The role of ventral stream areas for viewpoint-invariant object recognition.

Object constancy is one of the most crucial mechanisms of the human visual system enabling viewpoint invariant object recognition. However, the neuronal foundations of object constancy are widely unknown. Research has shown that the ventral visual stream is involved in processing of various kinds of object stimuli and that several regions along the ventral stream are possibly sensitive to the orientation of an object in space. To systematically address the question of viewpoint sensitive object perception, we conducted a study with stroke patients as well as an fMRI experiment with healthy participants applying object stimuli in several spatial orientations, for example in typical and atypical viewing conditions. In the fMRI experiment, we found stronger BOLD signals and above-chance classification accuracies for objects presented in atypical viewing conditions in fusiform face sensitive and lateral occipito-temporal object preferring areas. In the behavioral patient study, we observed that lesions of the right fusiform gyrus were associated with lower performance in object recognition for atypical views. The complementary results from both experiments emphasize the contributions of fusiform and lateral-occipital areas to visual object constancy and indicate that visual object constancy is particularly enabled through increased neuronal activity and specific activation patterns for objects in demanding viewing conditions.

Temporo-parietal brain regions are involved in higher order object perception.

Lesions to posterior temporo-parietal brain regions are associated with deficits in perception of global, hierarchical shapes, but also with impairments in the processing of objects presented under demanding viewing conditions. Evidence from neuroimaging studies and lesion patterns observed in patients with simultanagnosia and agnosia for object orientation suggest similar brain regions to be involved in perception of global shapes and processing of objects in atypical ('non-canonical') orientation. In a localizer experiment, we identified individual temporo-parietal brain areas involved in global shape perception and found significantly higher BOLD signals during the processing of non-canonical compared to canonical objects. In a multivariate approach, we demonstrated that posterior temporo-parietal brain areas show distinct voxel patterns for non-canonical and canonical objects and that voxel patterns of global shapes are more similar to those of objects in non-canonical compared to canonical viewing conditions. These results suggest that temporo-parietal brain areas are not only involved in global shape perception but might serve a more general mechanism of complex object perception. Our results challenge a strict attribution of object processing to the ventral visual stream by suggesting specific dorsal contributions in more demanding viewing conditions.

Tilted 3D visual scenes influence lateropulsion: A single case study of pusher syndrome.

INTRODUCTION: Hemiparetic stroke patients with so-called "pusher syndrome" (synonyms: contraversive lateropulsion, contraversive pushing) use their non-paretic extremities to push toward their paralyzed side and actively resist external posture correction. The disorder is associated with a distorted perception of postural vertical combined with a maintained, or little deviating perception of visual upright. With the aim of reducing this mismatch, and thus reducing contraversive lateropulsion, we manipulated the orientation of visual input in a virtual reality setup. METHOD: We presented healthy subjects and an acute stroke patient with severe pusher syndrome a 3D visual scene that was either upright or tilted in roll plane by 20°. By moving the sitting participants in roll plane to the left and right, we assessed the occurrence of contraversive lateropulsion, namely the active resistance to external posture manipulation. RESULTS: With the 3D visual scene oriented upright, the patient with pusher syndrome showed the typical active resistance against tilts toward the ipsilesional side. He used his non-paretic arm to block the examiner's attempt to move the body axis toward that side. With the visual scene tilted to the ipsiversive left, his pathological resistance was significantly reduced. Statistically, the tolerated body tilt angles no longer differed from those of healthy subjects. CONCLUSIONS: We conclude that even short presentations of tilted 3D visual input can reduce symptoms of severe contraversive lateropulsion. The technique provides potential for a new treatment method of pusher syndrome and offers a simple, straightforward approach that can be effortlessly integrated in clinical practice. TRIAL REGISTRATION: German Clinical Trials Register (DRKS00026700).

Hemifield-specific color perception deficits after unilateral V4α lesions.

Functional neuroimaging and patient studies demonstrated significant involvement of ventral area V4α, located in the anterior ventral pathway, in color vision. A low number of case studies reported lesions in close vicinity to this region leading to symptoms of hemiachromatopsia indicating hemifield-specific processing of color information. With the present study, we present the first group study investigating hemiachromatopsia after injury to anterior ventral brain areas. In lateral stimulus presentations with several color perception tasks, we observed symptoms of hemiachromatopsia, which were specific to patients with unilateral lesions to the ventral pathway. Particularly, we identified unilateral lesions to area V4α as an important contribution to color perception deficits under demanding viewing conditions. Our results suggest that color information processed along the anterior ventral path is hemifield-specific and that selective deficits in color perception cannot be fully compensated by the intact contralesional visual stream.

Changes in the perception of upright body orientation with age.

To determine own upright body orientation the brain creates a sense of verticality by a combination of multisensory inputs. To test whether this process is affected by aging, we placed younger and older adults on a motion platform and systematically tilted the orientation of their visual surroundings by using an augmented reality setup. In a series of trials, participants adjusted the orientation of the platform until they perceived themselves to be upright. Tilting the visual scene around the roll axis induced a bias in subjective postural vertical determination in the direction of scene tilt in both groups. In the group of older participants, however, the observed peak bias was larger and occurred at larger visual tilt angles. This indicates that the susceptibility to visually induced biases increases with age, possibly caused by a reduced reliability of sensory information.