Measuring the effects of age on foveal surround suppression of contrast.

Purpose: The apparent contrast of a visible central grating can be reduced by the presence of a surrounding grating-an effect known as surround suppression of contrast. Surround suppression is strong when the orientation of the surround matches that of the central probe and is strongest in the periphery. There is evidence that surround suppression at the fovea increases in strength with age, and that the orientation dependence of surround suppression is weakened. However, the range of visual stimuli that can produce this effect (and the implicated underlying mechanisms) requires further examination. This study aimed to characterise surround suppression at the fovea and its dependence on age. Visual stimuli with previously unexplored spatiotemporal parameters were used, designed to minimise contributions from spatially short-range overlay masking and temporally transient masking mechanisms.

Methods: In 20 younger (<30 years) and 17 older (>60 years) observers, psychophysical contrast-matching thresholds were measured using stimuli centred on the fovea. Grating stimuli were presented with either no surround, a collinearly oriented surround or an orthogonally oriented surround. Using a staircase procedure, observers matched the contrast of these central target stimuli to the contrast of a separate reference stimulus. The points of subjective equality between target and reference stimuli were compared between the two surround orientations and between younger and older age groups.

Results: Across all observers, weak foveal surround suppression was found that had little orientation tuning. No evidence for the strength of surround suppression increasing with age is reported.

Conclusions: These findings suggest that the age-related effects of surround suppression may be dependent on the spatiotemporal parameters of the stimulus used and encourage further exploration of the contrast masking mechanisms affected by age. The mild and weakly orientation-tuned suppression may have been produced by a weak and temporally sustained suppression mechanism.