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Relevance of the AC-A ratio.

It is obvious that a symposium of this kind on the subject of strabismus is concerned to a large extent with the growing-points of the specialty, and this is entirely appropriate when the meeting is held within the confines of St. John's College, Cambridge, a University centre of learning and academic progress for centuries, and abundantly emphasized by the link of this particular building with Pythagoras. The concept of the AC/A ratio, that is, the amount of accommodative-convergence (AC) which is induced by a change in accommodation (A), is a relatively recent one, but it must be appreciated that the justification to regard it as a growing point is essentially a measure of emphasis, not one of uniqueness, because the close link which exists between accommodation and convergence has been acknowledged since the time of Donders (I864). It should be mentioned at this stage that the use of the term "convergence" is a composite one because it includes true convergence (positive convergence) and divergence (negative convergence). It should also be mentioned that a discussion on the AC/A ratio is concerned with only one aspect ofreflex convergence, and that other reflex mechanismstonic, proximal, and fusional-play important roles in the vergence of the eyes, particularly the fusional mechanism which controls the amount of convergence which may be exerted or relaxed during the maintenance of a constant amount of accommodation (the relative convergence) and this has positive and negative components (positive relative fusional convergence and negative relative fusional convergence). In this description of relative convergence, it is logical to take note of the concept of relative accommodation which implies the amount of accommodation (positive and negative) which may occur without any apparent change in convergence. It is evident that something akin to relative accommodation must take place to some extent in the presence of an uncorrected refractive error, except in the rare event when the amount ofaccommodative change is equal to the amount of vergence change required to control an underlying heterophoria, but it has been suggested that there is no true relative accommodation because any change in accommodation inevitably initiates a change in vergence, although this is immediately blocked by the influence of the positive or negative fusional convergence mechanism in order to maintain bifoveal fixation. It must be conceded, however, that this somewhat controversial point is largely of academic interest. The concept of the AC/A ratio arose out of an attempt to obtain a precise understanding of the relationship between the two components of the ratio accommodative-convergence and accommodation-but this has proved to be a difficult task because of the complex nature of the relationship and it is scarcely surprising that several methods of determination have been put forward.