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Biomedical subjects

C Pantev

Publications and source records attributed to C Pantev.

At least 37 records · Page 2Linked to original sources
PubMed1997

Mismatch field to tone pairs: neuromagnetic evidence for temporal integration at the sensory level.

The mismatch field (MMF) to minor pitch changes in two experimental conditions was studied. Standard tones of 1000 Hz and deviant tones of 1050 Hz both of 50 ms duration were delivered in single tone condition. Paired tones of the same duration were used in the paired tone condition. The standard tone pair consisted of two 1000 Hz tones, whereas the deviant tone pair was composed of a 1000 Hz tone in the first position and a 1050 Hz tone in the second position with a silent interval of 15 ms between the two. Standards of 90% and deviants of 10% probability were presented in random order and with a randomized interstimulus interval between 600 and 900 ms. The source analysis showed a more lateral location for the MMF obtained in the paired tone condition (MMF.P) compared to the MMF elicited by the single deviants (MMF.S). The source location of both the MMF.P and MMF.S turned out to be significantly anterior relative to the sources of the M100. The increased stimulus repetition in the paired tone condition (two times more stimuli than in the single tone condition) lead to a strong suppression of the field amplitude and of the dipole moment of the M100, while this effect could not be seen for the MMF. The data demonstrate a fundamental difference between the processes reflected by the M100 and the MMF: while the M100 represents the processing of every individual tone, the MMF reflects the change detection of the paired stimuli as unitary events, forming a perceptual group. The different sources of the MMF.P and MMF.S also support an integrated processing of the paired stimuli.

Acoustic Stimulation↗
PubMed1996

Tonotopic organization of the sources of human auditory steady-state responses.

Steady-state responses (SSRs) or steady-state fields (SSFs) show maximum amplitude when tone pulses are presented at repetition rates near 40 Hz. This result has led to the hypothesis that the SSR/SSF consists of superimposed transient 'middle latency' responses which display wave periods near 40 Hz and summate with one another when phase locked by 40 Hz steady-state stimulation. We evaluated this hypothesis by comparing the cortical sources of the 40 Hz auditory SSF with sources of the middle latency Pa wave which is prominent in electrical and magnetic recordings, and with the cortical sources of the familiar N1 wave, at different carrier frequencies between 250 and 4000 Hz. SSF sources determined for the different carrier frequencies were found to display a 'medial' tendency tonotopy resembling that of the N1m (sources for the higher frequencies represented more deeply within the supratemporal sulcus), opposite the 'lateral' tendency tonotopy of the middle latency Pam (sources for the higher frequencies situated more laterally). A medial SSF tonotopy was observed in each of the subjects investigated, including three subjects for whom Pam and N1m maps were also available. These findings suggest that the 40 Hz SSF may not consist of summated or entrained middle latency responses, as has previously been proposed. Alternative mechanisms for the SSR are discussed.

Acoustic Stimulation↗
PubMed1996

High-frequency cortical responses reflect lexical processing: an MEG study.

Meaningful words and matched pseudowords, such as moon vs. noom, are of equal perceptual complexity, but invoke different cognitive processes. To investigate high-frequency cortical responses to these stimuli, biomagnetic signals were recorded simultaneously over both hemispheres of right-handed individuals listening to words and pseudowords. Consistent with earlier EEG studies, evoked spectral responses recorded from the left hemisphere revealed depression of spectral power in the low gamma band (around 30 Hz) after pseudowords but not after words. Similar differences between stimulus categories were present in the beta range. These results indicate that distinct patterns of high-frequency cortical responses correspond to the different cognitive processes invoked by words and pseudowords. It is hypothesized that differential high-frequency cortical responses signal the activation or activation failure of distributed Hebbian cell assemblies representing words and other elements of cognitive processing.

Adult↗
PubMed1996

Binaural fusion and the representation of virtual pitch in the human auditory cortex.

The auditory system derives the pitch of complex tones from the tone's harmonics. Research in psychoacoustics predicted that binaural fusion was an important feature of pitch processing. Based on neuromagnetic human data, the first neurophysiological confirmation of binaural fusion in hearing is presented. The centre of activation within the cortical tonotopic map corresponds to the location of the perceived pitch and not to the locations that are activated when the single frequency constituents are presented. This is also true when the different harmonics of a complex tone are presented dichotically. We conclude that the pitch processor includes binaural fusion to determine the particular pitch location which is activated in the auditory cortex.

Acoustic Stimulation↗
PubMed1996

Reorganizational and perceptional changes after amputation.

The demonstration of cortical representational shifts in adult animals subsequent to deafferentation from amputation or dorsal rhizotomy has spurred attempts to elucidate the perceptual correlates of reorganization. Because the hand is flanked by the face and the trunk on the cortical homunculus it has been suggested that cortical remapping in arm amputees leads to a mislocalization of sensations from these sites to the phantom arm in a systematic manner with modality specific one-to-one topographical correspondence. Therefore, we assessed shifts of representational zones by magnetic source imaging in eight arm-amputees and examined them for referred sensation by somaesthetic stimuli of different modalities at standardized sites. It was found that referred phantom sensations can be evoked from sites on the face and the trunk ipsilateral but also contralateral to the amputation and that the extent of physiological reorganization as revealed by magnetic source imaging strongly correlates with the number of sites, be it ipsi- or contralateral, from where painful stimuli evoke referred sensation. Thus, it seems that the extent of reorganization after amputation is closely related to nociceptive inputs. The mislocalization evoked from both sides of the body, suggesting involvement of bilateral pathways, demonstrates that the perceptual changes go beyond what can be explained by shifts in neighbouring cortical representational zones.

Adult↗
PubMed1996

The auditory evoked "off" response: sources and comparison with the "on" and the "sustained" responses.

OBJECTIVE: It is well known that tone bursts elicit a prominent N1/P2 complex in the auditory evoked potential (the on-response), but less is known about a morphologically similar complex (the off-response) that can be recorded under suitable stimulus conditions. The interaction between the two responses indicated that the responses were not physiologically independent. The present experiment employed neuromagnetic methods to determine the cortical sources of N1 and P2 on- and off-responses and their relation to other events observed in the auditory evoked field. DESIGN: Five female and five male subjects with no history of otologic or neurological disorders and with normal audiological status participated in this study. Tone bursts of 2 sec duration (10 msec rise and decay time, cosine function), carrier frequency of 1 kHz, and intensity of 60 dB nHL (normative hearing level) were presented 512 times to the subject's right ear (contralateral to the investigated hemisphere) with an interstimulus interval randomized between 5 and 7 sec. RESULTS: The present study is unique in that several components of the complex auditory evoked response (P1, N1on, P2on, sustained-field, N1off, P2off) were recorded and localized in the same subjects and in the same experiment. The source coordinates obtained for N1 and P2 on- and off-responses indicated that the two responses are generated by overlapping cortical regions. Sources for the P2 components were situated anterior and medial to sources for the N1 components and were indistinguishable from sources for the auditory sustained-field. An early P1on event preceded the N1on (but not the N1off) response and was spatially indistinguishable from the N1on. The equivalent source strength was greater for N1on and P2on sources compared with N1off and P2off sources. CONCLUSIONS: The recoding process signaled by on-and off-responses may be a dynamic form of plasticity in the auditory cortex with a time constant on the order of hundreds of milliseconds, corresponding to the duration of sustained-responses released by acoustic changes and to the duration of the acoustic foreperiod that is necessary before on-and off-responses to acoustic changes can be observed.

Adult↗
PubMed1996

Oscillatory neuromagnetic activity induced by language and non-language stimuli.

Event-related oscillatory brain activity during language perception differs from activity occurring during the processing of comparable non-language stimuli. This fact became apparent in the observation of changes in the normalized spectral power of magnetoencephalographic (MEG) signals during the subject's processing of these stimuli. MEG was recorded over the left and right hemispheres of 12 right-handed subjects. During the experimental session, bisyllablic content words and physically similar non-language stimuli were presented with equal probability in a randomized order in either the visual or auditory modality. Approximately 15% of these stimuli were marked and the subject's task was to detect these marked stimuli. As a major characteristic of language vs. non-language processing, we obtained an enhancement of the normalized spectral power around 240 ins in the 60-65-Hz band over the left hemisphere for the language condition and over the right hemisphere for the non-language condition, independent of the modality of stimulus presentation. Starting at approximately the same latency but in lower-frequency bands (15-45-Hz), an extended (250-600 ms) reduction of normalized spectral power was observed. This reduction, although it generally confirmed previous results, differed in the no hemisphere-specific reduction was found for the processing of words. A domain-specific enhancement of normalized spectral power was also evident around 800-1200 ms in the 15-30-Hz band. In the auditory condition, this enhancement of the normalized spectral power was larger after the presentation of language stimuli whereas in the visual condition a larger enhancement of the normalized spectral power was obtained after presentation of non-language stimuli. As this latter effect appears relatively late after the stimulus onset and differs in expression for both modalities of stimulus presentation, a simple relationship between language perception and oscillatory brain dynamics can be excluded for this enhancement. In contrast, the left hemispheric enhancement of the normalized spectral power present around 240 ms in the 60-65-Hz band seems to reflect oscillatory pattern specific to the processing of words.

Acoustic Stimulation↗
PubMed1995

Cortical reorganization in human amputees and mislocalization of painful stimuli to the phantom limb.

In human arm amputees, a significant relationship was found between the amount of reorganization in the primary somato-sensory cortex, and the amount of body surface from which painful stimuli evoked sensations that were perceived to be emanating from the now missing extremity, i.e. the phantom limb. This mislocalization could be evoked almost equally from stimulation of either side of the body. Based on these findings obtained by magnetic source imaging and psychophysical testing in eight amputees, it is concluded that the extent of the generally known cortical reorganization contralateral to the amputation is an indicator of more widespread plastic changes in the brain involving bilateral pathways.

Adult↗
PubMed1995

Increased cortical representation of the fingers of the left hand in string players.

Magnetic source imaging revealed that the cortical representation of the digits of the left hand of string players was larger than that in controls. The effect was smallest for the left thumb, and no such differences were observed for the representations of the right hand digits. The amount of cortical reorganization in the representation of the fingering digits was correlated with the age at which the person had begun to play. These results suggest that the representation of different parts of the body in the primary somatosensory cortex of humans depends on use and changes to conform to the current needs and experiences of the individual.

Adult↗
PubMed1995

Phantom-limb pain as a perceptual correlate of cortical reorganization following arm amputation.

Although phantom-limb pain is a frequent consequence of the amputation of an extremity, little is known about its origin. On the basis of the demonstration of substantial plasticity of the somatosensory cortex after amputation or somatosensory deafferentation in adult monkeys, it has been suggested that cortical reorganization could account for some non-painful phantom-limb phenomena in amputees and that cortical reorganization has an adaptive (that is, pain-preventing) function. Theoretical and empirical work on chronic back pain has revealed a positive relationship between the amount of cortical alteration and the magnitude of pain, so we predicted that cortical reorganization and phantom-limb pain should be positively related. Using non-invasive neuromagnetic imaging techniques to determine cortical reorganization in humans, we report a very strong direct relationship (r = 0.93) between the amount of cortical reorganization and the magnitude of phantom limb pain (but not non-painful phantom phenomena) experienced after arm amputation. These data indicate that phantom-limb pain is related to, and may be a consequence of, plastic changes in primary somatosensory cortex.

Adult↗
PubMed1995

Evoked and induced gamma-band activity of the human cortex.

The evoked gamma-band activity is an event related rhythmic response which persists within the first 100 ms after the stimulus onset. It shows spectral peaks between 30 and 40 Hz in the auditory, between 45 and 55 Hz in the somatosensory and between 100 and 110 Hz in the visual system. After separation of the wide-band activity in slow and gamma-band activity, a moving single equivalent current dipole model accounts for each activity almost completely. The induced gamma-band activity is not phase-locked to the stimulus or it is strongly gittering and thus it cannot be extracted in time domain. In this case we are using signal analysis methods in frequency domain. The evaluation of the induced brain gamma-band activity around 30 Hz shows differences to word and nonword stimuli. It was supposed that the induced gamma-band activity represents the synchronized activity of Hebbian cell assemblies correlated to words.

Brain Mapping↗
PubMed1995

Specific tonotopic organizations of different areas of the human auditory cortex revealed by simultaneous magnetic and electric recordings.

This paper presents data concerning auditory evoked responses in the middle latency range (wave Pam/Pa) and slow latency range (wave N1m/N1) recorded from 12 subjects. It is the first group study to report multi-channel data of both MEG and EEG recordings from the human auditory cortex. The experimental procedure involved potential and current density topographical brain mapping as well as magnetic and electric source analysis. Responses were compared for the following 3 stimulus frequencies: 500, 1000 and 4000 Hz. It was found that two areas of the auditory cortex showed mirrored tonotopic organization; one area, the source of N1m/N1 wave, exhibited higher frequencies at progressively deeper locations, while the second area, the source of the Pam/Pa wave, exhibited higher frequencies at progressively more superficial locations. The Pa tonotopic map was located in the primary auditory cortex anterior to the N1m/N1 mirror map. It is likely that N1m/N1 results from activation of secondary auditory areas. The location of the Pa map in A1, and its N1 mirror image in secondary auditory areas is in agreement with observations from animal studies.

Acoustic Stimulation↗
PubMed1995

Magnetic and electric brain activity evoked by the processing of tone and vowel stimuli.

Sustained magnetic and electric brain waves may reflect linguistic processing when elicited by auditory speech stimuli. In the present study, only in the latency interval subsequent to the N1m/N1 has a sensitivity of brain responses to features of speech been demonstrated. We conclude this from studying the auditory-evoked magnetic field (AEF) and the corresponding evoked potential (AEP) in response to vowels and a tone. Brain activity was recorded from the left and the right hemisphere of 11 subjects. Three aspects of transient activity were examined: (1) the amplitudes and source characteristics of the N1m component of the AEF; (2) the amplitudes and source characteristics of the sustained field (SF), and (3) the corresponding amplitude characteristics of the AEP. Sustained potential amplitudes and SF root mean square amplitudes, as well as the dipole strength of the SF source, were found to be larger for vowel-evoked signals than for signals elicited by the tone stimulus. The amplitude and dipole strength effects had an interaction with hemisphere, with larger interhemispheric differences for the vowel condition, as well as larger tone-vowel differences of these parameters in the speech-dominant left hemisphere. No statistically significant hemisphere-by-stimulus-type interactions were found in N1/N1m amplitudes and N1m source parameters.

Adult↗
PubMed1994

Comparison of magnetic and metabolic brain activity during a verb generation task.

The magnetic and metabolic activational patterns of the brain during the perception, generation and silent articulation of words overlap to some extent, yet also measure concrete activational patterns. In the present study, auditory evoked magnetic fields (MEG) and changes in regional cerebral blood flow (PET) were examined in healthy subjects during a verb generation task. The aim of the study was to determine whether the advantages of both recording techniques can be combined so as to identify distributed sources of brain activity during particular tasks such as language processing. Given the currently observed disparity of the results from the two types of brain imaging we conclude that PET data will most likely not provide physiologically meaningful constraints for the distributed source analysis of MEG data, and may not necessarily validate results of distributed source analyses.

Brain↗
PubMed1994

Intrasubject reliability and validity of somatosensory source localization using a large array biomagnetometer.

Neuromagnetic fields were evoked by tactile stimuli and detected with a multi-channel biomagnetometer through 72 independent repetitive measurements on a single subject. Each measurement consisted of a somatosensory evoked response (N = 256 stimuli) using a single probe placement. These fields were then analyzed for source localization using an equivalent current dipole model and demonstrated highly reliable localizations. The 3 major neuromagnetic somatosensory response components peaking at 35, 65 and 110 msec all localized to the same area of cortex. The relative contributions of intrinsic brain activity, habituation, probe placement, and choice of fiduciary points for headframe determination were quantified. Intrinsic factors were found to constitute the major source of inter-measurement error. Sources localized by magnetic source imaging (MSI) appeared valid relative to neuroanatomical estimation of the central fissure on MRI. Non-invasive presurgical biomagnetic localization of somatosensory cortex produces reliable and valid functional localizations which can be of potential value in risk assessment and may provide a useful guide for invasive functional mapping.

Adult↗
PubMed1994

The auditory evoked sustained field: origin and frequency dependence.

A sound lasting for several seconds is known to elicit a baseline shift in electrical and magnetic records. We have studied the dependence of the magnetic field distribution of this "per-stimulatory" sustained field (SF) on tone frequency. Tone bursts of 2 sec duration and 60 dB nHL intensity were presented to 11 subjects at varying interstimulus intervals between 5 and 7 sec. The carrier frequencies of 250, 1000 and 4000 Hz varied randomly from trial to trial. The field distributions obtained are consistent with the view that the auditory evoked sustained field activity originates in the supratemporal cortex. Differences in the locations of equivalent current dipoles of the SF from those of the M100 wave of the slow auditory evoked field are consistent across subjects. The SF source locations corresponding to stimulus frequencies over an extended frequency range are arranged in a tonotopic manner and support the idea that the sources of the M100 and the SF are current dipole sheets located on the superior surface of the primary auditory cortex.

Acoustic Stimulation↗
PubMed1994

The magnetic counterpart of the contingent negative variation.

The magnetic counterpart of the CNV, the contingent magnetic variation (CMV), was investigated in an Go/No Go design: subjects moved their index finger to the offset of a 4 sec tone of a certain frequency in the Go condition and were asked not to move during presentation of a 4 sec tone of different frequency in the No Go condition. During the preparatory interval, both the CMV and the electrical wave form followed a similar time course and both produced an equally pronounced statistical difference between conditions (Go and No Go). Compared to the variability in the auditory evoked fields, the CMV showed considerably more variance in the field distribution across subjects. The polarity reversal across the temporal surface of the head and the pronounced amplitudes over inferior temporal areas led us to conclude that a significant temporal activity contributes to both the late and the early CMV. However, neither for the early nor for the late CMV component did a single equivalent dipole prove to be a satisfying model. The data are consistent with the suggestion that the earlier as well as the later aspects of the CMV are fed through distributed sources in motoric, sensory and association areas, a distribution with considerable intersubject variability.

Acoustic Stimulation↗