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T Landis

Publications and source records attributed to T Landis.

At least 37 records · Page 2Linked to original sources
PubMed2002

Neuropathology of a case of dopa-responsive dystonia associated with a new genetic locus, DYT14.

Detailed autopsy findings are reported for a patient with dopa-responsive dystonia genetically related to the dopa-responsive dystonia locus DYT14 on chromosome 14q13. Substantia nigra and locus ceruleus showed a normal abundance of severely hypomelanized dopaminergic neurons and no Lewy body. In the nigra, the reduction of melanin pigment was found to be asymmetric between the two sides and uneven within neurons, and the lateral aspect of the nigra appeared more affected than the medial, in a pattern similar to the neuronal loss in PD. Dopa-responsive dystonia has a unique neuropathologic signature that seems to be independent of its genotype.

Aged↗
PubMed2002

Direction-specific motion blindness induced by focal stimulation of human extrastriate cortex.

Motion blindness (MB) or akinetopsia is the selective disturbance of visual motion perception while other features of the visual scene such as colour and shape are normally perceived. Chronic and transient forms of MB are characterized by a global deficit of direction discrimination (pandirectional), which is generally assumed to result from damage to, or interference with, the motion complex MT+/V5. However, the most characteristic feature of primate MT-neurons is not their motion specificity, but their preference for one direction of motion (direction specificity). Here, we report that focal electrical stimulation in the human posterior temporal lobe selectively impaired the perception of motion in one direction while the perception of motion in other directions was completely normal (unidirectional MB). In addition, the direction of MB was found to depend on the brain area stimulated. It is argued that direction specificity for visual motion is not only represented at the single neuron level, but also in much larger cortical units.

Adult↗
PubMed2002

[Variability of right hemisphere activation during semantic word processing in aphasic patients: an electrophysiologic study in three patients].

After stroke, the interhemispheric reorganisation of the neural network implicated in language is hypothesized to be a function not only of the site of lesion but also of the residual impairment. With a multiple case approach, we tested this hypothesis in three chronic aphasic patients. Two patients, GE (capsulo-lenticular stroke) and JHN (fronto-temporal stroke) showed formal residual semantic difficulties, while the third patient (EG, large sylvian lesion) did not. Brain electric activity was analysed during a categorisation task of tachistoscopically presented words in the left and the right visual field. The temporal analysis of brain activity showed that both patients with semantic residual difficulties activated the right hemisphere (RH) during some steps of word processing. In the third patient, without semantic impairment, the RH was activated only during a short time period. Further more, RH activation was shown to be dependent on the visual field of word presentation. Phonological impairment was not predictive of RH activation. These results suggest that RH activation, particularly anterior regions, can occur during semantic processing of words as a function of semantic residual impairment.

Adult↗
PubMed2001

CSF detection of the 14-3-3 protein in unselected patients with dementia.

OBJECTIVE: To determine the usefulness of the 14-3-3 test in patients with dementia of various causes. BACKGROUND: Recent reports have suggested that the detection of the 14-3-3 protein in the CSF of patients with Creutzfeldt--Jakob disease is a highly sensitive and specific marker of the disease that might be used as a diagnostic criterion. We examined the validity of this test when applied to a cohort of unselected patients prospectively examined for an ongoing dementing process. METHODS: One hundred patients underwent an extensive neurologic examination for dementia, including a CSF 14-3-3 protein immunoblotting assay. Final clinical diagnoses were compared with the qualitative results of the test, and statistical measures of test validity were carried out. RESULTS: We found a positive test in 14 of 100 patients, only two of whom had definite Creutzfeldt--Jakob disease. Positive results were found in patients with various degenerative dementias, including AD (4), frontotemporal dementia (2), and dementia with Lewy body (1), and in patients with vascular dementia (1), carcinomatous meningitis (1), and anoxic encephalopathy (1). In two other positive patients, the dementia could not be confidently classified. Sensitivity, specificity, and negative predictive value were fairly good, but positive predictive value was poor. Similar results were found independently of the disease duration. There was no correlation between intensity nor pattern of the 14-3-3 protein expression and diagnosis. CONCLUSIONS: The 14-3-3 test is not valid for discriminating between Creutzfeldt--Jakob disease and non-Creutzfeldt--Jakob disease in unselected patients with dementia. Positive results are found in various degenerative and secondary, prion-unrelated dementias.

14-3-3 Proteins↗
PubMed2001

So near yet so far: neglect in far or near space depends on tool use.

The study of unilateral spatial neglect has shown that space can be dissociated on a peripersonal versus extrapersonal basis. We report a novel type of dissociation based on tool use in a patient suffering from left neglect. Line bisection was carried out in near and far space, using a stick and a laser pointer. A rightward bias was always found for the former, but not for the latter. Neglect thus appears to be contingent not only on distance, but also on the motor action required by the task.

Aged↗
PubMed2001

Pure representational neglect after right thalamic lesion.

After a right thalamic stroke, an 86-year-old man presented an acute pure left representational neglect in the absence of any perceptual neglect. On spatial mental imagery tasks, the patient systematically omitted items located on his left side, but only when a vantage point was given. This suggests that (1) pure representational neglect is not just a residual finding after recovery from global (perceptual and representational) neglect; (2) space representation can be coded by two independent processes: in viewer-centered or world-based (allocentric) coordinates; and (3) the right thalamus serves as a relay in the processing of spatial visual imagery.

Aged↗
PubMed2001

Non-stationary distributed source approximation: an alternative to improve localization procedures.

Localization of the generators of the scalp measured electrical activity is particularly difficult when a large number of brain regions are simultaneously active. In this study, we describe an approach to automatically isolate scalp potential maps, which are simple enough to expect reasonable results after applying a distributed source localization procedure. The isolation technique is based on the time-frequency decomposition of the scalp-measured data by means of a time-frequency representation. The basic rationale behind the approach is that neural generators synchronize during short time periods over given frequency bands for the codification of information and its transmission. Consequently potential patterns specific for certain time-frequency pairs should be simpler than those appearing at single times but for all frequencies. The method generalizes the FFT approximation to the case of distributed source models with non-stationary time behavior. In summary, the non-stationary distributed source approximation aims to facilitate the localization of distributed source patterns acting at specific time and frequencies for non-stationary data such as epileptic seizures and single trial event related potentials. The merits of this approach are illustrated here in the analysis of synthetic data as well as in the localization of the epileptogenic area at seizure onset in patients. It is shown that time and frequency at seizure onset can be precisely detected in the time-frequency domain and those localization results are stable over seizures. The results suggest that the method could also be applied to localize generators in single trial evoked responses or spontaneous activity.

Adolescent↗
PubMed2001

Intracranial Neurophysiological Correlates Related to the Processing of Faces.

Face perception and recognition is an intriguing ability, already present in neonates. Numerous studies in patients with brain lesions identified the temporo-occipital cortex as the crucial structure for this capacity. Analysis of electrical signals (EEG) inside the brain of patients implanted with intracranial electrodes for diagnostic purposes allows researchers to describe the temporal and spatial organization of responses to various aspects of face processing in human subjects. Several findings have emerged and appear relevant for cerebral organization in general: (1) Selective face responses were obtained from the basal temporo-occipital cortex at around 200 ms (N200); however, other structures such as the lateral temporal lobe and frontal cortex also participate in face recognition and perception tasks. (2) Each structure has a distinct "response profile"; that is, with respect to a given task certain structures respond strongly, others less or not at all. This profile might change with a different task, although the physical parameters of the stimuli remain the same. (3) The right hemispheric predominance of face processing, as suggested by patient data and studies in healthy volunteers, seemed to be restricted to its early stages (i.e., before 100-150 ms). (4) Recognition of faces might be associated with differential intracranial responses, despite an incorrect overt response, reflecting neurophysiological correlates of implicit memory. (5) The more the stimulus resembled a complete human face, the earlier and larger the N200 response was found, in particular over the basal temporobasal cortex. Analysis of electrical signals from intracranial electrodes might help to improve our understanding of the underlying physiological and anatomical constraints of cognitive processes.

Journal Article↗
PubMed2001

Explicit and implicit perception of illusory contours in unilateral spatial neglect: behavioural and anatomical correlates of preattentive grouping mechanisms.

Studies of hemispatial neglect suggest that some perceptual processes still operate on contralesional stimuli independent from spatial attention or awareness. Here we examined whether preattentive processing in extrastriate areas may group unconnected elements inducing illusory contours despite neglect. While it has been debated whether illusory contours arise from preattentive grouping or higher cognitive processes, neurophysiological studies show that neurones in secondary visual cortex (V2) can code for illusory contours. Twelve patients with right hemisphere damage and left neglect were tested for implicit and explicit detection of illusory contours using, respectively: (1) a bisection task where patients were not explicitly required to attend to lateral elements and judged the midpoint of Kanizsa illusory stimuli, as well as other physically connected or unconnected stimuli of the same length; (2) a matching task where patients had to overtly attend to lateral elements and made same/different judgements on pairs of illusory stimuli with identical or different inducers on the right or left side. In some patients, bisection judgements were consistently similar on Kanizsa stimuli with illusory contours and connected stimuli with real contours but different on unconnected gap figures, regardless of their length, suggesting implicit grouping of inducing elements prior to processing stages where a spatial attentional bias arose. Their lesions centred on the inferior parietal cortex or thalamus. Other patients did not show a systematic bisection pattern and had lesions extending posteriorly in the lateral occipital cortex. However, both groups of patients failed to detect left-side inducers in explicit matching judgements, even though errors often revealed unconscious processing, and they showed similar neglect severity on other standard tests. These findings suggest that grouping by illusory contours can occur preattentively and influence bisection independently from the ability to detect contralateral inducers explicitly, severity of inattention, and other forms of unconscious processing. Implicit grouping may depend on the sparing of lateral occipital areas involved in figure-ground segmentation at early stages of visual processing.

Aged↗
PubMed2001

Electric source imaging of human brain functions.

We review recent methodological advances in electromagnetic source imaging and present EEG data from our laboratory obtained by application of these methods. There are two principal steps in our analysis of multichannel electromagnetic recordings: (i) the determination of functionally relevant time periods in the ongoing electric activity and (ii) the localization of the sources in the brain that generate these activities recorded on the scalp. We propose a temporal segmentation of the time-varying activity, which is based on determination of changes in the topography of the electric fields, as an approach to the first step, and a distributed linear inverse solution based on realistic head models as an approach to the second step. Data from studies of visual motion perception, visuo-motor transfer, mental imagery, semantic decision, and cognitive interference illustrate that this analysis allows us to define the patterns of electric activity that are present at given time periods after stimulus presentation, as well as those time periods where significantly different patterns appear between different stimuli and tasks. The presented data show rapid and parallel activation of different areas within complex neuronal networks, including early activity of brain regions remote from the primary sensory areas. In addition, the data indicate information exchange between homologous areas of the two hemispheres in cases where unilateral stimulus presentation requires interhemispheric transfer.

Brain Mapping↗
PubMed2001

The time course of semantic category processing in the cerebral hemispheres: an electrophysiological study.

Using visual half-field presentations of words to the right (RVF) and to the left visual field (LVF), this study investigated the time course of the hemispheric involvement in the processing of semantic category information. Multi-channel event related brain potentials (ERPs) were recorded from 15 healthy subjects during a categorisation task of sequentially presented word pairs. Subjects had to judge mentally after the appearance of the second word whether the words of a pair were semantically related (SR) or not (SU). ERPs were computed, from 100 ms before the onset of the second word to 600 ms, for SR and SU conditions in the LVF and in the RVF separately. The temporal segmentation of ERP map series into sequences of quasi-stable map configurations revealed a total of seven segments in each visual field of which only the first five (S1-S5, appearing between 70 and 400 ms) showed different map configurations as a function of visual field but presented a similar temporal sequence in both visual fields. By contrast, of the last two segments (S6 and S7) which appeared between approximately 400 and approximately 600 ms, only S7 differentiated SR and SU conditions in terms of its duration. Source localisation analysis of the segments showed that following the initial activation of posterior brain regions as a function of the visual field of presentation, a common neural network was activated in the left hemisphere (LH) although the dynamics of activation varied as a function of visual field. Concerning the role of the right hemisphere (RH) in lexico-semantic processing, the results presented here appear to be compatible with a 'callosal relay model' and suggest that, in healthy subjects, information is transferred rapidly ( approximately 150 ms) from the RH to the language dominant-LH.

Adult↗
PubMed2001

Space-oriented segmentation and 3-dimensional source reconstruction of ictal EEG patterns.

OBJECTIVES: Characterization of the EEG pattern during the early phase of a seizure is crucial for identifying the epileptic focus. The purpose of the present investigation was to evaluate a method that divides ictal EEG activity into segments of relatively constant surface voltage distribution, and to provide a 3-dimensional localization of the activity during the different segments. METHODS: For each timepoint the electrical voltage distribution on the scalp (the voltage map) was determined from the digitized EEG recording. Through a spatial cluster analysis time sequences where the maps did not change much (segments) were identified, and a 3-dimensional source reconstruction of the activity corresponding to the different mean maps was performed using a distributed linear inverse solution algorithm. RESULTS: Segments dominating early in seizure development were identified, and source reconstruction of the EEG activity corresponding to the maps of these segments yielded results which were consistent with the results from invasive recordings. In some cases a sequence of consecutive segments was obtained, which might reflect ictal propagation. CONCLUSIONS: Segmentation of ictal EEG with subsequent 3-dimensional source reconstruction is a useful method to non-invasively determine the initiation and perhaps also the spread of epileptiform activity in patients with epileptic seizures.

Adolescent↗
PubMed2001

Noninvasive localization of electromagnetic epileptic activity. I. Method descriptions and simulations.

This paper considers the solution of the bioelectromagnetic inverse problem with particular emphasis on focal compact sources that are likely to arise in epileptic data. Two linear inverse methods are proposed and evaluated in simulations. The first method belongs to the class of distributed inverse solutions, capable of dealing with multiple simultaneously active sources. This solution is based on a Local Auto Regressive Average (LAURA) model. Since no assumption is made about the number of activated sources, this approach can be applied to data with multiple sources. The second method, EPIFOCUS, assumes that there is only a single focal source. However, in contrast to the single dipole model, it allows the source to have a spatial extent beyond a single point and avoids the non-linear optimization process required by dipole fitting. The performance of both methods is evaluated with synthetic data in noisy and noise free conditions. The simulation results demonstrate that LAURA and EPIFOCUS increase the number of sources retrieved with zero dipole localization error and produce lower maximum error and lower average error compared to Minimum Norm, Weighted Minimum Norm and Minimum Laplacian (LORETA). The results show that EPIFOCUS is a robust and powerful tool to localize focal sources. Alternatives to localize data generated by multiple sources are discussed. A companion paper (Lantz et al. 2001, this issue) illustrates the application of LAURA and EPIFOCUS to the analysis of interictal data in epileptic patients.

Artifacts↗
PubMed2001

Beneficial effect of co-polymer 1 on cytokine production by CD4 T cells in multiple sclerosis.

Multiple sclerosis (MS) has been associated with an imbalance in the T helper type 1 (Th1) and Th2 subsets. We investigated, at the single-cell level, the synthesis of pro-inflammatory cytokines by CD4 and CD8 T cells from MS patients. We report the relationship between priming of CD4 and CD8 T cells for interleukin-2 (IL-2), interferon-gamma (IFN-gamma) and tumour necrosis factor-alpha (TNF-alpha) and disease evolution in MS patients, clinically subdivided into relapsing-remitting MS (RRMS) in remission, RRMS in relapse, or chronic progressive MS (CPMS). Moreover, we report the in vivo influence of co-polymer 1 (COP) treatment on the pattern of cytokine producers in RRMS patients. We show that the frequency of CD4 T cells primed for TNF-alpha synthesis increased in all stages of MS, including RRMS remitting, and was normalized to control values in COP-treated patients (43.2 +/- 11.8% in treated patients versus 47 +/- 7.3% in RRMS remitting versus 40.3 +/- 8% in controls). In addition, a significant decrease in the frequency of CD4 T cells primed for IL-2 was found in COP-treated patients as compared to the other groups of patients, reaching values below that of controls (59.1 +/- 9.9% in treated patients versus 70 +/- 11.6% in RRMS remitting versus 67.1 +/- 7.4% in controls). Unexpectedly, COP-treated patients also showed a significantly decreased priming for IFN-gamma at the CD4 T-cell level (9.1 +/- 3.4% in treated patients versus 18.8 +/- 0.6.4% in RRMS remitting versus 15.4 +/- 4.7% in controls), but not at the CD8 T-cell level. This bystander suppression on the inflammatory cells should be considered in the monitoring of MS patients submitted to COP treatment, in order to evaluate better its clinical efficacy.

Adult↗
PubMed2001

Associations to smell are more pleasant than to sound.

To assess and compare the quantitive and qualitative aspects of verbal associations of olfaction and audition, we conducted two verbal category fluency tasks, one consisting of the generation of 'smelling' associations and the other of 'auditory' associations. The responses of the 40 subjects on these fluency tasks were rated as pleasant or unpleasant by themselves as well as by an independent group of 40 subjects. In addition, all 80 healthy, right-handed subjects rated their momentary emotional state on a visual analog scale. The mean number of words generated by the 40 subjects did not differ between the two tasks, and in both tasks pleasant associations were more frequent than unpleasant associations. However, for all subjects, the proportion of pleasant associations was significantly higher in the olfactory compared to the auditory fluency task. The finding of more pleasant associations in both tasks confirms previous reports, but the pronounced effect in the olfactory task suggests that odors may be more hedonically coded than other sensory modalities, i.e., audition. Although there is evidence that the majority of odors are initially perceived as unpleasant, when retrieved from memory, pleasant connotations seem to dominate. The possible mechanisms for this dissociation are discussed.

Adult↗
PubMed2001

Functional neuroanatomical correlates of hysterical sensorimotor loss.

Hysterical conversion disorders refer to functional neurological deficits such as paralysis, anaesthesia or blindness not caused by organic damage but associated with emotional "psychogenic" disturbances. Symptoms are not intentionally feigned by the patients whose handicap often outweighs possible short-term gains. Neural concomitants of their altered experience of sensation and volition are still not known. We assessed brain functional activation in seven patients with unilateral hysterical sensorimotor loss during passive vibratory stimulation of both hands, when their deficit was present and 2-4 months later when they had recovered. Single photon emission computerized tomography using (99m)Tc-ECD revealed a consistent decrease of regional cerebral blood flow in the thalamus and basal ganglia contralateral to the deficit. Independent parametric mapping and principal component statistical analyses converged to show that such subcortical asymmetries were present in each subject. Importantly, contralateral basal ganglia and thalamic hypoactivation resolved after recovery. Furthermore, lower activation in contralateral caudate during hysterical conversion symptoms predicted poor recovery at follow-up. These results suggest that hysterical conversion deficits may entail a functional disorder in striatothalamocortical circuits controlling sensorimotor function and voluntary motor behaviour. Basal ganglia, especially the caudate nucleus, might be particularly well situated to modulate motor processes based on emotional and situational cues from the limbic system. Remarkably, the same subcortical premotor circuits are also involved in unilateral motor neglect after organic neurological damage, where voluntary limb use may fail despite a lack of true paralysis and intact primary sensorimotor pathways. These findings provide novel constraints for a modern psychobiological theory of hysteria.

Adolescent↗
PubMed2000

Visually induced activity in human frontal motor areas during simple visuomotor performance.

Visuomotor tasks elicit neuronal activity in primate motor areas at relatively short latencies. Although this early activity embodies features of visual responses (short latency, stimulus-dependency), its sensory nature has been questioned. We investigated neural correlates of visuomotor performance in human motor areas using scalp and intracranial event-related potential measures. A simple visuomanual reaction-time task evoked early potentials at 133-145 ms post-stimulus which occurred much earlier than the motor potentials of the same region. The amplitude of the early potentials covaried with stimulus location and was independent of parameters of the motor response. Because of their timing, stimulus-dependency and characteristics of our behavioral task, the early potentials are suggested to reflect neuronal responses of sensory nature rather than processing related to pure motor aspects of the task.

Adult↗