Abstract
The perceived contrast of a sinusoidal grating patch depends on its surrounding contrast. Normal aging alters these types of contrast related contextual effects, with older adults showing increased surround suppression relative to younger adults (Karas and McKendrick, 2009 Journal of Vision 9.5.11). This study aimed to determine whether age related increases in surround suppression manifest for particular contrast conditions, or specific contrast ratios between centre and surround. We also varied stimulus duration to determine whether group differences could arise due to between-group differences in surround adaptation. Fifteen younger (20–30 years) and 15 older adults (65–79 years) participated. Center-surround contrast discrimination (the Chubb contrast illusion) was measured for a 0.67 deg diameter circular patch of 4 c/deg grating, surrounded by a 4 deg diameter annulus of grating of the same orientation, phase, and spatial frequency. Nine contrast conditions were tested (all combinations of centre/surround for stimuli of 20, 40, and 80% contrast) with a stimulus duration of 500 ms. The 40% centre, 80% contrast surround was also tested with a stimulus duration of 100 ms. Surround suppression was increased in the older group [F(1,28) = 6.88, p = .01], particularly for low centre contrasts (interaction between group and centre contrast, p = .01). Increased surround suppression was present for both 500 ms and 100 ms presentations [main effect of group F(1,26) = 10.78, p= .01].