Early Neural Correlates of Conscious Somatosensory Perception

Abstract

The cortical processing of consciously perceived and unperceived somatosensory stimuli is thought to be identical during the first 100 -120 ms after stimulus onset. Thereafter, the electrophysiological correlates of conscious perception have been shown to be reflected in the N1 component of the evoked response as well as in later (Ͼ200 ms) nonstimulus-locked ␥-band (28 -50 Hz) oscillatory activity. To evaluate more specifically the time course and correlation of neuronal oscillations with conscious perception, we recorded neuromagnetic responses to threshold-intensity somatosensory stimuli. We show here that cortical broadband activities phase locked to the subsequently perceived stimuli in somatosensory, frontal, and parietal regions as early as 30 -70 ms from stimulus onset, whereas the phase locking to the unperceived stimuli was weak and primarily restricted to somatosensory regions. Such stimulus locking also preceded the perceived stimuli, indicating that the phase of ongoing cortical activities biases subsequent perception. Furthermore, the data show that the stimulus locking was present in the -(4 -8 Hz), ␣-(8 -14 Hz), ␤-(14 -28 Hz), and ␥-(28 -40 Hz) frequency bands, of which the widespread ␣-band component was dominant for the consciously perceived stimuli but virtually unobservable for the unperceived stimuli. Our results show that the neural correlates of conscious perception are already found during the earliest stages of cortical processing from 30 to 150 ms after stimulus onset and suggest that ␣-frequency-band oscillations have a role in the neural mechanisms of sensory awareness.

Synchronization of Neural Activity across Cortical Areas Correlates with Conscious Perception

“…Our results offer direct support for this notion. In addition, our results and the results from a previous study investigating the neural dynamics of perceived and unperceived somatosensory stimuli (Palva et al, 2005) are in line with a recent proposal (Dehaene et al, 2006) relating unconscious processing of information with local coordination of neural activity in resonant loops of medium range and relating conscious perception with global coordination of distant neural activity by long-range synchronization. Interestingly, the global long-distance synchronization found in the visible condition was very transient and the earliest event differentiating conscious from nonconscious processing.…”

“…The neural signature of unconscious perception would be local coordination of neural activity and propagation along sensory processing pathways, whereas conscious perception would require global coordination of widely distributed neural activity by long-distance synchronization (Dehaene et al, 2006). Indeed, ␤ and gamma frequency band phase synchrony is enhanced for consciously perceived stimuli (Meador et al, 2002;Gross et al, 2004;Nakatani et al, 2005;Palva et al, 2005) and correlates with conscious perception in binocular rivalry (Fries et al, 1997(Fries et al, , 2002Srinivasan et al, 1999;Doesburg et al, 2005). However, these studies have investigated synchronization in spatially restricted neural assemblies, or without distinguishing between local and global coordination.…”

Power modulation of electroencephalogram mu and beta frequency depends on perceived level of observed actions

“…In a MEG study, mu and beta suppression over the bilateral primary somatosensory cortices were more suppressed between perceived versus not perceived tactile stimulation, and nonperceived tactile stimulation was significantly suppressed relative to baseline (Palva et al. 2005). Similarly, EEG oscillations in the mid‐frequency range (12–20 Hz), have been reported to reflect auditory illusion intensity, with stronger power reductions corresponding with stronger percepts (Leske et al.…”

Neuronal Functional Diversity and Collective Behaviors

“…Distinct behavioral or perceptual brain states, including sensory perception, motor function, memory, and consciousness, are associated with different brain rhythms that reflect synchronization of neurons and their entrainment in a regular firing pattern. In this process, networks of reciprocally connected inhibitory neurons play a peculiar role [24,[79][80][81]. Oscillations at different frequencies interact, with precise phase-locking and functional hierarchy controlling neuronal excitability and information processing [82,83].…”

“…Oscillations in the gamma band are a peculiar specimen of this functional arrangement. Phase synchronization in the gamma band frequency (∼20-80 Hz) is thought to mediate in the organization of temporally coherent, but spatially segregated, neuronal activities in feature specification and perceptual/cognitive binding [42,81,[83][84][85][86][87][88][89][90]. Cortical gamma oscillations of neuronal membrane (local field) spiking rate are characteristic in frequency and depend on (pre)synaptic input with a common mean phase.…”