The effect of the Ebbinghaus illusion on grasping behaviour of children

Within the context of the Ebbinghaus illusion, adults regularly misjudge th e physical size of a centre disc, yet scale their hand aperture according t o its actual size. Separate visual pathways for perception and action are a ssumed to account for this finding. The dorsal visual stream is said to ela borate on egocentric (visuomotor), while the ventral stream is involved in allocentric transformations (object recognition). This study examines the o ntogenetic development of this dissociation between perception and action i n 35 children between the ages of 5 and 12 years. We report four major resu lts. First, when children judged object size without grasping the disc, the ir judgements were deceived by the illusion to the same extent as adults. H owever, when asked to estimate size and then to grasp the disc, young child ren's (5-7 years) perceptual judgements became unreliable, while adults wer e still reliably deceived by the illusion in 80% of their trials. Second, t he younger the children, the more their aperture was affected by the illusi onal surround. Discs of the same size were grasped with a smaller aperture when surrounded by a small annulus, although they were perceived as being l arger. Third, young children used the largest safety margin during grasping . Fourth, the reliance on visual feedback decreased with increasing age, wh ich was documented by shorter movement times and earlier maximum hand openi ng during grasping in the older children (feedforward control). Our results indicate that grasping behaviour in children is subject to an interaction between ventral and dorsal processes. Both pathways seem not to be function ally segregated in early and middle childhood. The data are inconclusive ab out whether young children predominantly use a specific visual stream for e ither a perceptual or motor task. However, our data demonstrate that childr en were relying on both visual processing streams during perceptual as well as visuomotor tasks. We found that children used egocentric cues to make p erceptual judgements, while their grasping gestures were not exclusively sh aped by viewer-centred but also by object-centred information.