The effect of speed on the typical and atypical development of motion-defined form perception

Abstract

Our previous work on motion-defined form perception showed: 1) slow maturation to adult levels by 7 years of age, and 2) large fellow-eye deficits in children with amblyopia. These developmental changes could reflect the functioning of the M/dorsal or the P/ventral pathway. The current study used motion speed to bias the relative contribution of the M/dorsal (fast motion) and P/ventral (slow motion) pathways to the processing of motion-defined form. Motion-defined rectangles (vertical or horizontal) were created by moving a proportion of dots coherently (up or down) while the remaining dots moved in random directions at the same speed within the rectangle. Outside the rectangle, dots moved in the opposite direction with the same speed and coherence. Monocular motion coherence thresholds, for discriminating rectangle orientation, were determined at slow (0.85 deg/s) and at fast speeds (5.1 deg/s) using a staircase procedure. First we examined normal development in children (4–6 years), adolescents (11–12 years) and young adults. Consistent with our previous results, coherence thresholds were higher in children than in adolescents and adults, with no difference between the two older groups. In addition, coherence thresholds were lower at the slow than at the fast speed for all three groups. Next we examined patients aged 11 years and older with anisometropic and/or strabismic amblyopia. A few individuals showed a deficit in either eye at both speeds, and thresholds were lower in those with compromised binocularity. As a group, however, performance was similar to that of the controls. This is contrary to our previous results, possibly due to the older age of the observers or to the use of coherence rather than minimum-speed thresholds. Our results provide no evidence for a developmental difference in M/dorsal versus P/ventral pathways. The speed effect may reflect the stronger contribution of the slow-motion system to motion-defined form perception.