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Thom Carney, Justin Ales, Stanley A. Klein; Extending the multi-focal VEP method to complex stimuli. Journal of Vision 2006;6(6):427. doi: 10.1167/6.6.427.
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The VEP multi-focal technique provides a method to study eccentricity dependent perceptual effects. It has efficiency advantages over traditional methods by being able to simultaneously record neural responses at multiple target locations. In addition, the multi-focal method in combination with the sparseness of cortical folding is an effective method for studying visual areas with retinotopic organization. The typical multi-focal cortically scaled stimulus patches are each independently flickered according to an m-sequence. Cross correlation of the EEG signal with the m-sequence results in a series of response kernels for each electrode and each stimulus patch. Commonly the number of target locations and target complexity has been graphics hardware limited.
To overcome these limitations we have replaced standard look-up-table animation with true color graphic sprites to present arbitrary spatial patterns and record simultaneously from 192 patches each activating small areas of V1 (∼4x4mm). Moreover, adjacent patch cross kernels effectively multiply the cortical sampling density. Our use of 96 recording electrodes and small stimulus patches enables us to identify rapid changes in cortical folding of early visual areas enabling accurate source localization when combined with MRI methods. Finally, using sprites we can multiplex stimuli such that at each patch location multiple stimuli can be presented thereby improving signal-to-noise over sequential methods. For example, we have presented patterns with different orientations (or spatial frequencies) at the same patch locations but independently temporally modulated to study cell population interactions.
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