Perception of global gestalt by temporal integration in simultanagnosia

Patients with bilateral parieto-occipital brain damage may show intact processing of individual objects, while their perception of multiple objects is disturbed at the same time. The deficit is termed 'simultanagnosia' and has been discussed in the context of restricted visual working memory and impaired visuo-spatial attention. Recent observations indicated that the recognition of global shapes can be modulated by the spatial distance between individual objects in patients with simultanagnosia and thus is not an all-or-nothing phenomenon depending on spatial continuity. However, grouping mechanisms not only require the spatial integration of visual information, but also involve integration processes over time. The present study investigated motion-defined integration mechanisms in two patients with simultanagnosia. We applied hierarchical organized stimuli of global objects that consisted of coherently moving dots ('shape-from-motion'). In addition, we tested the patients' ability to recognize biological motion by presenting characteristic human movements ('point-light-walker'). The data revealed largely preserved perception of biological motion, while the perception of motion-defined shapes was impaired. Our findings suggest separate mechanisms underlying the recognition of biological motion and shapes defined by coherently moving dots. They thus argue against a restriction in the overall capacity of visual working memory over time as a general explanation for the impaired global shape recognition in patients with simultanagnosia.     

Abnormal eye movement behaviour during text reading in neglect syndrome: a case study.

The eye movement behaviour of a patient suffering from a right basal ganglia infarction with a left-sided hemineglect but without any visual field defects was investigated during reading. The eye movements were registered by means of an i.r. light technique (pupil-corneal reflection method). The main findings were abnormal return sweeps. Whereas in normal readers the end of one line of text is linked to the beginning of the new line by a long leftward saccade, the return sweeps of the hemineglect patient stereotypically ended in the middle of the next line. They were followed by sequences of short saccades indicating silent backward reading until a linguistically plausible continuation of sentences from the previous line was found, irrespective of the actual beginning of text. The shortened return sweeps could not be attributed to a general oculomotor disturbance. The spatial border for the occurrence of the patient's abnormal scanning pattern (left half of texts) clearly did not depend on a retinal coordinate frame of reference but rather has to be attributed to a different body-centred reference system.     

Inflexibility of mental planning: a characteristic disorder with prefrontal lobe lesions?

Inflexibility of mental planning processes has repeatedly been discussed to represent a specific disorder resulting from human frontal lobe lesions. Patient groups suffering from acute or chronic unilateral frontal (medial, lateral) or retrorolandic (temporal, parietal) lesions and nonbrain-damaged controls were requested to mirror and to reverse mentally a maze structure that had been learned by the covered maze presentation technique (KARNATH et al., Neuropsychologia 29, 271-290, 1991). The patients with acute frontal lesions were impaired to adapt the acquired mental plan to the new (but related) problem and made more errors than those with acute temporal or parietal lesions and controls. However, no impaired plan modification was detected in patients with chronic frontal brain lesions. The difference of task performance in the acute and chronic (recovered) stage of frontal brain damage is discussed.     

Disturbed line bisection is associated with posterior brain lesions

Neglect patients classically fail to orient and respond to stimuli appearing on their contralesional side. Traditionally, the neglect syndrome has been associated with damage to the right inferior parietal lobule (IPL) and the right temporo-parietal junction (TPJ). Neglect is popularly assessed by two different tasks: line bisection and cancellation. In a previous study (S. Ferber, H.-O. Karnath, How to assess spatial neglect-line bisection or cancellation tasks. J. Clin. Exp. Neuropsychol. 23 (2001) 599-607), we observed that performance on the cancellation task correlates well with the characteristic behavioral disorders used to clinically diagnose spatial neglect, while line bisection was a poor predictor. This might indicate that the disability to correctly bisect lines is a distinct disorder separable from spatial neglect. Here, we assess the anatomy of the patients investigated in that study, and reveal that damage to the temporo-occipital junction correlates with poor performance in the line bisection task. This work extends previous work by Binder et al. (J. Binder, R. Marshall, R. Lazar, J. Benjamin, J.P. Mohr, Distinct syndromes of hemineglect. Arch. Neurol. 49 (1992) 1187-1194) suggesting that line bisection and cancellation identify distinct syndromes. The data suggest that these two tasks dissociate both in terms of behavior and anatomy. This anatomical distinction may help reconcile our recent finding that spatial neglect is associated with damage to the superior temporal cortex and insula, while others have identified the IPL and TPJ. Specifically, we note that our previous anatomical studies did not use the line bisection task to select neglect patients, while many others used this task. We suggest that anatomical studies that combine patients from both of these two distinct groups may result in misleading findings.     

An empirical evaluation of multivariate lesion behaviour mapping using support vector regression

Multivariate lesion behaviour mapping based on machine learning algorithms has recently been suggested to complement the methods of anatomo‐behavioural approaches in cognitive neuroscience. Several studies applied and validated support vector regression‐based lesion symptom mapping (SVR‐LSM) to map anatomo‐behavioural relations. However, this promising method, as well as the multivariate approach per se, still bears many open questions. By using large lesion samples in three simulation experiments, the present study empirically tested the validity of several methodological aspects. We found that (i) correction for multiple comparisons is required in the current implementation of SVR‐LSM, (ii) that sample sizes of at least 100–120 subjects are required to optimally model voxel‐wise lesion location in SVR‐LSM, and (iii) that SVR‐LSM is susceptible to misplacement of statistical topographies along the brain's vasculature to a similar extent as mass‐univariate analyses.     

Exploring the visual world: the neural substrate of spatial orienting

Inspecting the visual environment, humans typically direct their attention across space by means of voluntary saccadic eye movements. Neuroimaging studies in healthy subjects have identified the superior parietal cortex and intraparietal sulcus as important structures involved in visual search. However, in apparent contrast, spatial disturbance of free exploration typically is observed after damage of brain structures located far more ventrally. Lesion studies in such patients disclosed the inferior parietal lobule (IPL) and temporo-parietal junction (TPJ), the superior temporal gyrus (STG) and insula, as well as the inferior frontal gyrus (IFG) of the right hemisphere. Here we used functional magnetic resonance imaging to investigate the involvement of these areas in active visual exploration in the intact brain. We conducted a region of interest analysis comparing free visual exploration of a dense stimulus array with the execution of stepwise horizontal and vertical saccades. The comparison of BOLD responses revealed significant signal increases during exploration in TPJ, STG, and IFG. This result calls for a reappraisal of the previous thinking on the function of these areas in visual search processes. In agreement with lesion studies, the data suggest that these areas are part of the network involved in human spatial orienting and exploration. The IPL dorsally of TPJ seem to be of minor importance for free visual exploration as these areas appear to be equally involved in the execution of spatially predetermined saccades.     

Task-dependent differences in the exploratory behaviour of patients with spatial neglect

The present study analysed task-dependent effects on the exploratory behaviour of neglect patients during their spontaneous search of the surroundings. We were asking whether different tasks would be associated with different structuring of the visual display and, therefore, would result in different forms of neglect in one and the same brain-damaged subjects. Neglect patients' eye and head movements were recorded when they searched for a target within a homogeneous stimulus array surrounding the subjects. Subsequently, they explored the same array which was now segmented into different areas. When the patients' attention was allocated to the whole surrounding space, all patients completely neglected the left hemispace and spontaneously attended to the right hemispace. No significant left-right asymmetry was detected in a selected segment located in the periphery of the attended, right hemispace. However, all patients completely ignored the left part of this segment when they had to concentrate visual search on this segment alone. The results suggest an important influence of task-dependent effects on the exploratory behaviour of neglect patients. They show that one and the same physical stimulus at one and the same location in a scene might be attended or, in another situation, neglected, just depending on the behavioural goal of the subject. The findings support the idea that the brain organises and reorganises continuously the representation of the same physical input according to the changing task requirements.     

How efficient is a simple copying task to diagnose spatial neglect in its chronic phase?

Twenty-five patients with spatial neglect were tested in the acute phase and about 1.3 years after a right-hemisphere stroke. Ten patients had developed chronic spatial neglect. We investigated how sensitive a simple copying task is in detecting spatial neglect in the chronic phase. When the stroke was acute, all 10 patients omitted a considerable number of contralesionally located items in the copying task. In the chronic phase, 60% of the chronic neglect patients still demonstrated noticeable neglect in the copying task. The data indicate that a simple task such as the copying of a multi-object scene is a helpful tool to detect residual symptoms of spatial neglect even more than 1 year after the stroke.