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Biomedical subjects

T Landis

Publications and source records attributed to T Landis.

At least 55 records · Page 3Linked to original sources
PubMed2000

New insights into the Stroop effect: a spatio-temporal analysis of electric brain activity.

Recent clinical and imaging studies suggest the involvement of anterior brain regions in the Stroop effect without providing consensus on the hemisphere being involved. Here, we investigated the dynamics of brain activation during a modified Stroop task using behavioural, event-related potential map series, and source localization analysis. Behavioural analysis showed an increased RT in the interference (IC) as compared to the neutral (NC) and congruence conditions (CC). Map series analysis in these conditions displayed a similar sequence of 10 stable segments. From these, only segment S6, occurring at approximately 300 ms and displaying a dominant right anterior activation, was of increased duration in IC. Furthermore, in IC only, RT was shown to correlate with S6 duration. These results are discussed in terms of increased duration of an attentional process needed to solve the conflict.

Adult↗
PubMed2000

Location of the human frontal eye field as defined by electrical cortical stimulation: anatomical, functional and electrophysiological characteristics.

Electrical cortical stimulation of the human frontal gyri and the precentral gyrus has been shown to induce eye movements and it has classically been assumed that these stimulation-induced eye movements result from electrical interference with the human homologue of the monkey frontal eye field (FEF). However, amplitude of electrical current and induced type of eye movement, which are essential for the determination of eye fields in the monkey, have not been investigated systematically in man. We applied electrical cortical stimulation in the lateral frontal cortex in six epileptic patients. Sites whose stimulation resulted in eye movements were determined with respect to gyral and sulcal patterns, Talairach coordinates and neighboring functions as found by electrical cortical stimulation. Based on this approach, a restricted location of the electrically defined FEF is proposed within a larger oculomotor region on the posterior part of the middle frontal gyrus.

Adolescent↗
PubMed2000

Lack of anticipatory gaze-orienting responses in patients with right brain damage.

OBJECTIVE: To study eye movements during cervical proprioceptive stimulation by passive body rotation in darkness, with the head held stationary, in patients with right brain damage and hemineglect. BACKGROUND: At very low frequency, this stimulation is reported to produce an illusion of head turning in space and eye deviations directed opposite to trunk rotation (in the direction of the illusory head rotation). METHODS: Ten normal subjects and seven patients with unilateral cerebral lesions (five right brain-damaged patients with mild to moderate visuospatial neglect, two left brain-damaged patients without neglect) were included in the study. Subjects were seated on a rotating chair. Stimuli consisted of slow sinusoidal passive trunk rotations (+/-30 degrees, 0.01 Hz) while the head was fixed in space. RESULTS: Eye movements directed opposite to trunk rotation were typical for normal subjects and for left brain-damaged patients. In contrast, all right brain-damaged patients showed either eye movements in the direction of trunk rotation or no eye deviations at all. CONCLUSION: This result could characterize a lack of anticipatory coordinating gaze behavior in patients with right brain damage.

Aged↗
PubMed2000

Imaging the electrical activity of the brain: ELECTRA.

The construction of a tomography of neuronal sources is limited by a lack of information. A possible way around this problem is to change the biophysical model that underlies the statement of the inverse problem, i.e., searching for magnitudes that can be better determined from the available data. In this report, we describe a mathematical characterization of the type of currents that are actually able to produce the scalp-recorded EEG. Considering this characterization, we reformulate the bioelectric inverse problem. This approach, called ELECTRA, yields some advantages over the classical formulation in terms of the current density vector: (1) the number of unknowns can be reduced, which is equivalent to increasing the number of independent measurements, (2) the constraints used to reformulate the problem are undeniable since they do not imply any hypothesis about brain function but are instead based on the character of the measurements, and (3) existing experimental evidence suggests that the proposed source model characterizes the type of currents that arise in excitable tissues. We conclude that if the latter fact proves to be true for brain tissues, then no additional information is added to the inverse problem by using a more general source model than the one proposed here. Images obtained using this method for synthetic data, as well as early and middle components of human visual evoked responses to checkerboard stimuli, are presented to illustrate the characteristics of the reconstructed maps and their interpretation.

Brain↗
PubMed2000

Electrical cortical stimulation of the human prefrontal cortex evokes complex visual hallucinations.

Complex visual hallucinations are a well-known feature of electrical stimulation or epileptic discharge in the temporal lobe. It has been proposed that these visual hallucinations result from an electrical interference with the ventral visual processing stream in the lateral temporal lobe and the memory system in medial temporal structures, which explains their frequent visual and mnestic features. Even though recent studies have demonstrated visual and memory functions in the prefrontal cortex, up to now epileptic discharge or electrical stimulation of prefrontal structures has only rarely been reported to induce visual phenomena. We report on two patients undergoing invasive presurgical epilepsy evaluation in which electrical cortical stimulation of the left prefrontal cortex repeatedly induced complex visual hallucinations. Interestingly, the induced visual responses differed with respect to their spatial organization: whereas those evoked on the inferior frontal gyrus were perceived in the whole visual field, complex visual responses on the middle frontal gyrus were restricted to the contralateral hemispace. Based on the spatial organization of the visual experiences in our patients, animal work, and neuroimaging data it might be suggested that specific subregions of the human prefrontal cortex might contain separate visual and mnemonic processing mechanisms.

Journal Article↗
PubMed2000

Internally driven vs. externally cued movement selection: a study on the timing of brain activity.

Brain imaging studies in man and single cell recordings in monkey have suggested that medial supplementary motor areas (SMA) and lateral pre-motor areas (PMA) are functionally dissociated concerning their involvement in internally driven and externally cued movements. This dichotomy, however, seems to be relative rather than absolute. Here, we searched for further evidence of relative differences and aimed to determine by what aspect of brain activity (duration, strength, or both) these might be accounted for. Event-related potentials (ERPs) were recorded while healthy, right-handed subjects selected one of three possible right hand digit movements based either on 'internal' choice or 'external' cues. The results obtained from ERP mapping suggest that movement selection evokes the same electrical brain activity patterns in terms of surface potential configurations in the same order and at the same strength independent of the selection mode. These identical configurations, however, differed in their duration. Combined with the results of a distributed source localization procedure, our data are suggestive of longer lasting activity in SMA during the 'internal' and longer lasting activity in PMA during the 'external' condition. Our results confirm previous findings in showing that SMA and PMA are distinctively involved in the two tasks and that this functional dichotomy is relative rather than absolute but indicate that such a dissociation can result from differences in duration rather than pure strength of activation.

Adult↗
PubMed2000

Temporal and spatial determination of EEG-seizure onset in the frequency domain.

OBJECTIVE: A quantitative analysis of scalp electric fields in patients suffering from pharmacoresistant temporal lobe epilepsy was performed in order to study the development of rhythmic ictal activities over time. METHODS: A method that calculates phase-corrected voltage maps in the frequency domain (FFT-approximation) was applied to ictal multichannel recordings in 10 epileptic patients. The onset of the ictally dominant frequency was determined and its temporal evolution over a time period of 46 s around the ictal EEG onset was studied. The analysis was completed by a linear inverse solution that estimated the sources of the dominant frequency. RESULTS: This method permitted the identification of an ictally dominant frequency which started on the average prior to the onset of initial EEG signs as determined by visual inspection. The frequency incremented during the evolution of the seizure in all patients. The linear inverse solution algorithm localized the source of this frequency to the brain region which was clinically determined as the site of seizure onset and whose resection rendered all patients seizure-free. CONCLUSIONS: Our data suggest that the constant increase of the ictally dominant frequency is related to the amount of temporal lobe tissue generating the ictal discharges. Frequential analysis of ictal electric fields can be reliably used to detect focal pathological activity early during seizure onset arising in deep structures such as the mesial temporal lobe.

Adult↗
PubMed2000

Semantic category and rhyming processing in the left and right cerebral hemisphere.

In this study, the question of hemispheric abilities in processing explicit semantic and phonological information was addressed by measuring response latencies and performances in two judgement tasks. In the semantic task, word pairs were sequentially presented to the left or right visual field and subjects were asked to judge whether these words were categorically related or not. In the phonological task, the same subjects were asked to decide whether pairs of orthographically dissimilar words rhymed or not. Statistical analysis showed that reaction times (RT) were significantly shorter in both tasks when words were presented to the right visual field. Furthermore, in the semantic task, faster responses were observed in both visual fields when the words were related than when they were unrelated. This response facilitation tended to be stronger when words were presented to the left visual field. This result is in accordance with other results obtained by lexical decision studies showing that priming in the right hemisphere is due to controlled processing. By contrast, RT differences between rhyming and non-rhyming word pairs in the phonological task did not reach significance. This supports previous findings showing that phonological facilitation cannot occur when orthographic and phonological indices are in conflict.

Journal Article↗
PubMed2000

Electrophysiological evidence for fast visual processing through the human koniocellular pathway when stimuli move.

There is increasing evidence from cellular recordings in primates and behavioral studies in humans that motion can be processed by other than the magnocellular (M) pathway and the cortical dorsal stream. Little is known about cortical processing of moving stimuli when the information is conveyed by the third retinogeniculocortical pathway - the so-called koniocellular (K) pathway. We addressed this issue in humans by studying the spatio-temporal dynamics of the brain electrical fields evoked by tritan (S-cone isolating) and luminance-defined moving stimuli. Tritan and luminance stimuli are presumably carried by the K and M pathways respectively. We found two time intervals where significant stimulus-specific electric fields were evoked: an early period between 40 and 75 ms after stimulus onset, and a later period between 175 and 240 ms. Some of these fields were identical for tritanand luminance-motion, suggesting that the processing of moving stimuli share common cortical substrates when mediated via K and M pathway input. However, tritan-motion stimuli also evoked unique electric fields that appeared earlier in time than the common motion-specific fields, indicating very fast activation of cortical areas specific to input through the K pathway. A distributed source localization procedure revealed simultaneous activation of striate and extrastriate areas even at the early processing stages, strongly suggesting a very fast activation of the visual cerebral network.

Adult↗
PubMed2000

Monocular central dazzle after thalamic infarcts.

The authors observed a patient after he had ischemic strokes in both paramedian thalamic regions, which were more marked on the left side. Symptoms included dysphasia, vertical binocular diplopia, right-sided hemianopia, and a right-sided sensory and motor deficit, sparing the face. However, the most disturbing phenomenon was a painless, left monocular dazzle, which was the presenting symptom and also the only persisting symptom. This report shows that a thalamic lesion may be at the origin of central dazzle, and to the authors' knowledge, it is the first clinical observation of its monocular occurrence. It is conceivable that this dazzle was due to optic-trigeminal summation.

Brain Infarction↗
PubMed2000

Simple and complex vestibular responses induced by electrical cortical stimulation of the parietal cortex in humans.

The present study reports on a patient undergoing invasive monitoring for intractable epilepsy who experienced different vestibular sensations after electrical cortical stimulation of the inferior parietal lobule at the anterior part of the intraparietal sulcus. Types of vestibular response ranged from simple to complex sensations and depended on stimulation site and applied current. The findings suggest vestibular topography and hierarchical processing within the parietal vestibular cortex of humans.

Adult↗
PubMed2000

Facial palsy in cerebral venous thrombosis : transcranial stimulation and pathophysiological considerations.

BACKGROUND: Cranial nerve palsy in cerebral sinovenous thrombosis (CVT) is rare, its pathophysiology remains unclear, and data from electrophysiological examinations in such patients are missing. CASE DESCRIPTION: We report the case of a 17-year-old woman with familial protein S deficiency who was admitted with extensive multiple CVT. Two weeks after onset of symptoms, she developed isolated right peripheral facial palsy, and MR venography showed segmental occlusion of the ipsilateral transverse sinus. Complete recovery of facial palsy occurred concomitant with recanalization of the transverse sinus. Facial neurography, including transcranial magnetic stimulation of the facial nerve and related motor cortex, ruled out a coincidental idiopathic palsy and revealed conduction block proximal to the facial canal. CONCLUSIONS: Facial palsy in our patient was caused by transient neurapraxia in the intracranial segment of the nerve. We suggest that elevated venous transmural pressure in the nerve's satellite vein, which belongs to the affected drainage territory of the transverse sinus, might have caused venous blood-brain barrier dysfunction in the intrinsic vascular system of the nerve, with leakage of fluids and ions into the endoneurial space and thus an increase in interstitial resistance.

Adolescent↗
PubMed2000

Disruption of space perception due to cortical lesions.

Space control has for a long time been considered a unitary function. The structure associated with this function was the right parietal lobe. Hemispheric specialization for space appeared to make it automatically a human-specific function. However, recent primate research shows different regions of the parietal lobes to be differently involved with space control. A review of the literature, together with own cases shows that there is ample evidence of a modular organization of space control in humans on the basis of specific deficits subsequent to circumscribed cerebral lesions. Lesions influence differentially retinotopic, spatiotopic, egocentric, and allocentric frames of references. They also influence differentially the attention to far or near space, or to global or local features of space. Moreover, preattentive processes can be studied in the neglected hemispace of humans and prove to be sensible to the meaning of visual stimuli. Space representation and attentional mechanisms that seem to operate on these representations are organized in our brain in a very modular fashion, similar to the modularity of visual submodalities. There is probably not a unified space representation in the parietal lobes, but distributed functional modules. Thus, the study of visual optic recognition, either by the brain or by machines, is inconceivable without considering space, attention and awareness.

Brain Neoplasms↗