Sleep Spindles Revisited

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Abstract
A few additional points about sleep spindles are made in this complementary article to the excellent manuscript on spindles by Jankel and Niedermeyer. One of these is to discuss the improbability of a necessary close relationship between any two patterns just simply because they represent fast activity, waxing and waning in amplitude, therefore showing some morphological similarity to the sleep spindle. In this context, one example may be the delta brushes of the premature infant. One fascinating type of sleep spindle appears at approximately 3 to 5 months of age, the very long-lasting form that is usually less monophasic than the type that develops in the second half of the first year of life. The monophasic form often is asynchronous between the two sides during the first year and occasionally may appear in this form from 1 to 2 years of age, but such asynchrony should not appear in the normal 2 year old. Caution is urged in calling those records without any spindles abnormal, except when the interpreter can be certain that the “spindle stage” was actually achieved. Also, the normal 20–28/s rhythms of the drowsy child under 6 years of age should not be confused with the effect of medication, especially chloral hydrate, which likely has little or no fast activity associated with it. Relevant infrahuman data have revealed that adult monkeys do not usually show sleep spindles but show the closely related vertex sharp transients maximal on the cingulate area inferior to the supplementary motor region. Evidence for the thalamic control of spindles can be traced back over 40 years; attempts to relate thalamic and scalp activity must take into consideration the obvious points of amplitude ratio and electrode positioning. Spindles long in duration and large in amplitude can be seen in various conditions, including deaf children. Recent evidence has shown that the generators of spindles produce vertical, not tangential, current flow over the cortex. Also, the pathways producing the sleep spindle are likely to be the same as those that produce the bilaterally synchronous spike and wave complex.

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