Early age-related macular degeneration impairs tolerance to stimulus degradation

Pathologic changes of retinal photoreceptors associated with early age-related macular degeneration (AMD) have been well established, but the disease is usually asymptomatic at the early stage, and traditional suprathreshold clinical tests often fail to reveal functional deficiencies. The aim of this study is to demonstrate subtle changes of one suprathreshold visual function in early AMD eyes.

The quality of preattentively discriminable texture stimuli was systematically degraded through random deletion of texture checks. The task of the subject was to make a forced choice decision on whether two equally degraded patches contained samples of the same or different types of textures. Tolerance to texture stimulus degradation was measured in young and elderly normal controls and in patients with early AMD.

Subjects were trained to perform the texture discrimination task until they made few errors in discriminating intact textures. Texture discrimination deteriorated with increasing stimulus degradation in all subjects. There was no significant difference between performance of young and elderly normal controls. Early AMD eyes showed significantly less tolerance to stimulus degradation than age-similar normal controls at a range of degradation levels. After controlling for visual acuity, normal subjects still performed significantly better than early AMD eyes at approximately 22% check deletion. There was no significant difference between better eyes of early AMD patients and fellow eyes of late AMD eyes. Performance on the degraded texture task was not correlated with visual acuity. A mild blur of the stimulus had little effect on discrimination of degraded textures.

Early AMD may not directly affect suprathreshold visual functions when the stimuli are intact and contain redundant information but may manifest itself as a reduction of tolerance to stimulus degradation in the form of localized information loss. The performance of patients with early AMD may be compromised when the visual stimulus contains less redundant information.