TONIC ACCOMMODATION - A REVIEW .1. BASIC ASPECTS

In the absence of an adequate visual stimulus, accommodation adopts an intermediate position of almost-equal-to 1 D. Since this position was believed to reflect the level of tonic innervation to the ciliary mus cle, this response has been termed tonic accommodation (TA). Part I of this review will consider various aspects of this parameter, includin g its reference to closed-loop accommodative function and autonomic ph ysiology. In addition, both the methods of measurement and appropriate terminology for this function will be discussed. It is concluded that the response, which becomes apparent under so-called 'stimulus-free' conditions, in fact probably represents an aggregate response resultin g from multiple, non-optical stimuli. Thus the designation tonic accom modation may not be appropriate, since it fails to describe accurately the heterogeneous composition of the stimulus-free accommodative resp onse. An associated paper (to be published as part II of this review) will examine accommodative adaptation and both clinical aspects of TA and adaptation of TA.