The next level of analysis is the recording of field potentials. This method uses a recording electrode with a smaller tip and a higher resistance than that for EEG electrodes, and the electrophysiological measures are confined to a small population of neurons surrounding the electrode tip. This technique still depends on the simultaneous activation of a number of cells; however, because the electrode is inserted into the brain, the cells do not have to be at the surface of the skull as in the EEG recordings. Furthermore, the activation can consist of stimulation of an afferent pathway. Nonetheless, the array of neurons sampled must have a common orientation for the massed activity to be measurable. Therefore, such measures are typically restricted to cortical structures such as the neocortex and hippocampus. With this method, the current resulting from the parallel activation of excitatory and inhibitory afferents can be measured as well as the electrophysiological response of a population of neurons to such stimulation.
On the other hand, even in structures without a parallel orientation of neurons, field recordings have been useful in evaluating changes in afferent synaptic strength. For example, field recordings have been particularly effective in examining phenomena manifesting synaptic plasticity, such as long-term potentiation. By measuring the fields generated by stimulating afferent inputs, one can evaluate how effective synaptic pathways are in driving a region and how this effectiveness can be modified by experience. Therefore, learning paradigms, behavioral conditioning, and drug sensitization have all been shown to alter evoked field potentials within functional pathways in the brain (e.g., Goto and Grace 2005b).
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