Showing papers by "Gereon R. Fink published in 2000"

Line bisection judgments implicate right parietal cortex and cerebellum as assessed by fMRI

Abstract: Objective: To use functional MRI (fMRI) to determine which brain regions are implicated when normal volunteers judge whether pretransected horizontal lines are correctly bisected (the Landmark test). Background: Manual line bisection and a variant thereof involving perceptual judgments of pretransected lines (the Landmark test) are widely used to assess unilateral visuospatial neglect in patients with neurologic disease. Although unilateral (left) neglect most often results from lesions to right temporoparietal cortex, the normal functional anatomy of the Landmark test has not been convincingly demonstrated. Methods: fMRI was carried out in 12 healthy right-handed male volunteers who judged whether horizontal lines were correctly prebisected. In the control task, subjects detected whether the horizontal lines contained a transection mark irrespective of the position of that mark. Response was by two-choice key press: on half the trials, subjects used the right, and on half, the left hand. Statistical analysis of evoked blood oxygenation level-dependent responses, measured with echoplanar imaging, employed statistical parametric mapping. Results: Performing the Landmark task showed neural activity (p Conclusions: The right hemispheric dominance observed in inferior parietal cortex is consistent with the results of lesion studies. Right superior parietal cortex, vermis, and left cerebellar hemisphere have not been implicated in neglect, but all appear to play a cognitive role in the Landmark task.

Neural consequences of acting in near versus far space: a physiological basis for clinical dissociations

Abstract: We used PET to determine which brain regions are implicated when normal volunteers bisect horizontal lines and point to dots in near (peripersonal) or far (extrapersonal) space. Studies of line bisection in patients with right hemisphere lesions have shown that bisection performance can be severely impaired in either near or far space while remaining within normal limits in the other spatial domain. Likewise, clinical dissociations between pointing to objects in near and far space have been reported. The normal functional anatomy of these dissociations has not been demonstrated convincingly. Regional cerebral blood flow measurements using PET were carried out in 12 healthy right-handed male volunteers who bisected lines or pointed to dots in near or far space, using a laser pen. Subjects performing either task in near space showed neural activity in the left dorsal occipital cortex, left intraparietal cortex, left ventral premotor cortex and left thalamus. In far space, subjects performing either task showed activation of the ventral occipital cortex bilaterally and the right medial temporal cortex. These data provide physiological support for the clinically observed dissociations demonstrating that attending to and acting in near space differentially employs dorsal visuomotor processing areas, whereas attending to and acting in far space differentially draws on ventral visuoperceptual processing areas, even when the motor components of the tasks are identical when performed in the two spaces.

Diffusion- and Perfusion-Weighted MRI: Influence of Severe Carotid Artery Stenosis on the DWI/PWI Mismatch in Acute Stroke

Abstract: Background and Purpose—Diffusion-weighted imaging (DWI) and perfusion-weighted imaging (PWI) are relatively new MR techniques increasingly used in acute stroke. During the first hours of stroke evolution, the regions with abnormal perfusion are typically larger than the DWI lesions, and this mismatch region has been suggested to be “tissue at risk.” The aim of this study was to evaluate the PWI/DWI mismatch region in acute stroke patients and find parameters indicative of both infarct progression and functional impairment. Methods—Twenty patients with nonlacunar ischemic stroke were imaged with DWI, PWI, and conventional MRI within 24 hours of symptom onset and after 1 week; in addition, the European Stroke Scale (ESS) score was recorded. With PWI, the volumes of regions with “time-to-peak” (TTP) delays of ≥2, 4, 6, 8, and 10 seconds were measured; these volumes were compared with the acute DWI lesion volumes, final infarct size, and ESS score. Results—In 80% of patients the acute DWI lesion was surrounde...

Neural consequences of competing stimuli in both visual hemifields: a physiological basis for visual extinction.

Abstract: We used positron emission tomography in healthy volunteers to test hemispheric rivalry theories for normal and pathological spatial attention, which provide an influential account of contralesional extinction on bilateral stimulation after unilateral brain injury. Subjects reported visual characters presented either unilaterally or bilaterally. An extinction-like pattern was found behaviorally, with characters in one hemifield reported less accurately when competing characters appeared in the other hemifield. Differences in neural activity for unilateral minus bilateral conditions revealed greater activation of striate and extrastriate areas for stimuli presented without competing stimuli in the other hemifield. Thus, simultaneous bilateral stimulation led to a significant reduction in response by spatiotopic visual cortex contralateral to a particular stimulus. These data provide physiological support for interhemispheric rivalry in the intact human brain, and demonstrate that such competition impacts at early levels of perceptual processing.

Neuronal Activity in Early Visual Areas during Global and Local Processing: A Comment on Heinze, Hinrichs, Scholz, Burchert, and Mangun

Abstract: In a paper published in the 10:4 issue of JOCN, Heinze et al. 1998) studied a directed attention task using hierarchically organized letter Navon stimuli. On the basis of negative PET and ERP findings, they argue that``early sensory inputs are not modulated to gate global vs. local visual information differentially into the two hemispheres .'' Rather, based on positive ERP findings (an N2 effect) in a divided-attention task, they argue that``later stages of cortical processing may be asymmetrically organized in the left and right hemispheres and operate in parallel to process global and local aspects of complex stimuli.'' The authors conclude that their re-sults``support models proposing that spatial frequency analysis is only asymmetric at higher stages of perceptual processing and not at the earliest stages of visual cortical analysis.'' In three previous functional-neuroimaging studies of directed-visual attention to complex-hierarchical stimuli of various types (letters made of letters, objects made of objects, and rectangles made of rectangles), we have repeatedly demonstrated hemispheric differences between global and local processing both in early visual processing areas (V2/V3). We also find that higher stages of perceptual processing that activate temporo-parietal cortex are implicated in a divided-attention task using such stimuli (Fink et al., 1996; Fink et al., 1997a). In addition, such hemispheric-differential activations at the early stages of visual processing are influenced by sti-Our PET findings are thus consistent with neuropsy-chological studies demonstrating a critical role of the temporo-parietal cortex in global/local processing (Ro-1988), functional-imaging data using fMRI (Martinez et al., 1997) showing hemispheric differences between global and local processing, and other functional-imaging data (using both PET and fMRI) on the attentional gating of early visual processing areas (Corbetta, Miezin, Dobmeyer, Shulman, & Petersen, 1990). Furthermore, electrophysiological data obtained in monkeys directly demonstrate enhanced neural activity in early visual cortex resulting from changes in attentional set The N2 effect observed by Heinze et al. (1998) is consistent with the widely accepted view that the local aspects of hierarchical letter Navon figures are preferentially processed in the left hemisphere while the global aspects of such figures are preferentially processed in the right hemisphere. Leaving aside the problem of the exact localization of ERP sources, it is important to consider whether their negative PET findings (Heinze et al., 1998) can be taken as evidence against our findings of early visual processing differences in similar experiments (Fink et al., 1997a; Fink et al., 1999). The failure of …