The development of global motion discrimination in school aged children.

Global motion perception matures during childhood and involves the detection of local directional signals that are integrated across space. We examine the maturation of local directional selectivity and global motion integration with an equivalent noise paradigm applied to direction discrimination. One hundred and three observers (6-17 years) identified the global direction of motion in a 2AFC task. The 8° central stimuli consisted of 100 dots of 10% Michelson contrast moving 2.8°/s or 9.8°/s. Local directional selectivity and global sampling efficiency were estimated from direction discrimination thresholds as a function of external directional noise, speed, and age. Direction discrimination thresholds improved gradually until the age of 14 years (linear regression, p < 0.05) for both speeds. This improvement was associated with a gradual increase in sampling efficiency (linear regression, p < 0.05), with no significant change in internal noise. Direction sensitivity was lower for dots moving at 2.8°/s than at 9.8°/s for all ages (paired t test, p < 0.05) and is mainly due to lower sampling efficiency. Global motion perception improves gradually during development and matures by age 14. There was no change in internal noise after the age of 6, suggesting that local direction selectivity is mature by that age. The improvement in global motion perception is underpinned by a steady increase in the efficiency with which direction signals are pooled, suggesting that global motion pooling processes mature for longer and later than local motion processing.

Reduction in direction discrimination with age and slow speed is due to both increased internal noise and reduced sampling efficiency.

PURPOSE: Sensitivity to moving structure decreases with age and slow speeds may be selectively impaired. This loss could be caused by elevated internal noise in the responses of motion sensors or a reduction in the efficiency with which motion responses are integrated. We adapt an equivalent noise paradigm to analyze the perception of slow and fast speed motion as a function of normal aging. METHODS: A total of 70 observers (20 to 89 years) identified the direction of global motion in a two-alternative forced choice task. In a central 8° aperture, 100 dots of 10% Michelson contrast were moving at 1.6 or 5.5°/s. The direction of each dot was drawn from a Gaussian distribution whose mean and SD were adaptively changed. Internal noise and sampling efficiency were estimated from direction discrimination thresholds as a function of external direction noise, speed, and age. RESULTS: Direction sensitivity was significantly worse for slow speeds at all ages (paired t-test, P < 0.05) and decreased approximately 2% per year (linear regressions, P < 0.01). This aging deficit was due to significant changes in internal noise (5.5°/s) and sampling efficiency (1.6°/s) (linear regression, P < 0.05). CONCLUSIONS: There is motion sensitivity loss with age that arises from an increase in internal noise in the responses of directional sensors and a decrease in responses that contribute to the global decision. Differences in the rates of progression at each speed indicate that motion is processed by independent systems tuned to different speeds, and that the channel for slow speed may be more vulnerable to normal age-related changes.