Abstract
Our previous studies have demonstrated that Morlet-wavelet transform with an extra large sigma value could precisely determine the frequency signatures of neuromagnetic signals. Unfortunately, the increase of frequency sensitivity is associated with a decrease of temporal resolution. To solve this problem, we have developed wave-cross analysis to quantify neuromagnetic signals with both high temporal and frequency resolutions. The objective of the present study is to measure the time and frequency signatures of visual cortical activation in children using this new method. Twelve healthy children were studied with a whole-head magnetoencephalography (MEG) system. Visual evoked magnetic fields (VEFs) were evoked with full-field pattern-reversal checks. MEG data were transformed from time-domain to frequency domain using wave-cross spectrogram. Neuromagnetic sources were volumetrically localized with a wavelet-based beamformer. Three response peaks were identified at 73±7 ms (M75), 111±8 ms (M100) and 149±12 ms (M145). The latency of M75 and M100 decreased with age. The amplitude of M75 decreased with age while the amplitude of M100 increased with age. The amplitude ratio of M100/M75 increased significantly with age. The frequency bands up to 300 Hz have been identified. The frequency in the M73 appeared higher than M145, the later responses. In addition, the frequency signatures of the neuromagnetic signals also changed with age. The temporal and frequency signatures of the development of visual function in childhood are noninvasively quantifiable with MEG and Wave-cross. Our preliminary data have confirmed that wave-cross, a new time-frequency analysis method, could precisely determine the frequency and temporal signatures of brain activation. The results lay a foundation for quantitative identification of developmental delay and/or abnormalities of visual function in children with brain disorders.
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