Production of Threshold Levels of Conscious Sensation by Electrical Stimulation of Human Somatosensory Cortex

Transcranial magnetic stimulation of the visual cortex induces somatotopically organized qualia in blind subjects

“…To the best of our knowledge, there have been only two reports in the literature describing TMS-induced pure tactile sensations, produced by either stimulating the primary motor cortex (M1) (38) or S1 region (39). It has been argued that excitation of the postcentral gyrus requires prolonged repetitive stimulation for accessing the perceptual system (40).…”

The timing of brain events: Reply to the “Special Section” in this journal of September 2004, edited by Susan Pockett

“…Fast, purposeful responses can occur unconsciously before conscious awareness (Libet, 1985; see also Velmans, 1991). Pollen (p. 641) notes that with 8 per sec threshold stimuli to somatosensory cortex we had reported the likely required train duration is about 10 s (Libet et al, 1964). This stimulus produces a slow pulsatile sensation, different from the 0.5 s trains with higher frequencies.…”

“…Delay of about 500 ms for threshold sensation Libet et al (1964) and Libet (1973) had shown that a 500 ms train of repetitive stimuli to sensory cortex (at threshold intensity) is required to elicit a conscious sensory experience in a human subject. Pockett (2002) argued that this requirement was simply due to a facilitatory buildup in neuronal excitability that finally leads to neuronal firing at the end of a 500 ms train of stimuli.…”

The great subjective back-referral debate: Do neural responses increase during a train of stimuli?

“…As I have pointed out previously (Pockett, 2002a) the main line of evidence that neural responses did increase as trains of stimuli went on is actually that provided by LibetÕs own early experiments (Libet et al, 1964). Libet has repeatedly quoted this initial paper as showing that, to use the words of his present Reply, ''a 500 ms train of repetitive stimuli to sensory cortex (at threshold intensity) is required to elicit a conscious sensory experience in a human subject.''…”

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Evidence is summarised for and against the hypothesis that potentiation or facilitation of neural responses during a train of threshold-level stimuli occurred in the experiments reported by Libet et al (1964). It is concluded that such potentiation probably did occur.

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