Biomedical subjects

A A Ioannides

Publications and source records attributed to A A Ioannides.

At least 19 recordsLinked to original sources

PubMed2006

Localization of individual area neuronal activity.

A family of methods, collectively known as independent component analysis (ICA), has recently been added to the array of methods designed to decompose a multi-channel signal into components. ICA methods have been applied to raw magnetoencephalography (MEG) and electroencephalography (EEG) signals to remove artifacts, especially when sources such as power line or cardiac activity generate strong components that dominate the signal. More recently, successful ICA extraction of stimulus-evoked responses has been reported from single-trial raw MEG and EEG signals. The extraction of weak components has often been erratic, depending on which ICA method is employed and even on what parameters are used. In this work, we show that if the emphasis is placed on individual "independent components," as is usually the case with standard ICA applications, differences in the results obtained for different components are exaggerated. We propose instead the reconstruction of regional brain activations by combining tomographic estimates of individual independent components that have been selected by appropriate spatial and temporal criteria. Such localization of individual area neuronal activity (LIANA) allows reliable semi-automatic extraction of single-trial regional activations from raw MEG data. We demonstrate the new method with three different ICA algorithms applied to both computer-generated signals and real data. We show that LIANA provides almost identical results with each ICA method despite the fact that each method yields different individual components.

Adult↗

PubMed2006

Neural correlates of priming on occluded figure interpretation in human fusiform cortex.

The visual system rapidly completes a partially occluded figure. We probed the completion process by using priming in combination with neuroimaging techniques. Priming leads to more efficient visual processing and thus a reduction in neural activity in relevant brain areas. These areas were studied with high spatial resolution and temporal accuracy with focus on early perceptual processing. We recorded magnetoencephalographic responses from 10 human volunteers in a primed same-different task for test figures. The test figures were preceded by a sequence of two figures, a prime or control figure followed by an occluded figure. The prime figures were one of three possible interpretations of the occluded figures: global and local completions and mosaic interpretation. A significant priming effect was evident: in primed trials as compared with control trials, subjects responded faster and the latency was shorter in the magnetoencephalographic signal for the largest peak between 50 and 300 ms after the occluded figure onset. Tomographic and statistical parametric mapping analyses revealed stages of activation in occipitotemporal areas during occluded figure processing. Notably, we found significantly reduced activation in the right fusiform cortex between 120 and 200 ms after occluded figure onset for primed trials as compared with control trials. We also found significant spatiotemporal differences of local, global and mosaic interpretations for individual subjects but not across subjects. We conclude that modulation of activity in the right fusiform cortex may be a neural correlate of priming in the interpretation of an occluded figure, and that this area acts as a hub for different occluded figure interpretations in this early stage of perception.

Adult↗

PubMed2003

The human primary somatosensory cortex response contains components related to stimulus frequency and perception in a frequency discrimination task.

Somatosensory stimulation of primary somatosensory cortex (SI) using frequency discrimination offers a direct, well-defined and accessible way of studying cortical decisions at the locus of early input processing. Animal studies have identified and classified the neuronal responses in SI but they have not yet resolved whether during prolonged stimulation the collective SI response just passively reflects the input or actively participates in the comparison and decision processes. This question was investigated using tomographic analysis of single trial magnetoencephalographic data. Four right-handed males participated in a frequency discrimination task to detect changes in the frequency of an electrical stimulus applied to the right-hand digits 2+3+4. The subjects received approximately 600 pairs of stimuli with Stim1 always at 21 Hz, while Stim2 was either 21 Hz (50%) or varied from 22 to 29 Hz in steps of 1 Hz. Both stimuli were 1 s duration, separated by a 1 s interval of no stimulation. The left-SI was the most consistently activated area and showed the first activation peak at 35-48 ms after Stim1 onset and sustained activity during both stimulus periods. During the Stim2 period, we found that the left-SI activation started to differ significantly between two groups of trials (21 versus 26-29 Hz) within the first 100 ms and this difference was sustained and enhanced thereafter (approximately 600 ms). When only correct responses from the above two groups were used, the difference was even higher at later latencies (approximately 650 ms). For one subject who had enough trials of same perception to different input frequencies, e.g. responded 21 Hz to Stim2 at 21 Hz (correct) and 26-29 Hz (error), we found the sustained difference only before 650 ms. Our results suggest that SI is involved with the analysis of an input frequency and related to perception and decision at different latencies.

Adult↗

PubMed2003

Consistent and precise localization of brain activity in human primary visual cortex by MEG and fMRI.

The tomographic localization of activity within human primary visual cortex (striate cortex or V1) was examined using whole-head magnetoencephalography (MEG) and 4-T functional magnetic resonance imaging (fMRI) in four subjects. Circular checkerboard pattern stimuli with radii from 1.8 to 5.2 degrees were presented at eccentricity of 8 degrees and angular position of 45 degrees in the lower quadrant of the visual field to excite the dorsal part of V1 which is distant from the V1/V2 border and from the fundus of the calcarine sulcus. Both fMRI and MEG identified spatially well-overlapped activity within the targeted area in each subject. For MEG, in three subjects a very precise activation in V1 was identified at 42 ms for at least one of the two larger stimulus sizes (radii 4.5 and 5.2 degrees ). When this V1 activity was present, it marked the beginning of a weak wave of excitations in striate and extrastriate areas which ended at 50 ms (M50). The beginning of the next wave of activations (M70) was also marked by a brief V1 activation, mainly between 50 and 60 ms. The mean separation between V1 activation centers identified by fMRI and the earliest MEG activation was 3-5 mm.

Adult↗

PubMed2003

Single-trial variability in early visual neuromagnetic responses: an explorative study based on the regional activation contributing to the N70m peak.

Cortical activity evoked by repeated identical sensory stimulation is extremely variable. The source of this variability is often assigned to "random ongoing background activity" which is considered to be irrelevant to the processing of the stimuli and can therefore be eliminated by ensemble averaging. In this work, we studied the single-trial variability in neuromagnetic responses elicited by circular checkerboard pattern stimuli with radii of 1.8 degrees, 3.7 degrees, and 4.5 degrees. For most of the MEG sensors over the occipital areas, the averaged signal showed a clear early (N70m) response following the stimulus onset and this response was modulated by the checkerboard size. A data-driven spatial filter was used to extract one of the many possible composite time courses of single-trial activity corresponding to the complex of N70m generators. Pattern analysis principles were then employed to analyze, classify, and handle the extracted temporal patterns. We explored whether these patterns correspond to distinct response modes, which could characterize the evoked response better than the averaged signal and over an extended range of latencies around N70m. A novel scheme for detecting and organizing the structure in single-trial recordings was utilized. This served as a basis for comparisons between runs with different checkerboard sizes and provided a causal interpretation of variability in terms of regional dynamics, including the relatively weak activation in primary visual cortex. At the level of single trial activity, the polymorphic response to a simple stimulus is generated by a coupling of polymodal areas and cooperative activity in striate and extrastriate areas. Our results suggest a state-dependent response with a wide range of characteristic time scales and indicate the ongoing activity as a marker of the responsiveness state.

Adult↗

PubMed2002

Semantic geodesic maps: a unifying geometrical approach for studying the structure and dynamics of single trial evoked responses.

OBJECTIVES: A general framework for identifying and describing structure in a given sample of evoked response single-trial signals (STs) is introduced. The approach is based on conceptually simple geometrical ideas and enables the convergence of pattern analysis and non-linear time series analysis. METHODS: Classical steps for analyzing the STs by waveform are first employed and the ST-analysis is transferred to a multidimensional space, the feature space, the geometry of which is systematically studied via multidimensional scaling (MDS) techniques giving rise to semantic maps. The structure in the feature space characterizes the trial-to-trial variability and this is utilized to probe functional connectivity between two brain areas. The underlying dynamic process responsible for the emerged structure can be described by a multidimensional trajectory in the feature space. This in turn enables the detection of dynamical interareal coupling as similarity between the corresponding trajectories. RESULTS AND CONCLUSIONS: The utility of semantic maps was demonstrated using magnetoencephalographic data from a simple auditory paradigm. The coupling of ongoing activity and evoked response is vividly demonstrated and contrasted with the apparent deflection from zero baseline that survives averaging. Prototypes are easily identified as the end points of distinct paths in the semantic map representation, and their neighborhood is populated by STs with distinct properties not only in the latencies where the evoked response is expected to be strong, but also and very significantly in the prestimulus period. Finally our results provide evidence for interhemispheric binding in the (4-8 Hz) range and dynamical coupling at faster time scales.

Brain Mapping↗

PubMed2001

Contribution of the human superior parietal lobule to spatial selection process: an MEG study.

The magnetoencephalographic signal was collected in a visually guided response-finger selection GO/NOGO task. The minimum norm distributed source analysis identified the sources in bilateral superior parietal lobules (SPL), with stronger activity for contralateral finger movement. Our results suggest that the human SPL plays a role in the spatial selection in a visuomotor task similar to that identified recently in monkeys.

Adult↗

PubMed2001

Early (N70m) neuromagnetic signal topography and striate and extrastriate generators following pattern onset quadrant stimulation.

The MEG signal generated by sinusoidal grating pattern onset at 1 and 3 cpd, presented randomly to the four quadrants, was analyzed in terms of gross signal properties and current dipole modeling and for a subset of subjects with magnetic field tomography (MFT). In all subjects a prominent wave was identified with a peak latency around 70 ms (N70m), modulated by spatial frequency and varying systematically with the stimulation quadrant. Sensors over occipital areas recorded stronger responses with lower field quadrants, while the signal for sensors a few centimeters more superior was stronger with upper quadrant stimuli. A strong signal in inferior occipitotemporal areas was less sensitive to upper and lower field stimulation and was stronger in the left hemisphere with contralateral (right) visual field stimulation. For lower visual field stimulation a good fit to the average data was obtained with a single dipole for 3 cpd, but was less consistent across run repetitions for 1 cpd. Neither the single-dipole model nor the two-dipole model produced a good fit across runs with the upper field stimuli. MFT solutions identified overlapping activity in striate and extrastriate areas in all conditions. The MFT solutions in the V1/V2 at the N70m were highly reproducible across run repetitions for 1 and 3 cpd, and consistent with the cruciform model, even though they were often weaker than simultaneously activated extrastriate generators. Extrastriate generators in V5 and the human homologue of V6, which were variable across run repetitions at N70m, settled to highly reproducible activations between 100 and 200 ms.

Adult↗

PubMed2001

Exploratory data analysis of evoked response single trials based on minimal spanning tree.

OBJECTIVE: An exploratory data analysis framework, based on minimal spanning tree, is proposed as a means to support the analysis of single trial (ST) electrophysiological signals. The core of this framework is the compact description of the input ST sample in a form of content-dependent ordered lists. Based on the established hierarchies, efficient ways to increase the SNR, extract prototypical responses, visualize possible self-organization trends in the sample and track the course of evoked response along the trial-to-trial dimension, are proposed. METHOD: Magnetoencephalographic auditory evoked responses were used for demonstrating and validating the introduced framework. RESULTS AND CONCLUSION: The results demonstrate the benefits, from this intelligent manipulation of STs, in understanding and enhancing the actual evoked signal. Specifically we find support for stimulus-induced phase-resetting hypothesis in the 3-20 Hz band, the existence of trials void of the prototypical evoked response, and an order across the single trial set hinting at an underlying process with long time scale.

Adult↗

PubMed2001

Real time human brain function: observations and inferences from single trial analysis of magnetoencephalographic signals.

This paper brings together results obtained by applying Magnetic Field Tomography (MFT) to the analysis of Magnetoencephalography (MEG) data over the last decade. It emphasizes the most recent developments where the availability of helmet-like MEG probes with well over 100 sensing coils provide a full coverage of the head. The paper shows that it is possible to extract tomographic information from single trial, millisecond by millisecond MEG signal, and demonstrates two ways that this capability can be exploited. First, the single trial reconstructions are used to obtain robust statistical measures of changes of activity over small latency windows. Second, the interaction between areas is studied by computing the mutual information between short, time-lagged sections of the single trial time-courses. The usefulness of the computationally demanding approach is demonstrated by analyzing experiments using two widely used protocols, one for face and affect recognition and the other for Contingent Magnetic Variation (CMV). The results show foci of significant changes of activity, which are consistent with what is reported in the literature and provide a deeper understanding of their significance. Some new, but not all that unexpected, findings also emerge from the analysis.

Brain↗

PubMed2000

Neuromagnetic localization of CMV generators using incomplete and full-head biomagnetometer.

Contingent magnetic variation (CMV) data were recorded in three healthy male subjects using a 2 x 37 biomagnetometer system. The experiment was repeated for one of the subjects using a 151 whole-head biomagnetometer; the same auditory GO/NOGO choice reaction time paradigm as in the first experiment was used, extended to include repetitions of identical runs and additional control conditions. Magnetic field tomography was applied to the averaged data of each subject, for each run and condition (e.g., GO/NOGO). An independent estimate of the current density in the brain was obtained every few milliseconds. The slow components were emphasized by integrating the square of the current density vector, pixel by pixel, revealing in each subject activity in the auditory cortex, sensorimotor cortex, inferior prefrontal area, and posterior inferior parietal area. The intersubject variability was large, but looking across subjects the auditory and sensorimotor cortex (which were best covered by the two probes) were consistently identified in each subject as contributing to the generation of the early and late slow CMV components. These findings were confirmed by the whole-head single-subject experiment, in which slow activity was also identified in the supplementary motor area (SMA) and posterior cingulate cortex (PCC), areas very likely missed in the first experiment because of the limited view of the twin system. The PCC and particularly the SMA activations were substantially reduced when identical runs were repeated.

Adult↗

PubMed2000

Real time processing of affective and cognitive stimuli in the human brain extracted from MEG signals.

The magnetoencephalography (MEG) signal was recorded while subjects watched a video containing separate blocks of affective and cognitive advertisements and recalled slides extracted from the video a day later. An earlier behavioural study using the same video material showed that the affective advertisements were better recalled and that administration of propranolol (a beta-adrenergic blocker) abolished this effect. Magnetic field tomography (MFT) was used to extract tomographic estimates of activity millisecond by millisecond from the continuous MEG signal. Statistically significant differences between affective and cognitive blocks were identified in posterior and prefrontal areas. Cognitive blocks produced stronger activity in posterior parietal areas and superior prefrontal cortex in all three subjects. Affective blocks modulated activity in orbitofrontal and retrosplenial cortex, amygdala and brainstem. Individual contributions to the statistical maps were traced in real time from milliseconds to many seconds. Time-locked responses from the recall session were used to compare average and single trial MFT solutions and to combine activations from all subjects into a common anatomical space. The last step produced statistically significant increases in occipital and inferior ventral cortex between 100 and 200 ms compared to a prestimulus baseline.

Adult↗

PubMed2000

Temporal correlations versus noise in the correlation matrix formalism: an example of the brain auditory response.

We adopt the concept of the correlation matrix to study correlations among sequences of time-extended events occurring repeatedly at consecutive time intervals. As an application we analyze the magnetoencephalography recordings obtained from the human auditory cortex in the epoch mode during the delivery of sound stimuli to the left or right ear. We look into statistical properties and the eigenvalue spectrum of the correlation matrix C calculated for signals corresponding to different trials and originating from the same or opposite hemispheres. The spectrum of C largely agrees with the universal properties of the Gaussian orthogonal ensemble of random matrices, with deviations characterized by eigenvectors with high eigenvalues. The properties of these eigenvectors and eigenvalues provide an elegant and powerful way of quantifying the degree of the underlying collectivity during well-defined latency intervals with respect to stimulus onset. We also extend this analysis to study the time-lagged interhemispheric correlations, as a computationally less demanding alternative to other methods such as mutual information.

Acoustic Stimulation↗

PubMed2000

Coupling of regional activations in a human brain during an object and face affect recognition task.

Magnetic field tomography (MFT) was used to extract estimates for distributed source activity from average and single trial MEG signals recorded while subjects identified objects (including faces) and facial expressions of emotion. Regions of interest (ROIs) were automatically identified from the MFT solutions of the average signal for each subject. For one subject the entire set of MFT estimates obtained from unaveraged data was also used to compute simultaneous time series for the single trial activity in different ROIs. Three pairs of homologous areas in each hemisphere were selected for further analysis: posterior calcarine sulcus (PCS), fusiform gyrus (FM), and the amygdaloid complex (AM). Mutual information (MI) between each pair of the areas was computed from all single trial time series and contrasted for different tasks (object or emotion recognition) and categories within each task. The MI analysis shows that through feed-forward and feedback linkages, the "computation" load associated with the task of identifying objects and emotions is spread across both space (different ROIs and hemispheres) and time (different latencies and delays in couplings between areas)-well within 200 ms, different objects separate first in the right hemisphere PCS and FG coupling while different emotions separate in the right hemisphere FG and AM coupling, particularly at latencies after 200 ms.

Brain↗

PubMed1999

Neurophysiological correlates of the recognition of facial expressions of emotion as revealed by magnetoencephalography.

MEG correlates of the recognition of facial expressions of emotion were studied in four healthy volunteers. Subjects performed a facial emotion recognition task and a control task involving recognition of complex objects including faces. Facial emotion recognition activated inferior frontal cortex, amygdala and different parts of temporal cortex in a relatively consistent time sequence. The characteristics of these activations were clearly different from those recorded during the control task. Most interesting was the fact that faces evoked different MEG responses as a function of task demands, i.e., the activations recorded during facial emotion recognition were different from those recorded during simple face recognition in the control task. These findings support the assumption that MEG is able to specifically identify the activation pattern of the brain when recognition of the emotional expression of a face is performed.

Adult↗

PubMed1999

Single trial analysis of neurophysiological correlates of the recognition of complex objects and facial expressions of emotion.

In an earlier experiment, we have used the BTi twin MAGNES system (2 x 37 channels) to record the evoked magnetic field from five healthy right-handed male volunteers using two tasks: visual recognition of complex objects including faces and facial expressions of emotion. We have repeated the experiment with one of the five subjects using the BTi whole head system (148 channels). Magnetic field tomography (MFT) was used to extract 3D estimates of brain activity millisecond by millisecond from the recorded magnetoencephalographic (MEG) signals. Results from the MFT analysis of the average signals of the five subjects have been reported elsewhere (Streit et al. 1997; Streit et al. 1999). In this paper, we present results of the detailed single trial analysis for the subject recorded from the whole head system. We found activations in areas extending from the occipital pole to anterior areas. Regions of interest (ROIs) were defined entirely on functional criteria and confirmed independently by the location of the maximum activity on the MRI. Activation curves for each ROI were computed and objective statistical measures (Kolmogorov-Smirnov test) were then used to identify time segments for which the ROI activity showed significant differences both within the same and across different object/emotion categories. Emphasis is placed on the quantification of the activity from two ROIs, fusiform gyrus (FG) and amygdala (AM), which have been best studied in the context of processing of faces and facial expressions of emotion, respectively. We found no face-specific area as such, but instead areas like the FG was activated by all complex objects at roughly similar latencies and varying strengths. The amygdala activity was significantly different between 150 and 180 ms for fearful expression, and even earlier for happy expression.

Animals↗

PubMed1999

SOFIA: spatially optimal fast initial analysis of biomagnetic signals.

A method of analysing biomagnetic signals is presented which focuses attention on activity in a specific region of interest. The method is based on the construction of virtual sensors, corresponding to linear combinations of lead fields that are optimally localized within the region of interest. This method is fast and stable. It is tested against physiologically plausible computer generated signals and also against more elaborate analysis of real data. The results show that the method can be very effective as a fast scanning tool identifying activity from pre-defined regions of interest.

Algorithms↗

PubMed1999

Linear transformations of data space in MEG.

Magnetoencephalography (MEG) is a method which allows the non-invasive measurement of the minute magnetic field which is generated by ion currents in the brain. Due to the complex sensitivity profile of the sensors, the measured data are a non-trivial representation of the currents where information specific to local generators is distributed across many channels and each channel contains a mixture of contributions from many such generators. We propose a framework which generates a new representation of the data through a linear transformation which is designed so that some desired property is optimized in one or more new virtual channel(s). First figures of merit are suggested to describe the relation between the measured data and the underlying currents. Within this context the new framework is established by first showing how the transformation matrix itself is designed and then by its application to real and simulated data. The results demonstrate that the proposed linear transformations of data space provide a computationally efficient tool for analysis and a very much needed dimensional reduction of the data.

Brain↗