To the Editor.—
Robinson et al (Arch Neurol 32:98, 1975) have made a unique methodological contribution toward understanding the effects of chronic electrode implantation on surrounding tissue. It would be unwise, however, to generalize their results to other animals (including man) or to accept, without further considerations, their conclusion about the safety of chronic electrical stimulation. Implanted electrodes damage the brain through a combination of effects: (1) Mechanical disruption of tissue continues, after implantation, by virtue of rotational acceleration acting on the head.1,2 In a small animal, such as the rat, with low brain mass and a small CSF-brain volume ratio, an electrode implanted in a relatively deep structure (such as the hippocampus) has advantages that are not available in larger animals or in the cortex. (2) Passive material properties, such as susceptibility to encapsulation by body tissues, are also important. It is not clear that enamel is a