to describe the correct theme of the whole drawing. While the patient could not correctly depict any of the scenes from the drawings with weak connective elements in the centre, he was easily capable of properly describing the themes from the group of drawings with strong connective elements, which implies that he must have analysed and perceived the figural elements of both halves of the picture. Different possibilities are discussed to explain the similarity of the patient's exploratory eye movements but discrepancy in his verbal responses when dealing with the two different types of drawings.

The interactive contribution of neck muscle proprioception and vestibular stimulation to subjective "straight ahead" orientation in man

Seventeen normal subjects were asked to direct a laser point to the position they felt to lie exactly straight ahead of their body. Subjects were seated in complete darkness in an approximately spherical cabin in an upright position with the orientation of the trunk and head aligned. For both the horizontal and vertical plane, "straight ahead" judgements were closely scattered around the objective straight ahead body position. Posterior neck muscle vibration as well as caloric vestibular stimulation with ice water led to (1) an apparent motion and horizontal displacement of a stationary visual target to the side opposite to stimulation and (2) a horizontal deviation of subjective "straight ahead" perception toward the side of stimulation. Only those subjects who experienced an illusion of target motion also showed a deviation of their subjective body orientation. No systematic effect of a displacement of subjective body orientation in the vertical plane was detected. When vestibular stimulation and neck muscle vibration were combined their effects were additive, i.e. the horizontal deviation of subjective body orientation observed when either type of stimulation was applied in isolation, was linearly combined either by summation or by cancellation. The present results clearly support the assumption that afferent visual, vestibular and proprioceptive input converge to the neural generation of an egocentric, body-centred coordinate system that allows us to determine our body position with respect to visual space.

Decrease of contralateral neglect by neck muscle vibration and spatial orientation of trunk midline

Three patients with a right hemisphere lesion and marked left-sided neglect without visual field defects were asked to detect and identify stimuli which were tachistoscopically presented in the left or right visual half-field. Neglect of stimuli presented in the contralesional left visual field, which was observed when the patient's body was in a normal upright position with trunk, head and gaze oriented straight ahead to the middle of the projection screen, could be reduced by vibrating the left posterior neck muscles as well as by turning the trunk 15 degrees to the left. In contrast, unspecific stimulation on the left side of the patient's body, produced by vibrating the left hand muscles or the proprioceptive signal induced by turning the head 15 degrees to the left, had no compensatory effects. The results showed that the afferent information about real lengthening of the left posterior neck muscles (produced by turning the trunk) as apparent lengthening of these muscles (produced by their vibration), leads to a remission of contralateral neglect. Thus, the proprioceptive input from the neck muscles, i.e. the head-on-trunk signal, appears to influence the extension of the neglected part of space in patients with neglect. The signal seems to contribute substantially to the neural generation of the egocentric frame of reference that allows the determination of body position with respect to visual space. We hypothesize that the reduction of neglect by vibration of the contralateral posterior neck muscles is based on a shift of the subjective spatial localization of the sagittal midplane in the contralesional direction and a corresponding alteration of the egocentric coordinate system necessary for visuomotor coordination and exploration of space.

Inflexibility of mental planning: a characteristic disorder with prefrontal lobe lesions?

Inflexibility of mental planning processes has repeatedly been discussed to represent a specific disorder resulting from human frontal lobe lesions. Patient groups suffering from acute or chronic unilateral frontal (medial, lateral) or retrorolandic (temporal, parietal) lesions and nonbrain-damaged controls were requested to mirror and to reverse mentally a maze structure that had been learned by the covered maze presentation technique (KARNATH et al., Neuropsychologia 29, 271-290, 1991). The patients with acute frontal lesions were impaired to adapt the acquired mental plan to the new (but related) problem and made more errors than those with acute temporal or parietal lesions and controls. However, no impaired plan modification was detected in patients with chronic frontal brain lesions. The difference of task performance in the acute and chronic (recovered) stage of frontal brain damage is discussed.

Abnormal eye movement behaviour during text reading in neglect syndrome: a case study

The eye movement behaviour of a patient suffering from a right basal ganglia infarction with a left-sided hemineglect but without any visual field defects was investigated during reading. The eye movements were registered by means of an i.r. light technique (pupil-corneal reflection method). The main findings were abnormal return sweeps. Whereas in normal readers the end of one line of text is linked to the beginning of the new line by a long leftward saccade, the return sweeps of the hemineglect patient stereotypically ended in the middle of the next line. They were followed by sequences of short saccades indicating silent backward reading until a linguistically plausible continuation of sentences from the previous line was found, irrespective of the actual beginning of text. The shortened return sweeps could not be attributed to a general oculomotor disturbance. The spatial border for the occurrence of the patient's abnormal scanning pattern (left half of texts) clearly did not depend on a retinal coordinate frame of reference but rather has to be attributed to a different body-centred reference system.

Trunk orientation as the determining factor of the 'contralateral' deficit in the neglect syndrome and as the physical anchor of the internal representation of body orientation in space

The present study examines which egocentric coordinate system determines the border between the disturbed 'contralateral' and the normal 'ipsilateral' side in patients with hemineglect. Based on the observation of significantly longer reaction times for saccades towards stimuli presented in the left visual field (LVF) in right brain-damaged patients with hemineglect, stimuli were presented randomly to the LVF or RVF and the corresponding saccadic reaction times (SRTs) were compared. Beginning with the standard body position generally used for the investigation of neglect patients, where the midlines of head, trunk and visual field are parallel and oriented straight towards the middle of the projection screen, the spatial relation between orientation of head and trunk midlines and location of the target stimuli was systematically varied while holding the retinal projection of the stimuli constant. The deficit in SRTs towards the LVF in 4 right brain-damaged patients with left-sided hemineglect could be compensated for by turning the patients' trunk to the left, such that both LVF and RVF-stimuli were projected to the right, ipsilateral side of trunk space. The results suggest that the spatial orientation of the trunk midline divides our normal perception of space into an egocentric 'left' and an egocentric 'right' sector and seems to be the decisive factor for determining the neglected 'contralateral' part of space in patients with brain-damage. They indicate that the trunk midline constitutes the physical anchor for calculation of the internal egocentric coordinate frame for representing body position with respect to external objects. The hypothesis of Ventre et al. (1984) that deficient reactions to contralaterally located stimuli in neglect patients could be the result of a displacement of these egocentric coordinates towards the non-neglected, ipsilateral side is discussed.

Limitations of interhemispheric extracallosal transfer of visual information in callosal agenesis

A 57-year-old patient with callosal but not anterior commissure agenesis was investigated with a visual interfield comparison and a naming task. Stimuli were presented tachistoscopically either bilateral-simultaneously or unilaterally in the LVF and/or RVF. The stimuli presented bilaterally differed with respect to their degree of similarity. Whereas the patient was able to detect gross differences between stimuli presented in the left and right half-field, he was impaired at discriminating similar and identical stimuli across the half-fields. Identification by naming was correct on unilateral presentation of a stimulus, while with bilateral presentation of two stimuli, errors increased considerably with the stimulus named second. The data are thought to indicate a limited capacity of the extracallosal commissures, probably the anterior, for the transfer of visual information.