Sources of electric brain activity: intracortical current dipoles.

Intracortical evoked potentials after electrical stimulation of the optic nerve were simultaneously recorded from the different layers of the rabbit's visual cortex. The averaged potentials were subjected to current source density analyses. The results show that the first event after the stimulus is a current sink in layer VI according to the excitatory terminations of the specific afferent fibres of the geniculate body. Since the largest part of the specific afferents terminates in layers IV and V, the sink in those layers may be due to excitatory synaptic events there. The corresponding current sources are situated in the upper layers and are most probably passive sources due to the active sinks. This example demonstrates that the generation of intracortical electric potentials can be explained by vertically oriented current dipoles according to the anatomical peculiarities.