Biomedical subjects

P G Simos

Publications and source records attributed to P G Simos.

45 records · Page 3Linked to original sources

PubMed1997

Source localization of the N400 response in a sentence-reading paradigm using evoked magnetic fields and magnetic resonance imaging.

The aim of the present investigation was to localize the sources of the N400 response elicited in a sentence-reading paradigm. Eight neurologically healthy adults viewed sentences that were presented one word at a time in the center of a computer screen. Half of the sentences ended with a semantically inappropriate word, while the other half had appropriate endings. Event-related potentials recorded at Fz and Pz showed a negative-going deflection, the amplitude of which was strongly affected by semantic congruity (N400). Evoked magnetic fields that were simultaneously recorded over the left hemisphere showed clear magnetic field extrema in seven subjects during the time course of the N400. Underlying sources were successfully modeled as single equivalent current dipoles. Anatomical regions that contained the dipoles were localized by superimposing dipole coordinates onto magnetic resonance scans. Dipole regions were found in temporal lobe structures, in the vicinity of the hippocampus and the parahippocampal gyrus (in two subjects) and in posterior temporal neocortical regions (in the vicinity of the middle temporal gyrus; in five subjects). These findings are consistent with the view that posterior association cortices in the left hemisphere are involved in word recognition and semantic comprehension during reading.

Adolescent↗

PubMed1997

Behavioral and electrophysiological indices of voicing-cue discrimination: laterality patterns and development.

Voicing is an important phonetic dimension that distinguishes voiced (i.e., /b/) from voiceless-aspirated stop consonants (i.e.,/ph/) in English. Perception of discrete voicing categories is affected by a number of acoustic cues. The present paper reviews evidence from brain-damaged populations indicating that the perception of certain voicing cues is less dependent upon left hemisphere mechanisms than the ability to perceive place of articulation contrasts (e.g., /b/ vs./d/). In addition, electrophysiological and dichotic listening studies with neurologically normal individuals support the view that the right hemisphere may play a special role in the categorical processing of voicing. These findings are discussed in relation to current models of hemispheric specialization and laterality for language.

Adult↗

PubMed1997

Electrophysiological responses from a temporal order continuum in the newborn infant.

Auditory Evoked Responses (AERs) were used to investigate discrimination of temporal order differences in eight male and eight female newborn infants. The temporal lags in two-tone non-speech stimuli-known as tone-onset time (TOT)-modeled the temporal delay of voicing-onset time (VOT), an important cue for voicing contrasts. Analyses on peak amplitude measures and principal component scores indicated that the amplitude of the second negative AER deflection (N530) recorded from the left and right parietal leads changed abruptly as TOT values increased from +20 to +40 msec. However, no differences were noted between tokens that belonged in the same adult perceptual categories. A similar pattern of variability was observed for the amplitude of the first major negative peak (N200). Our findings indicate that the neurophysiological mechanisms underlying categorical-like distinctions of a temporal voicing cue used in speech perception may have an innate basis. Since both hemispheres produced similar responses at birth, the present data suggest that brain mechanisms involved in temporal order processing undergo significant reorganization during the first years of life.

Acoustic Stimulation↗

PubMed1997

Magnetoencephalographic evidence for common sources of long latency fields to rare target and rare novel visual stimuli.

This study used magnetoencephalography to examine the possibility that different generators account for the long-latency event-related potential (P300), evoked by rare target and by rare non-target, novel visual stimuli, in a visual oddball counting task performed by seven subjects. As expected, P300 peak latency was longer in response to rare targets compared to novel, non-target stimuli. Two main source regions were found for the Target- as well as for the Novel-P300, one in the temporal and one in the occipital lobe. Centers of neural activity were observed in the vicinity of the superior temporal sulcus, in the hippocampal formation and parahippocampal gyrus and in the occipital extrastriate cortex. It appears that the brain structures which contributed to the generation of the P300 response to both the target and the novel visual stimuli overlapped to a great extent.

Adult↗

PubMed1997

Event-related potentials in a two-choice task involving within-form comparisons of pictures and words.

Behavioral as well as electrophysiological evidence suggests that words are processed differently than pictures in a variety of tasks. In this study fifteen adult subjects were tested on a speeded "same-different" judgment task between printed names and drawings of common objects. In one condition, subjects decided on the identity between their internal image of the object that was named by S1 (word) and a subsequently presented drawing of an object (Word-Picture condition). In a second condition, comparisons were made on the basis of the name of the depicted object (Picture-Word trials). Event-related potentials (ERPs) were recorded from 12 scalp locations in response to the second item in each pair. No-match waveforms were characterized by larger N350 and P500 deflections compared to Match ERPs. The two responses could be distinguished on the basis of their lateral and anterior-posterior scalp distribution. Within-form comparisons involving pictures produced increased positivity between 150 and 600 ms poststimulus onset at posterior recording sites, whereas the opposite effect was noted at anterior sites during the early portion of the ERP. Decisions on word stimuli were associated with prolonged reaction time and longer N350 peak latency compared to decisions on pictures. These results demonstrate the existence of independent sources of ERP variability, each possibly reflecting a different aspect of cognitive comparisons. Latency and reaction time data provided valuable information regarding differences in the course of the comparison process when linguistic, as opposed to pictorial, stimuli/representations were involved.

Adolescent↗

PubMed1996

Nonlinear alignment and averaging for estimating the evoked potential.

This paper addresses the problems associated with averaging brain responses evoked through a repetitive application of an external stimulus. In order to improve the estimate of the evoked potential (EP) through signal averaging, a method which incorporates nonlinear alignment of the EP's into the averaging operation is developed. The method makes no prior assumptions about the properties of the EP or which response in the set best characterizes the EP to be estimated. The nonlinear alignment procedure is designed to pairwise generate optimally aligned EP's by backtracking along the optimal alignment path. The nonlinear alignment and averaging operations are systematically combined to develop methods to estimate the EP. Results from a series of experiments conducted on simulated and real sets of responses show that, through nonlinear alignment and averaging, the events in the EP's are preserved and the estimates of the EP are quite robust.

Adult↗

PubMedDate not supplied

Intraoperative neurophysiological monitoring. Part 1: General overview.

Intraoperative electrophysiological recordings are gradually becoming part of standard medical practice, mainly because they offer an objective and effective way to assess the functional integrity of the nervous system of a patient during the course of an orthopedic, neurological, or vascular surgery. Continuous monitoring of spontaneous and triggered bioelectrical activity not only can avert damage of neurological structures that are at risk during certain surgical maneuvers, but also allows identification of specific neuronal structures and landmarks that cannot be easily recognized on anatomical grounds only. This series on neurophysiological monitoring will introduce various techniques of monitoring available today, the rationale for their intraoperative use, and the main principles on which they are based. Recommendations for proper implementation and troubleshooting will also be given. The present article gives a general overview of the procedures.

Cost-Benefit Analysis↗

PubMedDate not supplied

Intraoperative neurophysiological monitoring. Part 2: Neurophysiological background.

Continuous intraoperative monitoring of neurophysiological signals provides a reliable way to determine the functional integrity of the nervous system during the course of neurological, orthopedic, or vascular surgery. To understand the use of electrophysiological recording in the operating room, the basic mechanisms that give rise to the recorded signals are briefly reviewed. Additionally, the equipment necessary for proper signal recording and presentation of the results is described. Finally, a short introduction to anesthesia management and some commonly induced neuroprotective conditions, such as hypothermia and hypotension, are provided, since these are the main factors affecting the signals, thus contributing to the difficulty of correct interpretation of the recordings during the course of an operation.

Anesthesia↗

PubMedDate not supplied

[Insights into brain function though magnetic source imaging: A review of research and clinical applications].

INTRODUCTION AND DEVELOPMENT: The paper presents a brief outline of the rationale behind the use of non invasive functional imaging and of the features that any imaging technique should display in order to make a substantial contribution to the search of the brain mechanisms responsible for cognitive functions. One such technique, magnetic source imaging (MSI), that meets these specifications, is described in more detail. Advantages of MSI include the capacity to provide direct measures of regional neurophysiological activity, a millisecond range temporal resolution, and the capacity to provide images of brain activity on an individual basis. We then describe applications of MSI to the study of brain mechanisms involved in various language functions such as oral comprehension and reading. Among these applications, the accuracy of MSI protocols in determining hemispheric dominance for language functions and in identifying the precise location and extent language specific cortex (Wernicke s area) has been verified through comparison with standard invasive techniques (Wada procedure and electrocortical stimulation mapping) in over 60 consecutive cases. In another series of studies we combined data from MSI and direct cortical stimulation to determine the role of temporoparietal areas in phonological analysis of spoken language and in phonological decoding of print. Finally, we have used MSI to gain unique insights into the brain mechanisms that support reading in developmental reading disability. CONCLUSION: Results from over 21 children diagnosed with this disorder suggest that impaired reading is associated with aberrant functional connections between temporal and temporoparietal areas of the left hemisphere that are normally engaged in reading.

Brain Mapping↗