Aging Effects on the Visual Span for Alphabetic Stimuli

Background: The visual span (i.e., an estimate of the number of letters that can be recognized reliably on a single glance) is widely considered to impose an important sensory limitation on reading speed. With the present research, we investigated adult age differences in the visual span for alphabetic stimuli (i.e., Latin alphabetic letters), as aging effects on span size may make an important contribution to slower reading speeds in older adulthood. Method: A trigram task, in which sets of three letters were displayed randomly at specified locations to the right and left of a central fixation point, was used to estimate the size of the visual span for young (18-30 years) and older (65+years) adults while an eye tracker was used to ensure accurate central fixation during stimulus presentation. Participants also completed tests of visual acuity and visual crowding. Results: There were clear age differences in the size of the visual span. The older adults produced visual spans which were on average 1.2 letters smaller than the spans of young adults. However, both young and older adults produced spans smaller than those previously reported. In addition, span size correlated with measures of both visual acuity and measures of visual crowding. Conclusion: The findings show that the size of the visual span is smaller for older compared to young adults. The age-related reduction in span size is relatively small but may make a significant contribution to reduced parafoveal processing during natural reading so may play a role in the greater difficulty experienced by older adult readers. Moreover, these results highlight the importance of carefully controlling fixation location in visual span experiments.