Because subliminal stimuli intensities are insufficient to initiate global network activity 14, 15 and these stimuli are not consciously perceived, the risk of perceptually confusing preceding subliminal stimuli with the subsequent target stimuli of the temporal discrimination task or masking the target stimuli is minimized. Operationally, the use of subliminal stimuli is advantageous compared to the use of suprathreshold stimuli. We assessed this relationship by using an electrotactile temporal discrimination task which was preceded by a subliminal (i.e., below perceptual threshold) stimulus. The causal approach would allow for the possibility to gather experimental evidence for or against the theory of discrete perception and shed light on the patterns of perceptual cycles. This causal approach stands in contrast to the simultaneous measurement of both variables without systematic variation, which would result in correlative evidence. Here, we use the term causal to define a process in which an independent variable (e.g., the onset of a putative perceptual cycle on behavioral level or the phase of neuronal oscillations on neurophysiological level) is experimentally and systematically modulated while measuring the corresponding changes on the dependent variable (i.e., rhythmic perception). To advance this discussion, it would be necessary to causally modulate the rhythmic patterns of perception (i.e., the perceptual cycles). ![]() Consequently, the theory of discrete perception remains controversially discussed 12, 13. However, current experimental evidence for discrete perception and its putative underlying neuronal mechanisms is mostly of correlative nature, while causal evidence remains scarce 10, 11. Said neuronal oscillations might thus form the neurophysiological basis of periodic modulations of perception, suggesting that neuronal oscillations in specific frequencies define perceptual cycles. These studies have shown that the periodic modulation of subjects’ perception was related to the phase of ongoing neuronal oscillations in the alpha and beta band located in the parieto-occipital or primary somatosensory cortex (S1) 7, 8, 9. While the ongoing debate whether perception is continuous or discrete has been put forward at least a century ago 5, 6, the hypothesis of discrete perception has only recently regained new support from neuroimaging studies. A snapshot or perceptual cycle, thus, forms the temporal unit of perceptual experience, leading to rhythmic or cyclic perception 4. Within such a framework, incoming sensory information would be grouped in consecutive separated perceptual cycles or snapshots 1, 2, 3. This can be compellingly explained by a model of discrete perceptual cycles.Īlthough perception appears smooth and continuous in our subjective experience, it has been discussed whether the nature of sensory information processing is intrinsically discrete. Importantly, perception was modulated rhythmically, in cycles corresponding to the beta-band (13–18 Hz). ![]() ![]() The present results confirm that, without being consciously perceived, the subliminal stimulus critically influenced perception in the discrimination task. We hypothesized that the subliminal stimulus indirectly, but systematically modulates the ongoing oscillatory phase in S1, thereby rhythmically shaping perception. Although not consciously perceived, subliminal stimuli are known to elicit neuronal activity in early sensory areas and modulate the phase of ongoing neuronal oscillations. We used in humans an electrotactile temporal discrimination task preceded by a subliminal (i.e., below perceptual threshold) stimulus. Here, we provide causal evidence that somatosensory perception is composed of discrete perceptual cycles. Despite being experienced as continuous, there is an ongoing debate if perception is an intrinsically discrete process, with incoming sensory information treated as a succession of single perceptual cycles.
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