Jyrki P. Mäkelä

Evidence for cortical origin of the 40 Hz auditory evoked response in man

We have recorded cerebral magnetic fields to trains of 40 Hz clicks in 6 subjects to study the neural generators of the 40 Hz auditory evoked response (AER). The largest deflection, N100m (magnetic counterpart of the electric N100), was preceded by a low voltage P40m and followed by P200m. A sustained field, SF, was seen at 300-500 msec. A rhythmic 40 Hz fluctuation was superimposed on the whole response. All deflections reversed polarity between the approximated ends of the sylvian fissure and the field patterns were dipolar during the peaks. The field patterns of the 40 Hz response suggested activation of the auditory cortex at the supratemporal plane, 2-3 cm beneath the scalp, close to the sources of P40m, N100m and SF. The equivalent source of SF was 0.5-1.5 cm anterior to the source of N100m in all subjects. The results suggest a cortical origin for the magnetic 40 Hz response, also suggesting that the electric 40 Hz AER is, at least in part, generated within the auditory cortex.

Whole-head mapping of middle-latency auditory evoked magnetic fields ☆

We recorded middle-latency auditory evoked magnetic fields from 9 healthy subjects with a 122-channel whole-head SQUID gradiometer. The stimuli were click triplets, 2.5 msec in total duration, delivered alternately to the two ears once every 333 msec. Contralateral clicks elicited P30m responses in 16 and P50m responses in 12 out of 18 hemispheres studied; ipsilateral clicks did so in 7 and 13 hemispheres, respectively. The field patterns were satisfactorily explained by current dipoles in 16 and 4 hemispheres for contra- and ipsilateral P30m, and in 4 and 10 hemispheres for contra- and ipsilateral P50m. The peak latencies of P30m and P50m were not affected by stimulation side. The results show that middle-latency auditory evoked responses receive a strong contribution from auditory cortical structures, and that differences of input latency to cortical auditory areas, evaluated from MLAEF latencies, do not explain the latency differences seen in late auditory evoked fields to contralateral vs. ipsilateral stimulation.

Selective listening modifies activity of the human auditory cortex

SummaryWe have studied the effect of selective listening on the neuromagnetic evoked activity of the human auditory cortex. In the word categorization experiment the stimuli were 5-letter words, each beginning with /k/. Half of them were targets, i.e., names of animals or plants, and half other meaningful Finnish words. In the duration discrimination experiment equiprobable tones of 425 ms (targets) or 600 ms duration were presented. In both experiments the interstimulus interval (ISI) was 2.3 s and the stimuli of the two classes were presented randomly. Subjects either ignored the stimuli (reading condition) or counted the number of targets (listening condition). The magnetic field over the head was measured with a 7-channel 1st-order SQUID-gradiometer. The stimuli evoked a transient response followed by a sustained field. The transient response did not differ between the two conditions but the sustained field was significantly larger in the listening than reading condition; the increase began 120–200 ms after stimulus onset and continued for several hundred milliseconds. The equivalent source locations of both transient and sustained responses agreed with activation of the supratemporal auditory cortex. In the dichotic listening experiment 25-ms square-wave stimuli were presented randomly and equiprobably either to the left or to the right ear at an ISI of 0.8–1 s, either alone or in presence of a speech masker. Counting the stimuli of either ear resulted in differences between responses to relevant and irrelevant sounds. The difference began 140–150 ms after stimulus onset and peaked at 200–240 ms. During monaural speech masking, N100m was larger for attended than ignored stimuli. The results suggest that neural mechanisms underlying direction of attention include modification of the activity of the auditory cortex and that the mechanisms are similar for words and tones.

Cortical origin of middle-latency auditory evoked responses in man

We have recorded middle-latency magnetic evoked responses to 50-ms noise bursts, presented once every 0.9 s, over the right hemisphere of healthy humans. The measurements were carried out with a sensitive 7-channel SQUID gradiometer with a passband of 0.5-2000 Hz. The response consisted of peaks at about 30, 50 and 65 ms. The location of the equivalent source of the 30-ms deflection agrees with activation of the supratemporal auditory cortex, slightly anterior to the source area of the well-known 100-ms deflection.

Modification of neuromagnetic responses of the human auditory cortex by masking sounds.

SummaryWe have studied the effects of masking sounds on auditory evoked magnetic fields (AEFs) of healthy humans. The AEFs were elicited by 25-ms tones presented randomly to the left or to the right ear, and the responses were recorded over the right auditory cortex. Without masking, the 100-ms deflection (N100m) was of somewhat higher amplitude and of shorter latency for contrathan ipsilateral stimuli. Continuous speech, music, or intermittent noise, delivered to the left ear, dampened N100m to stimulation of both ears without correlated changes in sensation. Intermittent noise had a weaker effect on N100m than speech or music. Continuous noise fed to the left ear dampened both the sensation of and the responses to the left-ear stimuli, with no significant effect on the responses to the right-ear stimuli. The results suggest that the masking effects of continuous noise, seen at the auditory cortex, derive mainly from the periphery whereas the effects of sounds with intensity and frequency modulations take place at more central auditory pathways.

Magnetoencephalography in neurosurgery

OBJECTIVE:To present applications of magnetoencephalography (MEG) in studies of neurosurgical patients. METHODS:MEG maps magnetic fields generated by electric currents in the brain, and allows the localization of brain areas producing evoked sensory responses and spontaneous electromagnetic activity. The identified sources can be integrated with other imaging modalities, e.g., with magnetic resonance imaging scans of individual patients with brain tumors or intractable epilepsy, or with other types of brain imaging data. RESULTS:MEG measurements using modern whole-scalp instruments assist in tailoring individual therapies for neurosurgical patients by producing maps of functionally irretrievable cortical areas and by identifying cortical sources of interictal and ictal epileptiform activity. The excellent time resolution of MEG enables tracking of complex spaciotemporal source patterns, helping, for example, with the separation of the epileptic pacemaker from propagated activity. The combination of noninvasive mapping of subcortical pathways by magnetic resonance imaging diffusion tensor imaging with MEG source localization will, in the near future, provide even more accurate navigational tools for preoperative planning. Other possible future applications of MEG include the noninvasive estimation of language lateralization and the follow-up of brain plasticity elicited by central or peripheral neural lesions or during the treatment of chronic pain. CONCLUSION:MEG is a mature technique suitable for producing preoperative “road maps” of eloquent cortical areas and for localizing epileptiform activity.

Human auditory cortex is activated by omissions of auditory stimuli

Cortical signals associated with infrequent tone omissions were recorded from 9 healthy adults with a whole-head 122 channel neuromagnetometer. The stimulus sequence consisted of monaural (left or right) 50-ms 1-kHz tones repeated every 0.2 or 0.5 s, with 7% of the tones randomly omitted. Tones elicited typical responses in the supratemporal auditory cortices. Omissions evoked strong responses over temporal and frontal areas, independently of the side of stimulation, with peak amplitudes at 145-195 ms. Response amplitudes were 60% weaker when the subject was not attending to the stimuli. Omission responses originated in supratemporal auditory cortices bilaterally, indicating that auditory cortex plays an important role in the brains modelling of temporal characteristics of the auditory environment. Additional activity was observed in the posterolateral frontal cortex and in the superior temporal sulcus, more often in the right than in the left hemisphere.

A novel approach for documenting naming errors induced by navigated transcranial magnetic stimulation

Transcranial magnetic stimulation (TMS) is widely used both in basic research and in clinical practice. TMS has been utilized in studies of functional organization of speech in healthy volunteers. Navigated TMS (nTMS) allows preoperative mapping of the motor cortex for surgical planning. Recording behavioral responses to nTMS in the speech-related cortical network in a manner that allows off-line review of performance might increase utility of nTMS both for scientific and clinical purposes, e.g., for a careful preoperative planning. Four subjects participated in the study. The subjects named pictures of objects presented every 2-3s on a computer screen. One-second trains of 5 pulses were applied by nTMS 300ms after the presentation of pictures. The nTMS and stimulus presentation screens were cloned. A commercial digital camera was utilized to record the subjects performance and the screen clones. Delays between presentation, audio and video signals were eliminated by carefully tested combination of displays and camera. An experienced neuropsychologist studied the videos and classified the errors evoked by nTMS during the object naming. Complete anomias, semantic, phonological and performance errors were observed during nTMS of left fronto-parieto-temporal cortical regions. Several errors were detected only in the video classification. nTMS combined with synchronized video recording provides an accurate monitoring tool of behavioral TMS experiments. This experimental setup can be particularly useful for high-quality cognitive paradigms and for clinical purposes.

Sources of Auditory Brainstem Responses Revisited: Contribution by Magnetoencephalography

Auditory brainstem responses provide diagnostic value in pathologies involving the early parts of the auditory pathway. Despite that, the neural generators underlying the various components of these responses have remained unclear. Direct electrical recordings in humans are possible only in limited time periods during surgery and from small regions of the diseased brains. The evidence of the generator sites is therefore fragmented and indirect, based strongly on lesion studies and animal models. Source modeling of EEG has been limited to grand averages across multiple subjects. Here, we employed magnetoencephalography (MEG) to shed more light on the neural origins of the auditory brainstem responses (ABR) and to test whether such deep brain structures are accessible by MEG. We show that the magnetic counterparts of the electric ABRs can be measured in 30 min and that they allow localization of some of the underlying neural sources in individual subjects. Many of the electric ABR components were present in our MEG data; however, the morphologies of the magnetic and electric responses were different, indicating that the MEG signals carry information complementary to the EEG data. The locations of the neural sources corresponding to the magnetic ABR deflections ranged from the auditory nerve to the inferior colliculus. The earliest cortical responses were detectable at the latency of 13 ms. Hum Brain Mapp, 2009.

Different analysis of frequency and amplitude modulations of a continuous tone in the human auditory cortex: A neuromagnetic study

We have measured auditory evoked magnetic fields to intermittent frequency and amplitude modulations (FMs and AMs) of a continuous tone in 6 healthy humans. The stimuli were presented in pairs separated by 500 ms in four different combinations (FM-AM, FM-FM, AM-FM and AM-AM). Both modulations elicited neuromagnetic responses of similar waveforms: the largest deflection, N100m (magnetic counterpart of the electric N100), was preceded by a low amplitude P60m and followed by P200m. For stimuli of different types, the decrease of N100m from the first to the second response was less than expected from the recovery cycle of the responses, estimated from the pairs of similar stimuli. We interpret these results as evidence for different processing of amplitude and frequency modulations in the auditory pathways up to the level of supratemporal auditory cortex.