Maturation of the Cortical Somatosensory Processing as Studied by MEG

Abstract

We present here a review of our studies on maturation of the somatosensory system during development. The data of most subjects have previously been published in several articles. Altogether 79 healthy newborns, 25 older infants (aged 6 months to 6 years), 13 adolescents (12 to 18 years), and 12 adults have participated in our studies. MEG was measured after electric stimulation of median nerve (MN) (some newborns and adults and all adolescents) or tactile stimulation of index finger (all age groups).

In newborns, the first response after MN stimulation peaks at 30 ms with anteriorly flowing underlying current, similar to the well-known adult N20m. The immaturity is seen in the subsequent prominent cortical responses from the primary somatosensory cortex (SI) both for tactile and electric stimulation: the direction of the equivalent current dipole (ECD) of the neonatal M60 continues to point anteriorly contrasting the posteriorly oriented ECD of P35m (MN stimulation) or M50 (tactile stimulation) in adults. In newborns, a later response, M200, localized more inferiorly and laterally than M60, most likely represents activity from the secondary somatosensory cortex (SII). M200 ECD is stronger in quiet than active sleep and with longer interstimulus intervals.

During the first 6 months of life, M60 develops into a Wshaped response with a notch separating two distinct peaks. In later infancy, this notch gradually increases in amplitude crossing the baseline and revealing an underlying ECD pointing posteriorly. By the age of 2 years it has developed into an adult-like M50 response, which then persists through childhood and adolescence.

MEG is a useful method to study the function and development of the somatosensory cortices from the newborn age, when SEFs are still characterized by several immature features. By 6 years of age the SEFs attain the adult-like pattern reflecting functional maturation of the somatosensory cortex.