The Development of Sensitivity to Motion

The purpose of this thesis is to determine the timeline of the development of sensitivity to first-order (luminance-defined) versus second-order (contrast-defined) motion and to compare how sensitivity to motion varies with temporal frequency (flicker rate), spatial frequency (stripe size), and velocity. In Chapter 2, I demonstrate that infants' sensitivity to drifting gratings is more adult-like for second-order than first-order stimuli. Moreover, the evidence suggests that infants choose a moving over a stationary grating based on their sensitivity to grating flicker rather than its direction. In Chapter 3, I demonstrate that sensitivity to second-order motion is more adult-like than sensitivity to first-order motion at all ages tested. Children reach adult-like levels of sensitivity to motion at 7 years when stimuli are second-order, but are still worse than adults at 10 years when stimuli are first-order. Furthermore, sensitivity to motion varies with temporal frequency when stimuli are first-order, but spatial frequency when stimuli are second-order. These results are consistent with the hypothesis that first-order motion is processed using low-level motion mechanisms, while second-order motion is processed using a feature tracking mechanism. In Chapter 4, I explored sensitivity to second-order motion in 5-year-olds and adults using directional and non-directional motion tasks. Children's sensitivity to motion is more immature when temporal frequency is relatively low, but only for the directional motion tasks. All participants perform better on the non-directional than the directional task, but only when temporal frequency is relatively high. These results are consistent with Seiffert and Cavanagh's (1998) findings that second-order motion is processed by a feature tracking mechanism. Combined, these findings are consistent with the idea that firstand second-order motion are processed, at least in part, by different neural mechanisms.