Variability of magnetoencephalographic sensor sensitivity measures as a function of age,brain volume and cortical area

Objective

To assess the feasibility and appropriateness of magnetoencephalography (MEG) for both adult and pediatric studies, as well as for the developmental comparison of these factors across a wide range of ages.

Methods

For 45 subjects with ages from 1 to 24 years (infants, toddlers, school-age children and young adults), lead fields (LFs) of MEG sensors are computed using anatomically realistic boundary element models (BEMs) and individually-reconstructed cortical surfaces. Novel metrics are introduced to quantify MEG sensor focality.

Results

The variability of MEG focality is graphed as a function of brain volume and cortical area. Statistically significant differences in total cerebral volume, cortical area, MEG global sensitivity and LF focality are found between age groups.

Conclusions

Because MEG focality and sensitivity differ substantially across the age groups studied, the cortical LF maps explored here can provide important insights for the examination and interpretation of MEG signals from early childhood to young adulthood.

Significance

This is the first study to (1) investigate the relationship between MEG cortical LFs and brain volume as well as cortical area across development, and (2) compare LFs between subjects with different head sizes using detailed cortical reconstructions.     

Speech‐in‐noise understanding in older age: The role of inhibitory cortical responses

Studies of central auditory processing underlying speech‐in‐noise (SIN) recognition in aging have mainly concerned the degrading neural representation of speech sound in the auditory brainstem and cortex. Less attention has been paid to the aging‐related decline of inhibitory function, which reduces the ability to suppress distraction from irrelevant sensory input. In a response suppression paradigm, young and older adults listened to sequences of three short sounds during MEG recording. The amplitudes of the cortical P30 response and the 40‐Hz transient gamma response were compared with age, hearing loss and SIN performance. Sensory gating, indicated by the P30 amplitude ratio between the last and the first responses, was reduced in older compared to young listeners. Sensory gating was correlated with age in the older adults but not with hearing loss nor with SIN understanding. The transient gamma response expressed less response suppression. However, the gamma amplitude increased with age and SIN loss. Comparisons of linear multi‐variable modeling showed a stronger brain–behavior relationship between the gamma amplitude and SIN performance than between gamma and age or hearing loss. The findings support the hypothesis that aging‐related changes in the balance between inhibitory and excitatory neural mechanisms modify the generation of gamma oscillations, which impacts on perceptual binding and consequently on SIN understanding abilities. In conclusion, SIN recognition in older age is less affected by central auditory processing at the level of sensation, indicated by sensory gating, but is strongly affected at the level of perceptual organization, indicated by the correlation with the gamma responses.     

磁源性影像对母语为汉语者语言皮质定位的研究

目的 确定磁源性影像(MSI)对汉字处理脑皮质的定位价值。方法 对8例右利手及1例左利手母语为汉语的健康受试者给予双耳纯音及词义相关和不相关的成对汉字刺激。由脑磁图(MEG)设备记录刺激后产生的听觉诱发磁场。对采集的数据分别按照纯音、词义相关成对汉字及词义不相关成对汉字产生的反应进行叠加。将MEG资料叠加到MRI上获得MSI。结果 所有受试者左、右半球均诱导出2个明显高的磁反应波M50、M100。M50及M100均定位于双侧颞横回。同一受试者同侧半球对词义相关的成对汉字及不相关成对汉字的反应波近似。1例左利手受试者右侧半球在300～600ms有明显高的波峰，而左侧半球无此波峰。其MSI显示语言区位于Wernicke区。2例右利手受试者双侧半球均出现明显的300—600ms磁反应波。提示这2例受试者语言区位于双侧半球。其MSI显示语言区位于Wernicke区。6例右利手受试者双侧半球均诱导出潜伏期300～600ms的反应波，但左侧半球波幅明显高于右侧半球。这6例受试者MSI显示，语言区均定位于颞上回后部及颞中回后部，即Wernicke区。结论 比较左、右两侧半球磁反应晚成分(潜伏期300～600ms的反应波)，优势半球波幅明显高于非优势半球。对词义相关及不相关的成对汉字进行判断均能够确定大脑优势半球语言皮质的位置。     

Confirmation of Two Magnetoencephalographic Epileptic Foci by Invasive Monitoring from Subdural Electrodes in an Adolescent with Right Frontocentral Epilepsy

PURPOSE: To report our evaluation of interictal two epileptic spike fields on magnetoencephalography (MEG) by using invasive intracranial monitoring in a patient without lesion on magnetic resonance imaging (MRI). METHODS: A 15-year-old left-handed boy with a 9-year history of refractory simple partial seizures, secondarily generalized, and a normal MRI, was studied with MEG to define magnetic spike sources, followed by invasive intracranial monitoring with subdural electrodes to delineate the epileptogenic zone and eloquent function pursuant to focal cortical excision. RESULTS: MEG demonstrated two spike foci on the right middle frontal and inferior rolandic areas adjacent to the sensory area. Ictal recordings during prolonged invasive monitoring from subdural electrodes revealed two epileptogenic zones in the same locations as those defined by MEG. Focal cortical excision was performed of each epileptogenic zone. The patient has been seizure free for 24 months without neurologic deficit. CONCLUSIONS: Magnetic source imaging is a valuable adjunct in the planning of subdural grid placement in epilepsy surgery, particularly in patients in whom conventional imaging fails to reveal a lesion.     

Minor structural abnormalities in the infant face disrupt neural processing: A unique window into early caregiving responses

Infant faces elicit early, specific activity in the orbitofrontal cortex (OFC), a key cortical region for reward and affective processing. A test of the causal relationship between infant facial configuration and OFC activity is provided by naturally occurring disruptions to the face structure. One such disruption is cleft lip, a small change to one facial feature, shown to disrupt parenting. Using magnetoencephalography, we investigated neural responses to infant faces with cleft lip compared with typical infant and adult faces. We found activity in the right OFC at 140 ms in response to typical infant faces but diminished activity to infant faces with cleft lip or adult faces. Activity in the right fusiform face area was of similar magnitude for typical adult and infant faces but was significantly lower for infant faces with cleft lip. This is the first evidence that a minor change to the infant face can disrupt neural activity potentially implicated in caregiving.     

Mismatch negativity (MMN), the deviance-elicited auditory deflection, explained

The current review constitutes the first comprehensive look at the possibility that the mismatch negativity (MMN, the deflection of the auditory ERP/ERF elicited by stimulus change) might be generated by so-called fresh-afferent neuronal activity. This possibility has been repeatedly ruled out for the past 30 years, with the prevailing theoretical accounts relying on a memory-based explanation instead. We propose that the MMN is, in essence, a latency- and amplitude-modulated expression of the auditory N1 response, generated by fresh-afferent activity of cortical neurons that are under nonuniform levels of adaptation.     

Explicit processing of verbal and spatial features during letter-location binding modulates oscillatory activity of a fronto-parietal network

The present study investigated the binding of verbal and spatial features in immediate memory. In a recent study, we demonstrated incidental and asymmetrical letter-location binding effects when participants attended to letter features (but not when they attended to location features) that were associated with greater oscillatory activity over prefrontal and posterior regions during the retention period. We were interested to investigate whether the patterns of brain activity associated with the incidental binding of letters and locations observed when only the verbal feature is attended differ from those reflecting the binding resulting from the controlled/explicit processing of both verbal and spatial features. To achieve this, neural activity was recorded using magnetoencephalography (MEG) while participants performed two working memory tasks. Both tasks were identical in terms of their perceptual characteristics and only differed with respect to the task instructions. One of the tasks required participants to process both letters and locations. In the other, participants were instructed to memorize only the letters, regardless of their location. Time-frequency representation of MEG data based on the wavelet transform of the signals was calculated on a single trial basis during the maintenance period of both tasks. Critically, despite equivalent behavioural binding effects in both tasks, single and dual feature encoding relied on different neuroanatomical and neural oscillatory correlates. We propose that enhanced activation of an anterior-posterior dorsal network observed in the task requiring the processing of both features reflects the necessity for allocating greater resources to intentionally process verbal and spatial features in this task.     

Acute tryptophan depletion decreases intensity dependence of auditory evoked magnetic N1/P2 dipole source activity

Abstract Rationale. Intensity dependence of the N1/P2 components may be regulated by serotonergic neurons in the primary auditory cortex, where low activity leads to a high intensity dependence and vice versa. Depletion of tryptophan (TRP), a precursor for serotonin has been described to reduce serotonin content in brain of animals and humans. Objective. We investigated the intensity dependence of magnetic and electric N1/P2 components in ten subjects in a double-blind, controlled, cross-over design study after oral mixture of amino-acids leading to acute tryptophan depletion (ATD) and control. Methods. Auditory evoked magnetic fields (AEF) and potentials (AEP) were recorded with 122-channel magnetoencephalography simultaneously with 64-channel EEG 5 h after ingestion of mixtures. The AEF sources and strength were estimated by a least-squares fit of a single equivalent current dipole. The amplitudes and latencies of N1 and P2 recorded with EEG were analyzed at frontal electrode site. Results. TRP depletion decreased the total and free TRP levels by 76 and 45% and control mixture increased it by 48 and 28%. ANOVA showed that ATD had a significant main effect on the N1m/P2m dipole moments at the contralateral (P=0.02), but failed significantly to influence the ipsilateral responses. A significant mixture ingestion-by-stimulus intensity interaction was observed on the N1m/P2m dipole moments at the contralateral hemisphere (P=0.01). The N1/P2 slope for intensity dependence function was decreased following ATD compared with the control experiment (P=0.01) at the contralateral hemisphere. For EEG, a significant mixture ingestion-by-stimulus intensity interaction on the N1 latencies at the Fz electrode position was observed (P=0.01). Conclusion. ATD decreased the intensity dependence of N1m/P2m source dipole moments in the primary auditory cortex at the hemisphere contralateral to the ear stimulated. These results suggest that serotonin participates in the regulation of intensity of auditory stimulation. Electronic Publication     

Functional connectivity between forearm and digits representations in human somatosensory area 3b.

OBJECTIVE: We compared the effects of tactile interference to the forearm on magnetic responses evoked by electric stimulation of the little finger (D5) and the thumb (D1). METHODS: Electric stimulation was delivered to D5 or D1 individually. In each stimulus session, magnetic recordings were conducted with or without concurrent tactile interference to the radial side of the anterior forearm. RESULTS: With forearm interference, the amplitude of the primary response (N20m) following D5 stimulation was reduced to 90.7% of the control value without interference, while that following D1 stimulation was not affected (100.7%). CONCLUSIONS: In human somatosensory area 3b, the representation of the forearm is immediately adjacent to that of the D5, and distant from that of the D1. Thus, the result suggests that the tactile interference effect on N20m depends on the cortical distance between electrically and mechanically activated 3b areas. SIGNIFICANCE: Intrinsic synaptic connections between the 3b hand representation and its surroundings have been hypothesized as a neural basis for plastic changes of the human brain, such as a phantom hand phenomenon. The present finding implies that these connections may play some physiological roles even in normal adult humans.