Egocentric body-centered coordinates modulate visuomotor performance

The effect of a trunk stabilization exercise program using weight loads on balance and gait in stroke patients: A randomized controlled study

BACKGROUND: The effects of trunk stabilization exercise programs and exercises applying weight loads for stroke patients are well presented. However, there is not enough objective research to prove the effects of trunk stabilization exercise programs using weight loads for stroke patients. OBJECTIVE: In this study, a trunk stabilization exercise program using weight loads was performed to investigate its impact on the balance and gait ability of stroke patients. METHODS: Thirty patients with stroke were randomly divided into three groups. Experiment group 1 followed a trunk stabilization exercise program using weight loads, experiment group 2 followed a trunk stabilization exercise program, and the control group followed general physiotherapy. Patients’ balance ability was assessed using balance measurement equipment BT-4 (postural sway area (PSA), postural sway length (PSL), limit of stability (LOS)), Berg Balance Scale (BBS), and Time Up and Go test (TUG). Gait speed was measured to examine gait ability. RESULT: After the intervention, the PSA, PFSL in experimental groups 1 and 2 decreased but that of the control group increased. BBS, TUG, and LOS scores of experimental group 1 were significantly improved compared to experimental group 2 and the control group. Also, gait speed was significantly improved in experimental group 1 compared to experimental group 2 and the control group. CONCULSION: This study may be used as a basic material for an effective trunk stabilization exercise method for stroke patients and might be of significance as an intervention method for stroke patients requiring long-term treatment.

The vestibular system modulates the contributions of head and torso to egocentric spatial judgements

Egocentric representations allow us to describe the external world as experienced from an individual's bodily location. We recently developed a novel method of quantifying the weight given to different body parts in egocentric judgments (the Misalignment Paradigm). We found that both head and torso contribute to simple alter-egocentric spatial judgments. We hypothesised that artificial stimulation of the vestibular system would provide a head-related signal, which might affect the weighting given to the head in egocentric spatial judgments. Bipolar Galvanic Vestibular Stimulation (GVS) was applied during the Misalignment Paradigm. A Sham stimulation condition was also included to control for non-specific effects. Our data show that the weight given to the head was increased during left anodal and right cathodal GVS, compared to the opposite GVS polarity (right anodal and left cathodal GVS) and Sham stimulation. That is, the polarity of GVS, which preferentially activates vestibular areas in the right cerebral hemisphere, influenced the relative weightings of head and torso in egocentric spatial judgments.

Auditory perception is influenced by the orientation of the trunk relative to a sound source

The study investigated how hearing depends on the whole body, head and trunk orientation relative to a sound source. In normal hearing humans we examined auditory thresholds and their ability to recognize logatomes (bi-syllabic non-sense words) at different whole body, head and trunk rotation relative to a sound source. We found that auditory threshold was increased and logatome recognition was impaired when the body or the trunk were rotated 40° away from a sound source compared to when the body or the trunk was oriented towards the sound source. Conversely, no effects were seen when only the head was rotated. Further, an increase of thresholds and impairment of logatome recognition were also observed after unilateral vibration of dorsal neck muscles that induces, per se, long-lasting illusory trunk displacement relative to the head. Thus, our findings support the idea that processing of acoustic signals depends on where a sound is located within a reference system defined by the subject's trunk coordinates.

Shared contributions of the head and torso to spatial reference frames across spatial judgments

Egocentric frames of reference take the body as the point of origin of a spatial coordinate system. Bodies, however, are not points, but extended objects, with distinct parts that can move independently of one another. We recently developed a novel paradigm to probe the use of different body parts in simple spatial judgments, what we called the misalignment paradigm. In this study, we applied the misalignment paradigm in a perspective-taking task to investigate whether the weightings given to different body parts are shared across different spatial judgments involving different spatial axes. Participants saw birds-eye images of a person with their head rotated 45° relative to the torso. On each trial, a ball appeared and participants made judgments either of whether the ball was to the person's left or right, or whether the ball was in front of the person or behind them. By analysing the pattern of responses with respect to both head and torso, we quantified the contribution of each body part to the reference frames underlying each judgment. For both judgment types we found clear contributions of both head and torso, with more weight being given on average to the torso. Individual differences in the use of the two body parts were correlated across judgment types indicating the use of a shared set of weightings used across spatial axes and judgments. Moreover, retesting of participants several months later showed high stability of these weightings, suggesting that they are stable characteristics of people.

Dissociating contributions of head and torso to spatial reference frames: The misalignment paradigm

When we represent someone's view of a scene as egocentrically structured, where do we represent the origin of the reference frame? By analysing responses in a spatial perspective-taking task as a function of spatial location with respect to both head and torso, we isolated the respective contribution of each part to spatial judgments. Both the head and the torso contributed to judgements, though with greater contributions from the torso. A second experiment manipulating visual contrast of the torso showed that this does not reflect low-level differences in visual salience between body parts. Our results demonstrate that spatial perspective-taking relies on a weighted combination of reference frames centred on different parts of the body.

Trunk rotation affects temporal order judgments with direct saccades: Influence of handedness

Manipulation of the trunk midline has been shown to improve visuospatial performance in patients with unilateral visual neglect. The goal of the present study was to disentangle motor and perceptual components of egocentric midline manipulations and to investigate the contribution of individual hand preference. Two versions of visual temporal order judgment (TOJ) tasks were tested in healthy right- and left-handed subjects while trunk rotation was varied. In the congruent version, subjects were required to execute a saccade to the first of two horizontal stimuli presented with different stimulus onset asynchronies (SOA). In the incongruent version, subjects were required to perform a vertical saccade to a pre-learned color target, thereby dissociating motor response from the perceptual stimulus location. The main findings of this study are a trunk rotation and response direction specific impact on temporal judgments in form of a prior entry bias for right hemifield stimuli during rightward trunk rotation, but only in the congruent task. This trunk rotation-induced spatial bias was most pronounced in left-handed participants but had the same sign in the right-handed group. Results suggest that egocentric midline shifts in healthy subjects induce a spatially-specific motor, but not a perceptual, bias and underline the importance of taking individual differences in functional laterality such as handedness and mode of perceptual report into account when evaluating effects of trunk rotation in either healthy subjects or neurological patients.

Parkinson's disease as a disconnection syndrome

Parkinson's disease (PD) is a major neurodegenerative disorder that is usually considered in terms of midbrain and basal ganglia dysfunction. Regarding PD instead as a disconnection syndrome may prove beneficial to understanding aspects of cognition, perception, and other neuropsychological domains in the disease. PD is usually of unilateral onset, providing evidence of intrahemispheric dissociations and an imbalance in the usual relative strengths of the right and left hemispheres. Hence, in order to appreciate the neuropsychology of PD, it is important to apply to this disease our understanding of hemispheric lateralization effects and within-hemisphere circuitry from brainstem to higher-order association cortex. The focus of this review is on the relevance of PD-related disconnections among subcortical and cortical structures to cognition, perception, emotion, and associated brainstem-based domains such as sleep and mood disturbance. Besides providing information on disease characteristics, regarding PD as a disconnection syndrome allows us to more completely understand normal brain-behavior relations in general.

Impact of optic flow perception and egocentric coordinates on veering in Parkinson's disease

Spatial navigation is a complex process requiring integration of visuoperceptual information. The present study examined how visuospatial function relates to navigational veering in Parkinson's disease, a movement disorder in which visuospatial cognition is affected by the degeneration of the basal ganglia and resulting dysfunction of the parietal lobes. We hypothesized that patients whose initial motor symptoms start on the left versus right side of the body (LPD, predominant right-hemisphere dysfunction; RPD, predominant left-hemisphere dysfunction) would display distinct patterns of navigational veering associated with the groups' dissimilar visuospatial profiles. Of particular interest was to examine the association of navigational veering (lateral deviation along the medio-lateral axis) with perception of egocentric coordinates and of radial optic flow patterns, both of which are mediated by the parietal lobes. Thirty-one non-demented Parkinson's disease patients (16 LPD, 15 RPD) and 18 healthy control (HC) adults received visuospatial tests, of whom 23 Parkinson's disease patients and 17 HC also underwent veering assessment. The participants were examined on three visual-feedback navigation conditions: none (eyes closed), natural, and optic flow supplied by a virtual-reality headset. All groups veered to the left when walking with eyes closed, women with Parkinson's disease more so than the other participants. On the navigation assessments with visual feedback, only LPD patients deviated right of centre. On tests of visuospatial function, the perceived midline was shifted rightward in LPD (men and women), increasingly so with the addition of visual input. In contrast, men with RPD showed leftward deviation. RPD patients and HC perceived optic flow in the left hemifield as faster than in the right hemifield, with a trend for the opposite pattern for LPD. Navigational veering in LPD was associated with deviation of the perceived egocentric midline and not with perception of optic flow speed asymmetries, and in RPD it was also associated with visual dependence, though in fact LPD subjects were more visually dependent than those with RPD. Our results indicate that (i) parietal-mediated perception of visual space is affected in Parkinson's disease, with both side of motor symptom onset and gender affecting spatial performance, and (ii) visual input affects veering.

Spatial span under translation: A study of reference frames

The nature of the reference frame used to remember location sequences in a computer-presented version of spatial span was investigated by moving the template (a rectangular frame enclosing nine target squares) across the screen during presentation and/or during recall. Movement of the display during presentation substantially impaired memory in comparison with a stationary display (Experiment 1). However, there was no effect of template movement during recall (Experiment 2). In Experiments 3 and 4, the template was moved through the same screen locations during presentation and recall. When the extrinsic, or screen location, of each position was repeated identically on each trial but the sequence on the template varied, learning was not facilitated (Experiment 3). When the template sequences were repeated across trials but extrinsic location varied, the sequences were rapidly learned (Experiment 4). In this version of spatial span, location sequences appear to be encoded in an intrinsic frame of reference that is based on the template. Movement of the template during encoding impairs this process, possibly because concurrent attention shifts prevent the encoding of locations. The results are discussed with respect to recent studies of positional encoding in which multiple reference frames were available.

Influence of subjective visual vertical misperception on balance recovery after stroke

BACKGROUND

Subjective visual vertical (SVV) perception can be perturbed after stroke, but its effect on balance recovery is not yet known.

AIM

To evaluate the influence of SVV perturbations on balance recovery after stroke.

METHODS

28 patients (14 with a right hemisphere lesion (RHL) and 14 with a left hemisphere lesion (LHL)) were included, 5 were lost to follow-up. SVV perception was initially tested within 3 months after stroke, then at 6 months, using a luminous line, which the patients adjusted to the vertical position in a dark room. Mean deviation (V) and uncertainty (U), defined as the standard deviation of the SVV, were calculated for eight trials. Balance was initially assessed by the Postural Assessment Scale for Stroke (PASS), and at 6 months by the PASS (PASS6), a force platform (lateral and sagittal stability limits (LSL6 and SSL6)), the Rivermead Mobility Index (RMI6) and gait velocity (v6). Functional outcome was also assessed by the Functional Independence Measure at 6 months (FIM6).

RESULTS

The scores for balance and for FIM6 were related to the initial V value: PASS6 (p = 0.01, tau = -0.38); RMI6 (p = 0.002, tau = -0.48), LSL6 (p = 0.06, tau = -0.29), SSL6 (p = 0.004, tau = -0.43), v6 (p = 0.01, tau = -0.36) and FIM6 (p = 0.001, tau = -0.49), as well as to the initial U value: PASS6 (p = 0.03, tau = -0.32), RMI6 (p = 0.02, tau = -0.35), SSL6 (p = 0.005, tau = -0.43) and FIM6 (p = 0.01, tau = -0.38).

CONCLUSIONS

Initial misperception of verticality was related to a poor score for balance after stroke. This relationship seems to be independent of motricity and neglect. Rehabilitation programmes should take into account verticality misperceptions, which could be an important factors influencing balance recovery after stroke.

Functional brain mapping of actual car-driving using [18F]FDG-PET

AIMS

This study aims at identifying the brain activation during actual car-driving on the road, and at comparing the results to those of previous studies on simulated car-driving.

METHODS

Thirty normal volunteers, aged 20 to 56 years, were divided into three subgroups, active driving, passive driving and control groups, for examination by positron emission tomography (PET) and [18F]2-deoxy-2-fluoro-D-glucose (FDG). The active driving subjects (n = 10) drove for 30 minutes on quiet normal roads with a few traffic signals. The passive driving subjects (n = 10) participated as passengers on the front seat. The control subjects (n = 10) remained seated in a lit room with their eyes open. Voxel-based t-statistics were applied using SPM2 to search brain activation among the subgroups mentioned above.

RESULTS

Significant brain activation was detected during active driving in the primary and secondary visual cortices, primary sensorimotor areas, premotor area, parietal association area, cingulate gyrus, the parahippocampal gyrus as well as in thalamus and cerebellum. The passive driving manifested a similar-looking activation pattern, lacking activations in the premotor area, cingulate and parahippocampal gyri and thalamus. Direct comparison of the active and passive driving conditions revealed activation in the cerebellum.

CONCLUSION

The result of actual driving looked similar to that of simulated driving, suggesting that visual perception and visuomotor coordination were the main brain functions while driving. In terms of attention and autonomic arousal, however, it seems there was a significant difference between simulated and actual driving possibly due to risk of accidents. Autonomic and emotional aspects of driving should be studied using an actual driving study-design.

The contribution of spatial remapping impairments to unilateral visual neglect

Left visual neglect following right hemisphere damage is a heterogeneous phenomenon, in which several underlying impairments have been identified. Despite recent advances in understanding the neural and cognitive bases of these impairments, current theories of neglect, particularly those that emphasise attentional deficits, do not explain a number of phenomena, including: 'Ipsilesional' neglect after left orienting. Positive or 'productive' manifestations. Spatial transposition errors. Mislocalisations. Revisiting behaviour during visual search. Lack of awareness for objects toward the contralesional side of space. We propose that these manifestations of neglect can be accounted for by an additional underlying disorder of spatial remapping due to parietal dysfunction. In primary visual areas, retinotopic maps are renewed and thus overwritten at each new ocular fixation. Remapping processes operating in higher-level oculocentric visual maps of the parietal cortex ensure visual integration of these successive retinal images over time and space, by creating a constantly updated representation of stimulus locations in terms of distance and direction from the fovea. They consist in the storage, refreshment and re-localization of the different components of the visual scene that are successively attended during its exploration, and provide spatial constancy of visual perception and a spatial buffer for working memory [Cereb Cortex 5 (1995) 470; Visual Cogn 7 (2000) 17]. We begin this article by reviewing theoretical and experimental arguments that have highlighted the importance of parietal remapping processes in maintaining an accurate representation of space across saccadic shifts. We then focus on findings from the double-step saccade task, [Ann Neurol 38 (1995) 739] as a basis for our model of the role of remapping impairments in many of the symptoms of neglect. From these results, remapping impairments would be demonstrated when a saccade has to be guided across the midline after having fixated an object in either the left or right visual field for patients with either left- or right-side parietal lesions. In addition, patients with right-side lesions will have remapping impairments within the left visual field following a saccade to a left-side target (see Fig. 5). In a large part of the article, we seek to build our hypothesis based on this basic model and more speculative assumptions supported with extensive evidence from the literature.

Right parietal lesions, unilateral spatial neglect, and the egocentric frame of reference

Recently, the hypothesis has been proposed that the crucial mechanism leading to neglect is the disturbed transformation of sensory input into a supramodal egocentric frame of reference (ER), which causes in turn a deviation of this reference frame toward the side ipsilateral to the brain lesion. This egocentric coordinate system is normally centered on the midsagittal plane, but a unilateral brain lesion may cause a deviation of the egocentric reference due to an imbalance between the differentially lateralized neural processes which build this representation. Although neglect signs are often defined in an egocentric frame, and an ER shift may be observed in some right-brain-damaged patients (RBD), I present here several data that rule out any causal link between the deviation of the egocentric reference and the presence and/or severity of left-neglect signs in RBD patients.