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Higher-order monochromatic aberrations in the human eye cause a difference in the appearance of stimuli at distances nearer and farther from best focus that could serve as a signed error signal for accommodation. We explored whether higher-order monochromatic aberrations affect the accommodative response to 0.5 D step changes in vergence in experiments in which these aberrations were either present as they normally are or removed with adaptive optics. Of six subjects, one could not accommodate at all for steps in either condition. One subject clearly required higher-order aberrations to accommodate at all. The remaining four subjects could accommodate in the correct direction even when higher-order aberrations were removed. No subjects improved their accommodation when higher-order aberrations were corrected, indicating that the corresponding decrease in the depth of field of the eye did not improve the accommodative response. These results are consistent with previous findings of large individual differences in the ability to accommodate in impoverished conditions. These results suggest that at least some subjects can use monochromatic higher-order aberrations to guide accommodation. They also show that some subjects can accommodate correctly when higher-order monochromatic aberrations as well as established cues to accommodation are greatly reduced.

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SUMMARY Objective infrared recording devices were used to measure simultaneously and continuously both accommodation and accommodative vergence to near stimuli in 4 subjects with intermittent exotropia of the divergence-excess type (2 simulated and 2 true). In addition standard clinically determined stimulus accommodative convergence to accommodation (AC/A) ratios were measured. Results showed the mean group response AC/A ratio to be 5-9/1 (range 4 5-8 0/1) with no differences between true and simulated divergence-excess. Similar AC/A ratios were found after 45 minutes of monocular occlusion. Our results clearly demonstrate relatively normal response AC/A ratios in these subjects. Thus, contrary to what is believed by many clinicians, the reduced ocular deviation at near compared with distance vision cannot be attributed primarily to an abnormally high AC/A ratio. We believe that fusional convergence after-effects and/or proximal convergence effects contribute to inflate the clinically determined stimulus AC/A ratios.The divergence excess type of intermittent exotrope, with the deviation being greater at distance than at near vision, ' is commonly believed to have a very high accommodative convergence to accommodation (AC/A) ratio.25 This notion has been based largely on distance/near measurements of the horizontal oculomotor exodeviation (i.e., tropia or phoria). The average deviation is 29 A at far and 9 A at near,6 resulting in an average stimulus AC/A ratio of approximately 14-0/1. However, reported values of the AC/A ratios in these patients show much variation. Ogle et al.7 found the mean stimulus AC/A ratio to be 3 0/1 using the fixation disparity method.8 Moore et al.,' grouping all exotropes (divergence excess, convergence insufficiency, and basic exotropes) together (45% were the divergence excess type), found the mean stimulus gradient AC/A ratio to be 4-7/1. Calculations from Brown' suggest presence of a stimulus AC/A ratio of 13-0/1 or greater based on the +3-00 dioptre gradient technique. Von Noorden'°found a wide range of stimulus AC/A ratios (3-3 to 9-0) using the +3-00 dioptre gradient technique in a large sample of patients. In 2 cases presented by Burian Correspondence to Dr Kenneth J. Ciuffreda.and Smith" gradient stimulus AC/A ratios were 6-0/1 and 8-0/1, while they were 14-0/1 by the distance/near phoria method.There are 2 basic problems with the above mentioned studies. First, only stimulus AC/A ratios were determined. One is never certain of the actual accommodative response but rather assumes it to be equal to the stimulus demand. This assumption results in an