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Biomedical subjects

S Johannes

Publications and source records attributed to S Johannes.

51 records · Page 3Linked to original sources
PubMed1996

Human evoked potentials to long duration vibratory stimuli: role of muscle afferents.

Tonic vibratory stimuli of 1000 ms duration and different frequencies were delivered to muscles of the forearms of young human subjects. Evoked potentials (EPs) were recorded from 29 scalp channels and revealed phasic highly lateralised and focally distributed EPs during the first 100 ms of the recording epoch that could be adequately modelled with a single point dipole source located in the vicinity of the central sulcus contralateral to the stimulated arm. A later negativity with an onset of about 400 ms and a duration of about 800 ms was found to be symmetrically distributed over fronto-central regions. This negativity is interpreted in terms of cortical activation beyond the primary sensory fields and could be related to the kinaesthetic phenomena experienced during muscle vibration.

Adult↗
PubMed1996

Transcranial Doppler assessment of cerebral blood flow velocity during visual spatial selective attention in humans.

Blood flow velocities of the posterior cerebral arteries were obtained while healthy subjects were engaged in a visual spatial attention task. Experimental runs consisted of series of stimuli comprised of four elements (two left and two right of a central fixation point) presented briefly in blue against a purple screen. After a period of passive viewing a left or right pointing arrow indicated the visual half-field to be attended by the subjects in order to detect identical symbols on the attended side. Relative to the passive viewing condition a marked increase of flow was seen in both posterior cerebral arteries during the attention period. No differential increase of flow as a function of attended field was detected. These results are discussed in comparison with recent positron-emission tomography (PET) and electrophysiological data obtained with the same task.

Adult↗
PubMed1996

Hierarchical visual stimuli: electrophysiological evidence for separate left hemispheric global and local processing mechanisms in humans.

Using reaction time measurements it has been shown that the neural processing of hierarchically composed visual stimuli may be mediated by partly distinctive mechanisms of global and local features analysis. Patients with focal left hemisphere brain lesions showed an impairment of local and patients with right hemisphere lesions of global stimulus perception. Our experiment was designed to extend the investigations of global/local feature analysis using the event-related brain potential (ERP) technique in healthy subjects. Thirteen subjects viewed a series of hierarchically composed non-linguistic stimuli in a divided attention condition in order to detect target stimuli which could be present at both local and global feature level, ERP analysis showed a posterior negative component (Ne) to be the correlate of early global/local information processing. When local targets were presented there was an effect of the global (non-target) level (termed 'distractor level') upon the onset of Ne but not vice versa. This supports the view of separate cortical mechanisms of global/local feature analysis. At odds with the previous patient data was the finding that both global and local level information was predominantly processed from the left hemisphere.

Adult↗
PubMed1996

Developmental dyslexia: passive visual stimulation provides no evidence for a magnocellular processing defect.

Livingstone et al. [Livingstone, M. S., Rosen, G. D., Drislane, F. W. and Galaburda, A. M. Physiological and anatomical evidence for a magnocellular defect in developmental dyslexia. Proceedings of the National Academy of Science U.S.A. 88, 7943-7947, 1991] presented evidence for a defect of the magnocellular visual processing stream in developmental dyslexia. We attempted to replicate this effect using transient and steady-state VEPs to checkerboard reversal stimuli in a group of adult developmental dyslexics. Several different reversal rates and contract levels were utilized. No differences were found between the dyslexic and control groups for the low-contrast, rapidly reversing patterns, nor for any combination of stimulus rate or contrast that was tested. Thus, these findings do not support a magnocellular processing deficit in developmental dyslexia.

Adult↗
PubMed1996

Mononeuropathia multiplex and idiopathic intracranial hypertension.

A young obese woman with menstrual irregularities presented with bilateral papilledema, abducens nerve paresis, visual obscurations and severe headache. Neuroimaging studies were normal but CSF opening pressure was elevated. The diagnosis of idiopathic intracranial hypertension was made. Insertion of a shunt quickly alleviated the symptoms. A few weeks later the patient developed a left femoral nerve paresis and an ulnar nerve paresis on the right. Electrodiagnostic tests confirmed the diagnosis of mononeuropathia multiplex. Bone marrow histology showed a vasculitis. We hypothesize that the vasculitis is underlying both, idiopathic intracranial hypertension and mononeuropathia multiplex.

Adult↗
PubMed1996

Maintaining the safe position in a palmar splint. The "double-T" plaster splint.

The injured and operated hand is routinely immobilized for a variable period of time in a palmar plaster splint. To minimize subsequent joint stiffness, the preferred position of immobilization of the hand is the "safe position". The true position of the hand in a splint was assessed on a lateral X-ray directly after application and just before removal 3 weeks later. It appears that in time the hand in a conventional palmar plaster splint loses its initial position because the bandage stretches. The "double-T" plaster splint maintains the safe position of the hand significantly better than the conventional palmar splint.

Adolescent↗
PubMed1996

Effects of alprazolam and bromazepam on visual search and verbal recognition memory in humans: a study with event-related brain potentials.

The effects of two benzodiazepines (alprazolam and bromazepam) on performance and event-related brain potentials (ERPs) in visual search and verbal recognition memory paradigms were tested in a placebo-controlled double-blind paradigm employing 12 young healthy subjects. Both drugs exerted similar effects on performance and electrophysiological measures. Reaction times and hit rates were adversely affected in both paradigms. The ERP pattern in the visual search experiment suggested an impairment in selective attention (reduction of N1 component) and of automatic feature registration (posterior N2 reduction). In the world recognition experiment a dramatic reduction of the N400 component was seen under benzodiazepines. This is interpreted as reflecting an impaired ability to perform context integration processes.

Adult↗
PubMed1996

[Choreoacanthocytosis. A neurologic-hematologic syndrome].

A 43 year old male patient is reported who presented at the age of 33 years with a hyperkinetic movement disorder. At the time of presentation orofacial dyskinesias, tic-like hyperkinesias with vocalisation and behavioural disturbance dominated the clinical picture. In the course of his illness he developed a marked truncal choreoathetosis and a symmetrical, distal, predominantly motor polyneuropathy with wasting of lower leg muscles. Serum creatinine kinase levels were markedly elevated. Serum lipids and lipoproteins were within normal limits. These clinical features in combination with an increased number of acanthocytes, clearly visible after dilution of whole blood with normal saline (1:1), led to the diagnosis of choreoacanthocytosis (CA). Both parents were neurologically and behaviourally normal, but were found to have acanthocytes in saline diluted whole blood. The literature concerning CA is reviewed.

Acanthocytes↗
PubMed1995

Brain potential correlates of number errors in the Turkish language.

Event-related potentials (ERPs) were recorded from 16 native speakers of Turkish while they watched Turkish sentences flashed upon the screen of a videomonitor. The sentences were three words long and contained a verb at the terminal position that was manipulated (1) to have either the correct or the incorrect number with regard to the subject of the sentence, and (2) to be semantically appropriate, inappropriate or to be a pseudoverb. The ERPs from 19 scalp channels revealed a more positive waveform at the left-fronto-temporal site with an onset latency of about 200 ms for the verbs having the wrong number regardless of the semantic appropriateness of the word, while at right fronto-temporal sites a more negative waveform was observed for plural words only. This number incongruency ERP-effect in Turkish is clearly at odds with findings from other languages.

Adult↗
PubMed1995

Brain potentials in developmental dyslexia: differential effects of word frequency in human subjects.

The differences of word processing between a group of adult developmental dyslexics and control subjects were examined with the event-related potential (ERP) technique. In particular, the effects of word frequency and word recognition were assessed. The subjects viewed a series of frequently and infrequently used words, most of which were repeated after some intervening items and they discriminated between first and second presentations of the words. It can be shown that in the range from 300 to 550 ms post stimulus the amplitude of the N400 component, an ERP measure of semantic processing, is reduced for high frequency words. This effect is more pronounced in the dyslexic group and the effects of word recognition are also reduced in the dyslexic group for high frequency words. These findings are discussed with respect to current concepts of dyslexia and of semantic processing.

Adult↗
PubMed1995

Cognitive functioning in experimental hypoglycaemia assessed with event-related potentials.

Cognitive effects of experimentally induced hypoglycaemia were investigated by event-related brain potentials (ERPs) in auditory and visual classification tasks. In a cross-over design eight healthy male subjects (age 22-27 years) were studied under control conditions or following administration of 0.15 IU insulin kg-1. The ERPs showed preserved early cortical components, while the N2 component was greatly reduced in both modalities. P3 components did not show an influence of hypoglycemia. These results suggest that the cognitive deficits are due to an impairment of automatic feature extraction from the stimuli.

Adult↗
PubMed1995

Luminance and spatial attention effects on early visual processing.

Event-related potentials (ERPs) were recorded from healthy subjects in response to unilaterally flashed high and low luminance bar stimuli presented randomly to left and right field locations. Their task was to covertly and selectively attend to either the left or right stimulus locations (separate blocks) in order to detect infrequent shorter target bars of either luminance. Independent of attention, higher stimulus luminance resulted in higher ERP amplitudes for the posterior N95 (80-110 ms), occipital P1 (110-140 ms), and parietal N1 (130-180 ms). Brighter stimuli also resulted in shorter peak latency for the occipital N1 component (135-220 ms); this effect was not observed for the N1 components over parietal, central or frontal regions. Significant attention-related amplitude modulations were obtained for the occipital P1, occipital, parietal and central N1, the occipital and parietal P2, and the parietal N2 components; these components were larger to stimuli at the attended location. In contrast to the relatively short latencies of both spatial attention and luminance effects, the first interaction between luminance and spatial attention effects was observed for the P3 component to the target stimuli (350-750 ms). This suggests that interactions of spatial attention and stimulus luminance previously reported for reaction time measures may not reflect the earliest stages of sensory/perceptual processing. Differences in the way in which luminance and attention affected the occipital P1, occipital N1 and parietal N1 components suggest dissociations among these ERPs in the mechanisms of visual and attentional processing they reflect.(ABSTRACT TRUNCATED AT 250 WORDS)

Adolescent↗
PubMed1994

Combined spatial and temporal imaging of brain activity during visual selective attention in humans.

Visual-spatial attention is an essential brain function that enables us to select and preferentially process high priority information in the visual fields. Several brain areas have been shown to participate in the control of spatial attention in humans, but little is known about the underlying selection mechanisms. Non-invasive scalp recordings of event-related potentials (e.r.ps) in humans have shown that attended visual stimuli are preferentially selected as early as 80-90 ms after stimulus onset, but current e.r.p. methods do not permit a precise localization of the participating cortical areas. In this study we combined neuroimaging (positron emission tomography) with e.r.p. recording in order to describe both the cortical anatomy and time course of attentional selection processes. Together these methods showed that visual inputs from attended locations receive enhanced processing in the extrastriate cortex (fusiform gyrus) at 80-130 ms after stimulus onset. These findings reinforce early selection models of attention.

Attention↗
PubMed1994

[Cerebral lateralization in constitutional dyslexia. Electrophysiologic findings].

Event-related brain potentials were recorded from a group of developmental dyslexics using a simple visual discrimination task and compared to the results of a control group. A large P3 component was elicited which did not differ in overall amplitude between groups. While the distribution was virtually symmetrical in the dyslexic group, there was a left hemispheric preponderance in the control group. The latencies of the P3 were longer in the dyslexic group than in the control group. The results are compared to findings of neuroanatomical and neuroimaging studies which revealed less hemispheric asymmetries in dyslexics compared to control subjects.

Adult↗