which can be recorded from a needle electrode inserted into the muscle when it is at rest. Polyphasic potentials are other electrical discharges that can be recorded from a previously denervated muscle that has become reinnervated by adjacent axons.

Vibrotactile Threshold

The vibrotactile threshold, a measure of sensory nerve function, is tested to evaluate the ability to perceive a vibrating stimulus. The test can be performed during a clinical examination with a tuning fork (128 Hz) placed over a toe or finger pad or joint of the foot, ankle, tibia, finger, or wrist. Subjects indicate to the examiner when they feel the vibration or when it diminishes and disappears. Testing can be performed with quantitative standardized methods; this is sometimes referred to as quantitative sensory testing (QST). One commonly used device, known as a Vibratron, assesses the function of large axons (fibers) of a peripheral nerve carrying the sensations of position and vibration. It has a stimulator that delivers vibrations of various amplitudes through two probes applied to the skin over a finger pad or an extremity joint with a constant frequency of vibration (100 Hz). With the so-called forced-choice method (choosing between two alternatives), subjects indicate when they perceive or do not perceive one or both probes vibrating. Testing of an exposed person soon after exposure and later, after removal from exposure, yields evidence of possible impairment and then recovery of vibrotactile sensation perception. Because height and age are known covariates of vibration threshold, epidemiologic studies control for height and age in the analysis and interpretation of results.

NEUROBEHAVIORAL EFFECTS

Neurobehavioral Tests

Neurobehavioral tests (also called neuropsychologic tests) are standardized tests designed to identify functional deficits associated with exposure to neurotoxicants. The tests also help to develop hypotheses about mechanisms of toxicity or localization of affected brain areas.

There are at least 250 distinct neurobehavioral tests, but they can be grouped under distinct domains of mental functioning. No test can be used alone to identify dysfunction as a result of a toxic exposure; rather, many tests are grouped into batteries to provide a broad characterization of a dysfunction (Fiedler et al., 1996). Some standardized test batteries have been developed (such as the WHO Neurobehavioral Core Test Battery and the Halstead-Reitan Battery). The batteries can be administered manually or by computer. Computer-administered tests consume less time and are less expensive to administer. The individual tests in a given battery are known as subtests. Summary scores for subtests in a battery can be used to determine the nature and degree of functional impairment.

One of the most important features of neurobehavioral testing is comprehensiveness of test selection. Given the wide array of tests available, batteries generally should include at least one test from each of the functional domains (Table F.1): overall cognitive ability; attention and concentration; motor skills; visuomotor coordination; visuospatial relations; memory; affect and personality. The functional domains to some extent overlap.



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