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 (20-89 years) identified the direction of global motion in a two-alternative forced choice task. In a central 8 degrees aperture, 100 dots of 10% Michelson contrast were moving at 1.6 or 5.5 degrees/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 degrees/s) and sampling efficiency (1.6 degrees/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.