Abstract
Purpose :
Studies have shown that humans perform in the hyperacuity range for detecting deformations of closed-contour shapes such as “radial frequency” (RF) patterns, an ability that is not affected by normal aging. Such tasks are thought to invoke global mechanisms that are highly efficient at integrating local information from around the entire contour. Shape discrimination has therefore the potential to provide useful information about visual performance in patients with glaucoma given the reduced sensitivity in certain areas of the visual field. The aim of this study was to determine how shape discrimination and global pooling are affected in glaucoma with a view to determining the potential of a shape discrimination task as a method of detecting early visual loss.
Methods :
Discrimination of RF patterns from circles was measured in 16 eyes with glaucoma (mean age 66.7±5.2 years) and in a control group of 12 age-matched normal eyes (65.2±4.3 years). An adaptive staircase procedure was employed in a two-interval forced choice design. RF patterns had 5 lobes (RF5) and discrimination was measured for pattern radii of 3, 5 and 10 degrees. Stimuli had a peak spatial frequency of 4 cycles per degree and were displayed at maximum contrast with a presentation time of 500ms. All participants underwent a full ophthalmic examination including IOP and perimetry (Central 24-2 SITA FAST, Humphrey VFA).
Results :
For all pattern sizes, discrimination thresholds were significantly higher (i.e. performance was poorer) in the glaucoma group compared to the control group (p <0.01). There was no significant difference in performance for the different pattern radii within either group (p =0.785 for glaucoma, p =0.219 for normals). For patterns with a 10 degree radius, there was a significant negative correlation between discrimination thresholds and visual field mean defect (i.e. performance reduced with increasing mean defect; r = -0.64, p <0.01). There was no significant correlation between these measures for the smaller pattern radii.
Conclusions :
Eyes with glaucoma exhibit impaired shape discrimination across a range of pattern sizes compared to age-matched controls. That performance is independent of pattern size indicates that this deficit can be detected even when testing a restricted area of the central visual field. This is a novel finding, which likely reflects a failure to engage global pooling mechanisms due to reduced local sensitivity across the visual field in glaucoma.
This is an abstract that was submitted for the 2016 ARVO Annual Meeting, held in Seattle, Wash., May 1-5, 2016.