A remarkable hypothesis has recently been advanced by Libet and promoted by Eccles which claims that there is standardly a backwards referral of conscious experiences in time, and that this constitutes empirical evidence for the failure of identity of brain states and mental states. Libet's neurophysiological data are critically examined and are found insufficient to support the hypothesis. Additionally, it is argued that even if there is a temporal displacement phenomenon to be explained, a neurophysiological explanation is most likely.

The original data reported by Benjamin Libet and colleagues are reinterpreted, taking into account the facilitation which is experimentally demonstrated in the first of their series of articles. It is shown that the original data equally well or better support a quite different set of conclusions from those drawn by Libet. The new conclusions are that it takes only 80 ms for stimuli to come to consciousness and that “subjective back-referral of sensations in time” to the time of the stimulus (...) does not occur. (shrink)

The major findings by Libet et al. are briefly summarized. The criticisms and alternative proposals by Trevena and Miller, Pockett, and Gomes are analyzed and found to be largely unwarranted.

Evidence that led to the hypothesis of a backwards referral of conscious sensory experiences in time, and the experimental tests of its predictions, is summarized. Criticisms of the data and the conclusion by Churchland that this hypothesis is untenable are analysed and found to be based upon misconceptions and faulty evaluations of facts and theory. Subjective referral in time violates no neurophysiological principles or data and is compatible with the theory of "mental" and "physical" correspondence.

An extended examination of Libet's works led to a comprehensive reinterpretation of his results. According to this reinterpretation, the Minimum Train Duration of electrical brain stimulation should be considered as the time needed to create a brain stimulus efficient for producing conscious sensation and not as a basis for inferring the latency for conscious sensation of peripheral origin. Latency for conscious sensation with brain stimulation may occurafterthe Minimum Train Duration. Backward masking with cortical stimuli suggests a 125-300 ms minimum value (...) for the latency for conscious sensation of threshold skin stimuli. Backward enhancement is not suitable for inferring this latency. For determining temporal relations between stimuli that correspond to subjects' reports, theendof cerebral Minimum Train Duration should be used as reference, rather than its onset. Results of coupling peripheral and cortical stimuli are explained by a latency after the cortical Minimum Train Duration, having roughly the same duration as the latency for supraliminal skin stimuli. Results of coupling peripheral stimuli and stimuli to medial lemniscus (LM) are explained by a shorter LM latency and/or a longer peripheral latency. This interpretation suggests a 230 ms minimum value for the latency for conscious sensation of somatosensory near-threshold stimuli. The backward referral hypothesis, as formulated by Libet, should not be retained. Long readiness potentials preceding spontaneous conscious or nonconscious movements suggest that both kinds of movement are nonconsciously initiated. The validity of Libet's measures of W and M moments (Libet et al., 1983a) is questionable due to problems involving latencies, training, and introspective distinction of W and M. Veto of intended actions may be initially nonconscious but dependent on conscious awareness. (shrink)

A commentary on articles by Klein, Pockett, and Trevena and Miller, in this issue, is given. Average shift in the point of subjective equality , calculated by Klein on Libet's data, and corresponding change in mean shift, calculated by Libet et al. , may be “corrected,” taking as a reference point the end of the minimum train duration. Values obtained, if significant, indicate a latency for conscious sensation of the skin stimulus of at least 230 ms. Pockett's main conclusions are (...) favored, but her explanation of peripheral–lemniscal couplings is found to be unconvincing. Trevena and Miller's article unsuccessfully tries to rescue a dualist interactionist view. Libet's method of timing intentions is thoroughly criticized. (shrink)

The identity of conscious states and brain states must remain a mystery until we find a way of characterising both sides of the equation in terms that have the same ‘dimensions’. In this paper I stress the need for ‘dual currency concepts’ that not only are but can be seen to be as appropriate for talking about, say, the experience of pain as for talking about the corresponding working of the brain. In the light of evolutionary theory I make a (...) radically new proposal for what the common dimension is, centred on the idea of ‘authorship’. (shrink)