Theodor Landis

Linking Out-of-Body Experience and Self Processing to Mental Own-Body Imagery at the Temporoparietal Junction

The spatial unity of self and body is challenged by various philosophical considerations and several phenomena, perhaps most notoriously the “out-of-body experience” (OBE) during which ones visual perspective and ones self are experienced to have departed from their habitual position within ones body. Although researchers started examining isolated aspects of the self, the neurocognitive processes of OBEs have not been investigated experimentally to further our understanding of the self. With the use of evoked potential mapping, we show the selective activation of the temporoparietal junction (TPJ) at 330-400 ms after stimulus onset when healthy volunteers imagined themselves in the position and visual perspective that generally are reported by people experiencing spontaneous OBEs. Interference with the TPJ by transcranial magnetic stimulation (TMS) at this time impaired mental transformation of ones own body in healthy volunteers relative to TMS over a control site. No such TMS effect was observed for imagined spatial transformations of external objects, suggesting the selective implication of the TPJ in mental imagery of ones own body. Finally, in an epileptic patient with OBEs originating from the TPJ, we show partial activation of the seizure focus during mental transformations of her body and visual perspective mimicking her OBE perceptions. These results suggest that the TPJ is a crucial structure for the conscious experience of the normal self, mediating spatial unity of self and body, and also suggest that impaired processing at the TPJ may lead to pathological selves such as OBEs.

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.

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 patients 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.

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.

Are unilateral right posterior cerebral lesions sufficient to cause prosopagnosia? Clinical and radiological findings in six additional patients

Controversy has arisen regarding the neuropathological basis of prosopagnosia. Some investigators suggest that bilateral lesions are needed to cause the deficit, whereas others felt that a unilateral right posterior lesion is sufficient. Six patients with prosopagnosia with clinical and radiological evidence of unilateral right posterior lesions are presented. Our observations together with evidence from similar cases described in the literature suggest that an appropriately placed right hemispheric lesion may be sufficient to produce prosopagnosia.

Electrical neuroimaging reveals early generator modulation to emotional words

Functional electrical neuroimaging investigated incidental emotional word processing. Previous research suggests that the brain may differentially respond to the emotional content of linguistic stimuli pre-lexically (i.e., before distinguishing that these stimuli are words). We investigated the spatiotemporal brain mechanisms of this apparent paradox and in particular whether the initial differentiation of emotional stimuli is marked by different brain generator configurations using high-density, event-related potentials. Such would support the existence of specific cerebral resources dedicated to emotional word processing. A related issue concerns the possibility of right-hemispheric specialization in the processing of emotional stimuli. Thirteen healthy men performed a go/no-go lexical decision task with bilateral word/non-word or non-word/non-word stimulus pairs. Words included equal numbers of neutral and emotional stimuli, but subjects made no explicit discrimination along this dimension. Emotional words appearing in the right visual field (ERVF) yielded the best overall performance, although the difference between emotional and neutral words was larger for left than for right visual field presentations. Electrophysiologically, ERVF presentations were distinguished from all other conditions over the 100-140 ms period by a distinct scalp topography, indicative of different intracranial generator configurations. A distributed linear source estimation (LAURA) of this distinct scalp potential field revealed bilateral lateral-occipital sources with a right hemisphere current density maximum. These data support the existence of a specialized brain network triggered by the emotional connotation of words at a very early processing stage.

Illusory Reduplication of One's Own Body: Phenomenology and Classification of Autoscopic Phenomena

Autoscopic phenomena involve the illusory reduplication of ones own body. The literature on the topic is widely scattered and suffers from considerable terminological and conceptual inconsistencies. This article proposes a classification scheme based on phenomenological criteria. Along with examples of illustrative cases, we outline the main features of autoscopic hallucinations, heautoscopy proper, the feeling of a presence, the out-of-body experience, and negative and inner forms of autoscopic phenomena. We also discuss the need for a differentiation of autoscopic phenomena from reduplicative paramnesias and the misidentification syndromes. Finally, the concept of a neuromatrix (Melzack, 1990) is proposed as a starting point for the understanding of the neuronal mechanisms underlying autoscopic phenomena.

128-channel EEG source imaging in epilepsy: Clinical yield and localization precision

The authors evaluated the feasibility, clinical yield, and localization precision of high-resolution EEG source imaging of interictal epileptic activity. A consecutive series of 44 patients with intractable epilepsy of various causes, who underwent a comprehensive presurgical epilepsy evaluation, were subjected to a 128-channel EEG recording. A standardized source imaging procedure constrained to the individual gray matter was applied to the averaged spikes of each patient. In 32 patients, the presurgical workup identified a focal epileptogenic area. The 128-channel EEG source imaging correctly localized this area in 30 of these patients (93.7%). Imprecise localization was explained by simplifications of the recordings and analysis procedure, which was accepted for the benefit of speed and standardization. In a subgroup of 24 patients who underwent operations, the sublobar precision of the 128-channel EEG source imaging was evaluated by calculating the distance of the source maximum to the resected area. This analysis revealed zero distance in 19 cases (79%). The authors conclude that high-resolution interictal EEG source imaging is a valuable noninvasive functional neuroimaging technique. The speed, ease, flexibility, and low cost of this technique warrant its use in clinical practice.

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.

Prediction of response speed by anticipatory high-frequency (gamma band) oscillations in the human brain

Response to a stimulus is faster when a subject is attending and knows beforehand how to respond. It has been suggested recently that this occurs because ongoing neuronal activity is spatially and temporally structured during states of expectancy preceding a stimulus. This mechanism is believed to mediate top‐down processing, facilitating the early grouping and selection of distributed neuronal ensembles implicated in ensuing sensory–motor processing. To validate this model, it must be shown that some features of this early ongoing neural activity are correlated with subsequent perceptual decisions or behavioral events. We investigated this hypothesis in an electrophysiologic study in 12 subjects carrying out a simple visuomotor reaction‐time task. Local field potentials (LFP) at each brain voxel were estimated using a linear distributed inverse solution termed “ELECTRA” for each single trial of each subject. The energy of oscillations for different frequency bands was computed for the period between the warning cue and visual stimuli by applying a time–frequency decomposition to the estimated LFP. A nonparametric correlation coefficient was then calculated between energy of oscillations and reaction times for each single sweep. Gamma band oscillatory activity in a frontoparietal network before stimulus onset significantly correlated with reaction time for a significant amount of subjects. These results provide direct evidence for the role of neural oscillations as a top‐down attentional control mechanism that mediates the speed of motor actions. Hum. Brain Mapping 24:50–58, 2005.