Beaming signal sources in measurement of focal visual evoked cortical potentials

VECPs (visual evoked cortical potential) after focal stimulation used in perimetry have weak amplitudes in comparison to the spontaneous EEG, thus the SNR (signal-to-noise ratio) falls off down to -20 dB and less. The shape of the VECP waves depends on several parameters and is unknown in general. Then for SNR enhancement and signal detection shape-independent methods can be used only. The most common of them is the stimulus synchronized averaging, which causes cumulative prolongation of the measurement time corresponding to the averaging order. For online measurements of VECP other ways in signal improvement are needed. In this paper a new method for SNR enhancement based on beam forming is introduced. While the anatomical structures of sources generating the focal VECP are known roughly and the electrode positions have sufficient density over the visual cortex, signal sources can be focused by controlling the channel delay.