Preserved perception-action dissociation but altered visuomotor behaviours in healthy aging.

The two visual pathways hypothesis posits distinct brain systems for vision-for-perception and vision-for-action. While this dissociation is well-established in younger adults, its integrity in healthy aging remains unclear. To address this, younger (n = 25, range: 18-25 years) and older adults (n = 25, range: 65-95 years) completed estimation and grasping tasks in two experiments. In Experiment 1, two rectangular objects with varying lengths (40 mm and 42 mm) were placed on the "far" and "close" surfaces of a Ponzo illusion. Despite age-related changes in grasping kinematics, the perception-action dissociation persisted. The Ponzo illusion influenced estimation such that objects placed on the "far" surface were perceived as longer. In contrast, grasping was not affected by the illusion in the same way, and showed effect in the opposite direction, with larger apertures for objects placed on the "close" surface of the illusion. Experiment 2 tested whether this reversed effect was mediated by the surface size on which the object was placed, rather than perceived distance. To this end, we removed the illusory distance cues and varied only the background surface size ("big" versus "small"). While perceptual estimations were unaffected, surface size modulated grasping in both age groups, with a stronger effect in older adults. Additionally, grasping times were slower for the far surface in both groups in Experiment 1, whereas this slowing was evident only in older adults in Experiment 2. These findings indicate that the perception-action dissociation is preserved in aging, but older adults rely more on contextual cues during action, potentially reflecting compensatory mechanisms to maintain visuomotor performance.