Searchlight mapping reveals brain regions whose activity patterns carry particular information
Subtle pattern information is sensitively detected by information-based searchlight mapping, but not by conventional activation mapping. The performance of activation- (black lines) and information-based mapping (colored lines) at detecting focally distributed effects (for color coding, see D) in simulated fMRI data. A shows an arrangement of four plots, where the top row displays average results obtained for large effect regions, and the bottom row displays average results obtained for small effect regions. The left and right columns display average results obtained for the two lower and the two higher functional contrast-to-noise ratios, respectively. A shows ROCs for the case of unsmoothed data. B shows the effect of spatial smoothing. The vertical axis here represents the area under the ROC. For the included case of no smoothing (i.e., full width at half maximum = 0), the areas under the ROC (marked as circles) correspond to the ROCs shown in A. Note that smoothing degrades performance for all techniques. For the crucial case of unsmoothed data, C summarizes the essential results by visually relating the detection performances afforded by the different techniques for small and large regions and low and high functional contrast-to-noise ratio. The searchlights yielding optimal performance in each case are shown in gray (4- or 5-mm radius). The circles in C replicate the circles in B reflecting the areas under the ROCs shown in A. In A and B, the line thickness measured vertically is 2 SEMs obtained by repeating the simulations and analyses 40 times with fresh noise. The shapes of the regions shown in green in A–C are exactly those used in the simulations. D illustrates the color coding in A–C and shows the searchlights used.