We examined a stroke patient (HWS) with a unilateral lesion of the right medial ventral visual stream, involving the right fusiform and parahippocampal gyri. In a number of object recognition tests with lateralized presentations of target stimuli, HWS showed significant symptoms of hemiagnosia with contralesional recognition deficits for everyday objects. We further explored the patient's capacities of visual expertise that were acquired before the current perceptual impairment became effective. We confronted him with objects he was an expert for already before stroke onset and compared this performance with the recognition of familiar everyday objects. HWS was able to identify significantly more of the specific (“expert”) than of the everyday objects on the affected contralesional side. This observation of better expert object recognition in visual hemiagnosia allows for several interpretations. The results may be caused by enhanced information processing for expert objects in the ventral system in the affected or the intact hemisphere. Expert knowledge could trigger top–down mechanisms supporting object recognition despite of impaired basic functions of object processing. More importantly, the current work demonstrates that top–down mechanisms of visual expertise influence object recognition at an early stage, probably before visual object information propagates to modules of higher object recognition. Because HWS showed a lesion to the fusiform gyrus and spared capacities of expert object recognition, the current study emphasizes possible contributions of areas outside the ventral stream to visual expertise.

The neuropsychological syndrome “simultanagnosia” is characterized by the inability to integrate local elements into a global entity. This deficit in Gestalt perception is mainly apparent for novel global structures administered in clinical tests or unfamiliar visual scenes. Recognition of familiar complex objects or well-known visual scenes is often unaffected. Recent neuroimaging studies and reports from simultanagnosia patients suggest a crucial involvement of temporoparietal brain areas in processing of hierarchically organized visual material. In this study, we investigated the specific role of the TPJ in Gestalt perception. On the basis of perceptual characteristics known from simultanagnosia, we hypothesized that TPJ is dominantly involved in processing of novel object arrangements. To answer this question, we performed a learning study with hierarchical stimuli and tested behavioral and neuronal characteristics of Gestalt perception pre- and posttraining. The study included 16 psychophysical training sessions and two neuroimaging sessions. Participants improved their behavioral performance for trained global stimuli and showed limited transfer to untrained global material. We found significant training dependent neuronal signal modulations in anterior right hemispheric TPJ regions. These activation changes were specific to trained global stimuli, whereas no systematic neuronal response changes were observed for recognition of untrained global stimuli, local elements and regular objects that served as control stimuli. In line with perceptual characteristics in simultanagnosia, the results argue for an involvement of TPJ in processing of novel global structures. We discuss the signal modulations in the context of a more efficient or different neuronal strategy to process familiar global stimuli.

In monkeys and humans, two functionally specialized cortical streams of visual processing emanating from V1 have been proposed: a dorsal, action-related system and a ventral, perception-related pathway. Traditionally, a separate organization of the two streams is assumed; the extent of functional interaction is unknown. After lesions of the dorsal stream in patients with optic ataxia, it has recently been shown that the ventral perception-related system might contribute to visuo-motor processing if movements rely on remembered target positions. The ventral pathway thus seemed to participate in goal-directed movements, a function that previously has been assigned exclusively to the dorsal stream. We wondered whether different types of pointing movements are controlled by switching between two separated cortical pathways or whether a variable interaction of interconnected systems should be assumed. Our study investigated two acute stroke patients with optic ataxia following lesions of the dorsal stream in a delayed pointing task. The delays ranged from 0 to 10 sec. The patients' pointing error decreased in a linear manner with the length of time. The finding suggests a gradual change between dorsal and ventral control of reaching behavior, rather than a sudden switch between two separated cortical processing streams. Although our observations with two patients require further validation, the results suggest that the ventral and dorsal systems interact closely in the sensorimotor control of reaching behavior.

Patients with spatial neglect exhibit a severe shift of spontaneous explorative movements to the right side, indicating a bias of long-living representations of space. Whether or not goal-directed movements likewise are affected by this rightward bias has been controversially discussed throughout the last decade. Unfortunately, substantial differences regarding patient selection and data analysis prevented a direct comparison of these results. We thus studied pointing movements in a new sample of subjects covering all different patient groups previously investigated on this issue. We analyzed all the different measures of hand path curvature used so far and, in addition, suggest a new measure that avoids the disadvantages of the previously used parameters. Despite their severe bias for exploratory movements, we did not find systematic, direction-specific deviations of goal-directed hand movements that were specific for the patients with spatial neglect. The results strongly suggest that the disturbance of long-living spatial representations underlying the bias of exploratory behavior in patients with neglect does not influence the performance of goal-directed movements. The data support the view of a dual mode of space representation in the posterior parietal and the superior temporal cortex.