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.     

Perception of active head rotation in patients with severe left unilateral spatial neglect

Unilateral spatial neglect is a common neurological syndrome following predominantly right hemisphere damage, and is characterized by a failure to perceive and report stimuli in the contralesional side of space. To test the reference shift hypothesis that contralesional spatial neglect in right-brain-damaged patients is attributed to a rightward deviation of the egocentric reference frame, we measured the final angular position to which controls and left-side neglect patients actively turned their head toward the left in response to a verbal instruction given from each of three locations—right, left, and front—in two conditions, with and without visual feedback. When neglect patients were asked to “look straight ahead”, they deviated about 30° toward the right in the eyes-open condition. However, the rightward deviation was markedly reduced in the eyes-closed condition. Regardless of visual feedback, there was no significant difference between controls and neglect patients in the final angular position of active head rotation when the verbal instruction came from the subject’s left or front side; however, the final angular position was significantly smaller in the neglect patients than in the controls when the verbal instruction was given from the right. These results support the contention that cervico-vestibular stimulation during active head rotation restores spatial remapping and sensori-motor correlations and so improves neglect without affecting the position of the egocentric reference; however, once left-side neglect patients respond to verbal instruction from the right side, they are unable to disengage attention from the hemispace, and the performance of head rotation is disturbed.     

Egocentric reference and represented space

The subjective estimate of the position of the egocentric reference was measured in normal right-handed subjects. They had to point manually in the straight ahead direction at what they thought was their body's sagittal plane. A systematic leftward bias of relatively small amplitude was observed for pointings with the right hand. No such bias was observed with the left hand. In addition, fixation of a visual target located within the right or left hemispace systematically deviated the estimated egocentric reference toward the side opposite to the target. These results contribute to the mechanisms responsible for directional coding of movements toward extrapersonal space.     

Is perception of visual verticality intact in patients with idiopathic cervical dystonia?

Idiopathic cervical dystonia (CD) is a focal dystonia characterized by an abnormal tilted or twisted head position. This abnormal head position could lead to a distorted perception of the visual vertical and spatial orientation. The aim of this cross-sectional study was to investigate whether the perception of the visual vertical is impaired in patients with CD. The subjective visual vertical test (SVV) was measured in 24 patients with CD and 30 controls. The SVV test is conducted in a completely darkened room. A laser bar is projected on an opposing white wall, which is deviated from the earth’s gravitational vertical. Participants were seated with their head unrestrained and were instructed to position this bar vertically. The deviations in degrees (°) are corrected for the side of laterocollis in order to measure the E-effect. We found that patients were able to position the laser bar as equally close to the earth’s gravitational vertical as controls (+ 0.67° SD ± 2.12 vs + 0.29° SD ± 1.08, p = 0.43). No E-effect was measured. Notwithstanding the abnormal position of the head, the perception of the visual vertical in patients with idiopathic CD is intact, possibly because of central neural compensatory mechanisms.     

Asymmetric Influence of Egocentric Representation onto Allocentric Perception

Objects in the visual world can be represented in both egocentric and allocentric coordinates. Previous studies have found that allocentric representation can affect the accuracy of spatial judgment relative to an egocentric frame, but not vice versa. Here we asked whether egocentric representation influenced the processing speed of allocentric perception. We measured the manual reaction time of human subjects in a position discrimination task in which the behavioral response purely relied on the target's allocentric location, independent of its egocentric position. We used two conditions of stimulus location: the compatible condition-allocentric left and egocentric left or allocentric right and egocentric right; the incompatible condition-allocentric left and egocentric right or allocentric right and egocentric left. We found that egocentric representation markedly influenced allocentric perception in three ways. First, in a given egocentric location, allocentric perception was significantly faster in the compatible condition than in the incompatible condition. Second, as the target became more eccentric in the visual field, the speed of allocentric perception gradually slowed down in the incompatible condition but remained unchanged in the compatible condition. Third, egocentric-allocentric incompatibility slowed allocentric perception more in the left egocentric side than the right egocentric side. These results cannot be explained by interhemispheric visuomotor transformation and stimulus-response compatibility theory. Our findings indicate that each hemisphere preferentially processes and integrates the contralateral egocentric and allocentric spatial information, and the right hemisphere receives more ipsilateral egocentric inputs than left hemisphere does.     

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.     

Verbal short-term memory and cerebellum: Evidence from a patient with congenital cerebellar vermis hypoplasia

When interacting with objects in their environment, patients with Parkinson's disease (PD) often make hypometric movements (e.g., micrographia). The purpose of this study was to learn if patients with PD, in the absence of overt actions or environmental stimuli, have egocentric (body-centered) conceptual (representational) hypometria. Actions can take place in either proximal or distal peri-personal space. Normally, the right hemisphere has a distal and the right a proximal attentional bias. We also want to learn if a patient with onset of disease in the right hemisphere would have a greater conceptual action hypometria than individuals with left hemispheric onset. Seventeen subjects with PD and 15 age-matched controls were given a questionnaire evaluating subjective perception of personal action space. The questionnaire asked individuals to rate, using a 0–5 scale, the distances between their body and hand when performing the various actions that take place in peri-personal space. When compared to control participants, participants with PD, and especially those with predominant left sided symptoms (right greater than left hemispheric dysfunction) had hypometric mental perceptions for actions that normally occur in far peri-personal space. Individuals with PD appear to demonstrate a conceptual hypometria, suggesting that there is a perturbation of their interactive representational maps and these maps appear to be more disordered by right than left hemisphere dysfunction. However, it is also possible that when making action distance computations, participants used covert actions and it was the hypometric covert actions that induced the computational hypometria.     

Hemianopia and neglect influence on straight-ahead perception

In spatial neglect, the pathological ipsilesional deviation of the subjective straight ahead (SSA) received both 'translational' and 'rotational' interpretations. Furthermore, hemianopia per se could also influence straight-ahead (SA) perception. Here, we aimed at disentangling the relative effects of neglect and hemianopia on the SSA by using a method analyzing translation and rotation in parallel. We included patients with a right hemisphere stroke. Ten had neglect and hemianopia, 6 neglect only, 3 hemianopia only, and 12 neither one nor the other. 15 were controls. Participants had to adjust a bar, movable in translation and rotation, SA of the navel, in darkness. Patients with spatial neglect showed systematic rightward translation, greater when hemianoptic. The rotation component appeared more variable and was influenced by the presence of a visual field defect, as most of the patients with hemianopia showed an anticlockwise rotation and most of the patients without hemianopia a clockwise rotation. Non-neglect patients and control subjects demonstrated a fair performance level. In conclusion, both neglect and hemianopia resulted in a translation error of the SSA to the ipsilesional side. In neglect patients, additional hemianopia resulted in a counterclockwise rotation. Furthermore, rotation and translation errors appeared dissociated, suggesting a different coding of these dimensions by the right posterior hemisphere.     

Early non-visual experience influences proprioceptive-spatial discrimination acuity in adulthood

The present study tested whether non-visual spatial experience affects later acuity of space perception. Congenitally blind adults who differed in the age acquired spatial knowledge via an orientation and mobility (O&M) training and matched sighted controls performed passive arm movements and judged the direction of the sensed movement. Proprioceptive-spatial discrimination acuity was assessed by an adaptive psychophysical procedure in two spatial coding conditions. In the egocentric coding condition, participants judged whether the hand trajectory felt left- or right-tilted in reference to a straight trajectory aligned to the transverse horizontal plane of the body midline axis. In the allocentric coding condition, they indicated whether the hand trajectory felt acute- or obtuse-angled in reference to a right angle. Proprioceptive-spatial acuity of congenitally blind participants significantly covaried with the age they attended an O&M training. The earlier the congenitally blind participants started the O&M training the more accurate was their space perception. Congenitally blind participants who underwent an O&M training after the age of 12 years showed poorer acuity than sighted controls. Congenitally blind participants with an earlier O&M training, however, approached the performance level of the sighted controls. The observed improvement in spatial acuity was more pronounced when judgements were given on the basis of an allocentric than an egocentric frame of reference. These findings suggest that proprioceptive-spatial acuity in adulthood depends on non-visual spatial experience during early development.     

Dissociation between egocentric and allocentric visuospatial and tactile neglect in acute stroke

Previous investigations provide evidence for distinction between egocentric (viewer-centered) and allocentric (stimulus- or object-centered) hemispatial neglect. However, it has not been determined whether this dissociation is modality-independent or modality-specific. We identify the incidence of egocentric and allocentric neglect in visual and tactile modalities, as well as the frequency of their co-occurrences in each modality. One-hundred patients with acute, right supratentorial ischemic stroke were administered tests for egocentric and allocentric hemispatial neglect in visual (n=98) and tactile (n=58) modalities. The visual test consisted of a page of 30 circles; 10 with no gap, 10 with a gap on the right side, and 10 with a gap on the left. Patients were asked to circle all complete circles and cross out all circles with gaps. A tactile version consisted of the same stimulus types presented as raised circles. Patients were asked to explore the board of circles with their dominant hand and report whether each circle had a gap. To determine the presence of egocentric or allocentric neglect, each test was analyzed for a significantly higher number of errors on the contralesional versus ipsilesional side of the page/board, or of the stimulus, using the Chi square analysis. On the visual test, 17 patients exhibited egocentric neglect; four exhibited allocentric neglect; and only two exhibited both. In the tactile modality, 19 exhibited egocentric neglect; one exhibited allocentric neglect; and none demonstrated both. Only four patients showed egocentric neglect on both visual and tactile tests. We found one patient with bilateral lesions who showed left egocentric visual neglect and right allocentric tactile neglect. These data provide strong evidence that egocentric and allocentric neglect are distinct syndromes that often dissociate and likely reflect damage to different brain areas. They also show that selective egocentric or allocentric neglect can occur in visual or tactile modalities.     

Visual,proprioceptive and tactile performance in left neglect patients

Patients with unilateral spatial neglect due to right-hemisphere lesions typically fail to attend to and explore left-sided stimulus objects. It has been postulated that in right-brain damaged (RBD) patients an ipsilesional displacement of the egocentric frame of reference (ER), whether visual or tactile, may be responsible for a contralesional supramodal spatial bias causing their left neglect behavior. However, this hypothesis had been proposed without testing, in the same patients, the position of the ER or their performance in the visual and tactile modalities. Thus, the aim of the present study was to test the hypothesis that an ipsilateral shift of the ER is responsible for a supramodal spatial bias in neglect.For this purpose, a within-subject design is required. Consequently, 12 left neglect patients and 12 control subjects were asked to perform a proprioceptive straight-ahead pointing task while blindfolded, as well as visual and tactile bisection tasks.In the left neglect patients, we found:no systematic deviation of the ER on the ipsilesional right side;a significant rightward bias in visual bisection, and normal performance in tactile bisection;no correlation among the three tasks;that only visual bisection correlated with the severity of neglect.These results are discussed with regard to the egocentric and attentional hypothesis of neglect.     

Object-based and action-based visual perception in children with spina bifida and hydrocephalus.

Children with spina bifida and hydrocephalus (SBH) have long been known to have difficulties with visual perception. We studied how children with SBH perform 12 visual perception tasks requiring object identification, multistable representations of visual space, or visually guided overt actions. Four tasks required object-based processing (visual constancy illusions, face recognition, recognition of fragmented objects, line orientation). Four tasks required the representation of visual space in egocentric coordinates (stereopsis, visual figure-ground identification, perception of multistable figures, egocentric mental rotation). Four tasks required the coupling of visual space to overt movement (visual pursuit, figure drawing, visually guided route finding, visually guided route planning). Effect sizes, measuring the magnitude of the difference between SBH children and controls, were consistently larger for action-based than object-based visual perception tasks. Within action-based tasks, effect sizes were large and roughly comparable for tasks requiring the representation of visual space and for tasks requiring visually guided action. The results are discussed in terms of the physical and brain problems of children with SBH that limit their ability to build effective situation models of space.     

Posterior parahippocampal gyrus lesions in the human impair egocentric learning in a virtual environment

Functional imaging studies have shown that the posterior parahippocampal gyrus (PHG) is involved in allocentric (world-centered) object and scene recognition. However, the putative role of the posterior PHG in egocentric (body-centered) spatial memory has received only limited systematic investigation. Thirty-one subjects with pharmacoresistant medial temporal lobe epilepsy (TLE) and temporal lobe removal were compared with 19 matched healthy control subjects on a virtual reality task affording the navigation in a virtual maze (egocentric memory). Lesions of the hippocampus and PHG of TLE subjects were determined by three-dimensional magnetic resonance imaging volumetric assessment. The results indicate that TLE subjects with right-sided posterior PHG lesions were impaired on virtual maze acquisition when compared with controls and TLE subjects with anterior PHG lesions. Larger posterior PHG lesions were significantly related to stronger impairments in virtual maze performance. Our results point to a role of the right-sided posterior PHG for the representation and storage of egocentric information. Moreover, access to both allocentric and egocentric streams of spatial information may enable the posterior PHG to construct a global and comprehensive representation of spatial environments.     

Egocentric body-centered coordinates modulate visuomotor performance

Parietal damage has been hypothesized to distort the body-centered coordinate frame and produce the ipsilesional spatial bias characteristic of unilateral neglect. The present studies investigated the role of the egocentric frame in normal visuomotor performance by manipulating the alignment of the body midline in neurologically-intact adults. The results from two experiments indicate that: (1) rightward rotation causes a right visual field advantage in detection times for lateralized targets; (2) rightward rotation evokes an increase in visual sensitivity to right visual field targets and a decrease in sensitivity to left visual field targets; and (3) leftward rotation does not affect response latency or sensitivity, however, response criterion is mildly affected. By demonstrating that the alignment of the body-centered frame can induce neglect-like asymmetries in visuomotor performance in neurologically-intact subjects, the results support a role for an altered body-centered representation in clinical neglect.     

Oblique spatial shifts of subjective visual straight ahead orientation in quadrantic visual field defects

Patients with postchiasmatic visual field defects often show a contralesional bias towards the scotoma in line bisection or when indicating their visual subjective straight ahead (VSSA). Recent evidence suggests a retinotopic misrepresentation of visual space in patients with homonymous quadrantanopia (HQ). We therefore assessed in the present study whether patients with HQ show an oblique shift of their VSSA towards their scotoma, in addition to the known bias in horizontal line bisection. Moreover, we examined whether eccentric fixation contributes to this shift. To this purpose, 15 non-neglecting stroke patients with HQ and 15 matched healthy control subjects were assessed in horizontal line bisection and in the horizontal and vertical dimension of their VSSA. Additionally, perimetric blind spot mapping was performed. Eight out of nine patients with left quadranopia showed the typical leftsided, horizontal line bisection error, while only one out of seven patients with rightsided quadranopia showed a rightsided shift. Normal subjects showed a non-significant leftward shift in line bisection (pseudoneglect). All 15 patients with HQ showed a large oblique shift of their VSSA towards the blind quadrants, while normal subjects showed no systematic left-rightward shift, but a small downward shift of the VSSA. The position of the blind spot was normal in all testable eyes of patients and control subjects, thus excluding eccentric fixation or cyclorotation of the eyes. In conclusion, our study reveals a hitherto unreported oblique spatial shift of subjective visual body orientation towards the blind quadrants in non-neglecting patients with quadranopia.     

Spatial orientation constancy is impaired in anorexia nervosa

In anorexia nervosa (AN), body distortions have been associated with parietal cortex (PC) dysfunction. The PC is also the anatomical substrate of a supramodal reference framework involved in spatial orientation constancy. Given the impaired spatial orientation constancy found in hemineglect, we sought to determine whether similar disturbances could be observed in anorexic patients. We investigated the effect of passive lateral body inclination on the tactile subjective vertical (SV). Fifty participants (25 AN patients and 25 healthy controls) were asked to manually set a rod into the vertical position under three postural conditions. For tilted conditions, we observed a significant deviation of the tactile SV towards the body. This effect was abnormally accentuated in AN patients and might be caused by higher weighting with respect to the egocentric frame of reference. Our findings reinforce the role of the PC in AN and suggest that this dysfunction affects spatial orientation constancy as well as body boundaries.     

In your place: neuropsychological evidence for altercentric remapping in embodied perspective taking

Humans are able to mentally adopt the spatial perspective of others and represent the visual world from their point of view. Here, we present neuropsychological evidence that information inaccessible from an egocentric perspective can be accessed from the perspective of another person. Patients affected by left neglect were asked to describe arrays of objects from their own egocentric perspective, from an opposite perspective (disembodied perspective taking), and from the point of view of another person actually seated in front of them (embodied perspective taking). Although disembodied perspective-taking ameliorated neglect severity, there was an even stronger positive effect of embodied perspective-taking: items presented on the left and neglected when reported from the egocentric perspective were instead recovered when patients assumed the perspective of the other. These findings suggest that perspective-taking entails an altercentric remapping of space, i.e. remapping of objects and locations coded with reference to the other person''s body.     

Convergent and divergent effects of neck proprioceptive and visual motion stimulation on visual space processing in neglect

Visual motion stimulation as well as neck muscle vibration are known to effectively modulate the subjective body orientation in spatial neglect. However, so far only motion stimulation has been demonstrated to substantially influence size and space distortion in neglect patients. The present study aimed to compare the two stimulation methods with respect to their potentially differential impact on subjective body orientation and on space and size distortion, in five neglect patients showing perceptual distortions. We found comparable beneficial effects during left motion stimulation and left neck vibration for the subjective straight ahead. Additionally, left motion stimulation significantly ameliorated the leftward overextension in size matching, line bisection and distance estimation in all five patients. In contrast, during neck vibration only two patients showed an improvement for line bisection and size estimation and none did so for distance estimation. Since these two patients differed from the others as they had either no visual field defects or a major visual field sparing, we suggest--based on recent anatomical and neuropsychological findings--that neck vibration only tends to improve pure neglect-related visuo-perceptual distortions whereas motion stimulation can additionally improve perceptual distortions in neglect associated with hemianopia.     

Both egocentric and allocentric cues support spatial priming in visual search

The perception-action model proposes that vision for perception and vision for action are subserved by two separate cortical systems, the ventral and dorsal streams, respectively [Milner, A. D., & Goodale, M. A. (1995). The visual brain in action (1st ed.). Oxford: Oxford University Press; Milner, A. D., & Goodale, M. A. (2006). The visual brain in action (2nd ed.). Oxford: Oxford University Press Inc.]. The dorsal stream codes spatial information egocentrically, that is, relative to the observer. Egocentric representations are argued to be highly transient; therefore, it might be expected that egocentric information cannot be used for spatial memory tasks, even when the visual information only needs to be retained for a few seconds. Here, by applying a spatial priming paradigm to a visual search task, we investigated whether short-term spatial memory can use egocentric information. Spatial priming manifests itself in speeded detection times for a target when that target appears in the same location it previously appeared in. Target locations can be defined in either egocentric or allocentric (i.e. relative to other items in the display) frames of reference; however, it is unclear which of these are used in spatial priming, or if both are. Our results show that both allocentric and egocentric cues were used in spatial priming, and that egocentric cues were in fact more effective than allocentric cues for short-term priming. We conclude that egocentric information can persist for several seconds; a conclusion which is at odds with the assumption of the perception-action model that egocentric representations are highly transient.