Visual acuity measured with luminance-modulated and contrast-modulated noise letter stimuli in young adults and adults above 50 years old

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.

Masking Effects of Similar and Dissimilar Letters: A Test of the Interactive Channels Model

Subjects identified which of two target letters was presented when the target was accompanied by a similar or dissimilar noise letter, the target appeared on the right or left of fixation, and the target was central or peripheral to the noise letter. Although performance deteriorated in the presence of noise letters compared to control conditions, masking was no greater with similar than dissimilar noise letters. Rather masking effects were specific to particular target-mask pairs, suggesting facilitation of target perception when mask and target shared features critical to the target-discrimination task. Thus, no evidence for Estes' interactive channels model was obtained. Neither the left vs right position of the target nor its centrality had any effect on accuracy or speed. Correct latencies to a target covaried with its accuracy of detection, but incorrect latencies were more strongly associated with the target identified than with the target presented.