FREEMAN, W. J. 
left intact. As soon as that was cut, hypothalam ically recorded elec- 
trical activity (before or after coagulation) was greatly diminished 
(Fig. 3, D, H). This implied that the tacit assumption we made, that 
the electrical changes were generated locally by the tissue in which 
the electrodes were located, was not tenable. 
This raised the question, then, where was this electrical activity 
coming from? By systematic mapping of these potentials in this 
volume of brain tissue, we found that the primary olfactory (pre- 
pyriform) cortex generated an electrical field accounting for them, 
and that the electromotive forces of the field lay in the molecular 
layer of the prepyriform cortex (Fig. 4). This was a dipole field, 
meaning that the surface and base of the cortex were at all times 
180 out of phase (Freeman, 1959). Such a dipole field existing in a 
volume conductor has no outside limits. In effect, this field spread 
throughout the brain and when its amplitude was high enough, or, 
correspondingly, when the amplitude of electrical activity of other 
structures was low enough, it could be detected in such distant points 
as the medulla, cerebellum, and temporal muscles. It was indeed a 
very powerful electrical field. The iso-potentials shown in Figure 
4 continued into the hypothalamus and indicated the existence of 
current flow from the cortex into this nuclear region. The spread 
of current is a passive process resulting from the occurrence of a 
difference in potential in the cortex with spread of currents in all 
directions, much as there is spread of sound from a source of noise 
throughout the limits of the confining space. These alternating cur- 
rents spread outwards in all directions, producing that gradient of 
potential seen in Figures 1 to 3. Examination of the lesions found 
to abolish activity recorded in the hypothalamus showed that the 
effective sections had severely damaged the prepyriform cortex in 
all cases. 
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