Neuromagnetic Responses to Tactile Stimulation of the Fingers: Evidence for Reduced Cortical Inhibition for Children with Autism and Children with Epilepsy
Objectives: Our objective was to compare TP50m responses from a group of 15 children with ASD (mean age 9.95 ±1.23 SD years; 3 female), and a separate group of 17 children with EPI (mean age 10.57 ±1.72 SD years; 4 female) with TP50m responses recorded from a group of 15 age matched typically developing (TD) controls (mean age 10.21±1.61 SD years; 2 female). We hypothesized that separately, for children with EPI and children with ASD, we would observe decreased SEFTP50m response amplitudes to tactile stimulation of the digits.
Methods: MEG recordings were performed at the Lurie Family Foundations’ MEG Imaging Center of the Department of Radiology at the Children’s Hospital of Philadelphia in a magnetically shielded room using a whole-cortex 275-channel MEG system. Somatosensory stimuli were presented to the left and right index fingers sequentially using pneumatic pulses of compressed air (30 p.s.i.) delivered via clip-on balloon diaphragms. Stimulation duration was 40ms and jittered between 0.5 and 0.7s random ISI. Data were collected in epochs of -0.1 to 0.3s for a total of 500 trials. Source localization of the TP50m response yielded a TP50m response strength parameter (i.e. equivalent current dipole (ECD) moment) as well as a peak latency which we considered in our linear mixed model using subject as a random effect, group and hemisphere as fixed effects, and age as a covariate.
Results: We observed a significant overall group effect of reduced TP50m dipole moment F(2,41)=3.99, P<0.05. Independent post-hoc pair wise comparisons for dipole moment showed marginal means were significantly different comparing TD (21.3±1.8nAm) vs. ASD (15.4±1.9nAm), P=0.027 as well as TD vs. EPI (14.9±1.7nAm), P=0.014. In addition, we observed a significant overall group effect for TP50m peak latency F(2,41)=4.66, P<0.05. Post-hoc pair wise comparisons for dipole latency showed this to be driven by a latency delay of ~8ms in the EPI group: marginal means were significantly different comparing TD (50±2ms) and EPI (58ms±2ms), P=0.014, and not significant for TD vs. ASD (50±2ms), P=0.9.
Conclusions: We observed significant decreases in TP50m dipole moment values from the ECD source localized TP50m response, for children with EPI and children with ASD. In addition, the latency of the TP50m peak was observed to be equivalent between TD and ASD groups but was significantly delayed in children with EPI by approximately 8 ms. The failure to observe a latency difference in TD vs ASD is interesting in relation to the well-established finding of delayed auditory responses known to occur in children with ASD and will be the subject of future investigation.