Biomedical subjects

T Landis

Publications and source records attributed to T Landis.

At least 91 records · Page 5Linked to original sources

PubMed1999

Hallucinatory experiences in extreme-altitude climbers.

OBJECTIVE: This study attempted a systematic investigation of incidence, type, and circumstances of anomalous perceptual experiences in a highly specialized group of healthy subjects, extreme-altitude climbers. BACKGROUND: There is anecdotal evidence for a high incidence of anomalous perceptual experiences during mountain climbing at high altitudes. METHOD: In a structured interview, we asked eight world-class climbers, each of whom has reached altitudes above 8500 m without supplementary oxygen, about hallucinatory experiences during mountain climbing at various altitudes. A comprehensive neuropsychological, electroencephalographic, and magnetic resonance imaging evaluation was performed within a week of the interview (8). RESULTS: All but one subject reported somesthetic illusions (distortions of body scheme) as well as visual and auditory pseudohallucinations (in this order of frequency of occurrence). A disproportionately large number of experiences above 6000 m as compared to below 6000 m were reported (relative to the total time spent at these different altitudes). Solo climbing and (in the case of somesthetic illusions) life-threatening danger were identified as probable triggers for anomalous perceptual experiences. No relationship between the number of reported experiences and neuropsychological impairment was found. Abnormalities in electroencephalographic (3 climbers) and magnetic resonance imaging (2 climbers) findings were likewise unrelated to the frequency of reported hallucinatory experiences. CONCLUSIONS: The results confirm earlier anecdotal evidence for a considerable incidence of hallucinatory experiences during climbing at high altitudes. Apart from hypoxia, social deprivation and acute stress seem to play a role in the genesis of these experiences.

Adult↗

PubMed1999

From cortical plasticity to unawareness of visual field defects.

It was long held that, following alterations in sensory input, structural changes in the primary visual cortex take place only in early life, during so-called "critical periods." Recently, however, it has been established that, in adults, cortical maps in the brain are not fixed, and the cortex does not perform stereotyped operations. Instead, neuronal receptive fields in the cortex can reorganize following deactivation or an altered pattern of activation. Plasticity is essential for the normal adjustment of the brain to modifications in the sensory environment, and for improving perceptual skills and sensorimotor performances. It also plays a crucial role in recovery from damage to the visual system. Cortical remapping generates a filling-in of visual field defects. Consequently, it alters the image perceived. Cortical rearrangement following lesions in the visual pathways does not restore function to the destroyed tissue, but it helps to compensate for gaps in perception. In this review article, we focus on effects of plasticity in the adult visual cortex which are of major importance in the daily practice of neuroophthalmology. Cortical reorganization, together with resulting filling-in, affects the early recognition and evaluation of visual field defects. The importance of brain remapping in these matters is still largely underestimated by clinicians.

Adult↗

PubMed1999

Psychosocial functioning in chronic epilepsy: relation to hippocampal volume and histopathological findings.

MRI volumetric measurements of the ipsi- and contralateral hippocampal volume (HV) were performed in a group of 37 patients with lesional and non-lesional temporal (TLE), and extratemporal lobe (ETLE) lobe epilepsy. Twenty-six patients underwent surgery and the resected tissues subjected to histopathological examination. Psychosocial scores were calculated on the basis of employment, partnership/family status and presence of a positive psychiatric history. Poorest values were obtained for the group with right lesional and non-lesional TLE (RTLE, p < 0.001). In this group, significant positive correlations between psychosocial functioning and the left hippocampal volume or age of onset were observed. We found that the left hippocampal volume in RTLE was smaller with increasing epilepsy duration. We conclude that patients with RTLE represent a particular group with regard to the structural and psychosocial consequences of chronic epilepsy. With regard to histopathological analysis, no particular type of lesion was identified as the major determinant of the results described above.

Adult↗

PubMed1999

Formant Pattern Ambiguity of Vowel Sounds.

The formant frequencies of a particular vowel vary according to the speaker group and to coarticulation. Therefore, overlapping formant patterns of different vowels are commonly related to sex and age differences and to coarticulation, and are considered to concern mainly the F(1) -F(2) pattern of adjacent vowels. However, several studies have reported indications of a correlation between the lower formant frequencies and F(0), as well as of the appearance of different formant numbers relevant to vowel identity. As a consequence, the overlap between formant patterns of different vowels might be more substantial than has traditionally been assumed. Within the present study, therefore, the extent to which a given formant pattern can represent different vowels was investigated for natural Swiss German vowels produced monotonously and in isolation by men, women and children at F(0) of 85-870 Hz. Similar formant patterns were found for vocalizations of different vowels with both small and large phonetic distances, and within the entire frequency ranges of the formants relevant for phoneme identity. For vowel sounds displaying ambiguous formant patterns, the main spectral characteristics related to differences in their perceptual identity were found to concern F(0) and relative formant amplitudes. Results are given in exemplary vowel series, and consequences for the psychophysics of the vowel are discussed.

Journal Article↗

PubMed1998

Illusory contours and spatial neglect.

It has been controversial whether the perception of illusory contours arise from higher level cognitive mechanisms that require attention or from early preattentive visual processes. We studied three patients with left spatial neglect who were unable to detect the left inducers of Kanizsa illusory figures in a same/different judgment task but nonetheless showed implicit perception of the figures in a midpoint judgment, in that they made identical bisection for figures with illusory or real contours but very different bisection for other spatially discontinuous figures that did not yield illusory filling-in. Grouping and filling-in mechanisms can thus occur without explicit detection of, or attention to, the inducing features, consistently with the hypothesis that they involve preattentive visual processes.

Aged↗

PubMed1998

Corticosteroid-responsive postmalaria encephalopathy characterized by motor aphasia, myoclonus, and postural tremor.

OBJECTIVES: To study the clinical spectrum of an acute severe encephalopathy occurring in 2 patients after recovery from falciparum malaria infection and to compare it with the reported clinical features of the postmalaria neurological syndrome. DESIGN: Case report. SETTING: Tertiary care hospital. PATIENTS: Two patients presented with acute onset of fluctuating motor aphasia, severe generalized myoclonus, and postural tremor. Additional signs were cerebellar ataxia, and in 1 patient, generalized epileptic seizures. Magnetic resonance imaging of the brain revealed patchy white matter lesions in 1 patient. Clinically, the patients' conditions continued to worsen until corticosteroids were introduced, the use of which induced a rapid, albeit incomplete, recovery. CONCLUSIONS: We describe a new, severe variant of the still poorly defined postmalaria neurological syndrome. We propose a preliminary classification of this syndrome, according to its clinical characteristics, as follows: a mild or localized form, characterized by isolated cerebellar ataxia or postural tremor; a diffuse, but relatively mild encephalopathic form, characterized by acute confusion or epileptic seizures; and a severe, corticosteroid-responsive encephalopathy that is characterized by motor aphasia, generalized myoclonus, postural tremor, and cerebellar ataxia.

Adrenal Cortex Hormones↗

PubMed1998

Near and far visual space in unilateral neglect.

A 24-year-old right-handed woman with a right temporal hematoma showed marked left visual neglect for far but not near space in a variety of tasks systematically given in near and far distance conditions. This case thus provides the dissociation opposite to Halligan and Marshall's patient, who had neglect for near but not far space after a right parietal stroke. Furthermore, although she made rightward errors in bisecting far-distant lines, our patient made smaller opposite leftward errors for near-distant lines. The evidence that unilateral neglect of far and near visual space may exist independently supports a division in the neural systems subserving attention to different compartments of the extrapersonal space in humans.

Adult↗

PubMed1998

Non-invasive epileptic focus localization using EEG-triggered functional MRI and electromagnetic tomography.

We present a new approach for non-invasive localization of focal epileptogenic discharges in patients considered for surgical treatment. EEG-triggered functional MR imaging (fMRI) and 3D EEG source localization were combined to map the primary electrical source with high spatial resolution. The method is illustrated by the case of a patient with medically intractable frontal lobe epilepsy. EEG obtained in the MRI system allowed triggering of the fMRI acquisition by the patient's habitual epileptogenic discharges. fMRI revealed multiple areas of signal enhancement. Three-dimensional EEG source localization identified the same active areas and provided evidence of onset in the left frontal lobe. Subsequent electrocorticography from subdural electrodes confirmed spike and seizure onset over this region. This approach, i.e. the combination of EEG-triggered fMRI and 3D EEG source analysis, represents a promising additional tool for presurgical epilepsy evaluation allowing precise non-invasive identification of the epileptic foci.

Adolescent↗

PubMed1998

Complete sparing of high-contrast color input to motion perception in cortical color blindness.

It is widely held that color and motion are processed by separate parallel pathways in the visual system, but this view is difficult to reconcile with the fact that motion can be detected in equiluminant stimuli that are defined by color alone. To examine the relationship between color and motion, we tested three patients who had lost their color vision following cortical damage (central achromatopsia). Despite their profound loss in the subjective experience of color and their inability to detect the motion of faint colors, all three subjects showed surprisingly strong responses to high-contrast, moving color stimuli--equal in all respects to the performance of subjects with normal color vision. The pathway from opponent-color detectors in the retina to the motion analysis areas must therefore be independent of the damaged color centers in the occipitotemporal area. It is probably also independent of the motion analysis area MT/V5, because the contribution of color to motion detection in these patients is much stronger than the color response of monkey area MT.

Adult↗

PubMed1998

[Visual perception of movement. A normative study].

BACKGROUND: A battery of tests has been developed to better define the variety of defects which can occur in the process of visual perception of motion. METHODS: The tests assessed the perception of 1) coherent motion, in each of the four cardinal directions; (2) form from motion; (3) flow in depth; and (4) motion of a target moving in depth, within or without an optic flow surround. A normative study was conducted with 34 subjects, 21 to 69 years of age. RESULTS: Recognition of form from motion was better with upwards than with downwards motion. Perception of motion in depth was better with backward than forwards motion. The presence of an optic flow altered the perception of a motion in depth. CONCLUSION: The recognition of form from motion demonstrated an asymmetry in threshold between the upward and downward movements. This asymmetry did not occur with global coherent motion, thus reflecting the variety of mechanisms involved in the visual process of motion. Moreover, alteration of perception of motion in depth within an optic flow reflected background effect.

Adult↗

PubMed1998

Motor perseverations: a function of the side and the site of a cerebral lesion.

Motor perseverations are commonly thought to be a sign of frontal lobe lesions, although they may also occur following lesions outside the frontal lobes. Moreover, differences in the type and frequency of motor perseverations between the two cerebral hemispheres have been suggested. Recurrent perseverations (repetition of a previous response to a subsequent stimulus) and continuous perseverations (abnormal prolongation of a current activity) are two different aspects of motor intentional disorders. The former have been associated with left hemispheric lesions, and the latter with right hemispheric lesions. To test this hypothesis, we examined 42 patients with single cerebral lesions of different locations: 22 right hemispheric (7 frontal and 15 extrafrontal) and 20 left hemispheric lesions (8 frontal and 12 extrafrontal), and healthy controls with a graphic task, allowing for analysis of these two types of motor perseverations. We found that (i) both types of perseverations increased significantly for all groups of patients compared to normal controls, except for recurrent perseverations in patients with right posterior lesions, (ii) there was no significant dissociation between the type of perseverations and the side of lesions, and (iii) there was a significant interaction between the type of perseveration, frontal versus extrafrontal location, and the side of a lesion, which is based on the tendency for recurrent perseverations to be increased in left posterior and right frontal lesions, and decreased in left frontal and right posterior lesions, as compared to continuous perseverations. These findings cast doubt about the predominant role of the frontal lobes for motor perseverations, but support the idea of hemispheric specialization in motor control which is, however, more complex than previously thought.

Adult↗

PubMed1998

Comprehensive postictal neuropsychology improves focus localization in epilepsy.

In recent years, new techniques such as single photon emission computed tomography (SPECT), positron emission tomography (PET) and magnetic resonance imaging (MRI) have been used to improve the localization of epileptic foci during the noninvasive evaluation procedure for epilepsy surgery. Since ictal/or immediate postictal SPECT studies were shown to localize epileptic foci better than interictal SPECT, we addressed the question of whether immediate postictal neuropsychological examination would show the same effect. Neuropsychological examinations were carried out postictally and interictally using a broad range of tests. Postictal results were analyzed with regard to lateralizing and localizing information about the epileptogenic region. Seventeen patients suffering from temporal and extratemporal pharmacoresistant epilepsy were investigated postictally with a subset of tests used for the interictal neuropsychological examination. A significant improvement in focus localization was seen in comparison with interictal neuropsychology (p = 0.014). We conclude that neuropsychology can yield lateralizing and sometimes localizing information, even for extratemporal foci, if carried out during the postictal period and based on a global analysis of the clinical neuropsychological picture.

Adolescent↗

PubMed1998

Ictal agraphia: a patient study.

We present a case of "pure" or "apraxic" agraphia, a recognized writing disturbance, caused by intermittent focal seizures from the left posterior parasagittal parieto-occipital region. Pure agraphia has been described in focal lesions or as part of a generalized confusional syndrome. The posterior cerebral vascular territory is the characteristic site for the transient neurologic syndromes associated with cyclosporine toxicity. Our patient was at risk for this complication while recovering from a lung transplant, with elevated cyclosporine levels.

Agraphia↗

PubMed1998

An unusual case of Creutzfeldt-Jakob disease.

A patient with histopathologically verified sporadic Creutzfeldt-Jakob disease (CJD) presented initially with diplopia, sleep disturbances, and L-dopa-responsive parkinsonism. After more than a year of slow progression, he did not become demented, and failed to fulfill the clinical criteria for possible CJD. No clinical examinations currently proposed to detect CJD showed the disease. CJD should be in the differential diagnosis of "parkinson plus" syndromes until a different etiology has been found or a histopathologic examination performed.

Creutzfeldt-Jakob Syndrome↗

PubMed1998

Periodic downbeat nystagmus.

The authors observed a periodic downbeat nystagmus with a cycle of 3 minutes 30 seconds, beating downward for a period of 90 seconds every 2 minutes. It lasted 86 hours in a patient with severe hypomagnesemia associated with complications from scleroderma. There was no lesion on brain imaging, and an EEG performed during the nystagmus was normal, indicating that the most likely explanation for this periodic nystagmus is metabolic. This form of nystagmus may have resulted from severe hypomagnesemia, possibly associated with thiamine deficiency.

Activity Cycles↗

PubMed1997

Evidence for rapid face recognition from human scalp and intracranial electrodes.

It is still generally believed that complex visual analysis is not carried out within the first 100 ms. Here we show that intra- and extracranial visual evoked potentials (VEPs) differentiate previously seen faces from novel faces as early as 50 ms after stimulus onset. EEG was recorded from scalp electrodes in 12 male healthy volunteers (group I) and intracranially from implanted depth electrodes in the temporal and frontal cortex of seven epilepsy patients (group II). Both groups were engaged in a face recognition task. All subjects showed significant differential responses which occurred very early (50-90 ms) and later (190-600 ms). In group II, the early responses were recorded more frequently in the right hemisphere, whereas the late differential VEPs were found in both hemispheres. Both types of VEPs were more frequent in the temporal neocortex, underlining its role as a major contributor to these fast recognition processes.

Adult↗

PubMed1997

Four legs. Illusory reduplication of the lower limbs after bilateral parietal lobe damage.

OBJECTIVE: To report an unusual disorder of body schema and its neurologic and neuropsychological correlates. DESIGN AND METHODS: We describe a patient with a reduplicative phantom illusion of her lower limbs. Motor and sensory functions, as well as mental representation of body and space, were studied during the reduplication experience until its resolution. SETTING: Clinical neurology department in a primary care hospital. PATIENT: A 64-year-old, left-handed woman who experienced the uncontrollable and distressing feeling of having 4 legs, without delusional belief, after surgical removal of a right-predominant parasagittal parietal meningioma. This phenomenon spontaneously resolved after 2 weeks. INTERVENTION: None. MAIN OUTCOME MEASURES: Clinical neurologic examinations and standardized neuropsychological tests, with emphasis on tests assessing orientation to body parts, right-left discrimination, and mental orientation in space. RESULTS: The patient had severe weakness and proprioceptive sensory loss in both lower limbs. She had no disturbances of body schema knowledge but a striking impairment in tasks requiring mental orientation in space, particularly for right-left laterality discrimination. Resolution of the reduplication experience correlated with improvement in the affected spatial abilities, while motor, sensory, and other cognitive functioning did not significantly change. CONCLUSION: This patient's reduplicative phantom illusion might be related to the combination of the severe somatosensory loss with an underlying impaired mental representation of relative positions in space.

Brain Neoplasms↗

PubMed1997

The anterior and posterior selective temporal lobe amobarbital tests: angiographic, clinical, electroencephalographic, PET, SPECT findings, and memory performance.

The techniques, results, and problems of three types of selective temporal lobe (TL) amobarbital procedures (balloon technique with temporary occlusion of the internal carotid artery distal to the origin of the anterior choroidal artery (acha) [n = 19]; selective anterior catheterization of the acha [n = 20]; and selective catheterization of the peduncular P2-segment of the posterior cerebral artery [n = 5]) are described in a group of 40 patients with medically refractory complex partial seizures of mesial TL origin. Selective amobarbital tests were carried out before surgery to predict the memory deficit after an intended selective amygdalohippocampectomy. The effects of selective anaesthetization of TL were correlated with clinical data, pattern and duration of amobarbital induced EEG changes, and performance on verbal and nonverbal memory tasks measured during the test. In 4 patients the effect of selective amobarbital injection on regional and global metabolism was studied with 18F-FDG-PET, with the PET tracer being injected intravenously immediately after amobarbital. More recently in 2 patients the vascular territory perfused by amobarbital in the acha test was studied with SPECT using 99m Tc ECD injected immediately prior to the amobarbital into the acha. Whereas the PET studies showed a rather widespread and bilateral amobarbital-induced decrease of metabolism, the SPECT studies confirmed the selective distribution of the tracer in the vascular territory of the acha, i.e., in amygdala and hippocampus. The comparison of selective TL amobarbital test performance with postoperative neuropsychological performance showed that the predictive value of this test is rather good for the postoperative verbal memory but underestimates postoperative nonverbal ("figural") memory performance.

Adult↗