Measuring the effects of age on contrast suppression
The apparent contrast of a suprathreshold central stimulus can be reduced by a surrounding stimulus - a phenomenon known as surround suppression. When stimuli are presented at the fovea, this effect reportedly increases in strength with age. The underlying mechanism proposed for this age dependence, a change in the balance of inhibition and excitation in cortex, makes this phenomenon potentially interesting as a biomarker of neurological dysfunction. Here, we attempt to repeat these measurements, but we use stimuli designed to control for untuned overlay masking, a different form of suppression thought to be of pre-cortical origin. We measured contrast matching thresholds in twenty younger (< 30) and seventeen older (>60) observers. Across all observers, we find weak suppression that has little or no orientation tuning and, importantly, no dependence on age. Our findings contradict those from earlier studies and suggest that effects relating to age may be dependent on the temporal parameters of this stimulus, and could arise from effects on other pre-cortical mechanisms.