A Subramanian, C M Dickinson
UMIST, Dept of Optometry and Neuroscience, Manchester, United Kingdom
Introduction:
In the presence of an absolute central scotoma, patients with macular pathologies may view a visual target centrally (when part will be obscured by the scotoma) or eccentrically (when performance may be reduced by the use of parafoveal retina). In reality the scotoma is more likely to be relative or patchy and it is then more common for the patient to persist with central fixation. This experiment was designed to find out which would be the best strategy for a two-line vernier acuity task in these different circumstances.
Methods:
Vernier acuity was measured in normal subjects with both an absolute and a patchy simulated scotoma from 2°to 25°in diameter created by obscuring the central part of the stimulus on the projection screen. Vernier acuity was also measured eccentrically by asking the observer to fixate in a position equivalent to placing the gap in the vernier lines at the edge of the scotoma: gaze could be shifted parallel to the target lines or orthogonal to them. The overall length of the two vernier line targets was always greater than the diameter of the scotoma, and the lines had a width of 30' and a gap separation of 30'.
Results and Conclusion:
Vernier acuity thresholds increased with increasing scotoma size for both the absolute and patchy scotoma, and with increasing eccentricity. Thresholds were lower with the patchy than an absolute scotoma of the same size, suggesting that seeing part of the target, even if incomplete, can be an advantage under some conditions. Thresholds were also better with central viewing, suggesting that if vernier acuity is considered as a measure of spatial localisation ability, patients performing such tasks may be better viewing centrally rather than eccentrically.
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