Right-hemispheric dominance for visual remapping in humans.

We review evidence showing a right-hemispheric dominance for visuo-spatial processing and representation in humans. Accordingly, visual disorganization symptoms (intuitively related to remapping impairments) are observed in both neglect and constructional apraxia. More specifically, we review findings from the intervening saccade paradigm in humans--and present additional original data--which suggest a specific role of the asymmetrical network at the temporo-parietal junction (TPJ) in the right hemisphere in visual remapping: following damage to the right dorsal posterior parietal cortex (PPC) as well as part of the corpus callosum connecting the PPC to the frontal lobes, patient OK in a double-step saccadic task exhibited an impairment when the second saccade had to be directed rightward. This singular and lateralized deficit cannot result solely from the patient's cortical lesion and, therefore, we propose that it is due to his callosal lesion that may specifically interrupt the interhemispheric transfer of information necessary to execute accurate rightward saccades towards a remapped target location. This suggests a specialized right-hemispheric network for visuo-spatial remapping that subsequently transfers target location information to downstream planning regions, which are symmetrically organized.

Correlated deficits of perception and action in optic ataxia.

Optic ataxia, following dorsal stream lesions, is characterised by impaired visuomotor guidance. Recent studies have found concurrent perceptual deficits, but it is unclear whether these are functionally related to the visuomotor symptoms. We studied the ability of a well-documented patient (IG) with bilateral optic ataxia to react to sudden target jumps by correcting ongoing reaches or by explicitly reporting the jump direction. IG showed deficient reach corrections, especially for target jumps to the visual periphery, and was similarly slow to discriminate the same jumps perceptually. Across six test conditions, in which the retinal locations of target jumps were varied, her perceptual slowing mirrored her reaching deficit precisely. These findings confirm perceptual impairments after dorsal stream lesions, and imply a shared functional basis with the classical visuomotor symptoms of optic ataxia. Additionally, we show that the online correction deficit is determined dually by the retinal location to which the reach must be diverted, and the location to which it is initially directed. We suggest that this deficit, and its perceptual counterpart, can be traced to a slowed contralesional orienting of attention in optic ataxia.

Parietal damage dissociates saccade planning from presaccadic perceptual facilitation.

A well-known theory in the field of attention today is the premotor theory of attention which suggests that the mechanisms involved in eye movements are the same as those for spatial attention shifts. We tested a parietal damaged patient with unilateral optic ataxia and 4 controls on a dual saccade/attentional task and show a dissociation between saccadic eye movements and presaccadic perceptual enhancement at the saccade goal. Remarkably, though the patient was able to make the appropriate saccades to the left, impaired visual field (undistinguishable from saccades to his right, intact visual field), he was unable to discriminate the letter at the saccade goal (whereas his performance was like controls for letter discrimination in his right visual field). This suggests that saccade planning and presaccadic perceptual facilitation are separable--planning a saccade to a location does not necessitate that the processing of this location is enhanced. Based on these results, we suggest that the parietal cortex is necessary for the coupling between saccade planning and presaccadic perceptual facilitation.

Parietal modules for reaching.

Optic ataxia (OA) is classically defined as a deficit of visually guided movements that follows lesions of the posterior part of the posterior parietal cortex (PPC). Since the formalisation of the double stream of visual information processing [Milner, A. D., & Goodale, M. A. (1995). The visual brain in action. Oxford: Oxford University Press] and the use of OA as an argument in favour of the involvement of the posterior parietal cortex (dorsal stream) in visually guided movements, many studies have looked at the visuomotor deficits of these patients. In parallel, the development of neuroimaging methods have led to increasing information about the role of the posterior parietal cortex in visually guided actions. In this article, we discuss the similarities and differences in the results that emerged from these two complementary viewpoints by combining a meta-analysis of neuroimaging data on reaching with lesion studies from OA patients and results of our own fMRI study on reaching in the ipsi- and contra-lateral visual field. We identified four bilateral parietal foci from the meta-analysis and found that the more posterior foci showed greater lateralisation for contralateral visual stimulation than more anterior ones Additionally, the more anterior foci showed greater lateralisation for the use of the contralateral hand than the more posterior ones. Therefore, we can demonstrate that they are organised along a postero-anterior gradient of visual-to-somatic information integration. Furthermore, from the combination of imaging and lesion data it can be inferred that a lesion of the three most posterior foci responsible for the target-hand integration could explain the hand and field effect revealed in OA reaching behaviour.

Optic ataxia is not only 'optic': impaired spatial integration of proprioceptive information.

Optic ataxia is considered to be a specific visuo-manual guidance deficit, which combines pointing errors due to the use of the contralesional hand ("hand effect") and to the presentation of the visual target in the contralesional field ("field effect"). The nature of the hand effect has not been identified. The field effect is acknowledged as an impaired spatial integration of visual target location. However, spatial integration of proprioceptive information from the arm has never been experimentally tested in these patients. Here, we specifically investigated the capacity of two patients with unilateral optic ataxia in tasks requiring different levels of proprioceptive integration from primary information processing to proprioceptivo-motor integration. In a first experiment -proprioceptive pointing with the ipsilesional hand toward the index finger of the contralesional hand- revealed a large mislocalisation of the ataxic hand accounting for the hand effect. In a second experiment -proprioceptive pointing with the ataxic arm toward the finger of the ipsilesional hand- revealed reaching errors for non-visual targets, i.e. optic ataxia is not specific to 'optic' targets. Altogether, the present results call for a redefinition of this neurological condition in the framework of parietal functions.