Spatial resolution and the neural correlates of sensory experience.

Freeman and Baird [5; Freeman WJ, Baird B. Behav Neurosci 1987;101:393-408] recorded from the surface of the brain in waking rabbits and found spatial patterns of voltage that covaried with sensory experience. We simulate mathematically the electric fields produced by radial dipoles in cortical gyri and show that patterns with the spatial frequencies observed by Freeman and Baird could be produced by cortical dipoles spaced 3 mm apart. We further calculate that to resolve the patterns produced by such dipole arrays, it is necessary to record less than 2.5 mm above the surface of the cortex. High-pass spatial filters increase this distance to 4.5 mm. Since the human scalp is 15-16 mm above the brain, we conclude that spatial patterns of voltage covarying with sensation are unlikely to be detectable in scalp records. If such patterns do exist in humans, dural or sub-dural electrode arrays, with an inter-electrode spacing of 1 mm or less, will be necessary to record them.