Päivi Nevalainen

Maturation of somatosensory cortical processing from birth to adulthood revealed by magnetoencephalography.

OBJECTIVE To evaluate the maturation of tactile processing by recording somatosensory evoked magnetic fields (SEFs) from healthy human subjects. METHODS SEFs to tactile stimulation of the left index finger were measured from the contralateral somatosensory cortex with magnetoencephalography (MEG) in five age groups: newborns, 6- and 12-18-month-olds, 1.6-6-year-olds, and adults. The waveforms of the measured signals and equivalent current dipoles (ECDs) were analyzed in awake and sleep states in order to separate the effects of age and vigilance state on SEFs. RESULTS There was an orderly, systematic change in the measured and ECD source waveforms of the initial cortical responses with age. The broad U-shaped response in newborns (M60) shifted to a W-shaped response with emergence of a notch by 6 months of age. The adult-type response with M30 and M50 components was present by 2 years. The ECDs of M60 and M30 were oriented anteriorly and that of M50 posteriorly. These maturational changes were independent of vigilance state. CONCLUSIONS The most significant maturation of short latency cortical responses to tactile stimulation takes place during the first 2 years of life. SIGNIFICANCE The maturational changes of somatosensory processing can noninvasively be evaluated with MEG already in infancy.

Immaturity of somatosensory cortical processing in human newborns.

The development of the early component of somatosensory evoked potentials (SEPs) from the neonatal N1 to adult N20 response has previously been described. The main emphasis has been on the change in the response latency during maturation. We used magnetoencephalography (MEG) to characterize the cortical generators of the N1 and the subsequent response in healthy human newborns. Furthermore, we studied the maturation of tactile processing according to responses evoked by tactile stimulation of the index finger in newborns, 6-month-old babies and adults. This study provides evidence of specific differences in the somatosensory processing in neonates compared to that in adults. Although the initial cortical response to electrical median nerve stimulation in the newborns was similar in field distribution to the corresponding N20m in adults, the subsequent major deflection in the response waveform had the opposite polarity. Similar immaturity in cortical processing was seen in the tactile evoked fields in both the newborns and the 6-month-old infants compared with the adults. Our results indicate that although the somatosensory pathway in full-term newborns is sufficiently developed to supply the brain with tactile information, the cortical neuronal networks for processing the input may not function in the same way as in adults.

Development of human somatosensory cortical functions - what have we learned from magnetoencephalography: a review

The mysteries of early development of cortical processing in humans have started to unravel with the help of new non-invasive brain research tools like multichannel magnetoencephalography (MEG). In this review, we evaluate, within a wider neuroscientific and clinical context, the value of MEG in studying normal and disturbed functional development of the human somatosensory system. The combination of excellent temporal resolution and good localization accuracy provided by MEG has, in the case of somatosensory studies, enabled the differentiation of activation patterns from the newborn’s primary (SI) and secondary somatosensory (SII) areas. Furthermore, MEG has shown that the functioning of both SI and SII in newborns has particular immature features in comparison with adults. In extremely preterm infants, the neonatal MEG response from SII also seems to potentially predict developmental outcome: those lacking SII responses at term show worse motor performance at age 2 years than those with normal SII responses at term. In older children with unilateral early brain lesions, bilateral alterations in somatosensory cortical activation detected in MEG imply that the impact of a localized insult may have an unexpectedly wide effect on cortical somatosensory networks. The achievements over the last decade show that MEG provides a unique approach for studying the development of the somatosensory system and its disturbances in childhood. MEG well complements other neuroimaging methods in studies of cortical processes in the developing brain.

Somatosensory evoked magnetic fields from the primary and secondary somatosensory cortices in healthy newborns

Although brain development has been actively investigated in animals, maturation of the cerebral cortex in human newborns is still poorly understood. This study aimed at characterizing the cortical areas participating in tactile processing in human neonates. Somatosensory-evoked magnetic fields were recorded from 21 healthy full-term newborns during natural sleep. Altogether, four cortical areas were activated by tactile stimulation: the contra- and ipsilateral primary (SI) and secondary (SII) somatosensory cortices. The contralateral SI was activated first in all the newborns. This early activity was not affected by the interstimulus interval or the sleep stage. The contralateral SII activation at around 200 ms was prominent in quiet sleep (QS) but attenuated in active sleep (AS). Activity in this area was strongly depressed by a faster rate of stimulation. Ipsilateral activity was seen in most subjects: more often in QS than AS. The ipsilateral activity originated from SII in most babies, but in some the ipsilateral SI was also activated. We conclude that both the contra- and ipsilateral SI and SII can participate in the processing of somatosensory information in human neonates.

Does very premature birth affect the functioning of the somatosensory cortex? — A magnetoencephalography study

Increased survival of extremely low birth weight infants has led to a need for new prognostic methods to predict possible future neurological impairment. We investigated the early development of the somatosensory system by recording the somatosensory evoked magnetic fields (SEFs) during natural sleep at fullterm age in 16 very prematurely born infants and 16 healthy newborns born at term. The purpose was to determine possible changes in the function of the somatosensory cortex in the prematurely born infants by comparing the latency, strength, location and morphology of the SEFs with those of healthy fullterm newborns. We recorded reliable SEFs in all patients and controls. The equivalent current dipole (ECD) strength of the first cortical response, M60, was significantly lower in the patients. Otherwise, the general morphology and latency of the SEFs were similar in the two groups of babies. The similar response latencies in the two groups indicate normally developed conduction in the somatosensory system of the prematurely born infants. The attenuated ECD strength may reflect weaker synchrony in firing or a smaller number of the cortical neurons activated by the somatosensory stimulation. At the individual level, in four of the preterm infants, a later M200 response was not present or could not be modeled: all of them had lesions of the underlying hemisphere depicted by ultrasound and magnetic resonance imaging.

Bilateral alterations in somatosensory cortical processing in hemiplegic cerebral palsy

Aim  In individuals with cerebral palsy (CP), cerebral insults during early development may induce profound reorganization of the motor representation. This study determined the extent of alterations in cortical somatosensory functions in adolescents with hemiplegic CP with subcortical brain lesions.

Cortical somatosensory processing measured by magnetoencephalography predicts neurodevelopment in extremely low-gestational-age infants

Background:Higher cortical function during sensory processing can be examined by recording specific somatosensory-evoked magnetic fields (SEFs) with magnetoencephalography (MEG). We evaluated whether, in extremely low-gestational-age (ELGA) infants, abnormalities in MEG-recorded SEFs at term age are associated with adverse neurodevelopment at 2 y of corrected age.Methods:SEFs to tactile stimulation of the index finger were recorded at term age in 30 ELGA infants (26.5 ± 1.2 wk, birth weight: 884g ± 181 g). Neurodevelopment was evaluated at 2 y of corrected age. Controls were 11 healthy term infants.Results:In nine of the ELGA infants (30.0%), SEFs were categorized as abnormal on the basis of lack of response from secondary somatosensory cortex (SII). At 2 y, these infants had a significantly worse mean developmental quotient and locomotor subscale on the Griffiths Mental Development Scales than the ELGA infants with normal responses. Mild white matter abnormalities in magnetic resonance imaging at term age were detected in 21% of infants, but these abnormalities were not associated with adverse neurodevelopment.Conclusion:Abnormal SII responses at term predict adverse neuromotor development at 2 y of corrected age. This adverse development may not be foreseen with conventional neuroimaging methods, suggesting a role for evaluating SII responses in the developmental risk assessment of ELGA infants.

Spatiotemporal Dynamics of the Processing of Spoken Inflected and Derived Words: A Combined EEG and MEG Study

The spatiotemporal dynamics of the neural processing of spoken morphologically complex words are still an open issue. In the current study, we investigated the time course and neural sources of spoken inflected and derived words using simultaneously recorded electroencephalography (EEG) and magnetoencephalography (MEG) responses. Ten participants (native speakers) listened to inflected, derived, and monomorphemic Finnish words and judged their acceptability. EEG and MEG responses were time-locked to both the stimulus onset and the critical point (suffix onset for complex words, uniqueness point for monomorphemic words). The ERP results showed that inflected words elicited a larger left-lateralized negativity than derived and monomorphemic words approximately 200 ms after the critical point. Source modeling of MEG responses showed one bilateral source in the superior temporal area ∼100 ms after the critical point, with derived words eliciting stronger source amplitudes than inflected and monomorphemic words in the right hemisphere. Source modeling also showed two sources in the temporal cortex approximately 200 ms after the critical point. There, inflected words showed a more systematic pattern in source locations and elicited temporally distinct source activity in comparison to the derived word condition. The current results provide electrophysiological evidence for at least partially distinct cortical processing of spoken inflected and derived words. In general, the results support models of morphological processing stating that during the recognition of inflected words, the constituent morphemes are accessed separately. With regard to derived words, stem and suffix morphemes might be at least initially activated along with the whole word representation.

Reactivity of sensorimotor oscillations is altered in children with hemiplegic cerebral palsy: A magnetoencephalographic study

Cerebral palsy (CP) is characterized by difficulty in control of movement and posture due to brain damage during early development. In addition, tactile discrimination deficits are prevalent in CP. To study the function of somatosensory and motor systems in CP, we compared the reactivity of sensorimotor cortical oscillations to median nerve stimulation in 12 hemiplegic CP children vs. 12 typically developing children using magnetoencephalography. We also determined the primary cortical somatosensory and motor representation areas of the affected hand in the CP children using somatosensory‐evoked magnetic fields and navigated transcranial magnetic stimulation, respectively. We hypothesized that the reactivity of the sensorimotor oscillations in alpha (10 Hz) and beta (20 Hz) bands would be altered in CP and that the beta‐band reactivity would depend on the individual pattern of motor representation. Accordingly, in children with CP, suppression and rebound of both oscillations after stimulation of the contralateral hand were smaller in the lesioned than intact hemisphere. Furthermore, in two of the three children with CP having ipsilateral motor representation, the beta‐ but not alpha‐band modulations were absent in both hemispheres after affected hand stimulation suggesting abnormal sensorimotor network interactions in these individuals. The results are consistent with widespread alterations in information processing in the sensorimotor system and complement current understanding of sensorimotor network development after early brain insults. Precise knowledge of the functional sensorimotor network organization may be useful in tailoring individual rehabilitation for people with CP. Hum Brain Mapp 35:4105–4117, 2014.

Evaluation of somatosensory cortical processing in extremely preterm infants at term with MEG and EEG

OBJECTIVE Prior studies on extremely preterm infants have reported long-term prognostic value of absent secondary somatosensory cortex (SII) responses in magnetoencephalography (MEG) at term. The present work (i) further examines the potential added value of SII responses in neonatal neurological evaluation of preterm infants, and (ii) tests whether SII responses are detectable in routine neonatal electroencephalogram complemented with median nerve stimulation (EEG-SEP). METHODS Altogether 29 infants born <28 gestational weeks underwent MEG, MRI, and neonatal neurological examination at term age, and Hempel neurological examination at 2-years corrected age. Term-age EEG-SEP was available for seven infants. RESULTS While in neonatal neurological examination severely abnormal finding predicted unfavorable outcome in 2/2 infants, outcome was unfavorable also in 3/9 (33%) moderately abnormal and in 5/18 (28%) mildly abnormal/normal infants. Of these eight infants four had unilaterally absent SII responses in MEG, compared with only two of the 24 infants with favorable outcome. Furthermore, SII responses (when present in MEG) were also usually detectable in EEG-SEP. CONCLUSIONS Complementing clinical EEG recording with SEP holds promise for valuable extension of neonatal neurophysiological assessment. SIGNIFICANCE Multimodal study of EEG and sensory evoked responses is informative, safe, and cheap, and it can be readily performed at bedside.