title: Subliminal stimulation and somatosensory signal detection.
abstract: Only a small fraction of sensory signals is consciously perceived. The brain's perceptual systems may include mechanisms of feedforward inhibition that protect the cortex from subliminal noise, thus reserving cortical capacity and conscious awareness for significant stimuli. Here we provide a new view of these mechanisms based on signal detection theory, and gain control. We demonstrated that subliminal somatosensory stimulation decreased sensitivity for the detection of a subsequent somatosensory input, largely due to increased false alarm rates. By delivering the subliminal somatosensory stimulus and the to-be-detected somatosensory stimulus to different digits of the same hand, we show that this effect spreads across the sensory surface. In addition, subliminal somatosensory stimulation tended to produce an increased probability of responding "yes", whether the somatosensory stimulus was present or not. Our results suggest that subliminal stimuli temporarily reduce input gain, avoiding excessive responses to further small inputs. This gain control may be automatic, and may precede discriminative classification of inputs into signals or noise. Crucially, we found that subliminal inputs influenced false alarm rates only on blocks where the to-be-detected stimuli were present, and not on pre-test control blocks where they were absent. Participants appeared to adjust their perceptual criterion according to a statistical distribution of stimuli in the current context, with the presence of supraliminal stimuli having an important role in the criterion-setting process. These findings clarify the cognitive mechanisms that reserve conscious perception for salient and important signals.
authors.affiliation: Institute of Cognitive Neuroscience, University College London, Alexandra House, 17 Queen Square, London WC1N 3AR, UK; Department of Psychology, Royal Holloway University of London, Egham, Surrey TW20 0EX, UK. Electronic address: e.ferre@ucl.ac.uk.
authors.forename: Elisa Raffaella
authors.initials: ER
authors.lastname: Ferrè
authors.affiliation: Gatsby Computational Neuroscience Unit, University College London, 25 Howland Street, London, W1T4JG, UK.
authors.forename: Maneesh
authors.initials: M
authors.lastname: Sahani
authors.affiliation: Institute of Cognitive Neuroscience, University College London, Alexandra House, 17 Queen Square, London WC1N 3AR, UK. Electronic address: p.haggard@ucl.ac.uk.
authors.forename: Patrick
authors.initials: P
authors.lastname: Haggard
mesh_terms: D000328:Adult; D003243:Consciousness / Q000502:physiology; D004558:Electric Stimulation; D005260:Female; D005385:Fingers / Q000502:physiology; D006801:Humans; D008297:Male; D012684:Sensory Thresholds / Q000502:physiology; D017603:Signal Detection, Psychological / Q000502:physiology*; D013003:Somatosensory Cortex / Q000502:physiology*; D013360:Subliminal Stimulation; D055815:Young Adult
keywords: Consciousness; Gain control; Signal detection theory; Signal/noise ratio; Somatosensory detection; Subliminal stimulation