Awake state-specific suppression of primary somatosensory evoked response correlated with duration of temporal lobe epilepsy

Abstract

Epilepsy is a network disease. The primary somatosensory cortex (S1) is usually considered to be intact, but could be subclinically disturbed based on abnormal functional connectivity in patients with temporal lobe epilepsy (TLE). We aimed to investigate if the S1 of TLE is abnormally modulated. Somatosensory evoked magnetic fields (SEFs) evoked by median nerve stimulation were recorded in each hemisphere of 15 TLE patients and 28 normal subjects. All responses were separately averaged in the awake state and light sleep using background magnetoencephalography. Latency and strength of the equivalent current dipole (ECD) was compared between the groups for the first (M1) and second peaks. Latencies showed no significant differences between the groups in either wakefulness or light sleep. ECD strengths were significantly lower in TLE patients than in controls only during wakefulness. The reduction of M1 ECD strength in the awake state is significantly correlated with duration of epilepsy. SEFs of TLE patients showed pure ECD strength reduction without latency delay. The phenomenon occurred exclusively during wakefulness, suggesting that a wakefulness-specific modulator of S1 is abnormal in TLE. Repetitive seizures may gradually insult the modulator of S1 distant from the epileptogenic network.

Measurement of somatosensory evoked magnetic fields using an adjustable magnetoresistive sensor array

“…In SEF measurements using SQUID sensors with similar filter conditions, the maximum M20 amplitude was approximately 300 fT [14], [16]- [18]. The amplitude of 725 fT (519-1,134 fT) in this study was larger than that of the SEF measurements using SQUID sensors.…”

Scalp attached tangential magnetoencephalography using tunnel magneto-resistive sensors

“…We have already reported successful measurement of chest-attached magnetocardiography (MCG) without signal averaging techniques and scalp-attached MEG of the alpha wave, awake brain background rhythm, by signal averaging techniques 10 . Our combination of magnetic ux concentrators has nally reached to the sensitivity of the TMR sensor at the level of 940 fT/√Hz at 1 Hz and 50 fT/√Hz at 1 kHz, which are adequate to measure somatosensory evoked magnetic elds (SEFs), which are one of the weakest signals but used in most common applications of MEG [11][12][13][14] . Our present success with scalp-attached tangential MEG measurement will allow non-invasive imaging of human brain functions at millimeter and millisecond resolutions.…”