Reduced Suppression in the Aging Visual System
It is common knowledge that vision worsens in older adulthood. The visual deficits often go beyond the normal degradation of the optical system, implicating age-related changes in the neural mechanisms of visual perception. In particular, it has been suggested that the balance of inhibition and excitation is altered with age, which may affect very basic visual functions such as motion perception. In a series of studies we investigated the impact of age on visual tasks that engage suppressive (i.e., inhibitory) visual mechanisms. In our psychophysical task observers were required to discriminate the direction of motion of drifting Gabor gratings that varied in size and contrast. Not surprisingly, older observers performed worse than their younger counterparts when the stimuli were small, regardless of stimulus contrast. When the stimuli were large and of high contrast, younger observers’ sensitivity to motion direction declined, an effect that is commonly attributed to the engagement of suppressive mechanisms. Older observers did not show the same decline in sensitivity, and actually performed better than the younger observers. The age advantage for large, high contrast stimuli was evident over a range of spatial frequencies and speeds, for both translational and radial motion. This counterintuitive result is consistent with an age-related decline in cortical inhibitory mechanisms that control the basic receptive field properties of visual neurons. Ongoing fMRI work is focused on measuring the effect of age on suppression of the BOLD response within early visual areas.