Pia Schroeder | Neural basis of somatosensory target detection: From local interactions to frontoparietal networks

The scientific study of somatosensory awareness has yielded highly diverse findings with putative neural correlates ranging from local interactions within somatosensory cortices to activation of widely distributed frontoparietal networks. A potential source of these divergent results may reside in latent cognitive processes that often coincide with stimulus awareness in experimental settings. In fact, the most commonly employed experimental paradigm, the near-threshold somatosensory detection task, may conflate somatosensory awareness with processing of stimulus uncertainty, overt reports, and motor planning. To elucidate the contribution of these processes to neural activity commonly assumed to reflect somatosensory awareness, we employed a novel somatosensory detection task in combination with functional magnetic resonance imaging (fMRI). Participants received electrical target stimuli at intensities spanning the full range of their psychometric functions to vary stimulus uncertainty while maintaining spontaneous fluctuations in stimulus awareness. Instead of directly reporting target detection, participants assessed the congruence of their somatosensory percepts with simultaneously presented visual reference cues and reported a match or mismatch by making saccadic eye movements, such that target detection was decorrelated from overt reports and motor planning. Using a Bayesian analysis approach, we track the transformation from physical to perceptual stimulus representations along the somatosensory hierarchy. We show that the emergence of stimulus awareness is largely restricted to somatosensory regions, whereas activity in frontal and parietal areas is best explained by stimulus uncertainty and overt reports. Our results emphasize the role of early sensory cortex for conscious perception and dissect the contribution of the frontoparietal network to classical detection tasks.