The Slope of Frequency-of-Seeing Curves in Normal, Amblyopic and Pathologic Vision

It has long been known that threshold variability is increased in glaucoma.1 Furthermore, it has been suggested that increased variability may be one of the earliest signs of visual dysfunction in this disease.1-3 There is debate, as to the mechanisms responsible for the increased variability. One theory suggests that increased variability is due to a reduced signal to noise ratio in diseased neurons. Another theory asserts that increased variability is due in part to eye movements, whereby small fixation shifts allow retinal areas with vastly different sensitivities to contribute to the measured threshold.4-6 The current study controlled the effect of fixation shifts by producing retinally stabilized stimuli. Psychometric variability was defined in terms of the slope of frequency-of-seeing (FOS) curves generated using a method of constant stimuli (MOCS) paradigm.