Age-related differences in a delayed pointing of a Müller-Lyer illusion.

It has been suggested that movements to visible or remembered targets are differently sensitive to the Müller-Lyer (ML) illusion. Indeed, when the target is continuously visible, movements rely on the veridical object characteristics, whereas remembered movements are thought to reflect the perceived characteristics of the object. The aim of the present study was to determine how movements to visible or remembered targets are influenced by the ML illusion in children aged 7 to 11 years old. Participants were asked to make a perceptual judgment or to point a shaft extremity of the ML configurations (Closed, Control, and Open) in three visual conditions (Closed Loop, Open Loop-0-s delay, and 5-s delay). Perceptual (Perceived Length, PL) and motor (Movement Magnitude, MM, and Peak velocity, PV) variables were measured. Results showed that PL was influenced to the same extent by the ML illusion in the three visual conditions. Moreover, it appears that in subjects as young as 7 years old, the activation of the ventral system features may give rise to the perceptual illusion effect observed in all three experimental conditions. However, regardless of the subject's age, MM and PV were only sensitive in the delay condition, suggesting that delayed movements are also mediated by the ventral stream. These data suggested that the distinction between perception and motor visual pathways appears quite early during childhood (before 7 years). Our data also demonstrated that children were relying on both visual processing streams during perceptual as well as visuomotor tasks during remembered movements.