Spatial Resolution and the Neural Correlates of Sensory Experience

Abstract

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.

Consciousness Is a Thing, Not a Process

“…All of the empirical reports cited above involve the back-action of brain-generated fields on the very neurons that generated those fields in the first place. This gels with the fact that the brain-generated fields proposed by the EM field theory of consciousness as being conscious [70] involve electric dipoles, which means that the strength of the field falls off not with the square of distance from their site of generation, but with the cube of distance from their site of generation [71]; which in turn means that these fields essentially cease to exist as spatial patterns within a few mm of their site of generation [72]. This fact has two important consequences: (1) in practical terms, in order to measure such patterned EM fields the measuring electrode has to be located not more than a couple of mm away from the site of generation of the field; but also (2) given that the neurons that initiate bodily actions generally reside much further than a couple of mm from the site of generation of putatively conscious EM patterns, it is hard to see how such patterns could be the proximal cause of bodily actions.…”

“…This led [72] to the prediction that the three-dimensional LFP pattern that is produced by cortico-cortical synapses on motor cortical pyramidal cells might differ from that which is produced by cortico-cortical synapses on sensory pyramidal cells as shown in Figure 2. …”

“…In order to measure the spatial patterns that are proposed above as constituting consciousness, physics dictates that it is necessary to record from microelectrodes that are positioned less than a mm from the synapses producing any given section of the pattern [72]. In practice, this means recording directly from the neocortex of waking subjects, using 3-dimensional arrays of electrodes consisting of a large number of multi-contact shanks that are arranged in a closely spaced grid.…”

Difficulties with the Electromagnetic Field Theory of Consciousness: An Update

“…The frequency of these spatial patterns was approximately 0.2 cycles/mm. We have now applied the standard math of electromagnetic fields to show that, if the patterns we are looking for in humans have a spatial frequency similar to that of the rabbit patterns (which is not unlikely, given that both the anatomical types of neuron in the brain and the diameter of the columnar unit of corticocortical afferent terminations remain roughly the same over a wide range of species (Szentagothai, 1977)) then the patterns we are looking for are likely to be generated by dipoles spaced approximately 3 mm apart in the human cortex (Pockett et al, 2007). Further calculations show that in order to resolve the spatial electromagnetic patterns produced by dipoles spaced 3 mm apart, the array of recording electrodes has to be less than 2.5 mm above the dipoles.…”

“…In fact, the more I learn about motor control, the less likely it seems that motor acts could be controlled in any way at all by an instrument as blunt as a field. As shown by the calculations mentioned above (Pockett et al 2007), the spatial pattern characterizing a local field smears and spreads with distance according to Maxwell's Laws so rapidly that the pattern is unrecognizable even a few mm away from its neuronal source. Certainly there is zero chance that a spatial electromagnetic pattern generated in the frontal lobes (where decisions are made) could directly affect motor neurons in the spinal cord, as suggested by McFadden (2002).…”