Biomedical subjects

C Lammertmann

Publications and source records attributed to C Lammertmann.

6 recordsLinked to original sources

PubMed2003

Localization of primary auditory cortex in humans by magnetoencephalography.

Brief auditory stimuli activate the primary auditory cortex (PAC) earlier than any other cortical area so, within a certain latency range, the PAC is the only cortical source contributing to the auditory evoked field (AEF). Nevertheless, there is no AEF component specific to PAC that can be reliably detected in all individuals. The present study suggests that a peak in the first temporal derivative of the magnetic field at about 20 ms (dP20m) is a genuine correlate of PAC activity. AEFs in response to clicks presented to the right ear were recorded with a 37-channel axial gradiometer system positioned over the left hemisphere in nine normal-hearing subjects. More than 8500 stimuli were presented in each of two independent sessions at a rate of approximately 3/s. The dipole coordinates for the dP20m derived from the two sessions typically differed by only a few millimeters. Coregistration of the dipoles with structural magnetic resonance images suggests that dP20m arises from an area close to the retroinsular origin of Heschl's gyrus. Although the dP20m is simply the point of steepest slope on the well-known middle-latency peak, P30m or Pam, it would appear that dP20m and P30m do not have the same cortical origin. Evidence is provided that P30m receives major contributions from at least two distinct cortical areas, only one of which is PAC.

Acoustic Stimulation↗

PubMed2003

Neuromagnetic evidence for a pitch processing center in Heschl's gyrus.

There have been several attempts to use the neuromagnetic response to the onset of a tonal sound (N100m) to study pitch processing in auditory cortex. Unfortunately, a large proportion of the N100m is simply a response to the onset of sound energy, independent of whether the sound produces a pitch. The current study describes a novel stimulus paradigm designed to circumvent the energy-onset response and thereby isolate the response of those neural elements specifically involved in pitch processing. The temporal resolution of magnetoencephalography enables us to show that the latency and amplitude of this pitch-onset response (POR) vary with the pitch and pitch strength of the tone. The spatial resolution is sufficient to show that its source lies somewhat anterior and inferior to that of the N100m, probably in the medial part of Heschl's gyrus.

Acoustic Stimulation↗

PubMed2002

Magnetoencephalographic correlates of audiotactile interaction.

To seek for correlates of an interaction between auditory and somatosensory processing, the brain's magnetic field in response to simultaneously presented auditory (A) and tactile (T) stimuli was compared with the sum of the respective unimodal responses (A+T). The stimuli were binaural 1047-Hz tone bursts of 60 dB sensation level and tactile pressure pulses to the right thumb. The mean interval between two stimuli of the same modality was 1.95 s. The magnetic field was recorded using a 306-channel whole-scalp neuromagnetometer. A clear audiotactile interaction was revealed in the hemisphere contralateral to the side of tactile stimulation in six of eight subjects, whereas in the ipsilateral hemisphere an interaction was noticed in only three subjects. The time courses of these audiotactile interaction fields typically showed major deflections of opposite polarities around 140 and 220 ms. The first deflection appeared to arise in the region of the secondary somatosensory cortex (SII). The polarity of this interaction was consistent with the view that the auditory stimulus resulted in a partial inhibition in SII. In two subjects, strong indications of auditory contributions to the interaction were available, although in different hemispheres. The relatively high interindividual variability of the observed interaction, which represents potential neural substrates for multisensory integration, could indicate that the way subjects perceive the simultaneous presentation of auditory and tactile stimuli differs.

Attention↗

PubMed2001

Near-DC magnetic fields following a periodic presentation of long-duration tonebursts.

OBJECTIVES: The purpose of this study was to determine the time course of low-frequency (<0.1 Hz) magnetic field components evoked by long-duration tonebursts. The following two questions were of central interest. Does the sustained field (SF) show adaptation as described before for the sustained potential (SP)? How does the field amplitude return to the pre-stimulus baseline after stimulus offset? METHODS: Neuromagnetic measurements were done with a 37-channel first-order gradiometer system. The stimulus was a 1 kHz toneburst of 10 s duration presented at fixed 20 s intervals. The averaged data (high-pass filtered, 0.03 Hz cut-off) were analyzed using the model of an equivalent current dipole with time-invariant location and orientation (fixed dipole). RESULTS: In the grand average of the subjects with the best signal-to-noise ratio, the SF exhibited adaptation with a time constant of 3.6 s. After stimulus offset, the amplitude of the dipole moment dropped to a lower level within 300 ms and decayed exponentially to the baseline thereafter (time constant 2.7 s). CONCLUSIONS: A two-component model is proposed: One component roughly follows the envelope of the stimulus, the other behaves like a leaky integrator. A better understanding of near-DC fields appears to be crucial for the understanding of the relationship between magnetoencephalography and other functional imaging techniques like functional magnetic resonance imaging and positron emission tomography.

Adaptation, Physiological↗

PubMed2000

Brain stem auditory evoked fields in response to clicks.

Magnetoencephalographic correlates of brain stem auditory evoked potentials (BAEP) have been identified by applying the model of a current dipole with invariant location and orientation (fixed dipole) to data recorded with a 37-channel first-order gradiometer system. In all three subjects studied, the dominating wave of the recorded brain stem auditory evoked field (BAEF) coincided exactly with wave V in the contralateral BAEP (maximal field amplitude of the order of 2-3 fT). In one subject, also a wave preceding wave V was observed, which was associated with basically the same spatial pattern as wave V itself, but had an opposite polarity. The study suggests that the supplementary information provided by BAEF measurements could be decisive for a better understanding of human auditory evoked brain stem activity.

Acoustic Stimulation↗

PubMedDate not supplied

Latency of auditory evoked field deflection N100m ruled by pitch or spectrum?

The auditory evoked field (AEF) in response to pure tones of 250 and 1000 Hz and a complex tone with a periodicity of 4 ms (composed of the frequencies 1000, 1250, 1500, 1750, and 2000 Hz), corresponding to a pitch of 250 Hz, was recorded with a 37-channel neuromagnetometer system. The intensity was 60 dB sensation level (SL). Two different stimulus durations were examined in 12 subjects: 500 ms (long tones) and 100 ms (short tones). The stimulus onset asynchrony (SOA) was uniformly distributed between 3 and 4 s for the long tones and between 0.8 and 1.2 s for the short tones. Each subject was investigated four times, to assess the intraindividual variability. The mean latency of the AEF deflection N100m turned out to be similar for the long and the short tones: about 98 and 87 ms for the pure tones of 250 Hz and 1000 Hz, respectively, and 95 ms for the complex tone with a pitch of 250 Hz. However, a great interindividual variability was observed, exhibiting no consistent relationship between the N100m latencies for the three different tones, except that the response to the pure tone of 1000 Hz generally occurred earlier. In conclusion, this study does not support the proposal that the N100m latency represents a code for pitch, although a low pitch appears to be a factor favoring a longer N100m latency.

Adult↗