Search PubMed⌕ Search

Biomedical subjects

Hans-Otto Karnath

Publications and source records attributed to Hans-Otto Karnath.

At least 19 recordsLinked to original sources
PubMed2005

Processing of auditory spatial cues in human cortex: an fMRI study.

The issue of where in the human cortex coding of sound location is represented still is a matter of debate. It is unclear whether there are cortical areas that are specifically activated depending on the location of sound. Are identical or distinct cortical areas in one hemisphere involved in processing of sounds from the left and right? Also, the possibility has not been investigated so far that distinct areas have a preference for processing of central and eccentric sound locations. The present study focussed on these issues by using functional magnetic resonance imaging (fMRI). Activations evoked by left, right and central sounds were analysed separately, and contrasts were computed between these conditions. We did not find areas, which were involved in the processing of exclusively left, right or central sound positions. Large overlapping areas rather were observed for the three sound stimuli, located in the temporal, parietal and frontal cortices of both hemispheres. This result argues for the idea of a widely distributed bilateral network accessing an internal representation of the body to encode stimulus position in relation to the body median plane. However, two areas (right BA 40 and left BA 37) also were found to have preferences for sound position. In particular, BA 40 turned out to be significantly more activated by processing central positions, compared to eccentric stimuli. In line with previous findings on visual perception, the latter observation supports the assumption that the right inferior parietal cortex may be preferentially involved in the perception of central stimulus positions in relation to the body.

Acoustic Stimulation↗
PubMed2005

Spontaneous eye and head position in patients with spatial neglect.

The most prominent deficit in patients with spatial neglect is a bias of their active behaviour, i. e. a deviation of exploratory movements towards the right. When searching for targets, copying, or reading, the patients direct their eye and hand movements towards the ipsilesional side, leading to neglect of the contralesional side. The present study investigated whether spatial neglect is predominantly linked with such active behaviour or if it is obvious also without any explicit requirements, namely in the patients' spontaneous eye and head position. To address this issue we investigated the patients' spontaneous resting position while "doing nothing", i. e. just sitting and waiting for an experiment to start. Using magnetic search coil technique, we recorded spontaneous eye-in-head and head-on-trunk orientation in that waiting period in 24 patients with and without spatial neglect. In contrast to controls, neglect patients showed a marked deviation of spontaneous eye and head orientation of about 30 degrees (= gaze position) towards the right. The findings strengthen the view that one component of the behaviour in neglect patients is due to a very elementary disturbance of spatial information processing. The deviation of eye and head may be understood as a pathological adjustment of the subject's normal resting position to a more rightward position. While the position in healthy subjects is in line with trunk orientation, this "default position" is shifted to a new origin in patients with spatial neglect.

Adult↗
PubMed2005

Posterior thalamic hemorrhage induces "pusher syndrome".

BACKGROUND: Recent findings argue for a pathway in humans for sensing the orientation of gravity and controlling upright body posture, separate from the one for orientation perception of the visual world. Stroke patients with contraversive pushing were shown to experience their body as oriented upright when actually tilted about 20 degrees to the ipsilesional side, in spite of normal visual-vestibular functioning. A recent study suggested the involvement of posterolateral thalamus typically associated with the disorder. OBJECTIVE: To evaluate the relationship between pushing behavior and thalamic function. METHODS: Over a 3-year period the authors prospectively investigated 40 patients with left- or right-sided thalamic strokes. RESULTS: Twenty-eight percent showed contraversive pushing. The authors found a strong relationship between etiology, vascular territory, lesion size, and neurologic disorders associated with contraversive pushing. Pusher patients had larger lesions that typically were caused by hemorrhage (vs infarcts) located in the posterior thalamus (vs anterior thalamic lesions in those patients without pushing behavior). A paresis of the contralesional extremities was more frequent and more severe in pusher patients. Further, these patients showed more additional spatial neglect with right thalamic lesions, while they tended to be more aphasic with left thalamic lesions. CONCLUSIONS: Posterior thalamus seems to be fundamentally involved in our control of upright body posture. Higher pressure, swelling, and other secondary pathologic processes associated with posterior thalamic hemorrhage (vs thalamic infarction) may provoke contraversive pushing in combination with additional neurologic symptoms.

Aged↗
PubMed2005

Cortical control of visually guided reaching: evidence from patients with optic ataxia.

The dorsal stream of visual information processing connecting V1 to the parietal cortex is thought to provide a fast control of visually guided reaching. Important for this assumption was the observation that in both the monkey and the human, parietal lesions may provoke disturbance of visually goal-directed hand movements. In the human, severe misreaching termed 'optic ataxia' has been ascribed to lesions of the superior parietal lobule (SPL) and/or the intraparietal sulcus. Using new tools for lesion analysis, here we re-evaluated this view investigating the typical lesion location in a large group of unilateral stroke patients with optic ataxia, collected over a time period of 15 years. We found no evidence for the assumption that disruption of visually guided reaching in humans is associated with a lesion typically centering on the SPL on the convexity. In both left and right hemispheres, we found optic ataxia associated with a lesion overlap that affected the lateral cortical convexity at the occipito-parietal junction, i.e. the junction between the inferior parietal lobule (IPL) and superior occipital cortex and--in the left hemisphere even more posteriorly--also the junction between occipital cortex and the SPL. Via the underlying parietal white matter, the lesion overlap extended in both hemispheres to the medial cortical aspect, where it affected the precuneus close to the occipito-parietal junction. These lateral and medial structures seem to be integral to the fast control of visually guided reaching in humans.

Aged↗
PubMed2005

NOA-03 trial of high-dose methotrexate in primary central nervous system lymphoma: final report.

The NOA-03 trial explored high-dose methotrexate alone in 37 patients with primary central nervous system lymphoma. The overall median survival was 25 months. After 4 years, the rate of leukoencephalopathy in patients surviving more than 12 months was 58% with and 10% without whole-brain radiotherapy given at relapse (p = 0.11). Attention deficits were found in all six tested patients, and memory deficits in four patients. Two patients had normal, three had moderately restricted, and one had markedly restricted quality of life. Thus, high-dose methotrexate with deferred radiotherapy had only moderate efficacy and was associated with significant neurotoxicity in long-term surviving patients.

Antimetabolites, Antineoplastic↗
PubMed2005

Dorsal and ventral stream interaction: contributions from optic ataxia.

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 visuomotor 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.

Aged↗
PubMed2004

The anatomy of spatial neglect based on voxelwise statistical analysis: a study of 140 patients.

A major challenge for any anatomical study of spatial neglect in neurological patients is that human lesions vary tremendously in extent and location between individuals. Approaches to this problem used in previous studies were to focus on subgroups of patients that are more homogeneous either with respect to the branch territory affected by the stroke or with respect to existing additional neurological symptoms (e.g. additional visual field defects). It could be argued that such strategies might bias the conclusions on the critical substrate associated with spatial neglect. The present study thus addressed the high variability inherent in naturally occurring lesions by using an unselected, but very large sample size and by comparing a neglect group with a non-neglect group using voxelwise statistical testing. We investigated an unselected 7 year sample of 140 consecutively admitted patients with right hemisphere strokes. Seventy-eight had spatial neglect, 62 did not show the disorder. The incidence of visual field defects was comparable in both groups. For assessing lesion location, in a first step, we used conventional lesion density plots together with subtraction analysis. Moreover, due to the large size of the sample voxelwise statistical testing was possible to objectively estimate which brain regions are more frequently compromised in neglect patients relative to patients without neglect. The results demonstrate that the right superior temporal cortex, the insula and subcortically putamen and caudate nucleus are the neural structures damaged significantly more often in patients with spatial neglect.

Adult↗
PubMed2004

Using human brain lesions to infer function: a relic from a past era in the fMRI age?

Recent technological advances, such as functional imaging techniques, allow neuroscientists to measure and localize brain activity in healthy individuals. These techniques avoid many of the limitations of the traditional method for inferring brain function, which relies on examining patients with brain lesions. This has fueled the zeitgeist that the classical lesion method is an inferior and perhaps obsolescent technique. However, although the lesion method has important weaknesses, we argue that it complements the newer activation methods (and their weaknesses). Furthermore, recent developments can address many of the criticisms of the lesion method. Patients with brain lesions provide a unique window into brain function, and this approach will fill an important niche in future research.

Brain↗
PubMed2004

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.

Aged↗
PubMed2004

Is there a role of visual cortex in spatial hearing?

The integration of auditory and visual spatial information is an important prerequisite for accurate orientation in the environment. However, while visual spatial information is based on retinal coordinates, the auditory system receives information on sound location in relation to the head. Thus, any deviation of the eyes from a central position results in a divergence between the retinal visual and the head-centred auditory coordinates. It has been suggested that this divergence is compensated for by a neural coordinate transformation, using a signal of eye-in-head position. Using functional magnetic resonance imaging, we investigated which cortical areas of the human brain participate in such auditory-visual coordinate transformations. Sounds were produced with different interaural level differences, leading to left, right or central intracranial percepts, while subjects directed their gaze to visual targets presented to the left, to the right or straight ahead. When gaze was to the left or right, we found the primary visual cortex (V1/V2) activated in both hemispheres. The occipital activation did not occur with sound lateralization per se, but was found exclusively in combination with eccentric eye positions. This result suggests a relation of neural processing in the visual cortex and the transformation of auditory spatial coordinates responsible for maintaining the perceptual alignment of audition and vision with changes in gaze direction.

Acoustic Stimulation↗
PubMed2003

Goal-directed hand movements are not affected by the biased space representation in spatial neglect.

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.

Adult↗
PubMed2003

Neglect-like behavior in healthy subjects: dissociation of space exploration and goal-directed pointing after vestibular stimulation.

Evidence has been reported favoring the view of a dual mode of space representation for action and spatial cognition. While the dorsal system seems to be mainly involved in direct coding of space for action by means of several effector-specific representations, the ventral system appears to be responsible for more enduring and conscious representations underlying spatial cognition and awareness. In accordance with this view are recent studies documenting dissociations between exploratory and goal-directed movements in patients with brain damage. Patients with neglect exhibit a spatial bias of exploratory movements to the ipsilesional side, while goal-directed movements land precisely on target. The exploratory bias was found susceptible to asymmetric sensory stimulation such as caloric vestibular stimulation, inducing transient reduction of contralateral neglect. The present study compared exploratory and goal-directed hand movements in healthy subjects following cold caloric stimulation of the right vestibular organ. We observed a rightward shift of tactile exploration, while goal-directed pointing remained unaffected. Asymmetric vestibular stimulation in healthy subjects thus produced a neglect-like behavior with a similar dissociation between impaired exploratory and nonimpaired goal-directed hand movements. The stimulation provoked a further, very characteristic symptom of neglect patients: a deviation of spontaneous head orientation toward the right. The present observations strengthen substantially the assumption of different modes of space representation for action and spatial cognition in humans.

Adolescent↗
PubMed2003

Disturbed sound lateralization in patients with spatial neglect.

Previous studies on auditory space perception in patients with neglect have investigated localization of free-field-sound stimuli or lateralization of dichotic stimuli that are perceived intracranially. Since those studies in part revealed contradictory results, reporting either systematic errors to the left or systematic errors to the right, we reassessed the ability of auditory lateralization in patients with right hemispheric lesions with and without neglect. Unexpectedly, about half of the patients with neglect showed erratic judgments on sound position, that is, they were completely unable to lateralize sounds. The remaining neglect patients only showed a small deviation of the auditory median plane to the left side, indicating that they perceived the sounds as slightly shifted to the right side. The comparison between both groups revealed higher severity of neglect in the group of neglect patients who were unable to perform the task, suggesting that the inability of sound lateralization was associated with the strength of clinical neglect. However, we also observed 1 out of 9 patients with left brain damage who was not able to lateralize spatial sounds. This patient did not show any symptoms of spatial neglect. Thus, it may be that a spatial auditory deficit, such as that observed here in right-brain-damaged patients, only co-occurs with spatial neglect if the right superior temporal cortex is lesioned.

Adult↗
PubMed2003

The cortical substrate of visual extinction.

Neuroimaging studies investigated the attentional systems of the human brain revealing two networks, one for voluntary allocation of attention and another for stimulus-driven attentional processes. Whereas lesions of the latter system were supposed to lead to spatial neglect, we show that such lesions rather are typical for the occurrence of visual extinction. Extinction describes the inability of brain-damaged patients to detect a contralesional target in the presence of a competing ipsilesional stimulus. In a sample of consecutively admitted patients with right hemisphere stroke, we found dissociable cortical substrates for spatial neglect and visual extinction. There was a surprising congruency between the typical lesion site in patients with extinction and the activation clusters found in previous neuroimaging studies of healthy subjects. The results show that the temporo-parietal junction (TPJ), considered to be a crucial part of the stimulus-driven attentional network, is the neural substrate of visual extinction.

Aged↗