Neurodynamic Studies on Emotional and Inverted Faces in an Oddball Paradigm

The detection of a change in a face stimulus was studied in an oddball paradigm. Event-related potentials (ERPs) and MEG responses to face stimuli were recorded in four conditions: 1) happy standard, neutral deviant; 2) neutral standard, neutral deviant; 3) inverted happy standard, inverted neutral deviant; 4) inverted neutral standard, inverted neutral deviant. In all conditions, the target was a face with glasses. Neutral deviants elicited a negative deflection (with a maximum around 280 ms) in ERP and MEG responses, an effect similar to auditory mismatch negativity. Face inversion diminished deviance-related negativity, implying an important role of face recognition in the observed effect. Emotional content and larger physical differences between stimuli in conditions 1 and 3 compared to conditions 2 and 4 did not show statistically significant effect on the neutral-deviant-related negativity.     

Alcohol reduces prefrontal cortical excitability in humans: a combined TMS and EEG study.

The effects of alcohol (0.8 g/kg) on the prefrontal cortex were studied in nine healthy subjects using the technique of transcranial magnetic stimulation (TMS) combined with electroencephalography (EEG). A total of 120 magnetic pulses were delivered with a figure-of-eight coil to the left prefrontal cortex at the rate of 0.4-0.7 Hz. The EEG was recorded simultaneously with 60 scalp electrodes (41 electrodes were used for analysis); the TMS-evoked activation was estimated by the area under the global mean field amplitude (GMFA) time curve. TMS caused changes in EEG activity lasting up to 270 ms poststimulus. Alcohol decreased GMFA at 30-270 ms poststimulus (713+/-303 vs 478+/-142 microV ms; p=0.007). Alcohol-induced differences were most pronounced at anterior electrodes. These results suggest that alcohol reduces the excitability in the prefrontal cortex.     

Auditory cortex evoked magnetic fields and lateralization of speech processing

Potential use of different auditory evoked brain responses for determining cerebral lateralization of speech function was evaluated. Cortical magnetic fields elicited by plosive syllables or complex non-speech sounds analogous to them were recorded with 122-channel magnetometer. We estimated parameters of magnetic P1, N1 and P2 responses to both stimuli in the two hemispheres and found no hemispheric asymmetry for any of the responses. No correlation between the right-ear advantage, determined with dichotic listening test, and any of asymmetry indexes, calculated for the speech-elicited responses, was observed. These results suggest that P1, N1 and P2 responses to speech signals do not indicate lateralization of speech function in the brain. The results are discussed in relation to previous studies suggesting that the mismatch negativity (MMN) seems to be the only early auditory cortex response sensitive to the lateralization of speech function.     

Comparison of spherical and realistically shaped boundary element head models for transcranial magnetic stimulation navigation

Objective

MRI-guided real-time transcranial magnetic stimulation (TMS) navigators that apply electromagnetic modeling have improved the utility of TMS. However, their accuracy and speed depends on the assumed volume conductor geometry. Spherical models found in present navigators are computationally fast but may be inaccurate in some areas. Realistically shaped boundary-element models (BEMs) could increase accuracy at a moderate computational cost, but it is unknown which model features have the largest influence on accuracy. Thus, we compared different types of spherical models and BEMs.

Methods

Globally and locally fitted spherical models and different BEMs with either one or three compartments and with different skull-to-brain conductivity ratios (1/1–1/80) were compared against a reference BEM.

Results

The one-compartment BEM at inner skull surface was almost as accurate as the reference BEM. Skull/brain conductivity ratio in the range 1/10–1/80 had only a minor influence. BEMs were superior to spherical models especially in frontal and temporal areas (up to 20 mm localization and 40% intensity improvement); in motor cortex all models provided similar results.

Conclusions

One-compartment BEMs offer a good balance between accuracy and computational cost.

Significance

Realistically shaped BEMs may increase TMS navigation accuracy in several brain areas, such as in prefrontal regions often targeted in clinical applications.     

The Effect of Stimulus Parameters on TMS–EEG Muscle Artifacts

BackgroundWhen transcranial magnetic stimulation (TMS) is delivered close to the lateral aspects of the head, large-amplitude (～10–1000 μV) biphasic electroencephalographic (EEG) deflections, peaking at around 4–10 and 8–20 ms, appear.ObjectiveTo characterize the spatiotemporal features of these artifacts, to quantify the effect of stimulus parameters on them, and thus, to study the feasibility of different measurement procedures to decrease the artifacts online. Furthermore, to show that these deflections, when measured with a sample-and-hold system, mainly result from excitation of cranial muscles.MethodsThree subjects received TMS to 16 sites over the left hemisphere. TMS-compatible EEG was recorded simultaneously. Four other subjects received TMS to M1 with different coil rotation and tilt angles and stimulation intensities. We also stimulated a conductive phantom and recorded simultaneous EEG to exclude the possibility of residual electromagnetic artifacts.ResultsThe artifacts were largest when the stimulator was placed above cranial muscles, whereas stimulation of relatively central sites far from the muscles produced muscle artifact-free data. The laterally situated EEG channels were most severely contaminated. The artifacts were significantly reduced when reducing the intensity or when tilting or rotating the coil so that coil wings moved further away from the temporal muscle, while brain responses remained visible. Stimulation of the phantom did not produce such large-amplitude biphasic artifacts.ConclusionAltering the stimulation parameters can reduce the described artifact, while brain responses can still be recorded. The early, laterally appearing, large biphasic TMS-evoked EEG deflections recorded with a sample-and-hold system are caused by cranial muscle activation.     

Prefrontal TMS produces smaller EEG responses than motor-cortex TMS: implications for rTMS treatment in depression

Rationale The stimulus intensity of prefrontal repetitive transcranial magnetic stimulation (rTMS) during depression treatment is usually determined by adjusting it with respect to the motor threshold (MT). There is some evidence that reactivity of the prefrontal cortex to transcranial magnetic stimulation (TMS) is lower than that of the motor cortex at MT stimulation. However, it is unknown whether this is true when other stimulus intensities are used. We investigated whether the magnitude and shape of the overall TMS-evoked electroencephalographic (EEG) responses differ between prefrontal and motor cortices. Methods Magnetic pulses to the left motor and prefrontal cortices (the middle frontal gyrus identified from magnetic resonance images) were delivered at four intensities (60, 80, 100, and 120% of MT of the right abductor digiti minimi muscle) for six subjects. Simultaneously, EEG was recorded with 60 scalp electrodes. Results Global mean-field amplitudes (GMFAs) reflecting overall cortical activity were significantly smaller after prefrontal- than after motor-cortex TMS. A significant positive correlation (r _s=0.84, p<0.01) was found between GMFAs of motor- and prefrontal-cortex TMS across the experiments. However, when correlation between the responses of motor and prefrontal cortices was examined, significant positive correlations were found at 80 and 100% intensities only. Conclusions This study provides further evidence that the prefrontal and motor cortices have different reactivity to TMS, but the MT may be used for determining the stimulus intensity of prefrontal rTMS treatment in depression, at least at motor threshold intensities or near to it.     

Brain signatures of meaning access in action word recognition

The brain basis of action words may be neuron ensembles binding language- and action-related information that are dispersed over both language- and action-related cortical areas. This predicts fast spreading of neuronal activity from language areas to specific sensorimotor areas when action words semantically related to different parts of the body are being perceived. To test this, fast neurophysiological imaging was applied to reveal spatiotemporal activity patterns elicited by words with different action-related meaning. Spoken words referring to actions involving the face or leg were presented while subjects engaged in a distraction task and their brain activity was recorded using high-density magnetoencephalography. Shortly after the words could be recognized as unique lexical items, objective source localization using minimum norm current estimates revealed activation in superior temporal (130 msec) and inferior frontocentral areas (142-146 msec). Face-word stimuli activated inferior frontocentral areas more strongly than leg words, whereas the reverse was found at superior central sites (170 msec), thus reflecting the cortical somatotopy of motor actions signified by the words. Significant correlations were found between local source strengths in the frontocentral cortex calculated for all participants and their semantic ratings of the stimulus words, thus further establishing a close relationship between word meaning access and neurophysiology. These results show that meaning access in action word recognition is an early automatic process ref lected by spatiotemporal signatures of word-evoked activity. Word-related distributed neuronal assemblies with specific cortical topographies can explain the observed spatiotemporal dynamics reflecting word meaning access.     

Activation in the anterior left auditory cortex associated with phonological analysis of speech input: localization of the phonological mismatch negativity response with MEG

The spatio-temporal dynamics of cortical activation underlying auditory word recognition, particularly its phonological stage, was studied with whole-head magnetoencephalography (MEG). Subjects performed a visuo-auditory priming task known to evoke the phonological mismatch negativity (PMN) response that is elicited by violations of phonological expectancies. Words and non-words were presented in separate conditions. In each of the 318 trials, the subjects first saw a word/non-word (e.g., 'cat') that was soon followed by a prime letter (e.g., 'h'). Their task was to replace mentally the sound of the first letter of the word/non-word with the prime letter, thus resulting in a new word/non-word (e.g., 'hat'). Finally, an auditory word/non-word either matching or mismatching with the anticipated item was presented. In most subjects, a PMNm followed by a later, N400m-like negativity was obtained in the left hemisphere to the mismatching auditory stimuli. A similar response pattern was obtained in the right hemisphere only in a few subjects. Source localization of the N1m, an index of acoustic analysis, and the PMNm and N400m-like responses was performed using L1 minimum-norm estimation. In the left hemisphere, the PMNm source for the words was significantly more anterior than the source of the N400m-like response; for the non-words, the PMNm source was significantly more anterior than the sources of the N1m and the N400m-like response. These results suggest that the left-hemisphere neuronal networks involved in sub-lexical phonological analysis are at least partly different from those responsible for the earlier (acoustic) and later (whole item) processing of speech input.     

Tryptophan depletion effects on EEG and MEG responses suggest serotonergic modulation of auditory involuntary attention in humans

Involuntary attention shifting, i.e., detecting and orienting to unexpected stimulus changes, may be altered at low brain serotonin (5-hydroxytryptamine; 5-HT) levels. This was studied in 13 healthy subjects (21–30 years old; 6 females) by using a dietary challenge, acute tryptophan depletion (ATD), which decreases 5-HT synthesis in the brain. Five hours after ingestion of either ATD or control mixture (randomized, double-blinded, crossover design), brain responses indexing involuntary attention were measured with simultaneous 64-channel electroencephalography (EEG) and 122-channel magnetoencephalography (MEG). During the measurement, the subjects were instructed to discriminate equiprobable 200- and 400-ms tones by pressing one of two buttons rapidly. Occasionally, the frequency of the tones changed (10% increase/decrease), causing involuntary attention shifting. ATD significantly lowered plasma tryptophan concentrations (total tryptophan decreased by 75%, free tryptophan decreased by 35%). As compared to the control condition, ATD reduced the amplitude of the deviant-tone N2 wave, including the overlapping mismatch negativity (MMN) and N2b subcomponents, which are suggested to reflect change detection in the brain. The EEG results were accompanied by a significant increase in the peak latency of the magnetic counterpart of MMN. However, no ATD effects were observed in P3 to task-irrelevant frequency change. Reaction time (RT) to deviants per se was not significantly affected, but RT in trials succeeding the deviant-frequency tones was increased by ATD, which suggested impaired reorienting to the task-relevant activity. In conclusion, the results suggest that decreased level of central 5-HT function after ATD may decrease involuntary attention shifting to task-irrelevant sound changes and thus modulate resource allocation to the task-relevant activity.     

Mismatch negativity indexes auditory temporal resolution: evidence from event-related potential (ERP) and event-related field (ERF) recordings

This study examined auditory temporal resolution as indexed by gap detection using the mismatch negativity (MMN) component of the auditory event-related potential (ERP) and its magnetic counterpart (MMNm). ERPs were recorded in 10 subjects who were presented with auditory stimuli. These stimuli were presented in sequences of repetitive continuous 'standard' sinusoidal tones interspersed with infrequently occurring 'deviant' stimuli that differed from standards only in that they contained a silent gap midway in the stimulus. The gap size varied in separate stimulus blocks and was either 3, 5 or 7 ms. The stimuli were presented monaurally either to the left or the right ear. In a separate session, event-related magnetic fields (ERFs) were recorded from eight subjects using a similar paradigm but with gap sizes of 3, 7 or 11 ms and with binaural stimulation. Both ERP and ERF recordings showed that the smallest gap size (3 ms) did not elicit as large or reliable MMN or MMNm as did the larger ones. There were no differences in the laterality of the MMN as might be predicted on the basis of previous behavioural studies, but this result is likely a reflection of differences in task requirements. Nonetheless, the findings suggest that MMN and MMNm successfully index auditory temporal resolution thresholds, as measures that are independent of attention.