Age-related differences in the automatic processing of single letters: implications for selective attention.

Older adults exhibit diminished ability to inhibit the processing of visual stimuli that are supposed to be ignored. The extent to which age-related changes in early visual processing contribute to impairments in selective attention remains to be determined. Here, 103 adults, 18-85 years of age, completed a color selective attention task in which they were asked to attend to a specified color and respond to designated target letters. An optimal approach would be to initially filter according to color and then process letter forms in the attend color to identify targets. An asymmetric N170 ERP component (larger amplitude over left posterior hemisphere sites) was used as a marker of the early automatic processing of letter forms. Young and middle-aged adults did not generate an asymmetric N170 component. In contrast, young-old and old-old adults produced a larger N170 over the left hemisphere. Furthermore, older adults generated a larger N170 to letter than nonletter stimuli over the left, but not right hemisphere. More asymmetric N170 responses predicted greater allocation of late selection resources to target letters in the ignore color, as indexed by P3b amplitude. These results suggest that unlike their younger counterparts, older adults automatically process stimuli as letters early in the selection process, when it would be more efficient to attend to color only. The inability to ignore letters early in the processing stream helps explain the age-related increase in subsequent processing of target letter forms presented in the ignore color.