Unilateral posterior parietal lobe lesions affect representation of visual space

Perception of Upright: Multisensory Convergence and the Role of Temporo-Parietal Cortex

We inherently maintain a stable perception of the world despite frequent changes in the head, eye, and body positions. Such "orientation constancy" is a prerequisite for coherent spatial perception and sensorimotor planning. As a multimodal sensory reference, perception of upright represents neural processes that subserve orientation constancy through integration of sensory information encoding the eye, head, and body positions. Although perception of upright is distinct from perception of body orientation, they share similar neural substrates within the cerebral cortical networks involved in perception of spatial orientation. These cortical networks, mainly within the temporo-parietal junction, are crucial for multisensory processing and integration that generate sensory reference frames for coherent perception of self-position and extrapersonal space transformations. In this review, we focus on these neural mechanisms and discuss (i) neurobehavioral aspects of orientation constancy, (ii) sensory models that address the neurophysiology underlying perception of upright, and (iii) the current evidence for the role of cerebral cortex in perception of upright and orientation constancy, including findings from the neurological disorders that affect cortical function.

Functional neuro-anatomy of egocentric versus allocentric space representation

INTRODUCTION

The functional neuroanatomy of the egocentric and allocentric representations of space remains poorly studied with neuroimaging. Here we aim to determine brain structures subserving two different kinds of spatial representations centred on the main axis of either the body or the external scene.

METHOD

Sixteen healthy participants evaluated the alignment of a bar relative to the middle of their body (Ego) or relative to another stimulus (Allo) during functional MRI. In a control task (Ctrl), they had to judge the colour of the bar.

RESULTS

Correct response rates and response times were similar in the three tasks. fMRI data revealed a predominant role of the right hemisphere in the egocentric task (Ego vs. Allo): selective activity was found in the occipital, superior parietal, and inferior frontal cortices, as well as in the precuneus and supplementary motor area. On the left side, the insula, thalamus, and cerebellum were also activated. Conversely, the allocentric task (Allo vs. Ctrl) showed selective activity centred on the left temporal gyrus.

DISCUSSION

This study demonstrates a right hemisphere dominance for representations centred on the longitudinal body axis, but more left-sided activity for scene/object-centred representations of space. These new data shed light on the unique role of several regions involved in spatial perception and help better understand spatial deficits in patients with right hemispheric lesions.

Spatial perception errors do not predict pointing errors by individuals with brain lesions

We examined the relationship between errors in sensorimotor transformations (SMT) for reaching to external targets and visual and kinesthetic spatial perception of those targets by participants with damage to the posterior parietal lobule (PPL) and adjacent areas. A total of 12 participants had focal PPL lesions, 7 had with brain lesions outside the PPL, and 10 had no brain lesions. Brain lesions were due to stroke (12) or surgical resection (7). Participants made visually and kinesthetically guided pointing movements to 16 targets with the ipsilesional hand (to minimize effects of motor deficits). Head and eye movements were unrestricted. When full vision was allowed, all participants pointed accurately to visually specified targets. However, participants with PPL lesions made larger vertical plane errors than did neurologically intact control participants in visual and kinesthetic perceptual tasks and in SMT tasks (in which participants reached without vision to targets that had been visually presented). However, across all 29 participants, the size and direction of SMT errors showed low correlation with visual and kinesthetic perceptual errors. Moreover, participants with PPL lesions who made large visual and/or kinesthetic perceptual errors to individual targets also showed weak correlations with SMT errors to those targets. The results are compatible with theories that different neural pathways support perception and action for both the visual and the kinesthetic systems.

Unilateral posterior parietal lobe lesions disrupt kinaesthetic representation of forearm orientation

OBJECTIVE

To apply the lesion method to assess neuroanatomical substrates for judgments of forearm orientation from proprioceptive cues in humans.

METHODS

Participants were 15 subjects with chronic unilateral brain lesions and stable behavioural deficits, and 14 neurologically normal controls. Subjects aligned the forearm to earth fixed vertical and trunk fixed anterior-posterior (A-P) axes ("straight ahead"), with the head aligned to the trunk and with head and shoulder orientations varied on each trial.

RESULTS

Most subjects with posterior parietal lobe lesions made larger variable errors than controls in aligning the forearm to the earth fixed vertical axis and the trunk A-P axes, whether the head was held upright or oriented in different positions. Lesion subjects and controls made similar constant errors for aligning the forearm to gravitational vertical. Variable error magnitude correlated positively with greater lesion volume of right and left superior parietal lobules (SPL), but not with lesions in other brain areas. Larger variable errors for aligning the forearm to the trunk fixed A-P axis were also correlated with the volume of SPL lesions, but constant error magnitude correlated with larger volume lesions in premotor areas, inferior parietal lobules, and posterior regions of the superior temporal gyri, but not with SPL lesion volume.

CONCLUSIONS

The findings suggest that the right and left superior and inferior parietal lobules, posterior superior temporal gyri, and premotor areas play a role in defining higher level coordinate systems for specifying orientation of the right and left forearm.