EPV 11. Subliminal Tactile Stimuli Modulate Perception Rhythmically

Abstract

Perception of near-threshold stimuli varies rhythmically over trials depending on the time point of attentional cues ( Landau and Fries, 2012 , Landau et al., 2015 ). Neuronal oscillatory phase in sensory cortices predicts the perception of visual ( Busch et al., 2009 ) and tactile ( Baumgarten et al., 2015 ) stimuli. This gives rise to the question if oscillatory phase accounts for rhythmic variations in perception. Sensory stimulation can reset ongoing oscillatory phase in sensory cortices ( Lakatos et al., 2007 ), thereby allowing to externally influence perception. Here, we investigated if perception of suprathreshold electrotactile stimuli is influenced by a preceding subliminal electrotactile stimulus. We hypothesized that the time interval between the preceding stimulus and the target stimuli rhythmically determines the perception of the target stimuli. 18 right-handed subjects (6 male, 28 ± 5 y) were measured in a behavioral paradigm assessing the perception of electrotactile stimuli applied to the left index finger. First, one subliminal stimuli with subthreshold amplitude was presented. Subsequently, either zero, one or two suprathreshold target stimuli separated by specific stimulus onset asynchronies (SOAs) were presented. SOAs included an individually determined SOA (32 ± 17.6 ms) intended to achieve a 1:1 response distribution for perceived one vs. two stimuli. The temporal interval between subliminal stimulus and target stimuli varied from 20 to 600 ms in steps of 20 ms. Subjects were asked to report if they perceived zero, one or two stimuli. Perception rates were analyzed as a function of the temporal interval between subliminal stimulus and target stimuli. To determine rhythmical dependencies, Fourier-analysis was performed on behavioral data. We found that temporal perceptual discrimination rates varied rhythmically with the temporal interval between subliminal stimulus and target stimuli. Fourier analysis revealed significant rhythms in the delta/theta ( ∼ 3 Hz) and beta band ( ∼ 15 Hz). The present results demonstrate a rhythmic relationship between the perception of two subsequently presented electrotactile stimuli and the temporal interval between subliminal stimulus and target stimuli. This relationship was most prominent in the delta/theta and beta band. These results illustrate that subliminal stimulation which is not consciously perceived can influence the perception of suprathreshold stimuli. Perceptual fluctuations at 3 Hz can be interpreted as a result of attentional sampling ( Landau and Fries, 2012 , Landau et al., 2015 ). The 15 Hz component can be explained by discrete perceptual sampling in the somatosensory domain ( Baumgarten et al., 2015 ). Presumably, subliminal stimulation resulted in a phase reset within the somatosensory system. This determined the position of the target stimuli relative to ongoing perceptual cycles, consequently, modulating stimulus perception.