The method they used was to expose

The method they used was to expose a portion of the monkey’s cortex and flood it with a voltage-sensitive dye. The amount of dye taken up into a cell depends directly on that cell’s activity; the more impulses the cell fires, the more strongly it will be marked by the dye. They then filmed the pattern of the dye in the cortex as the monkey watched stripes of different orientations presented to either eye.