An invisible touch: Body-related multisensory conflicts modulate visual consciousness

Cerebral Projection of Mirrored Touch via sLORETA Imaging

Touch is one of the primary communication tools. Interestingly, the sensation of touch can also be experienced when observed in another person. Due to the system of mirror neurons, it is, in fact, being mapped on the somatosensory cortex of the observer. This phenomenon can be triggered not only by observing touch in another individual, but also by a mirror reflection of the contralateral limb. Our study aims to evaluate and localize changes in the intracerebral source activity via sLORETA imaging during the haptic stimulation of hands, while modifying this contact by a mirror illusion. A total of 10 healthy volunteers aged 23-42 years attended the experiment. The electrical brain activity was detected via scalp EEG. First, we registered the brain activity during resting state with open and with closed eyes, each for 5 min. Afterwards, the subjects were seated at a table with a mirror reflecting their left hand and occluding their right hand. The EEG was then recorded in 2 min sequencies during four modifications of the experiment (haptic contact on both hands, stimulation of the left hand only, right hand only and without any tactile stimuli). We randomized the order of the modifications for each participant. The obtained EEG data were converted into the sLORETA program and evaluated statistically at the significance level of p ≤ 0.05. The subjective experience of all the participants was registered using a survey. A statistically significant difference in source brain activity occurred during all four modifications of our experiment in the beta-2, beta-3 and delta frequency bands, resulting in the activation of 10 different Brodmann areas varying by modification. The results suggest that the summation of stimuli secured by interpersonal haptic contact modified by mirror illusion can activate the brain areas integrating motor, sensory and cognitive functions and further areas related to communication and understanding processes, including the mirror neuron system. We believe these findings may have potential for therapy.

Unconscious integration: Current evidence for integrative processing under subliminal conditions

Integrative processing is traditionally believed to be dependent on consciousness. While earlier studies within the last decade reported many types of integration under subliminal conditions (i.e. without perceptual awareness), these findings are widely challenged recently. This review evaluates the current evidence for 10 types of subliminal integration that are widely studied: arithmetic processing, object-context integration, multi-word processing, same-different processing, multisensory integration and 5 different types of associative learning. Potential methodological issues concerning awareness measures are also taken into account. It is concluded that while there is currently no reliable evidence for subliminal integration, this does not necessarily refute 'unconscious' integration defined through non-subliminal (e.g. implicit) approaches.

Seeing food fast and slow: Arousing pictures and words have reverse priorities in accessing awareness

Previous studies have shown that stimuli triggering higher arousal (e.g., attractiveness) can access awareness faster than those triggering lower arousal, yet no studies have examined the effect of food calories. Since food brings us energy, satiety, and positive emotions, food stimuli bringing higher arousal would likely have higher priority in accessing awareness over those with lower arousal. We used high-calorie and low-calorie food stimuli as representatives for high and low arousal stimuli, respectively, based on the tight relationship between calorie and arousal. By adopting the breaking continuous flash suppression (b-CFS) paradigm, we had high-calorie and low-calorie food pictures or words presented dichoptically with dynamic Mondrian masks and measured the time for food stimuli to be released from suppression. Our results showed that high-calorie food pictures could access visual awareness faster than low-calorie food pictures (Experiment 1), and the reverse pattern was observed for food words (Experiment 2). We ruled out the possibility of the difference in low-level features (Experiment 3) and post-perceptual response bias (Experiment 4) as the causes for the observed b-CFS time differences. This study revealed the dissociation of the unconscious processing of pictures and words, which may rely on mechanisms related to attentional capture. High-arousing stimuli do not always enjoy priority in accessing visual awareness.

The body-ownership is unconsciously distorted in the brain: An event-related potential study of rubber hand illusion

Many studies have reported that bottom-up multisensory integration of visual, tactile, and proprioceptive information can distort our sense of body-ownership, producing rubber hand illusion (RHI). There is less evidence about when and how the body-ownership is distorted in the brain during RHI. To examine whether this illusion effect occurs preattentively at an early stage of processing, we monitored the visual mismatch negativity (vMMN) component (the index of automatic deviant detection) and N2 (the index for conflict monitoring). Participants first performed an RHI elicitation task in a synchronous or asynchronous setting and then finished a passive visual oddball task in which the deviant stimuli were unrelated to the explicit task. A significant interaction between Deviancy (deviant hand vs. standard hand) and Group (synchronous vs. asynchronous) was found. The asynchronous group showed clear mismatch effects in both vMMN and N2, while the synchronous group had such effect only in N2. The results indicate that after the elicitation of RHI bottom-up integration could be retrieved at the early stage of sensory processing before top-down processing, providing evidence for the priority of the bottom-up processes after the generation of RHI and revealing the mechanism of how the body-ownership is unconsciously distorted in the brain.

Effects of Multimodal Association on Ambiguous Perception in Binocular Rivalry

When two eyes view dissimilar images, an observer typically reports ambiguous perception called binocular rivalry where the subjective perception fluctuates between the two inputs. This perceptual instability is often comprised of exclusive dominance of each image and a transition state called piecemeal state where the two images are intermingled in patchwork manner. Herein, we investigated the effects of multimodal association of sensory congruent pair, arbitrary pair, and reverse pair on piecemeal state in order to see how each level of association affects the ambiguous perception during binocular rivalry. To induce the multisensory associations, we designed a matching task with audiovisual feedback where subjects were required to respond according to given pairing rules. We found that explicit audiovisual associations can substantially affect the piecemeal state during binocular rivalry and that this congruency effect that reduces the amount of visual ambiguity originates primarily from explicit audiovisual association training rather than common sensory features. Furthermore, when one information is associated with multiple information, recent and preexisting associations work collectively to influence the perceptual ambiguity during rivalry. Our findings show that learned multimodal association directly affects the temporal dynamics of ambiguous perception during binocular rivalry by modulating not only the exclusive dominance but also the piecemeal state in a systematic manner.

Interferences between breathing, experimental dyspnoea and bodily self-consciousness

Dyspnoea, a subjective experience of breathing discomfort, is a most distressing symptom. It implicates complex cortical networks that partially overlap with those underlying bodily self-consciousness, the experience that the body is one's own within a given location (self-identification and self-location, respectively). Breathing as an interoceptive signal contributes to bodily self-consciousness: we predicted that inducing experimental dyspnoea would modify or disrupt this contribution. We also predicted that manipulating bodily self-consciousness with respiratory-visual stimulation would possibly attenuate dyspnoea. Twenty-five healthy volunteers were exposed to synchronous and asynchronous respiratory-visual illumination of an avatar during normal breathing and mechanically loaded breathing that elicited dyspnoea. During normal breathing, synchronous respiratory-visual stimulation induced illusory self-identification with the avatar and an illusory location of the subjects' breathing towards the avatar. This did not occur when respiratory-visual stimulation was performed during dyspnoea-inducing loaded breathing. In this condition, the affective impact of dyspnoea was attenuated by respiratory-visual stimulation, particularly when asynchronous. This study replicates and reinforces previous studies about the integration of interoceptive and exteroceptive signals in the construction of bodily self-consciousness. It confirms the existence of interferences between experimental dyspnoea and cognitive functions. It suggests that respiratory-visual stimulation should be tested as a non-pharmacological approach of dyspnoea treatment.

The Insula Mediates Access to Awareness of Visual Stimuli Presented Synchronously to the Heartbeat

The processing of interoceptive signals in the insular cortex is thought to underlie self-awareness. However, the influence of interoception on visual awareness and the role of the insular cortex in this process remain unclear. Here, we show in a series of experiments that the relative timing of visual stimuli with respect to the heartbeat modulates visual awareness. We used two masking techniques and show that conscious access for visual stimuli synchronous to participants' heartbeat is suppressed compared with the same stimuli presented asynchronously to their heartbeat. Two independent brain imaging experiments using high-resolution fMRI revealed that the insular cortex was sensitive to both visible and invisible cardio-visual stimulation, showing reduced activation for visual stimuli presented synchronously to the heartbeat. Our results show that interoceptive insular processing affects visual awareness, demonstrating the role of the insula in integrating interoceptive and exteroceptive signals and in the processing of conscious signals beyond self-awareness.

SIGNIFICANCE STATEMENT

There is growing evidence that interoceptive signals conveying information regarding the internal state of the body influence perception and self-awareness. The insular cortex, which receives sensory inputs from both interoceptive and exteroceptive sources, is thought to integrate these multimodal signals. This study shows that cardiac interoceptive signals modulate awareness for visual stimuli such that visual stimuli occurring at the cardiac frequency take longer to access visual awareness and are more difficult to discriminate. Two fMRI experiments show that the insular region is sensitive to this cardio-visual synchrony even when the visual stimuli are rendered invisible through interocular masking. The results indicate a perceptual and neural suppression for visual events coinciding with cardiac interoceptive signals.

Choice of analysis pathway dramatically affects statistical outcomes in breaking continuous flash suppression

Breaking Continuous Flash Suppression (bCFS) has been adopted as an appealing means to study human visual awareness, but the literature is beclouded by inconsistent and contradictory results. Although previous reviews have focused chiefly on design pitfalls and instances of false reasoning, we show in this study that the choice of analysis pathway can have severe effects on the statistical output when applied to bCFS data. Using a representative dataset designed to address a specific controversy in the realm of language processing under bCFS, namely whether psycholinguistic variables affect access to awareness, we present a range of analysis methods based on real instances in the published literature, and indicate how each approach affects the perceived outcome. We provide a summary of published bCFS studies indicating the use of data transformation and trimming, and highlight that more compelling analysis methods are sparsely used in this field. We discuss potential interpretations based on both classical and more complex analyses, to highlight how these differ. We conclude that an adherence to openly available data and analysis pathways could provide a great benefit to this field, so that conclusions can be tested against multiple analyses as standard practices are updated.

Consciousness is more than meets the eye: a call for a multisensory study of subjective experience

Over the last 30 years, our understanding of the neurocognitive bases of consciousness has improved, mostly through studies employing vision. While studying consciousness in the visual modality presents clear advantages, we believe that a comprehensive scientific account of subjective experience must not neglect other exteroceptive and interoceptive signals as well as the role of multisensory interactions for perceptual and self-consciousness. Here, we briefly review four distinct lines of work which converge in documenting how multisensory signals are processed across several levels and contents of consciousness. Namely, how multisensory interactions occur when consciousness is prevented because of perceptual manipulations (i.e. subliminal stimuli) or because of low vigilance states (i.e. sleep, anesthesia), how interactions between exteroceptive and interoceptive signals give rise to bodily self-consciousness, and how multisensory signals are combined to form metacognitive judgments. By describing the interactions between multisensory signals at the perceptual, cognitive, and metacognitive levels, we illustrate how stepping out the visual comfort zone may help in deriving refined accounts of consciousness, and may allow cancelling out idiosyncrasies of each sense to delineate supramodal mechanisms involved during consciousness.

Interplay between Narrative and Bodily Self in Access to Consciousness: No Difference between Self- and Non-self Attributes

The construct of the “self” is conceived as being fundamental in promoting survival. As such, extensive studies have documented preferential processing of self-relevant stimuli. For example, attributes that relate to the self are better encoded and retrieved, and are more readily consciously perceived. The preferential processing of self-relevant information, however, appears to be especially true for physical (e.g., faces), as opposed to psychological (e.g., traits), conceptions of the self. Here, we test whether semantic attributes that participants judge as self-relevant are further processed unconsciously than attributes that were not judged as self-relevant. In Experiment 1, a continuous flash suppression paradigm was employed with “self” and “non-self” attribute words being presented subliminally, and we asked participants to categorize unseen words as either self-related or not. In a second experiment, we attempted to boost putative preferential self-processing by relation to its physical conception, that is, one’s own body. To this aim, we repeated Experiment 1 while administrating acoustic stimuli either close or far from the body, i.e., within or outside peripersonal space. Results of both Experiment 1 and 2 demonstrate no difference in breaking suppression for self and non-self words. Additionally, we found that while participants were able to process the physical location of the unseen words (above or below fixation) they were not able to categorize these as self-relevant or not. Finally, results showed that sounds presented in the extra-personal space elicited a more stringent response criterion for “self” in the process of categorizing unseen visual stimuli. This shift in criterion as a consequence of sound location was restricted to the self, as no such effect was observed in the categorization of attributes occurring above or below fixation. Overall, our findings seem to indicate that subliminally presented stimuli are not semantically processed, at least inasmuch as to be categorized as self-relevant or not. However, we do demonstrate that the distance at which acoustic stimuli are presented may alter the balance between self- and non-self biases.

Touch Accelerates Visual Awareness

To efficiently interact with the external environment, our nervous system combines information arising from different sensory modalities. Recent evidence suggests that cross-modal interactions can be automatic and even unconscious, reflecting the ecological relevance of cross-modal processing. Here, we use continuous flash suppression (CFS) to directly investigate whether haptic signals can interact with visual signals outside of visual awareness. We measured suppression durations of visual gratings rendered invisible by CFS either during visual stimulation alone or during visuo-haptic stimulation. We found that active exploration of a haptic grating congruent in orientation with the suppressed visual grating reduced suppression durations both compared with visual-only stimulation and to incongruent visuo-haptic stimulation. We also found that the facilitatory effect of touch on visual suppression disappeared when the visual and haptic gratings were mismatched in either spatial frequency or orientation. Together, these results demonstrate that congruent touch can accelerate the rise to consciousness of a suppressed visual stimulus and that this unconscious cross-modal interaction depends on visuo-haptic congruency. Furthermore, since CFS suppression is thought to occur early in visual cortical processing, our data reinforce the evidence suggesting that visuo-haptic interactions can occur at the earliest stages of cortical processing.

Neuronal correlates of continuous manual tracking under varying visual movement feedback in a virtual reality environment

To accurately guide one's actions online, the brain predicts sensory action feedback ahead of time based on internal models, which can be updated by sensory prediction errors. The underlying operations can be experimentally investigated in sensorimotor adaptation tasks, in which moving under perturbed sensory action feedback requires internal model updates. Here we altered healthy participants' visual hand movement feedback in a virtual reality setup, while assessing brain activity with functional magnetic resonance imaging (fMRI). Participants tracked a continually moving virtual target object with a photorealistic, three-dimensional (3D) virtual hand controlled online via a data glove. During the continuous tracking task, the virtual hand's movements (i.e., visual movement feedback) were repeatedly periodically delayed, which participants had to compensate for to maintain accurate tracking. This realistic task design allowed us to simultaneously investigate processes likely operating at several levels of the brain's motor control hierarchy. FMRI revealed that the length of visual feedback delay was parametrically reflected by activity in the inferior parietal cortex and posterior temporal cortex. Unpredicted changes in visuomotor mapping (at transitions from synchronous to delayed visual feedback periods or vice versa) activated biological motion-sensitive regions in the lateral occipitotemporal cortex (LOTC). Activity in the posterior parietal cortex (PPC), focused on the contralateral anterior intraparietal sulcus (aIPS), correlated with tracking error, whereby this correlation was stronger in participants with higher tracking performance. Our results are in line with recent proposals of a wide-spread cortical motor control hierarchy, where temporoparietal regions seem to evaluate visuomotor congruence and thus possibly ground a self-attribution of movements, the LOTC likely processes early visual prediction errors, and the aIPS computes action goal errors and possibly corresponding motor corrections.