Selective activation of a parietofrontal circuit during implicitly imagined prehension

It is generally held that motor imagery is the internal simulation of movements involving one's own body in the absence of overt execution. Consistent with this hypothesis, results from numerous functional neuroimaging studies indicate that motor imagery activates a large variety of motor-related brain regions. However, it is unclear precisely which of these areas are involved in motor imagery per se as opposed to other planning processes that do not involve movement simulation. In an attempt to resolve this issue, we employed event-related fMRI to separate activations related to hand preparation-a task component that does not demand imagining movements-from grip selection-a component previously shown to require the internal simulation of reaching movements. Our results show that in contrast to preparation of overt actions, preparation of either hand for covert movement simulation activates a large network of motor-related areas located primarily within the left cerebral and right cerebellar hemispheres. By contrast, imagined grip selection activates a distinct parietofrontal circuit that includes the bilateral dorsal premotor cortex, contralateral intraparietal sulcus, and right superior parietal lobule. Because these areas are highly consistent with the frontoparietal reach circuit identified in monkeys, we conclude that motor imagery involves action-specific motor representations computed in parietofrontal circuits.

Practicability of magnetoencephalography-guided neuronavigation

Magnetoencephalography (MEG) is a noninvasive option for localizing electroneurophysiological activity on the human cortex. The purpose of this study was to evaluate the practicability and reliability of MEG imaging integrated into a neuronavigation system to identify the sensorimotor cortex intraoperatively in patients with brain tumors in or near the central motor strip. It was performed prior to surgery in 30 patients with space-occupying lesions in or around the central region to localize the primary somatosensory cortex. These functional brain maps were superimposed on MR images obtained prior to surgery and transferred in the operating room for intraoperative functional neuronavigation. During surgery, the phase reversal technique identified a generator which coincided with the somatosensory cortex as displayed by the MEG-based functional neuronavigation system. Following surgery, the motor deficit improved in seven patients, was unchanged in five, and showed a slight transient deterioration in five. One patient suffered a deterioration of motor function with incomplete recovery. The MEG-based functional neuronavigation was found to be practicable and useful in finding a safe approach to tumors in or adjacent to the central region. The accuracy of MEG was concluded to be reliable as verified by the phase reversal technique.

Deconvolution of event-related fMRI responses in fast-rate experimental designs: tracking amplitude variations

Recent developments towards event-related functional magnetic resonance imaging has greatly extended the range of experimental designs. If the events occur in rapid succession, the corresponding time-locked responses overlap significantly and need to be deconvolved in order to separate the contributions of different events. Here we present a deconvolution approach, which is especially aimed at the analysis of fMRI data where sequence- or context-related responses are expected. For this purpose, we make the assumption of a hemodynamic response function (HDR) with constant yet not predefined shape but with possibly variable amplitudes. This approach reduces the number of variables to be estimated but still keeps the solutions flexible with respect to the shape. Consequently, statistical efficiency is improved. Temporal variations of the HDR strength are directly indicated by the amplitudes derived by the algorithm. Both the estimation efficiency and statistical inference are further supported by an improved estimation of the noise covariance. Using synthesized data sets, both differently shaped HDRs and varying amplitude factors were correctly identified. The gain in statistical sensitivity led to improved ratios of false- and true-positive detection rates for synthetic activations in these data. In an event-related fMRI experiment with a human subject, different HDR amplitudes could be derived corresponding to stimulation at different visual stimulus contrasts. Finally, in a visual spatial attention experiment we obtained different fMRI response amplitudes depending on the sequences of attention conditions.

Integrating electrophysiology and neuroimaging of spatial selective attention to simple isolated visual stimuli

Visual-spatial attention involves modulations of activity in human visual cortex as indexed by electrophysiological and functional neuroimaging measures. Prior studies investigating the time course and functional anatomy of spatial attention mechanisms in visual cortex have used higher-order discrimination tasks with complex stimuli (e.g. symbol matching in bilateral stimulus arrays, or letter discrimination), or simple detection tasks but in the presence of complex distracting information (e.g. luminance detection with superimposed symbols as distractors). Here we tested the hypothesis that short-latency modulations of incoming sensory signals in extrastriate visual cortex reflect an early spatially specific attentional mechanism. We sought evidence of attentional modulations of sensory input processing for simple, isolated stimuli requiring only an elementary discrimination (i.e. size discrimination). As in prior studies using complex symbols, we observed attention-related changes in regional cerebral blood flow in extrastriate visual cortex that were associated with changes in event-related potentials at a specific latency range. These findings support the idea that early in cortical processing, spatially-specific attentional selection mechanisms can modulate incoming sensory signals based on their spatial location and perhaps independently of higher-order stimulus form.

Alterations of continuous MEG measures during mental activities

In a pilot study, we investigated the topography of 11 continuous MEG measures for the eyes-opened and eyes-closed condition together with three simple mental tasks (mental arithmetic, visual imagery, word generation). One-minute recordings for each condition from 16 right-handed subjects were analyzed. The electrophysiological measures consisted of 6 spectral band measures together with spectral edge frequency and spectral entropy, plus the time-domain-based entropy of amplitudes (ENA) and the nonlinear measures correlation dimension D2 and Lyapunov exponent L1. In summary, our results indicate a pronounced task-dependent difference between the anterior and the posterior region, but no lateralization effects. Although the nonlinear measures ranged in the middle field with respect to the number of significant contrasts, they were the only ones to be partially successful in discriminating the mental tasks from each other. The most efficient measure turned out to be the ENA. Under mental activation the ENA was larger than in both no task conditions (eyes opened and eyes closed). This finding reflects lower variations of the maximum amplitude during performance of mental tasks than during no task states.

Lateralized auditory spatial perception and the contralaterality of cortical processing as studied with functional magnetic resonance imaging and magnetoencephalography

Functional magnetic resonance imaging (fMRI) and magnetoencephalography (MEG) were used to study the relationships between lateralized auditory perception in humans and the contralaterality of processing in auditory cortex. Subjects listened to rapidly presented streams of short FM-sweep tone bursts to detect infrequent, slightly deviant tone bursts. The stimulus streams consisted of either monaural stimuli to one ear or the other or binaural stimuli with brief interaural onset delays. The onset delay gives the binaural sounds a lateralized auditory perception and is thought to be a key component of how our brains localize sounds in space. For the monaural stimuli, fMRI revealed a clear contralaterality in auditory cortex, with a contralaterality index (contralateral activity divided by the sum of contralateral and ipsilateral activity) of 67%. In contrast, the fMRI activations from the laterally perceived binaural stimuli indicated little or no contralaterality (index of 51%). The MEG recordings from the same subjects performing the same task converged qualitatively with the fMRI data, confirming a clear monaural contralaterality, with no contralaterality for the laterally perceived binaurals. However, the MEG monaural contralaterality (55%) was less than the fMRI and decreased across the several hundred millisecond poststimulus time period, going from 57% in the M50 latency range (20-70 ms) to 53% in the M200 range (170-250 ms). These data sets provide both quantification of the degree of contralaterality in the auditory pathways and insight into the locus and mechanism of the lateralized perception of spatially lateralized sounds.

Circular dichroism as a detection method in the screening of enantioselective catalysts

The combination of liquid chromatography (HPLC), UV/Vis-spectroscopy and circular dichroism (CD) can be used to construct a high-throughput screening system to determine the enantioselectivity of enzyme- or metal-catalyzed reduction of acetophenone with formation of (S)- and (R)-1-phenylethanol. Prerequisite for the viability of this system is the experimental finding that the anisotropy factor g is linearly related to the enantiomeric excess (ee) and that it is independent of concentration, thereby excluding possible aggregation effects.

Linear inverse filtering improves spatial separation of nonlinear brain dynamics: a simulation study

We examined topographic variations in nonlinear measures based on scalp voltages, which were generated by two simulated current dipoles each placed in a different hemisphere of a spherical volume conductor (three-shell model). Dipole dynamics were that of a three-torus and the x-component of the Lorenz-system and scalp voltage were calculated for a configuration of 29 electrode positions. Although estimates for correlation dimension D2 and Lyapunov exponent L1 were close to the theoretical values for the original time series, the simulated scalp voltage data showed almost no topographic resolution of dipole positions. In order to enhance topographic differentiation, we constructed linear inverse filters, to focus on brain activity from a specified brain region. It turned out that the nonlinear measures for the inversely filtered time series were much closer to the expected values (with respect to the location of the dipoles used in the simulation) than when using unfiltered data. Our preliminary results indicate that inverse filtering can improve the topographic resolution of nonlinear scalp EEG estimates.

A magnetoencephalographic study of brain activity related to recognition memory in healthy young human subjects

Neural activity associated with recognition memory was investigated using magnetencephalography (MEG) in healthy young subjects. At sensor sites overlying frontal and temporoparietal cortices, magnetic evoked fields (MEFs) revealed a difference between studied and unstudied stimuli, which onset about 400 ms following stimulus onset and lasted about 600 ms. MEG yielded reliable source information revealing the activity of three independent dipoles, located in the right medial temporal lobe (MTL), the right inferior frontal and the left inferior parietal cortices. Our findings suggest that neural activity underlying recognition memory from both superficial and deep brain structures can be monitored by MEG.

Functional dissociation between medial and lateral prefrontal cortical spatiotemporal activation in negative and positive emotions: a combined fMRI/MEG study

The orbitofrontal cortex has been cytoarchitectonically and connectionally subdivided into a medial and a lateral part which are assumed to subserve distinct functions in emotional processing. However the exact spatiotemporal mechanisms of negative and positive emotional processing in medial and lateral orbitofrontal cortex remain unclear. We therefore investigated spatiotemporal orbitofrontal and prefrontal cortical activation patterns during emotional stimulation in a combined fMRI/MEG study. We investigated 10 healthy subjects, 5 women and 5 men. Positive and negative pictures from the International Affective Picture system (IAPS) were used for emotional stimulation, whereas neutral and gray pictures were taken as control conditions. fMRI/MEG measurements covered the whole frontal lobe and a time window between -2000 and +200 ms around motor responses (right index finger extension) associated with each picture. Positively and negatively correlated activities were determined in various prefrontal/frontal cortical regions in fMRI. Isocontour maps and single dipoles in MEG were analyzed in 50 ms time windows ranging from -2000 to +200 ms. Dipoles and fMR images were mapped on three-dimensional anatomical MRI so that anatomical localization of single dipoles and regional fMRI activity could be compared. Both negative and positive emotional conditions differed from non-emotional control conditions by strong orbitofrontal and lateral prefrontal activation as well as by the presence of early magnetic fields (-1700 to +1100 ms). Negative emotional processing was characterized by strong medial orbitofrontal activation and earlier (-1700 ms), stronger and more medially oriented orbitofrontal dipoles. In contrast positive emotional processing showed a rather strong activation in lateral prefrontal cortex with later (-1500 ms), weaker and more laterally oriented orbito and prefrontal dipoles. Negative emotional processing can be characterized by strong and early medial orbitofrontal cortical activation, whereas positive emotional processing showed rather later and weaker activation in lateral orbitofrontal/prefrontal cortex. Such a functional dissociation between medial and lateral orbito-frontal/prefrontal cortex during negative and positive emotional processing lends further support to the assumption of a functional subdivision in the orbitofrontal cortex.

Computerized classification of corpus cavernosum electromyogram signals by the use of discriminant analysis and artificial neural networks to support diagnosis of erectile dysfunction

Corpus cavernosum electromyogram (CC-EMG) provides diagnostic information on cavernous autonomic innervation and a measure of the degree to which the cavernous smooth muscle cells are intact. The complicated CC-EMG is evaluated and used in the diagnosis of patients suffering from erectile dysfunction. The evaluation procedure has been simplified by applying digital signal processing techniques. Since mathematically-based interpretations require quantitative data, spectral analysis was performed. The derived biosignals were analyzed by fast Fourier transform (FFT). Besides various other spectral parameters, specific frequency bands were determined in the power spectrum using factor analysis. The parameters were used for the computerized classification of normal and pathological CC-EMG data and the classification was performed using two independent methods: discriminant analysis (DA) and artificial neural networks (ANN). A medical expert analyzed a total of 200 CC-EMG recordings from patients with and without erectile dysfunction and separated these into normal (136) and pathological (64) cases. Although each independent method had already resulted in a relatively high number of correct classifications, the classification success rate could be slightly improved by using a combination of both classification methods. A total of 72.79% and 77.94% were successfully classified using DA and ANN, respectively. The combination of both methods increased the classification success to 80.15%. The results of this study enabled impartial evaluation of the CC-EMG signals for clinical diagnostic purposes of erectile dysfunction. This method provided an objective and easy way to analyze the CC-EMG. Furthermore, this results in patient diagnosis becoming an easier task for less experienced doctors, since little knowledge of the raw signal is needed.

Effects of the amplitude threshold on the separability of neuropathic and myopathic from normal EMG using parameters of the turns/amplitude analysis

OBJECTIVE

In this study, the relationship between the amplitude threshold used for the determination of the turns of the electromyographic (EMG) interference pattern and the parameters of the turns/amplitude analysis was examined. It was investigated whether the discrimination of myopathic and neuropathic from normal muscles could be optimized by an appropriate amplitude threshold.

METHODS

The interference patterns of the tibialis anterior muscle of 15 patients with myopathies, 30 patients with neuropathies and 56 controls were recorded, using concentric needle electrodes. A computer program performed the Willison analysis, systematically varying the amplitude threshold between 10 microV and 200 microV.

RESULTS

Amplitudes as well as the number of turns per second were non-linearly related to the amplitude threshold. The reduction of the amplitude threshold to 30 microV resulted in a clearly better separation of the distributions of the number of turns of neuropathic, myopathic and normal EMG, compared to the traditional threshold value of 100 microV. The distributions of amplitude values, however, were not affected. The distance between the turns parameter distributions of neuropathic patients and controls and between the distributions of myopathic patients and controls, expressed by the Kolmogoroff-Smirnov distance, had a maximum at 30 microV.

CONCLUSIONS

For the turns/amplitude analysis of the tibialis anterior muscle an amplitude threshold of 30 microV should be selected.

Digital speckle-pattern interferometry as a full-field fluid-velocimetry technique

We present a novel fluid-velocimetry technique based on speckle interferometry. The light scattered from an illuminated plane is recorded with a CCD camera at the same time as a speckled reference beam. Substraction of two nonsimultaneous frames provides information about the velocity field for an out-of-plane component. An application to a Rayleigh-Bénard convective flow is given.

Involvement of striate and extrastriate visual cortical areas in spatial attention

We investigated the cortical mechanisms of visual-spatial attention while subjects discriminated patterned targets within distractor arrays. Functional magnetic resonance imaging (fMRI) was used to map the boundaries of retinotopic visual areas and to localize attention-related changes in neural activity within several of those areas, including primary visual (striate) cortex. Event-related potentials (ERPs) and modeling of their neural sources, however, indicated that the initial sensory input to striate cortex at 50-55 milliseconds after the stimulus was not modulated by attention. The earliest facilitation of attended signals was observed in extrastriate visual areas, at 70-75 milliseconds. We hypothesize that the striate cortex modulation found with fMRI may represent a delayed, re-entrant feedback from higher visual areas or a sustained biasing of striate cortical neurons during attention. ERP recordings provide critical temporal information for analyzing the functional neuroanatomy of visual attention.

Neural mechanisms of global and local processing. A combined PET and ERP study

The neural mechanisms of hierarchical stimulus processing were investigated using a combined event-related potentials (ERPs) and positron emission tomography (PET) approach. Healthy subjects were tested under two conditions that involved selective or divided attention between local and global levels of hierarchical letter stimuli in order to determine whether and where hemispheric differences might exist in the processing of local versus global information. When attention was divided between global and local levels, the N2 component of the ERPs (260- to 360-msec latency) elicited by the target stimuli showed asymmetries in amplitude over the two hemispheres. The N2 to local targets was larger over the left hemisphere, but the N2 to global targets tended to be slightly larger over the right hemisphere. However, the shorter-latency, sensory-evoked P1 component (90- to 150-msec latency) was not different for global versus local targets under conditions of divided attention. In contrast, during selective attention to either global or local targets, asymmetries in the N2 component were not observed. But under selective attention conditions, the sensory-evoked P1 components in the extrastriate cortex were enlarged for global versus local attention. Increased regional cerebral blood flow in the posterior fusiform gyrus bilaterally was observed in the PET data during selective attention to either global or local targets, but neither these nor the P1 component showed any tendency toward hemispheric difference for global versus local attention. Neither were there any activations observed in the parietal lobe during selective attention to global versus local targets. Together these data indicate that early sensory inputs are not modulated to gate global versus local information differentially into the two hemispheres. Rather, later stages of processing that may be asymmetrically organized in the left and right hemispheres operate in parallel to process global and local aspects of complex stimuli (i.e., the N2 effect of the ERPs). This pattern of results supports models proposing that spatial frequency analysis is only asymmetric at higher stages of perceptual processing and not at the earliest stages of visual cortical analysis.

Successful verbal encoding into episodic memory engages the posterior hippocampus: a parametrically analyzed functional magnetic resonance imaging study

The medial temporal lobe (MTL) is essential for episodic memory encoding, as evidenced by memory deficits in patients with MTL damage. However, previous functional neuroimaging studies have either failed to show MTL activation during encoding or they did not differentiate between two MTL related processes: novelty assessment and episodic memory encoding. Furthermore, there is evidence that the MTL can be subdivided into subcomponents serving different memory processes, but the extent of this functional subdivision remains unknown. The aim of the present functional magnetic resonance imaging (fMRI) study was to investigate the role of the MTL in episodic encoding and to determine whether this function might be restricted to anatomical subdivisions of the MTL. Thirteen healthy volunteers performed a word list learning paradigm with free recall after distraction. Functional images acquired during encoding were analyzed separately for each participant by a voxel-wise correlation (Kendall's tau) between the time series of the T2*-signal intensity and the number of subsequently recalled words encoded during each particular scan. Of the 13 participants, 11 showed voxel clusters with statistically significant, positive correlations in the posterior part of the hippocampus. Across participants, an ANOVA on the number of voxels with significant, positive correlations within individually defined volumes of interest confirmed a statistically significant difference in activation for anterior versus posterior regions of the hippocampus. However, no differences between left and right hippocampal activation were revealed. Thus, these findings demonstrate that successful encoding into episodic memory engages neural circuits in the posterior part of the hippocampus.

Time course of human 40 Hz EEG activity accompanying P3 responses in an auditory oddball paradigm

In order to quantify the time course of auditory P3-related gamma activity, root mean square (RMS) values were calculated from band-filtered (30-45 Hz) target and non-target responses in an auditory oddball experiment. Evoked (phase locked) gamma activity was evaluated from the time domain averages, whereas induced (not necessarily phase locked) activity was analyzed on the basis of single trials. Gamma RMS values were integrated across different time windows, namely the prestimulus, N50/P50, N100, pre P3, P3 and post P3 window. The single trial P3 window hereby was defined by a maximum amplitude criterion. In accordance with other studies, we found a pronounced increase of evoked gamma activity in the time window up to 80 ms after stimulus onset. In contrast, induced gamma activity as revealed by single trial analysis decreased markedly after stimulus presentation. Starting with the P3 window, induced activity recovered to baseline level for the non-target trials, whereas it remained significantly suppressed for the target responses.

[Use of automatic, computer-assisted EEG analysis in clinical practice]

Digital EEG-recorders are being increasingly accepted for clinical routine application, thereby offering the possibility for an automated computerised EEG evaluation. This paper presents the results of a corresponding computer programme developed in our group. Based on 313 clinical routine-EEG we compared the computer reports to the visual EEG-interpretations and obtained the following result: Background activity is reliably detected with a rate comparable to a human interpreter. Similar results were observed with focussed pathological activity. Intermittent activity (Parenrhythmia, dysrhythmia) however lacks a sufficiently high score of correct evaluation and requires further development. Epileptiform potentials, especially spikes, are detected with high sensitivity, however, at the cost of low specificity, and are therefore still in need of further improvement.

Light-in-flight holography for visualization and velocimetry in three-dimensional flows

Flow visualization and particle image velocimetry of deep volumes are achieved by holographic recordings. Light-in-flight holography is applied to avoid noise from out-of-focus regions during interrogation of the reconstructed image by use of a ruby laser source of small coherence. The scheme permits reconstruction of thin layers in depth without disturbance by the rest of the field. The location of the layers is selected by the position of a reconstructing slit aperture on the hologram. Applications to tracer particles in water and visualization by smoke of an air flow are given.