Sensory events with variable central latencies provide inaccurate clocks

Impaired awareness of motor intention in functional neurological disorder: implications for voluntary and functional movement

BACKGROUND

Functional neurological disorders (FNDs), also known as conversion disorder, are unexplained neurological symptoms unrelated to a neurological cause. The disorder is common, yet poorly understood. The symptoms are experienced as involuntary but have similarities to voluntary processes. Here we studied intention awareness in FND.

METHOD

A total of 26 FND patients and 25 healthy volunteers participated in this functional magnetic resonance study using Libet's clock.

RESULTS

FND is characterized by delayed awareness of the intention to move relative to the movement itself. The reporting of intention was more precise, suggesting that these findings are reliable and unrelated to non-specific attentional deficits. That these findings were more prominent with aberrant positive functional movement symptoms rather than negative symptoms may be relevant to impairments in timing for an inhibitory veto process. Attention towards intention relative to movement was associated with lower right inferior parietal cortex activity in FND, a region early in the processing of intention. During rest, aberrant functional connectivity was observed with the right inferior parietal cortex and other motor intention regions.

CONCLUSIONS

The results converge with observations of low inferior parietal activity comparing involuntary with voluntary movement in FND, emphasizing core deficiencies in intention. Heightened precision of this impaired intention is consistent with Bayesian theories of impaired top-down priors that might influence the sense of involuntariness. A primary impairment in voluntary motor intention at an early processing stage might explain clinical observations of slowed effortful voluntary movement, heightened self-directed attention and underlie functional movements. These findings further suggest novel therapeutic targets.

The timing of the conscious intention to move

The foundation of modern neuroscience and psychology about intention for action was laid by B. Libet et al. [(1983) Brain 106, 623-642]. They reported the time of awareness of wanting to move to be about 0.2 s before voluntary movement onset. However, despite repeated confirmation of the result, their method has been criticised for its dependence on self-reported timing and subjective memory, and the interpretation has been widely debated without general consensus. Here, we show that the mean time of the conscious intention to move was 1.42 s before movement, estimated based on the subject's real-time decision of whether or not there was a thought to move when a tone occurred. This event is after the onset of the bereitschaftspotential, an electroencephalographic activity preceding voluntary movement, but about 1 s earlier than the timing of intention reported previously based on the subject's recall. Our result solves some problems of the conventional method, thus giving a clearer answer to the controversies. The difference between the conventional result and our result suggests that the perception of intention rises through multiple levels of awareness, starting just after the brain initiates movement.

The interpretation of Libet's results on the timing of conscious events: a commentary

A commentary on articles by Klein, Pockett, and Trevena and Miller, in this issue, is given. Average shift in the point of subjective equality (PSE), calculated by Klein on Libet's data, and corresponding change in mean shift, calculated by Libet et al. (1983), 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.

The timing of conscious experience: a critical review and reinterpretation of Libet's research

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 occur after the 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, the end of 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.