Benjamin Libet compared the perceived time of direct brain stimulation to the perceived time of skin stimulation. His results are among the most controversial experiments at the interface between psychology and philosophy. The new element that I bring to this discussion is a reanalysis of Libet's raw data. Libet's original data were difficult to interpret because of the manner in which they were presented in tables. Plotting the data as psychometric functions shows that the observers have great uncertainty about the (...) relative timing of events, as seen the shallow psychometric slopes. A second indication of uncertainty comes from Libet's use of three response categories, A first; B first; and A and B simultaneous. The large number of “perceptually simultaneous” responses provides a further measure of the difficulty of the judgment. There are thus a very broad range of stimulus delays in which the subject is unable to make an accurate ordering response. These points provide evidence that there is no compelling reason to invent exotic or ad hoc mechanisms to account for Libet's data since the uncertainty window is large enough to allow simple mechanism such as memory shifts. Libet argued that his data provide evidence for a backward referral in time. I argue that even though Libet's own data are weak, there are good arguments for a backward referral mechanism to help the subject make sense out of the tangled chaos of asynchronous information associated with experienced events. (shrink)

The idea that our conscious decisions determine our actions has been challenged by a report suggesting that the brain starts to prepare for a movement before the person concerned has consciously decided to move . Libet et al. claimed that their results show that our actions are not consciously initiated. The current article describes two experiments in which we attempted to replicate Libet et al.'s comparison of participants' movement-related brain activity with the reported times of their decisions to move and (...) also the reported times of their decisions of which hand to move. We also looked at the distribution of participants' reports over time to evaluate an alternative explanation of Libet et al.'s results. Although the Readiness Potential was usually present before all of the decisions to move, consistent with the findings of Keller and Heckhausen and Libet et al. , we found that many reported decision times were before the onset of the Lateralized Readiness Potential, which measures hand-specific movement preparation. The latter finding is consistent with the conclusion that the LRP always started after the conscious decision to move. We conclude that even though activity related to movement anticipation may be present before a conscious decision to move, the cortical preparation necessary for the movement to happen immediately may not start until after the conscious decision to move. (shrink)

Trevena and Miller provide further evidence that readiness potentials occur in the brain prior to the time that participants claim to have initiated a voluntary movement, a contention originally forwarded by Libet, Gleason, Wright, and Pearl . In their examination of this issue, though, aspects of their data lead them to question whether their measurement of the initiation of a voluntary movement was accurate. The current article addresses this concern by providing a direct analysis of biases in this task. This (...) was done by asking participants to make subjective timing decisions regarding a stimulus that could be measured objectively. Our findings suggest that their timing task was indeed biased such that participants' tend to report events as happening approximately 70 ms later than they actually happened. Implications for the original Libet et al. claims are discussed. (shrink)

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. (shrink)

The conclusions drawn by Benjamin Libet from his work with collegues on the timing of somatosensorial conscious experiences has met with a lot of praise and criticism. In this issue we find three examples of the latter. Here I attempt to place the divide between the two opponent camps in a broader perspective by analyzing the question of the relation between physical timing, neural timing, and experiential timing. The nervous system does a sophisticated job of recombining and recoding messages from (...) the sensorial surfaces and if these processes are slighted in a theory, it might become necessary to postulate weird operations, including subjective back-referral. Neuroscientifically inspired theories are of necessity still based on guesses, extrapolations, and philosophically dubious manners of speech. They often assume some neural correlate of consciousness as a part of the nervous system that transforms neural activity in reportable experiences. The majority of neuroscientists appear to assume that the NCC can compare and bind activity patterns only if they arrive simultaneously at the NCC. This leads to a search for synchrony or to theories in terms of the compensation of differences in neural delays . This is the main dimension of the Libet discussion. Examples from vision research, such as “temporal-binding-by-synchrony” and the “flash-lag” effect, are then used to illustrate these reasoning patterns in more detail. Alternatively one could assume symbolic representations of time and space that are not coded in their own dimension . Unless such tags are multiplexed with the quality message , one gets a binding problem for tags. One of the hidden aspects of the discussion between Libet and opponents appears to be the following. Is the NCC smarter than the rest of the nervous system, so that it can solve the problems of local sign and timing on its own, or are these pieces of information coded symbolically early on in the system? A supersmart NCC appears to be the assumption of Libet's camp . The wish to distribute the smartness evenly across all stages of processing in the nervous system appears to motivate the opponents. I argue that there are reasons to side with the latter group. (shrink)