Temporal summation and phenomenal simultaneity: Experiments with the radius display

This study explores the phenomenon of subjective simultaneity between multiple successive positions of a target, which consisted here of an illuminated radial line in both stepwise and continous rotation around a fixation point. In four experiments the span of phenomenal simultaneity (M) was measured under varying intensities of target and surround, and with two different estimation procedures. M was found to change as a power function of target intensity, with an exponent of -0.13 or -0.16, for both stroboscopic and continuous illumination of the target. Variation of surround intensity affected M only for 9 out of 15 subjects (Experiment III.) For these subjects M varied inversely with log I of the surround; for the remaining 6 subjects M remained approximately constant over a range of 5 log units of I. Auditory noise tended slightly to decrease M in the former group and to increase it in the latter. Under simultaneous continuous and stroboscopic illumination of the target a paradoxical, single-valued M was obtained (Experiment IV). Higher values of M were found when subjects attempted to estimate the number of apparently simultaneous radii (Experiment I). The function relating M to target intensity closely parallels that for the critical duration of temporal brightness summation. The possibility that both measures reflect the same underlying process is discussed.