The rotating spot method of timing subjective events

Abstract

The rotating spot method of timing subjective events involves the subject’s watching a rotating spot on a computer and reporting the position of the spot at the instant when the subjective event of interest occurs. We conducted an experiment to investigate factors that may impact on the results produced by this method, using the subject’s perception of when they made a simple finger movement as the subjective event to be timed. Seven aspects of the rotating spot method were investigated, using a factorial experiment. Four of these aspects altered the physical characteristics of the computer generated spot or clock face and the remaining three altered the instructions given to the participant. We found compelling evidence that one factor, whether the subject was instructed to report the instant when the finger movement was initiated or the instant when it was completed, resulted in a systematic shift in the response. Evidence that three other factors affect the observed variability in the response was also found. In addition, we observed that there are substantial systematic differences in the responses made by different subjects. We discuss the implications of our findings and make recommendations about the optimal way of conducting future experiments using the rotating spot method. Our overall conclusion is that our results strongly validate the rotating spot method of timing at least the studied variety of subjective event.

Introduction

The question of how to assign an objective time to internal subjective events has tested the ingenuity of scientists since the 19th century (Spence, Shore, & Klein, 2001). One well-known recent incarnation of Wundt’s ‘komplikationspendl’ method of timing subjective events is the rotating spot method invented by Benjamin Libet and colleagues (Libet, Gleason, Wright, & Pearl, 1983). This method involves the subject’s watching a spot of light (or in later versions sometimes a clock hand) rotating on an experimenter-generated clock face and then reporting the position of the spot at the instant when the subjective event in question occurred. Although Libet’s version of the technique first appeared in the literature a little over 20 years ago, it presently remains essentially the only widely used method of timing subjective events.

One problem with the rotating spot method is that the results it produces have been somewhat variable. Consider, for example, the various estimates it has produced of Libet’s “time M”—the time at which a subject reports being aware of moving their finger to press a key or button. Libet’s original averaged estimate of M (Libet et al., 1983) was −86 ms in relation to the onset of EMG activity. According to Haggard and Eimer (1999), EMG onset typically occurs 30–50 ms before the key or button is finally pressed, so in relation to the button press Libet’s −86 ms would translate to about −126 ms. Later papers report M in relation to the button or key press at: −89 ms (Haggard & Eimer, 1999), −10 ± 34 ms (Tsakiris & Haggard, 2003), −1 ± 43 ms (Haggard & Clark, 2003), +5 ± 40 ms (Wohlschlager, Haggard, Gesierich, & Prinz, 2003) and +19.8 ± 39 ms (Sirigu et al. (2004)). The authors of these publications usually choose to render the ‘accuracy’ of the method relatively unimportant by using repeated measures/within subjects designs, which simply compare two experimental conditions within the same subject. However, it would be useful to know the source of the variation between the baseline results in different reports, so that the method might be refined to provide more reliable absolute timings for subjective events.

Thus, the aims of the present paper are to elucidate firstly what, if any, features of the rotating spot method might have played into the production of the fairly wide range of results in the literature and secondly how best to arrange the details of the rotating spot method to increase the accuracy and reduce the variability of subjective reports. To achieve these aims, we investigate the effects of altering seven different experimental factors on the results our subjects produced when asked to estimate Libet’s time M.