It is possible, by using single-pulse high-amplitude magnetic stimulation, to directly activate the motor cortex (transcranial magnetic stimulation) and cervical spine segments, and to detect delays or lack of conduction in descending motor pathways. This technique, introduced by Marsden and associates, painlessly stimulates only the largest motor neurons (presumably Betz cells) and the fastest-conducting axons. Cervical magnetic stimulation is believed to activate the anterior roots. The difference in time between the motor cortical and cervical activation of hand or forearm muscles reflects the conduction velocity of the cortical–cervical cord motor neurons. The technique has been used to understand the organization, function, and recovery of the motor cortex and the pathophysiology of stroke, multiple sclerosis, and amyotrophic lateral sclerosis. Although the degree of functional deficit does not precisely correlate with the degree of electrophysiologic change, one expects that refinements of this technique may be useful in evaluating the status of the corticospinal motor system as well as other cortically based functions.
It is also possible to activate the motor (anterior) roots by magnetic stimulation and to measure the time required to elicit a muscle contraction (see review by Cros and Chiappa). These root stimulation tests can be quite uncomfortable for the patient as a result of the contraction of muscles surrounding the stimulation site. This technique finds its main use in diseases of the motor neuron, roots, and plexus.
Among the very late brain electrical potentials (>100-ms latency) that can be extracted from background activity by computer methods, are a group that cannot be classified as sensory or motor but rather as psychophysical responses to environmental stimuli. These responses are of very low voltage, often fleeting and inconsistent, and of unknown anatomic origin. The most studied types occur approximately 300 ms (P300) after an attentive subject identifies an unexpected or novel stimulus that has been inserted into a regular train of stimuli. Almost any stimulus modality can be used and the potential occurs even when a stimulus has been omitted from a regular pattern. The amplitude of the response depends on the difficulty of the task and has an inverse relationship to the frequency of the unexpected or “odd” event; the latency depends on the task difficulty and other features of testing. There is therefore no single P300; instead, there are numerous types, depending on the experimental paradigm. Prolongation of the latency is found with aging and in dementia as well as with degenerative diseases such as Parkinson disease, progressive supranuclear palsy, and Huntington chorea. The amplitude is reduced in schizophrenia and depression. The potential has been interpreted by some as a reflection of the subject’s orienting behavior or attention and by others, including Donchin, who discovered the phenomenon, as related to an updating of the brain’s representation of the environment. The P300 remains a curiosity for the clinical neurologist because abnormalities are detected only when large groups are compared to normal individuals, and the technique is not as standardized as the conventional evoked potentials. A review of the subject can be found in sections by Altenmüller and Gerloff and by Polich in the Niedermeyer and Lopes DaSilva text on electroencephalography.