Figure 3. SCP and consciousness.

(a) Schematic illustration of our hypothesis. Superficial layers of the cerebral cortex (shown in purple) are the only layers containing extensive long-range horizontal connections (thick blue lines); they are also the main target for nonspecific thalamocortical inputs (black lines) in addition to long-range inter-areal feedback connections (thick red arrows). We propose that long-lasting synaptic activities in superficial layers, manifesting as SCPs in surface recording or low-frequency current source density (CSD) activity in superficial layers, carry large-scale information integration in the brain and contribute directly to conscious awareness. Neuronal circuits in deep layers (thin blue lines) provide specialized local processing that assist superficial-layer computations and send output to subcortical structures. Two specific predictions made by this hypothesis are provided in ‘concluding remarks’ (b) Subjects performed a target detection task in which a visual grating stimulus at threshold was briefly presented. Following a variable delay, the subject was prompted by an auditory cue to press one of two buttons to indicate whether they saw the stimulus. A small percentage of catch trials in which no grating was presented were randomly interleaved. EEG potential from the left parietal electrode (P3, using Laplacian derivation, which emphasizes local vertical currents underneath the electrode) was averaged around the onset of grating stimulus (left panel), or around the motor response (right panel). The evoked potentials for ‘Yes, I saw’, ‘No, I did not see’, and catch trials are shown in black, dark gray and light gray respectively. The inter-subject s.d. for catch trials are shown as dotted lines. A negative slow potential builds up between stimulus onset and motor response during ‘Yes’ trials but not catch trials nor the trials during which the stimulus was present but not perceived. Adapted, with permission, from Pins and ffytche et al. [50] (c) Average evoked-potentials (EPs) in response to single stimulus pulses at the skin, recorded from the surface of somatosensory cortex. EPs to 500 stimulus presentations were averaged for each condition. Abbreviations: Sub T, subthreshold stimuli, none of the 500 stimuli were felt by the subject; T, threshold stimuli, subject reporting feeling some of the 500 stimuli. Each recording trace is 500 ms long; Primary EP, a transient, surface-positive deflection that occurs ~30 ms after the stimulus, was present in both cases; Secondary EP, a later slower surface-negative component, only occurs when the stimulus was at times felt. Adapted, with permission, from Libet et al. [52] (d) The Bereitschaft potential (BP) is a negative SCP shift preceding the onset of a voluntary movement [56]. It was shown by Libet [57] that the onset of the BP also precedes the subject’s awareness of the intention to make the movement by a few hundred milliseconds. Adapted, with permission, from Haggard [58].