Abstract
This series of experiments investigated the neural basis of conscious vision in humans using a form of transcranial magnetic stimulation (TMS) known as continuous theta burst stimulation (cTBS). Previous studies have shown that occipital TMS, when time-locked to the onset of visual stimuli, can induce a phenomenon analogous to blindsight in which conscious detection is impaired while the ability to discriminate ‘unseen’ stimuli is preserved above chance. Here we sought to reproduce this phenomenon using offline occipital cTBS, which has been shown to induce an inhibitory cortical aftereffect lasting 45–60 minutes. Contrary to expectations, our first experiment revealed the opposite effect: cTBS enhanced conscious vision relative to a sham control. We then sought to replicate this cTBS-induced potentiation of consciousness in conjunction with magnetoencephalography (MEG) and undertook additional experiments to assess its relationship to visual cortical excitability and levels of the inhibitory neurotransmitter γ-aminobutyric acid (GABA; via magnetic resonance spectroscopy, MRS). Occipital cTBS decreased cortical excitability and increased regional GABA concentration. No significant effects of cTBS on MEG measures were observed, although the results provided weak evidence for potentiation of event related desynchronisation in the β band. Collectively these experiments suggest that, through the suppression of noise, cTBS can increase the signal-to-noise ratio of neural activity underlying conscious vision. We speculate that gating-by-inhibition in the visual cortex may provide a key foundation of consciousness.
Highlights
1.1 Initial behavioural experiment Blindsight has been one of the most informative conditions in recent investigations of consciousness (e.g.[1,2,3,4])
The primary hypothesis of Experiment 1 was that occipital continuous theta burst stimulation (cTBS), being a cortically suppressive intervention, should cause an effect resembling blindsight that includes two concurrent behavioural profiles: a reduction in DPrC following active cTBS compared to sham, together with residual ‘unseen’ capacity (PcU) that is greater than chance and unaffected by cTBS
The exclusion of data acquired from the two authors did not appreciably alter the significant effect of cTBS on conscious detection (DPrC for cTBS vs. sham F(1,12) = 11.55, p = 0.005, d = 1.00, B(PrC cTBS.sham) = 15.81) or the absence of an effect of cTBS on ‘unseen’ discrimination (DPcU for cTBS vs. sham F(1,12) = 0.02, p = 0.90, d = 0.05, B(PcU cTBS.sham) = 0.08)
Summary
1.1 Initial behavioural experiment Blindsight has been one of the most informative conditions in recent investigations of consciousness (e.g.[1,2,3,4]). Analysis of the nature of the disruption and the residual capacities has furthered our understanding of the processes that contribute to both conscious and unconscious perception. Previous demonstrations of transcranial magnetic stimulation (TMS)-induced blindsight have largely involved applying single or short bursts of TMS to the occipital lobe to interfere with subjects’ awareness of stimuli (e.g.[5,6,7,8]). This has informed our understanding of the causal temporal dynamics of occipital processing in visual consciousness. The demonstration of above-chance perceptual capacity despite such interference is informative with respect to unconscious processing, leading to the suggestion that it may be supported by pathways that bypass the main geniculostriate route
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