Pre-stimulus oscillatory signatures of tactile detection and attention

Oscillatory neuronal activity in the alpha band has been associated with both conscious perception and attention. Firstly, conscious perception of a weak sensory stimulus is preceded by alpha power decreases. Secondly, attention to a sensory event reduces alpha activity in the corresponding sensory regions. According to the widely accepted functional inhibition hypothesis, oscillatory neural activity in the alpha band reflects cortical excitability; in other words, a sensory region with low alpha power levels is more excitable. Several questions regarding the relationship between conscious perception, attention and alpha band activity have not been addressed so far. Firstly, it remained unclear whether brain states predisposing consciousness only comprise local pre-stimulus alpha power decreases, or also global network states. Secondly, it remained unclear whether alpha power decreases prior to conscious perception are confounded by fluctuations of attention or not. The goal of the current thesis is to address these two open questions in the tactile modality with two magnetoencephalography studies. The first study explored brain states predisposing conscious tactile perception, with a particular focus on functional connectivity patterns in addition to alpha power modulations. To this end, a simple near-threshold detection paradigm was conducted, with weak tactile stimuli to the participants’ left index finger. Findings revealed that conscious perception is preceded by a) a relative alpha power decreases in the somatosensory cortex contralateral to stimulation, and b) a spectrally specific pattern of functional connectivity in the primary somatosensory cortex. Based on the first study, it can be concluded that brain states predisposing consciousness comprise local cortical excitability changes as well as frequency-specific network patterns. The second study focused on alpha power changes prior to conscious perception in the context of spatial attention. To this end, a near-threshold detection paradigm with a double-pulse target stimulus was combined with a spatial attention task. The results showed a) that spatial tactile attention modulates pre-stimulus alpha power, and b) that spontaneous alpha power fluctuations not explained by attention influence perception. These findings indicate that – while attention does affect pre-stimulus alpha power levels – spontaneous alpha power fluctuations predispose consciousness. Taken together, we conclude that brain states predisposing conscious perception comprise spectrally specific functional connectivity patterns, and alpha power fluctuations distinct from attention-induced alpha power modulations.