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Andrew James, Xin-Lin Goh, Simo Vanni; Pattern-pulse multifocal MEG mapping of human visual cortex using the general linear model. Journal of Vision 2007;7(9):232. doi: 10.1167/7.9.232.
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© ARVO (1962-2015); The Authors (2016-present)
Objective: Multifocal analysis allows characterization of responses to stimulation of multiple visual field regions by using concurrent stimulation with independent modulation sequences, and decomposition of the resulting compound responses into components due to each region. The introduction of multifocal pattern-pulse stimuli, and the efficient estimation of response components by the general linear model rather than cross-correlation, has increased the flexibility and power of multifocal analysis in the domain of visual evoked potentials . This study presents a multifocal pattern-pulse analysis for the magneto-encephalographic mapping of activity in human visual cortex.
Methods: An 84 region multifocal stimulus comprising 7 rings of 12 sectors extending to 24o eccentricity was defined, with pattern-pulse presentation of checkerboard stimuli at a mean rate of 2 per second to each region, driven by linearly independent sequences. Visual evoked fields were recorded with a Neuromag Vectorview whole-head device which houses 306 SQUID sensors in a helmet array. Ten subjects were recorded, for two repeats of a design presenting 484 pattern-pulses to each of the 84 regions, over 240 seconds, divided into 4 one-minute segments. Fitting of each response signal with the general linear model estimated the elementary response waveforms due to each of the 84 stimulus regions for each of the 306 recording channels.
Results & Discussion: Good signal to noise ratio was obtained from this design for all subjects, with magnetometer responses to single regions achieving peak values of over 200 fT on occipital channels, and gradiometers achieving peak values over 50 fT/cm on occipital channels. Replicability between repeats, measured as peak value divided by standard deviation between repeats indicated amplitude signal to noise ratios that often exceeded 10 on occipital channels, for both magnetometers and gradiometers.  James, AC (2003) IOVS
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