Search

The so-called action vs. perception model represents one of the currently dominating models addressing visual processing in primates. One of the crucial cornerstones of the action vs. perception model of visual processing is the dissociation of impaired perception versus intact visuomotor control in neurological patients with visual form agnosia (VFA). In fact, virtually all evidence related to VFA supporting the model was reported from only one patient: patient D.F. Through the last two decades D.F. became as important as only very few other exemplar cases in the neurosciences. However, a large corpus of experiments with this individual used methods that were insufficient to reveal less obvious impairments on a single subject level. We reanalysed the data of D.F. and identified basic visuomotor impairments that had been overlooked so far. Our reanalysis underlines the fact that the widespread and popular presentation of strong dissociations between distinct visual systems seems to be exaggerated.

The integration of visual elements into global perception seems to be implemented separately to single object perception. This assumption is supported by the existence of patients with simultanagnosia who can identify single objects but are incapable of integrating multiple visual items. We investigated a case of simultanagnosia due to posterior cortical atrophy without structural brain damage who demonstrated an incomplete simultanagnosia. The patient successfully recognized a global stimulus in one trial but failed to do so just a few seconds later. Using event-related fMRI, we contrasted post hoc selected trials of successful global perception with trials of global recognition failure. We found circumscribed clusters of activity at the right and left primary intermediate sulci and a bilateral cluster at the ventral precuneus. The integration of multiple visual elements resulting in a conscious perception of their gestalt seems to rely on these bilateral structures in the human lateral and medial inferior parietal cortex.

Lesions of the parieto-occipital junction (POJ) in humans cause gross deviations of reaching movements and impaired grip formation if the targets are located in the subjects’ peripheral visual field. Movements to central targets are typically less impaired. This disorder has been termed “optic ataxia”. It has been suggested that a general deficit of online corrections under central as well as peripheral viewing conditions might be sufficient to explain this discrepancy. According to this hypothesis, patients with optic ataxia should demonstrate an impaired online correction of grip aperture under central viewing conditions if the target object changes its size during the grasping movement. We investigated this prediction in a patient with optic ataxia (I.G.) in a virtual visuo-haptic grasping task. We imposed an isolated need for online corrections of the hand aperture independently of positional changes of the target object. While we found some general inaccuracies of her grasping movements, the patient d id not show a specific impairment of online adjustment of grip aperture. On the contrary, I.G. smoothly adjusted her grip aperture comparable to healthy subjects. A general deficit of fast movement correction affecting targets in peripheral as well as central visual fields thus does not appear to account for the overt visuomotor deficits in optic ataxia. Rather, it seems more likely that an anatomical dissociation between visuomotor pathways related to actions in the central and in the peripheral visual field underlies the dissociation of visuomotor performance depending on the retinotopic target position in optic ataxia.

M. Himmelbach a,∗, H.-O. Karnath a, M.-T. Perenin b,c a Section Neuropsychology, Center for Neurology, Hertie-Institute for Clinical Brain Research, University of Tubingen, Hoppe-Seyler-Str. 3, 72076 Tubingen, Germany b Department of Integrative Neurosciences, INSERM U846, Stem Cell and Brain Research Institute, 18 Avenue Doyen Lepine, 69675 Bron Cedex, France c Universite Claude Bernard Lyon I, Lyon, France

Sensory representations in the postcentral gyrus are supposed to be strictly lateralised and to provide spatially unbiased representations of limb positions. However, electrophysiological and behavioural measurements in humans and non-human primates tentatively suggested some degree of bilateral processing even in early somatosensory areas. We report a patient who suffered a small and confined lesion of the hand area in the postcentral gyrus that resulted in a proprioceptive deficit without any concomitant primary motor impairment. We performed a finger position-matching task with target locations being defined proprioceptively. Without visual feedback of either hand, the patient demonstrated a significant leftward shift of perceived locations when reaching with the ipsilesional right hand to her contralesional left hand and an opposite rightward shift when reaching with the left hand to the position of the right hand. Although these directional errors improved when vision of the active hand was allowed, errors were still significantly larger than those of age-matched healthy controls with unconstrained view of the active contralesional hand. Reaching to visual targets without visual online feedback the patient revealed comparable errors with both hands. Reaching to visual targets with full visual feedback, she was as accurate as controls with either hand. In summary, our data demonstrate an effect of the right postcentral lesion on proprioceptive information processing for both hands. The results suggest an integration of contralateral and ipsilateral proprioceptive information already at this early processing stage possibly mediated by callosal connections.

Fast goal-directed actions are supposed to be controlled almost exclusively by bottom-up visual control. This mode of processing has been identified with the so-called dorsal visual stream. It is generally accepted that object recognition, mediated by the ventral stream, must be important for deciding what action to execute depending on the specific object to be grasped and the particular purpose. In contrast, the kinematic parameters of the actual movement itself are supposed to be unaffected by recognition processes. This view was recently challenged by the demonstration of a significant impact of object familiarity on grasping kinematics under binocular visual control (McIntosh & Lashley, 2008). This effect was observed for very well known everyday objects. However, it remained unclear whether the effect was really due to long-term, everyday familiarity of the target objects or whether it was simply mediated by short-term learning during the experiment. Therefore, we examined whether the same effect could also be found with objects that were geometrically identical to the ones used by McIntosh and Lashley (2008) and could be distinguished by a pictorial cue but were not associated with long-term, everyday experience. We only found an effect of familiarity under monocular but not under binocular control. Our observation suggests that indeed familiarity exerts an effect on movements under binocular control only if explicit knowledge about the objects is very stable and salient, e.g. after long-term experience.

Patients with optic ataxia after lesions of the occipito-parietal cortex demonstrate gross deviations of movements to visual targets in their peripheral visual field. When the same patients point to remembered target locations their accuracy improves considerably. Taking into account opposite findings in a single patient suffering from visual form agnosia due to bilateral occipito-temporal lesions (D.F.), this paradoxical improvement was attributed to brain structures outside the dorsal stream, and supposed to be specifically associated with delayed movement execution. This conclusion was based on the still unverified assumption that the dorsal system is almost completely lacking any localization function in patients with optic ataxia who demonstrate the paradoxical delay effect. We thus investigated brain activity associated with immediately executed and delayed movements in a patient with optic ataxia due to extensive bilateral lesions (I.G.) and in 16 healthy subjects using functional magnetic resonance imaging. Our analysis revealed robust and indistinguishable activation of intact dorsal occipital and parietal areas adjacent to the patient's lesions for both types of movements. In healthy subjects, we found the same visuomotor network activated during immediate and delayed movements as well as additionally higher signal increases for movements to visible targets than for delayed movements in bilateral occipito-parietal and occipito-temporal areas. Our results suggest that in healthy subjects as well as in the optic ataxia patient I.G. dorsal areas are not only involved in immediate but also in delayed reaching. This observation questions the hypothesis that residual visuospatial abilities in patients with optic ataxia could only be mediated by a system outside of the dorsal stream.