ABSTRACT
OBJECTIVE: The source of somatosensory evoked high-frequency activity at about 600 Hz is still not completely clear. Hence, we aimed to study the influence of double stimulation on the human somatosensory system by analyzing both the low-frequency activity and the high-frequency oscillations (HFOs) at about 600 Hz. METHODS: We used median nerve stimulation at seven interstimuli intervals (ISIs) with a high time resolution between 2.4 and 4.8 ms to investigate the N15, N20 and superimposed HFOs. Simultaneously, the electroencephalogram and the magnetoencephalogram of 12 healthy participants were recorded. Subsequently, the source analysis of precortical and cortical dipoles was performed. RESULTS: The difference computations of precortical dipole activation curves showed in both the low- and high-frequency range a correlation between the ISI and the latency of the second stimulus response. The cortical low-frequency response showed a similar behavior. Contrarily, in the second response of cortical HFOs this latency shift could not be confirmed. We found amplitude fluctuations that were dependent on the ISI in the low-frequency activity and the HFOs. These nonlinear interactions occurred at ISIs, which differ by one full HFO period (1.6 ms). CONCLUSIONS: Low-frequency activity and HFOs originate from different generators. Precortical and cortical HFOs are independently generated. The amplitude fluctuations dependent on ISI indicate nonlinear interference between successive stimuli. SIGNIFICANCE: Information processing in human somatosensory system includes nonlinearity.
Subject(s)
Biological Clocks/physiology , Evoked Potentials, Somatosensory/physiology , Nonlinear Dynamics , Somatosensory Cortex/physiology , Thalamus/physiology , Action Potentials , Brain Mapping , Dose-Response Relationship, Radiation , Electric Stimulation/methods , Female , Functional Laterality , Humans , Magnetoencephalography/methods , Male , Neural Pathways/physiology , Reaction Time/physiology , Reaction Time/radiation effects , Somatosensory Cortex/cytology , Thalamus/cytology , Time FactorsABSTRACT
Directed information transfer in the human brain occurs presumably by oscillations. As of yet, most approaches for the analysis of these oscillations are based on time-frequency or coherence analysis. The present work concerns the modeling of cortical 600 Hz oscillations, localized within the Brodmann Areas 3b and 1 after stimulation of the nervus medianus, by means of coupled differential equations. This approach leads to the so-called parameter identification problem, where based on a given data set, a set of unknown parameters of a system of ordinary differential equations is determined by special optimization procedures. Some suitable algorithms for this task are presented in this paper. Finally an oscillatory network model is optimally fitted to the data taken from ten volunteers.
Subject(s)
Cerebral Cortex/physiology , Evoked Potentials, Somatosensory/physiology , Nerve Net/physiology , Thalamus/physiology , Electric Stimulation , Electromagnetic Fields , Humans , Magnetic Resonance Imaging/statistics & numerical data , Models, Neurological , Models, Statistical , Nonlinear Dynamics , Reproducibility of ResultsABSTRACT
Biomagnetic measurement performed outside a magnetically shielded room is subject to distortion by strong magnetic fields. Reducing such disturbances can enhance and stabilize biomagnetic measurement conditions in the absence of passive shielding. We have developed an active magnetic shielding system that produces both homogeneous and spatial gradient magnetic fields. The system is composed of anisotropic magnetoresistive sensors, a digital signal processor controller and two different coil systems. In order to improve the measurement environment for a first-order gradient coil SQUID system, the disturbing vertical magnetic fields and vertical field gradients are reduced, thus achieving a shielding factor of approximately 6 at 100 Hz. Our system provides a more flexible and less costly alternative to magnetically shielded rooms.
