Isolating neural signatures of conscious speech perception with a no-report sine-wave speechparadigm

Abstract

Identifying neural correlates of conscious perception is a fundamental endeavor of cognitive neuroscience. Most studies so far have focused on visual awareness along with trial-by-trial reports of task relevant stimuli, which can confound neural measures of perceptual awareness with post-perceptual processing. Here, we used a three-phase sine-wave speech paradigm that dissociated between conscious speech perception and task relevance while recording EEG in humans of both sexes. Compared to tokens perceived as noise, physically identical sine-wave speech tokens that were perceived as speech elicited a left-lateralized, near-vertex negativity, which we interpret as a phonological version of a perceptual awareness negativity. This response appeared between 200 and 300 ms after token onset and was not present for frequency-flipped control tokens that were never perceived as speech. In contrast, the P3b elicited by task-irrelevant tokens did not significantly differ when the tokens were perceived as speech versus noise, and was only enhanced for tokens that were both perceived as speech and relevant to the task. Our results extend the findings from previous studies on visual awareness and speech perception, and suggest that correlates of conscious perception, across types of conscious content, are most likely to be found in mid-latency negative-going brain responses in content-specific sensory areas. Significance StatementHow patterns of brain activity give rise to conscious perception is a fundamental question of cognitive neuroscience. Here, we asked whether markers of conscious speech perception can be separated from task-related confounds. We combined sine-wave speech - a degraded speech signal that is heard as noise by naive individuals but can readily be heard as speech after minimal training - with a no-report paradigm that independently manipulated perception (speech versus non-speech) and task (relevant versus irrelevant). Using this paradigm, we were able to identify a marker of speech perception in mid-latency responses over left frontotemporal EEG channels that was independent of task. Our results demonstrate that the "perceptual awareness negativity" is present for a new type of perceptual content (speech).