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Problems in the Measurement of Cortical Evoked Potentials

The term "evoked potential" denotes a rather loosely defined class of electrical phenomena in the central nervous system that follow impulse stimulation either of sensory receptors, as by clicks and light flashes, or of the brain itself, electrically. These tantalizing phenomena can be recorded from the reasonably intact brains of otherwise normal animals including man during controlled sequences of behavior. They are tantalizing because they come closer to direct measurement of activity in large pools of central neurons than do the results of any other mode of observation yet devised, and yet they have so far defied systematic analysis. No one knows what they mean. THE NEED FOR MEASUREMENT Basically, an evoked potential is a test for excitability. If a controlled stimulus is applied to a given structure, what is the electrical output, and, further, what does the electrical output imply about the physiological state of the structure? If the wave form changes during delivery of a series of stimuli to the system, has the excitability changed, and, if so, how is it different and what made it differ? The task of finding answers to these and other questions consists of making measurements on gross changes of potential with time, and then making inferences from these about the operations of a neuron pool in terms of its contribution to behavioral function. The problems here are as difficult as any faced in modern science. The major requirement for analysis of evoked potentials is the availability of a precise system for measurement. The techniques used by electronics specialists for "signal" identification [1,6] are admirably suited to this task. All that is required is the use of a general purpose computer such as the IBM 7090 or its equivalent in electronic hardware. These techniques have now been applied to evoked potentials recorded from the olfactory (prepyriform) cortex of cats following electrical stimulation of the lateral olfactory tract. This is an appropriate structure for this first attempt for a number of reasons. This cortex has three layers in contrast to neocortex, and its electrical responses are comparatively uncomplicated. The response usually takes the form of a dipole field of potential with a clean-cut zero isopotential or "turnover" at the junction of the molecular and pyramidal cell layers [4]. This permits the response to be isolated electrically by placing one electrode in the molecular layer and another electrode in the cell layer. Recording between these …