Stability of individual psychophysical functions for perceived distance in natural indoor and outdoor settings.

Exp. 1 examined correlations of the power functions exponents for individual observers, for apparent distance in a natural indoor setting, obtained for a group of 9 observers over 5 experimental sessions separated by intersession intervals that varied from 1 day to 9 mo. Stable individual differences in the exponent of the psychophysical power law were shown. Intercorrelations of subjects' exponents were positive and reliable with 1 day between sessions and 9 mo. In Exp. 2 the estimates of apparent distance were made by 5 independent groups of 10 observers each, tested and retested at different intersession intervals. There were 5 intervals of 1 day to 9 mo. Fair temporal stability of psychophysical power function exponents was observed in most groups. In Exp. 3 psychophysical power functions were obtained from magnitude estimates of apparent distance in a natural outdoor setting. The same 10 observers were tested and then retested after 1 day and again after 1 yr. Significant correlations for both intersessions intervals indicated consistent and persisting individual differences. Taken together Exps. 1, 2, and 3, regardless of the environment in which distance estimates were performed, clearly demonstrate that power functions are repeatable, enduring, and characteristic of individuals. In addition, the mean exponents were in reasonable agreement with other reported apparent distance exponents obtained in both settings.