Search PubMed⌕ Search

Biomedical subjects

T Landis

Publications and source records attributed to T Landis.

At least 109 records · Page 6Linked to original sources
PubMed1997

What is the role of the corpus callosum in intermanual transfer of motor skills? A study of three cases with callosal pathology.

Intermanual transfer for a skilled motor task was studied in two patients with total callosal agenesis, and one with an acquired partial callosal lesion and clinical evidence for disturbed transfer of motor signals. Patients had to draw meaningless figures with one upper extremity (original learning, OL) and to reproduce their mirror-reversals thereafter with the other side (transfer learning, TL). Both directions of intermanual transfer were tested in two conditions, that is, between either proximal or distal muscle groups. Transfer was evaluated by comparing OL and TL performance at the same effector. The main variable of interest was movement time during the first eight trials of OL and TL. All three patients displayed a significant benefit for transfer from the dominant to the non-dominant hand but not vice versa during proximal motor activity. When compared with the performance of healthy subjects tested in almost identical conditions in a previously reported study, the proximal transfer behavior was found to be similar for all patients and the normal group. Although patients exhibited no significant benefit for distal transfer, their non-dominant-to-dominant distal transfer was above the normal range. The similar transfer pattern of the patients and healthy subjects when using proximal musculature suggests that proximal transfer may be subserved by identical extracallosal pathways, most probably by the ipsilaterally descending motor systems. Since non-dominant-to-dominant distal transfer was found to be disadvantageous in healthy subjects, the patients' relative superiority in this condition may reflect missing callosal influences of an inhibitory nature.

Adult↗
PubMed1997

Intermanual transfer of training: blood flow correlates in the human brain.

In a previous study, we found that relearning of a task with one hand might negatively be influenced by previous, opposite hand training of the analogue task, Thut G., et al., Exp. Brain Res., 108 (1996) 321-327. Drawing of a figure with the right hand, following left hand training, was slower than right hand drawing of an unknown figure. These conditions were termed right hand transfer learning (rTL) and right hand original learning (rOL). The present study aimed to identify the cerebral areas associated with these influences by measuring regional cerebral blood flow (rCBF) in 16 right-handed, healthy subjects during rTL and rOL. Positron emission tomography and statistical parametric mapping were used. Compared with rOL, rTL was associated with increased rCBF in the left medial prefrontal cortex and the right prefrontal convexity. Individual rCBF changes in the area homotopic to the right prefrontal convexity furthermore correlated with individual changes in rTL performance. While the smallest rCBF increases were found in subjects with weakest slowing of rTL relative to rOL, highest rCBF increases were present when rTL slowing dominated. Comparisons between rTL and rOL, however, revealed on average no performance differences. Our data suggest that relearning after previous opposite hand training activates neural mechanisms within the prefrontal convexity which might have an inhibitory function but that inhibition does not have to be the net final behavioral result.

Adult↗
PubMed1997

Extracranial localization of intracranial interictal epileptiform activity using LORETA (low resolution electromagnetic tomography).

Besides the standard clinical methods of EEG waveshape analysis, mathematical models for reconstruction of dipolar sources from the digitized surface EEG have been introduced in epilepsy research. Although useful for localizing focal sources, these methods are inadequate for analyzing widespread epileptiform activity. A recently introduced alternative method called LORETA (low resolution electromagnetic tomography, Pascual-Marqui et al., 1994), directly computes the current distribution throughout the full brain volume, assuming that neighboring neuronal populations are simultaneously and synchronously activated. In mathematical terms the method selects the smoothest of all possible 3-dimensional current distributions, inherently introducing a certain amount of dispersion. In 7 patients, undergoing simultaneous EEG recording from 10 intracranial (subdural) and 22 extracranial electrodes, 111 subdural discharges (61 subtemporal and 50 lateral temporal) were identified. The exact time point of maximal intracranial activity was automatically identified, and the LORETA solution at that timepoint was computed from the surface EEG. Statistical comparison revealed significantly higher LORETA current density in the area corresponding to the subdurally recorded spike compared to other areas, and a more anterior and more medial LORETA location for subtemporal compared to lateral temporal spikes. This study indicates that the LORETA technique may become a useful method to localize electrical activity in the brain.

Adolescent↗
PubMed1997

"Gourmand syndrome": eating passion associated with right anterior lesions.

We present a new benign eating disorder associated with lesions involving parts of the right anterior cerebral hemisphere. This "gourmand" syndrome describes a preoccupation with food and a preference for fine eating. Two exemplary case reports illustrate this new syndrome. Analysis of the clinical and anatomical data of 36 patients who displayed this behavior revealed, in 34, a strong association with lesion location in the right anterior part of the brain involving cortical areas, basal ganglia, or limbic structures. Our finding provides further evidence of a correlation between right hemispheric damage, eating, and other impulse control disorders. We conjecture that the serotonergic system subserves different functions in the left and right hemisphere.

Brain Diseases↗
PubMed1996

Human colour discrimination based on a non-parvocellular pathway.

BACKGROUND: Traditionally, colour information is assumed to be carried by neural channels in the parvocellular pathway and to be encoded in an opponent manner, while other, non-parvocellular, spectrally non-opponent channels are thought to play no part in colour vision. But is the parvocellular pathway the only way that colours can be discriminated in human vision? We studied two patients with cerebral achromatopsia, who lack conscious colour perception but are nevertheless able to make use of colour information. In particular, we investigated whether, in these patients, colour discrimination is mediated by the parvocellular pathway. RESULTS: The achromatopsic patients carried out a forced-choice colour- and luminance-discrimination task, and showed clear evidence of unconscious colour processing, consistent with previous studies. We added different types of luminance noise to see when this unconscious colour information could be masked. The results of the colour-discrimination-with-noise and the brightness-non-additivity experiments showed a double-dissociation between patients. This indicates that, in one patient, unconscious colour discrimination may be subserved by a spectrally non-opponent mechanism, which does not have the characteristics of the parvocellular pathway and which is responsive to fast flicker. Spectral sensitivity, contrast sensitivity and motion perception experiments confirmed that this patient lacks a working opponent parvocellular system. The second achromatopsic patient showed evidence of a residual parvocellular system. CONCLUSIONS: Our results show that chromatic discrimination need not be mediated by neural mechanisms, the parvocellular system in particular, normally assumed to subserve conscious colour perception. Such discrimination may be mediated by a neural subsystem which responds to fast flicker, is spectrally non-opponent, and supports normal motion perception.

Aged↗
PubMed1996

Intermanual transfer of proximal and distal motor engrams in humans.

We studied intermanual motor transfer for right-to-left or left-to-right direction of transfer between either proximal or distal upper extremity muscle groups. The influence of previously acquired motor engrams (original learning, OL) on learning efficiency of the contralateral side (transfer learning, TL) was examined in 26 right-handed healthy subjects. The task consisted of the drawing of meaningless figures. During TL, OL figures had to be reproduced as vertical mirror reversals. Data revealed a benefit for right-to-left but not left-to-right direction of transfer for time to complete a figure as well as a left-to-right transfer benefit for spatial motor precision. Furthermore, a benefit for intermanual transfer of training between proximal but not distal muscle groups was found when movement time to complete a figure was evaluated. Of special interest was the observation of a disadvantage due to prior contralateral learning for performance at right distal effectors. The asymmetrical transfer benefits with respect to side are in line with previous findings and support the proficiency model and the cross-activation model. Results further showed that intermanual transfer of training might differ with respect to muscle group involvement and suggest that, although primarily facilitating, previous opposite hand training may lead to inhibitory influences on subsequent contralateral reproduction.

Adult↗
PubMed1996

Three- to five-dimensional biomedical multisensor imaging for the assessment of neurological (dys) function.

This report describes techniques and protocols implemented at the Geneva Canton University Hospitals (HUG) for the combination of various biomedical imaging modalities and sensors including electromagnetic tomography, to study, assess, and localize neurological (dys) function. The interest for this combination stems from the broad variety of information brought out by (functional) magnetic resonance imaging, magnetic resonance spectroscopy, computed tomography, single-photon emission tomography, positron emission tomography, and electromagnetic tomography. Combining these data allows morphology, metabolism, and function to be studied simultaneously, the complementary nature of the information from these modalities becoming evident when studying pathologies reflected by metabolic or electrophysiologic dysfunctions. Compared with other current multimodality approaches, the one at the HUG is totally compatible with both clinical and research protocols, and efficiently addresses the multidimensional registration and visualization issues. It also smoothly integrates electrophysiology and related data as fully featured modalities.

Computer Graphics↗
PubMed1996

Plasticity in the adult visual cortex: implications for the diagnosis of visual field defects and visual rehabilitation.

It has recently been shown that, contrary to long-held beliefs, sensory and motor maps are not stable in the adult cerebral cortex. Alteration of input from the periphery results in changes in topography in the cortex, including the primary visual cortex. Mechanisms involved consist mainly of reshaping the receptive field of cortical cells and increasing the sensitivity of deprived cells in the visual cortex. Cortical plasticity allows the brain to adapt to background modifications or to damage of the nervous system. It also underlies learning and attention processes. Cortical changes occurring after focal visual differentiation modify visual perception by filling in visual field defects with information from the area surrounding the scotoma. This modification causes affected subjects to ignore or underestimate their defects. With visual field defects, cortical plasticity also causes distortion in spatial perception. Although the effects of cortical plasticity are prominent in neuro-ophthalmological daily practice, they are usually unrecognized or greatly underestimated. These effects cause delay in recognizing visual field defects, and hence in receiving therapy, while affecting the results of some procedures for testing the visual field. Affected individuals who are unaware of their defects may have increased difficulty in coping with activities in everyday life. Up to now, phenomena related to plasticity in the visual system have been investigated mainly by psychophysicists and neurophysiologists. It is essential to start considering the various effects of cortical reorganization in clinical practice. It is especially important to introduce into clinics the concept of dissociation between actual and perceived defects in the visual field, resulting from the filling-in process, and the need to measure it. This dissociation should also be demonstrated to the affected subjects.

Adult↗
PubMed1996

Intelligibility and spectral differences in high-pitched vowels.

The first formant frequency of most German vowels can be 'oversung' in the sense of vocalizations with pitch frequencies above F1 of normal speech. Investigations of sung and synthesized vowels suggested that, with rising F0, either the vowel loses its identity and its spectral characteristics, or changes in the vocal effort and the speaker group are perceived. This study presents high-pitched vocalizations by untrained men, women and children, apart from singing or shouting. Three main results were found: (i) vowel identity can be maintained at high pitches (F0 = 660-870 Hz); (ii) clear spectral differences in high-pitched vowels are demonstrated; (iii) high pitched vowels can be found within one speaker group, apart from changes in the perceived speaker group. Both the F0 dependence of the lower formants and a possible relationship between phonation and articulation are discussed.

Adult↗
PubMed1995

The cerebral hemispheres and neural network simulations: design considerations.

The conclusions concerning hemispheric specializations based on neural network simulations, which were previously reported by Kosslyn, Chabris, Marsolek, and Koenig (1992), are shown not to be valid. Differences in network performance on tasks said to be "categorical" and "coordinate spatial" in nature were due to imbalances in the input stimuli and cannot, in principle, be related to differences in performance on such tasks in human subjects. The use of truth tables and correlation coefficients in the design of neural networks is discussed.

Brain↗
PubMed1995

EEG reactivity in the prognosis of severe head injury.

We compared reactivity of EEG to external stimuli--an easily and quickly available measure--with the central conduction time (CCT) of the somatosensory evoked potentials, currently the most-used electrophysiologic method to predict outcome in severe head injury (SHI), and with the initial Glasgow Coma Scale (GCS) score. In 50 patients, comatose subsequent to SHI, we measured EEG reactivity and CCT within 48 to 72 hours and compared them with the outcome after 1.5 years. Using discriminant analysis, EEG reactivity correctly classified 92%, CCT classified 82%, and both measures together classified 98% of the patients into globally good or bad outcome groups. GCS allowed a correct classification in only 72% and, combined with either of the two electrophysiologic measures, did not further increase predictability. EEG reactivity is an excellent long-term global outcome predictor in SHI, superior to CCT and GCS. When the two electrophysiologic measures are combined, a prognostic accuracy is achieved that is better than that of any other reported method.

Adult↗
PubMed1995

FO-dependence, number alteration, and non-systematic behaviour of the formants in German vowels.

In acoustic theory, formant pattern differences within one vowel identify are related to differences in the speaker groups or in the types of vocalization. Yet, studies of vowel synthesis indicate that formant patterns can also vary strongly in relation either to pitch or to formant number alterations. Within this study, natural vocalizations of nine German vowels were investigated with regard to different formant patterns representing the same vowel identity, apart from differences in speaker groups and vocalization types. The results show i) FO-dependence of the lower formants < 1.5-2 kHz; ii) occurrence of natural one-formant back and two-formant front vowels; iii) a non-systematic relationship between FO and the formants, and a non-systematic relationship between formant patterns of different formant numbers. The implications for psychophysics, physiology and perception of speech are discussed.

Adult↗
PubMed1994

Lupus erythematosus and Miller-Fisher syndrome.

OBJECTIVE: To compare the clinical course of an unusual case of Miller-Fisher syndrome in systemic lupus erythematosus with therapeutic interventions, in particular with plasma exchanges. DESIGN: The clinical state and laboratory and electrophysiologic parameters were controlled for over a year and related to therapeutic attempts with immunoglobulins, steroids, and plasma exchanges. SETTING: Medical intensive care unit of a university hospital. PATIENT: A 17-year-old black female student with known systemic lupus erythematosus who developed ataxia, are flexia, and ophthalmoplegia (Miller-Fisher syndrome) and later became tetraplegic and required full mechanical ventilatory support. RESULTS: High-dose immunoglobulin treatment combined with corticosteroid pulse therapy was not beneficial. However, plasma exchange (performed five times over a period of 4 months) was followed by a striking clinical improvement within hours after each plasma exchange. CONCLUSIONS: Plasma exchange appears to remove a yet unknown agent producing a distal motor nerve conduction block and is efficacious in severe neuropathy associated with Miller-Fisher syndrome in lupus erythematosus.

Adolescent↗
PubMed1994

Hemispheric cooperation in visuospatial rotations: evidence for a manipulation role for the left hemisphere and a reference role for the right hemisphere.

A survey of mental rotation strategies in 210 normal subjects showed a strong tendency for right-handers to prefer rotating an object on the right and vice versa for left-handers. The differential functioning of the cerebral hemispheres during mental rotation was then assessed in 42 subjects by means of tachistoscopic presentation of two geometrical figures separately to the left and right visual fields--one of which was gravitationally stable and the other unstable. Performance was better when the unstable object was presented to the right visual field and the stable object to the left. This finding is interpreted as indicating more efficient hemispheric cooperation when the active manipulation of a mental image is performed by the left hemisphere, while the reference role is carried out by the right hemisphere.

Adolescent↗
PubMed1994

Dissociation of local and global processing in visual agnosia.

Subsequent to strokes in the right and left inferomedial occipito-temporal lobes, two patients became prosopagnosic and alexic, respectively. They also show a complementary dissociation of the analysis of handwritten text. The patient with the right posterior stroke can read it but not recognize whose handwriting it is; the patient with the left posterior stroke cannot read the text but knows who wrote it. The analysis of spatial vision revealed that the prosopagnosic patient has no problem with seeing texture elements when presented in isolation. Yet she performs poorly with Moiré and texture perception, i.e. she suffers from a selective loss of global visual perception. The alexic patient performs well with Moiré patterns but neither with (complex) texture elements nor with textures. She seemingly can locally and globally process patterns composed of simple figural elements but fails with stimuli that require the integration of features. This finding of a concomitant dissociation of local and global visual processes in the two patients supports the view that prosopagnosia as well as alexia are the most conspicuous aspects of more general alterations of visual perception.

Agnosia↗