Abstract
The human visual system is sensitive in detecting objects that have different luminance level from their background, known as first-order or luminance-modulated (LM) stimuli. We are also able to detect objects that have the same mean luminance as their background, only differing in contrast (or other attributes). Such objects are known as second-order or contrast-modulated (CM), stimuli. CM stimuli are thought to be processed in higher visual areas compared to LM stimuli, and may be more susceptible to ageing. We compared visual acuities (VA) of five healthy older adults (54.0±1.83 years old) and five healthy younger adults (25.4±1.29 years old) with LM and CM letters under monocular and binocular viewing. For monocular viewing, age had no effect on VA [F(1, 8)= 2.50, p> 0.05]. However, there was a significant main effect of age on VA under binocular viewing [F(1, 8)= 5.67, p< 0.05]. Binocular VA with CM letters in younger adults was approximately two lines better than that in older adults. For LM, binocular summation ratios were similar for older (1.16±0.21) and younger (1.15±0.06) adults. For CM, younger adults had higher binocular summation ratio (1.39±0.08) compared to older adults (1.12±0.09). Binocular viewing improved VA with LM letters for both groups similarly. However, in older adults, binocular viewing did not improve VA with CM letters as much as in younger adults. This could reflect a decline of higher visual areas due to ageing process, most likely higher than V1, which may be missed if measured with luminance-based stimuli alone.
Highlights
Visual acuity (VA) measurement is one of the clinical routines for ocular examination
VA with LM and CM letters in visually normal older and younger adults were investigated in this study
Similar results were shown in our study, where VA with LM letters was better than that with CM letters in visually normal older and younger adults (Table 1 and Figure 2)
Summary
Visual acuity (VA) measurement is one of the clinical routines for ocular examination. VA is the capacity for seeing distinctly the details of an object It can be described as the eye’s ability to discriminate or resolve spatially organised details. It is usually represented in two ways, which are the reciprocal of the minimum angle of resolution and Snellen fraction. Clinical letter acuity charts rely on the ability of the patient to discriminate between different letters or optotypes. A common feature of letter charts is having black letters on a white background, resulting in maximum difference in luminance or brightness between them. As such, these letters can be classified as luminance-based or first order stimuli
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