Formulas for Measuring Recovery from Figural Aftereffects

Abstract

There has recently been some confusion about the measurement of the persistence of kinesthetic and visual figural aftereffects. Wertheimer and Leventhal (1957) have correctly pointed out that if, for example, it takes a person with a large initial aftereffect (displacement) longer to dissipate the aftereffect than a person with a small initial afcereffect, this by itself tells us nothing about the relative rate of dissipation. That is, a person with a large initial aftereffect may, after 5 min. rest, still display a substantial residual aftereffect. A person with a very small initial aftereffect may, after 5 min. rest, display only a small residual afcereffect. W e could then say that the person with the large initial aftereffect showed greater total persistence of aftereffects, but this would still tell us nothing about his relative rate of dissipation. For investigators interested in brain processes, it is important to know the relative recovery rate of each of the hypothetical persons in the above example. This is particularly true if we attempt to correlate the size of the initial aftereffect with the speed of recovery after specified periods of rest. The first formula presented here, essentially a measure of the rate of recovery from displacement aftereffects, is the Recovery Ratio (R) as a function of time ( t ) between the last post-inspection measurement taken immediately after fixation on an Inspection Figure and the first measurement taken some period later. R , = ( P P i ) / ( C P i ) , where, C = Control Score. (The point of subjective judgment prior to presentation of the Inspection Figure. Usually obtained by taking an average of four judgments.) Pi = Post-inspection Score. (The point of subjective judgment immediately after the presentation of the Inspection Figure. Usually the average of four judgments.) P = Persistence Score. (The point of subjective judgment a prescribed length of time after Pi. Usually the average of four judgments.) N o matter whether the aftereffect is scored in terms of plus or minus, the Rt score will always be positive unless S continues in the direction of the displacement. That is, only if an S shows an even greater displacement on the persistence trial than on the post-inspection trial will his score be negative. The formula for Total Persistence (S) as a function of time (t, as in the above formula), on the other hand, may be stated simply as: