Figure 1. Electrode penetration through the external and internal regions of the globus pallidus and the optic tract. As the electrode advances from top to bottom, the activity of individual neurons (black dots) is recorded and analyzed within seconds. Neurons located in the external part of globus pallidus, just above the target, exhibit interrupted pattern of activity (short and long burst charts). They can fire at a rate of 30-100 times per second for a period of 0.5-1.0 seconds and then pause for a similar period of time. However, when the electrode advances to the internal part of the globus pallidus, neurons exhibit a continuous pattern of activity (continuous burst charts). They now fire at a rate of 30-180 times per second. These distinct differences enable identification of the desired site to be destroyed. (Courtesy of R. Burstein.)