Contrast sensitivity testing.

SIR, A recent article' reports an important methodological improvement for contrast sensitivity testing with printed gratings derived from the Arden plates.2 Vaegan and Halliday' used a 4-alternative forced-choice procedure to reduce the influence of response bias, and presented results obtained in glaucoma. However, in discussing these results they inaccurately characterised our previous findings on contrast sensitivity defects in glaucoma.3-5 The inaccuracies have implications which concern not only testing methodology but also the pathophysiology of vision changes in glaucoma. Vaegan and Halliday' found that 'glaucoma patients ... have a contrast sensitivity deficit of about 6 db in relation to aged matched normal persons at all spatial frequencies when tested with a conventional oscilloscope display of static gratings' (p. 481).' They erroneously conclude (p. 481)' that their results 'disagree with the findings of Atkin et al. I,' and speculate (p. 490) that 'It is possible that the sample was too small or unrepresentative in many studies which do not find significant differences' (citing Atkin et al. 3 5). There actually is no disagreement. Our results with static patterns (unpublished) were similar to Veagan and Halliday's. ' The static stimuli, which were similar to those used in conventional contrast sensitivity methods,' 2 had in fact yielded significant differences between group means but had not shown a high enough ability to discriminate individual patients from normals to offer promise of a clinically useful test. Therefore in our original abstract we had said that 'sensitivity to the (non-flickering) grating . . did not distinguish between patients and controls',3 meaning that the groups showed such a large amount of overlap that the static-target method provided little useful information about individual patients. Consequently our subsequent papers4' reported only the results with flickering targets. With targets flickering at 8 Hz (a method not used by Vaegan and Halliday'), our studies revealed dynamic components of early glaucomatous vision changes4' which appeared to be more clinically useful than the static