Early Age-Related Macular Degeneration Impairs Tolerance to Stimulus Degradation

Abstract

Purpose 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. Methods The quality of pre-attentively discriminable texture stimuli was systematically degraded through random deletion of texture checks. The subjects’ task 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. Results 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 around 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. Conclusions 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.

Cone photoreceptor macular function and recovery after photostress in early non-exudative age-related macular degeneration

“… 28 , 29 Finally, results have shown that there is a loss of cone-mediated psychophysical visual function and electrophysiological function in early AMD. Contrast and color sensitivity, 30 – 33 texture discrimination, 34 flicker sensitivity, 35 , 36 retinal sensitivity, 6 , 37 – 39 and electroretinogram response 40 , 41 have been shown to be affected prior to changes in VA. The relevance of cone-mediated visual function recovery following photostress in early AMD has been shown in numerous studies.…”

Relationships of orientation discrimination threshold and visual acuity with macular lesions in age-related macular degeneration

“…Although the Amsler grid is simple to use and inexpensive, it lacks sensitivity and only provides a qualitative description of the perceived distortion [ 13 ]. Functional tests developed later, which include preferential hyperacuity perimetry (PHP), M-charts, and shape discrimination [ 14 – 16 ], can provide quantitative measurements of the distortion. However, none of those methods are firmly established or clinically routine.…”

Improving patient outcomes: role of the primary care optometrist in the early diagnosis and management of age-related macular degeneration

“…Although pathological changes associated with early AMD can interfere with the normal functioning of retinal neural cells, 34 these changes are usually in the form of small-scale lesions randomly scattered in the macula. 35 These lesions do not usually affect high-contrast visual acuity or impair one's ability to perform tasks of daily living, thus can easily elude the patient's attention. Therefore, detection is most likely to occur when the patient seeks help from eye-care professionals for reasons other than AMD -for example, for a new pair of spectacles.…”

“…59 Liu and White found that while early AMD subjects could discriminate two intact textures with great accuracy based on their structural difference, that ability dropped off much more quickly than in normal controls when the textures were systematically degraded by random texture-check deletion (degraded-texture discrimination test). 35 One challenge in primary care AMD detection is to develop "non-standard, yet clinically-practical" vision tests. 55 Some of the tests mentioned require dedicated equipment or are long and arduous, thus may not fit in a busy office schedule.…”