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AGE AND BEHAVIOR: PROCESSING SENSE INFORMATION - SEQUENTIAL INTEGRATION - STIMULUS FUSION - CRITICAL FLICKER FUSION

A classic and frequently used measure of visual efficiency is critical flicker fusion (CFF). A flashing light is presented to the viewer and the duration and the frequency of the flashes are very well controlled. If the flashes follow one another in rapid order they appear to fuse for the viewer, and are perceived as a steady light. The exact rate of flashing at which they will appear fused depends on a variety of factors including the level of illumination, the duration of both the on and off periods of the lights, and of course, the frequency of the nicker, i.e., how quickly the light flashes come one after the other. Among the better replicated findings is that the fusing point comes sooner for older people than it does for younger people. It takes a higher rate of on-off flickering for the young to perceive a steady state than it does for the old. Weale (1965) reviewed several studies and concluded that between 20 and 60 years the mean decline in CFF is about seven cycles per second. Young adults perceive fusion when the light flickers about 40 cycles per second (Misiak, 1947) and old people perceive it at slower rates. There is little doubt that CFF threshold is related to peripheral, pre-retinal factors, especially the thickening of the lenses of the eyes and the reduction in pupil size. But, largely on the basis of a study by Coppinger (1955), who showed that an increase in illumination makes for fusion sooner in the young than in the old, Weiss (1959) concluded central factors are implicated in CFF thresholds as well as the peripheral. Other evidence implicates central factors too. In reviewing the literature, Corso (1971) concluded, "whenever the general efficiency of the central nervous system is reduced due to pathology or age, both CFF and certain intellectual functions will also be depressed." Stimulus persistence theory may explain these results. If the trace of the first flash persists longer in the older person's nervous system than in that of the younger person, it has the same effect as a longer duration flash, and, as such, will fuse with the subsequent trace of the external flash which soon comes. McFarland, Warren, and Karis (1958) presented data which could be interpreted in this manner. As they varied the procedure so that the on-off interval was taken up more and more with the light on, fusion of the flashes came about more readily, i.e., thresholds for CFF were lowered. When there was long light-on time, the old and young were relatively similar in CFF threshold; it is as if the stimulus made for a persisting trace in all subjects regardless of age. But, when there was little light-on time, and fusion was not readily perceived by any age group, it was the old group, much more than the young group, that perceived fusion. For the old, when the light was on only 10 percent of the time, stimulus persistence made the effective stimulus duration longer and their CFF thresholds relatively lower than those of the young. *181\220\8*