PURPOSE. Several conventional clinical procedures such as the Jackson Cross Cylinder and dynamic cross cylinder (DCC) tests involve accommodation to astigmatic stimuli. In the presence of a finite interval of Sturm, the resulting accommodative response may either place one of the focal lines close to the retina, or alternatively position the circle of least confusion (COLC) at the retinal plane. While it has been suggested that having the COLC coincident with the retina is preferable, there is little evidence that this actually occurs. Accordingly, the present study examined the accommodative response to a range of targets in subjects having induced astigmatism. METHOD. The study was carried out on 26 young, visually-normal subjects. Astigmatism was induced using toric soft contact lenses having cylindrical powers up to 1.75D. Monocular, steady-state accommodation was measured objectively with an infra-red optometer, while subjects viewed one of 4 targets located at a viewing distance of 40cms, namely: (i) a starburst, (ii) Verhoeff circle, (iii) a row of letters or (iv) an orthogonal line target similar to that used conventionally for the DCC procedure.
RESULTS. For all of the targets tested, the mean accommodative response resulted in the anterior focal line being no more than 0.46D behind the retina. However, the mean COLC position was always more hyperopic than the anterior focal line. No significant difference between the mean accommodative responses for the 4 targets was found.
CONCLUSIONS. Subjects appear to invoke the minimum accommodative response necessary to place the anterior focal line within the depth-of-focus of the eye. Under none of the conditions tested was the COLC positioned on or close to the retina. Accordingly, the theory underlying those clinical procedures which necessitate having the COLC on the retina may need to be re-evaluated.