Weighing the evidence for a dorsal processing bias under continuous flash suppression

No effect of attentional modulation by spatial cueing in a masked numerical priming paradigm using continuous flash suppression (CFS)

One notion emerging from studies on unconscious visual processing is that different "blinding techniques" seem to suppress the conscious perception of stimuli at different levels of the neurocognitive architecture. However, even when only the results from a single suppression method are compared, the picture of the scope and limits of unconscious visual processing remains strikingly heterogeneous, as in the case of continuous flash suppression (CFS). To resolve this issue, it has been suggested that high-level semantic processing under CFS is facilitated whenever interocular suppression is attenuated by the removal of visuospatial attention. In this behavioral study, we aimed to further investigate this "CFS-attenuation-by-inattention" hypothesis in a numerical priming study using spatial cueing. Participants performed a number comparison task on a visible target number ("compare number to five"). Prime-target pairs were either congruent (both numbers smaller, or both larger than five) or incongruent. Based on the "CFS-attenuation-by-inattention" hypothesis, we predicted that reaction times (RTs) for congruent prime-target pairs should be faster than for incongruent ones, but only when the prime was presented at the uncued location. In the invisible condition, we observed no priming effects and thus no evidence in support of the "CFS-attenuation-by-inattention" hypothesis. In the visible condition, we found an inverse effect of prime-target congruency. Our results agree with the notion that the representation of CF-suppressed stimuli is fractionated, and limited to their basic, elemental features, thus precluding semantic processing.

Probing the attentional modulation of unconscious processing under interocular suppression in a spatial cueing paradigm

The debate about the scope and limits of unconscious visual processing under continuous flash suppression (CFS) has created a heterogeneous set of divergent findings that are yet to be reconciled. Attention has been suggested as an important factor in modulating the processing of suppressed visual information under CFS. Specifically, Eo et al. (2016) reported that semantic processing under CFS can be significantly facilitated when spatial attention is diverted away from the suppressed stimulus. Based on event-related potential (ERP) findings involving the N400, they proposed that inattention attenuates interocular suppression and thereby makes semantic processing available unconsciously, potentially reconciling conflicting evidence in the literature. In this study, we aimed to further investigate the "CFS-attenuation-by-inattention" hypothesis using functional magnetic resonance imaging (fMRI) and multivariate pattern analysis (MVPA). We tested whether the decodability of object category increases under CFS when attention is diverted away from the suppressed stimulus in a spatial cueing task. Our results provide no evidence for the "CFS-attenuation-by-inattention" hypothesis, but show higher decoding accuracies for visible stimuli than for invisible stimuli. We discuss the implications of our findings for the important endeavor of trying to reconcile the divergent reports of unconscious processing under CFS.

Informative neural representations of unseen contents during higher-order processing in human brains and deep artificial networks

A framework to pinpoint the scope of unconscious processing is critical to improve models of visual consciousness. Previous research observed brain signatures of unconscious processing in visual cortex, but these were not reliably identified. Further, whether unconscious contents are represented in high-level stages of the ventral visual stream and linked parieto-frontal areas remains unknown. Using a within-subject, high-precision functional magnetic resonance imaging approach, we show that unconscious contents can be decoded from multi-voxel patterns that are highly distributed alongside the ventral visual pathway and also involving parieto-frontal substrates. Classifiers trained with multi-voxel patterns of conscious items generalized to predict the unconscious counterparts, indicating that their neural representations overlap. These findings suggest revisions to models of consciousness such as the neuronal global workspace. We then provide a computational simulation of visual processing/representation without perceptual sensitivity by using deep neural networks performing a similar visual task. The work provides a framework for pinpointing the representation of unconscious knowledge across different task domains.

Unconscious social relation threats: Invisible boss face biases attention

Threatening stimuli as a kind of salient information often guide attentional orienting. Besides physically threatening stimuli, social threats can also strongly bias attention, even in the absence of conscious awareness. However, the available evidence mainly came from studies on an emotional face. It is unclear whether social relation threats, such as a boss face without emotional expressions, can also direct attentional orienting unconsciously. This study aimed to reveal the extent to which the attentional system has developed to process threatening stimuli by exploring whether invisible social relation threats unconsciously biased attention. We asked graduate and undergraduate students to perform a modified Posner's cue-target task, in which the probe was preceded by a pair of competitive face cues (an advisor's face and another faculty member's face), rendered invisible through continuous flash suppression. Experiment 1a's results showed that the advisor's face reflexively oriented graduate students' spatial attention, which was significantly correlated with subjective social threat evaluation. However, Experiment 1b showed that an invisible advisor's face did not induce the same effect in undergraduate students, as they reported significantly fewer threats from their advisors than graduates. To ensure the robustness of this new effect, we preregistered a replicate study and successfully replicated the above results in Experiments 2a and 2b. Our findings provide evidence for the existence of an attentional orienting bias toward invisible social relation threats. These results suggest that the attentional system evolved to promote the exploration of our visual environment for threatening social relation signals.

Continuous flash suppression: Known and unknowns

Studies utilizing continuous flash suppression (CFS) provide valuable information regarding conscious and nonconscious perception. There are, however, crucial unanswered questions regarding the mechanisms of suppression and the level of visual processing in the absence of consciousness with CFS. Research suggests that the answers to these questions depend on the experimental configuration and how we assess consciousness in these studies. The aim of this review is to evaluate the impact of different experimental configurations and the assessment of consciousness on the results of the previous CFS studies. We review studies that evaluated the influence of different experimental configuration on the depth of suppression with CFS and discuss how different assessments of consciousness may impact the results of CFS studies. Finally, we review behavioral and brain recording studies of CFS. In conclusion, previous studies provide evidence for survival of low-level visual information and complete impairment of high-level visual information under the influence of CFS. That is, studies suggest that nonconscious perception of lower-level visual information happens with CFS, but there is no evidence for nonconscious high-level recognition with CFS.

Conscious awareness modulates processing speed in the redundant signal effect

Evidence for the influence of unaware signals on behaviour has been reported in both patient groups and healthy observers using the Redundant Signal Effect (RSE). The RSE refers to faster manual reaction times to the onset of multiple simultaneously presented target than those to a single stimulus. These findings are robust and apply to unimodal and multi-modal sensory inputs. A number of studies on neurologically impaired cases have demonstrated that RSE can be found even in the absence of conscious experience of the redundant signals. Here, we investigated behavioural changes associated with awareness in healthy observers by using Continuous Flash Suppression to render observers unaware of redundant targets. Across three experiments, we found an association between reaction times to the onset of a consciously perceived target and the reported level of visual awareness of the redundant target, with higher awareness being associated with faster reaction times. However, in the absence of any awareness of the redundant target, we found no evidence for speeded reaction times and even weak evidence for an inhibitory effect (slowing down of reaction times) on response to the seen target. These findings reveal marked differences between healthy observers and blindsight patients in how aware and unaware information from different locations is integrated in the RSE.

Color Can Shorten Breakthrough Times in Continuous Flash Suppression through Increased Salience and Task Relevance

Color can enhance the perception of relevant stimuli by increasing their salience and guiding visual search towards stimuli that match a task-relevant color. Using Continuous Flash Suppression (CFS), the current study investigated whether color facilitates the discrimination of targets that are difficult to perceive due to interocular suppression. Gabor patterns of two or four cycles per degree (cpd) were shown as targets to the non-dominant eye of human participants. CFS masks were presented at a rate of 10 Hz to the dominant eye, and participants had the task to report the target’s orientation as soon as they could discriminate it. The 2-cpd targets were robustly suppressed and resulted in much longer response times compared to 4-cpd targets. Moreover, only for 2-cpd targets, two color-related effects were evident. First, in trials where targets and CFS masks had different colors, targets were reported faster than in trials where targets and CFS masks had the same color. Second, targets with a known color, either cyan or yellow, were reported earlier than targets whose color was randomly cyan or yellow. The results suggest that the targets’ entry to consciousness may have been speeded by color-mediated effects relating to increased (bottom-up) salience and (top-down) task relevance.

Dynamic face mask enhances continuous flash suppression

In continuous flash suppression (CFS), an image presented to one eye is suppressed from awareness by a dynamic image masker presented to the other eye. Previous studies report that face stimuli break out of CFS more readily when they are oriented upright and contain ecologically relevant information such as facial expressions or direct eye gaze, potentially implicating face processing in the mechanisms of interocular competition. It is unknown, however, whether face content helps to drive interocular suppression when incorporated into the dynamic masker itself, either by engaging higher-level visual mechanisms that underlie face detection or due to lower-level image features that the faces happen to contain. To investigate this, we devised a dynamic mask composed of upright faces and tested how well it suppressed detection of face or grating targets presented to the other eye. Relative contributions of higher-level and lower-level features were compared by manipulating the image properties of the mask. Results show that the dynamic face mask is strikingly effective at suppressing sensory input presented to the opposing eye, but its effectiveness is largely attributable to image texture, which can be quantified in terms of image entropy and edge density. This is because strong suppression was still observed following phase-scrambling or spatial inversion of the face elements, and while a target-selective effect was observed for the face mask, inverting the face elements to interfere with configural processing did not significantly diminish this effect. Thus, visual properties of faces, such as their image entropy and complex phase structure, predominate in driving interocular suppression rather than face detection per se.

Novel procedure for generating continuous flash suppression: Seurat meets Mondrian

Continuous flash suppression (CFS) entails presentation of a stationary target to one eye and an animated sequence of arrays of geometric figures, the mask, to the other eye. The prototypical CFS sequence comprises different sized rectangles of various colors, dubbed Mondrians. Presented as a rapid, changing sequence to one eye, Mondrians or other similarly constructed textured arrays can abolish awareness of the target viewed by the other eye for many seconds at a time, producing target suppression durations much longer than those associated with conventional binocular rivalry. We have devised an animation technique that replaces meaningless Mondrian figures with recognizable visual objects and scenes as inducers of CFS, allowing explicit manipulation of the visual semantic content of those masks. By converting each image of these CFS sequences into successively presented objects or scenes each comprised of many small, circular patches of color, we create pointillist CFS sequences closely matched in terms of their spatio-temporal power spectra. Randomly rearranging the positions of the pointillist patches scrambles the images so they are no longer recognizable. CFS sequences comprising a stream of different objects produces more robust interocular suppression than do sequences comprising a stream of different scenes, even when the two categories of CFS are matched in root mean square contrast and spatial frequency content. Factors promoting these differences in CFS potency could range from low-level, image-based features to high-level factors including attention and recognizability. At the same time, object- and scene-based CFS sequences, when themselves suppressed from awareness, do not differ in their durations of suppression, implying that semantic content of those images comprising CFS sequences are not registered during suppression. The pointillist technique itself offers a potentially useful means for examining the impact of high-level image meaning on aspects of visual perception other than interocular suppression.

Invisible Flashes Alter Perceived Sound Location

Information integration across the senses is fundamental for effective interactions with our environment. The extent to which signals from different senses can interact in the absence of awareness is controversial. Combining the spatial ventriloquist illusion and dynamic continuous flash suppression (dCFS), we investigated in a series of two experiments whether visual signals that observers do not consciously perceive can influence spatial perception of sounds. Importantly, dCFS obliterated visual awareness only on a fraction of trials allowing us to compare spatial ventriloquism for physically identical flashes that were judged as visible or invisible. Our results show a stronger ventriloquist effect for visible than invisible flashes. Critically, a robust ventriloquist effect emerged also for invisible flashes even when participants were at chance when locating the flash. Collectively, our findings demonstrate that signals that we are not aware of in one sensory modality can alter spatial perception of signals in another sensory modality.

More than Action: The Dorsal Pathway Contributes to the Perception of 3-D Structure

An evolving view in cognitive neuroscience is that the dorsal visual pathway not only plays a key role in visuomotor behavior but that it also contributes functionally to the recognition of objects. To characterize the nature of the object representations derived by the dorsal pathway, we assessed perceptual performance in the context of the continuous flash suppression paradigm, which suppresses object processing in the ventral pathway while sparing computation in the dorsal pathway. In a series of experiments, prime stimuli, which were rendered imperceptible by the continuous flash suppression, still contributed to perceptual decisions related to the subsequent perceptible target stimuli. However, the contribution of the prime to perception was contingent on the prime's structural coherence, in that a perceptual advantage was observed only for targets primed by objects with legitimate 3-D structure. Finally, we obtained additional evidence to demonstrate that the processing of the suppressed objects was contingent on the magnocellular, rather than the parvocellular, system, further linking the processing of the suppressed stimuli to the dorsal pathway. Together, these results provide novel evidence that the dorsal pathway does not only support visuomotor control but, rather, that it also derives the structural description of 3-D objects and contributes to shape perception.

Ventral and Dorsal Pathways Relate Differently to Visual Awareness of Body Postures under Continuous Flash Suppression

Visual perception includes ventral and dorsal stream processes. However, it is still unclear whether the former is predominantly related to conscious and the latter to nonconscious visual perception as argued in the literature. In this study upright and inverted body postures were rendered either visible or invisible under continuous flash suppression (CFS), while brain activity of human participants was measured with functional MRI (fMRI). Activity in the ventral body-sensitive areas was higher during visible conditions. In comparison, activity in the posterior part of the bilateral intraparietal sulcus (IPS) showed a significant interaction of stimulus orientation and visibility. Our results provide evidence that dorsal stream areas are less associated with visual awareness.

Unconscious response priming during continuous flash suppression

Continuous flash suppression (CFS) has become a popular tool for studying unconscious processing, but the level at which unconscious processing of visual stimuli occurs under CFS is not clear. Response priming is a robust and well-understood phenomenon, in which the prime stimulus facilitates overt responses to targets if the prime and target are associated with the same response. We used CFS to study unconscious response priming of shape: arrows with left or right orientation served as primes and targets. The prime was presented near the limen of consciousness and each trial was followed by subjective rating of visibility and a forced-choice response concerning the orientation of the prime in counterbalanced order. In trials without any reported awareness of the presence of the prime, discrimination of the prime's orientation was at chance level. However, priming was elicited in such unconscious trials. Unconscious priming was not influenced by the prime-target onset-asynchrony (SOA)/prime duration, whereas conscious processing, as indicated by the enhanced discriminability of the prime's orientation and conscious priming, increased at the longest SOAs/prime durations. These results show that conscious and unconscious processes can be dissociated with CFS and that CFS-masking does not completely suppress unconscious visual processing of shape.

Unaware Processing of Tools in the Neural System for Object-Directed Action Representation

The hypothesis that the brain constitutively encodes observed manipulable objects for the actions they afford is still debated. Yet, crucial evidence demonstrating that, even in the absence of perceptual awareness, the mere visual appearance of a manipulable object triggers a visuomotor coding in the action representation system including the premotor cortex, has hitherto not been provided. In this fMRI study, we instantiated reliable unaware visual perception conditions by means of continuous flash suppression, and we tested in 24 healthy human participants (13 females) whether the visuomotor object-directed action representation system that includes left-hemispheric premotor, parietal, and posterior temporal cortices is activated even under subliminal perceptual conditions. We found consistent activation in the target visuomotor cortices, both with and without perceptual awareness, specifically for pictures of manipulable versus non-manipulable objects. By means of a multivariate searchlight analysis, we also found that the brain activation patterns in this visuomotor network enabled the decoding of manipulable versus non-manipulable object picture processing, both with and without awareness. These findings demonstrate the intimate neural coupling between visual perception and motor representation that underlies manipulable object processing: manipulable object stimuli specifically engage the visuomotor object-directed action representation system, in a constitutive manner that is independent from perceptual awareness. This perceptuo-motor coupling endows the brain with an efficient mechanism for monitoring and planning reactions to external stimuli in the absence of awareness.SIGNIFICANCE STATEMENT Our brain constantly encodes the visual information that hits the retina, leading to a stimulus-specific activation of sensory and semantic representations, even for objects that we do not consciously perceive. Do these unconscious representations encompass the motor programming of actions that could be accomplished congruently with the objects' functions? In this fMRI study, we instantiated unaware visual perception conditions, by dynamically suppressing the visibility of manipulable object pictures with mondrian masks. Despite escaping conscious perception, manipulable objects activated an object-directed action representation system that includes left-hemispheric premotor, parietal, and posterior temporal cortices. This demonstrates that visuomotor encoding occurs independently of conscious object perception.

Size Aftereffects Are Eliminated When Adaptor Stimuli Are Prevented from Reaching Awareness by Continuous Flash Suppression

Size aftereffects are a compelling perceptual phenomenon in which we perceive the size of a stimulus as being different than it actually is following a period of visual stimulation of an adapter stimulus with a different size. Here, we used continuous flash suppression (CFS) to determine if size aftereffects require a high-level appraisal of the adapter stimulus. The strength of size aftereffects was quantified following a 3-s exposure to perceptually visible and invisible adapters. Participants judged the size of a target that followed the adapter in comparison to a subsequent reference. Our experiments demonstrate that the adapter no longer influenced the perceived size of the subsequent target stimulus under CFS. We conclude that the perception of size aftereffects is prevented when CFS is used to suppress the conscious awarness of the adapting stimulus.

Differential modulation of visual object processing in dorsal and ventral stream by stimulus visibility

As a functional organization principle in cortical visual information processing, the influential 'two visual systems' hypothesis proposes a division of labor between a dorsal "vision-for-action" and a ventral "vision-for-perception" stream. A core assumption of this model is that the two visual streams are differentially involved in visual awareness: ventral stream processing is closely linked to awareness while dorsal stream processing is not. In this functional magnetic resonance imaging (fMRI) study with human observers, we directly probed the stimulus-related information encoded in fMRI response patterns in both visual streams as a function of stimulus visibility. We parametrically modulated the visibility of face and tool stimuli by varying the contrasts of the masks in a continuous flash suppression (CFS) paradigm. We found that visibility - operationalized by objective and subjective measures - decreased proportionally with increasing log CFS mask contrast. Neuronally, this relationship was closely matched by ventral visual areas, showing a linear decrease of stimulus-related information with increasing mask contrast. Stimulus-related information in dorsal areas also showed a dependency on mask contrast, but the decrease rather followed a step function instead of a linear function. Together, our results suggest that both the ventral and the dorsal visual stream are linked to visual awareness, but neural activity in ventral areas more closely reflects graded differences in awareness compared to dorsal areas.

Visual Awareness Is Limited by the Representational Architecture of the Visual System

Visual perception and awareness have strict limitations. We suggest that one source of these limitations is the representational architecture of the visual system. Under this view, the extent to which items activate the same neural channels constrains the amount of information that can be processed by the visual system and ultimately reach awareness. Here, we measured how well stimuli from different categories (e.g., faces and cars) blocked one another from reaching awareness using two distinct paradigms that render stimuli invisible: visual masking and continuous flash suppression. Next, we used fMRI to measure the similarity of the neural responses elicited by these categories across the entire visual hierarchy. Overall, we found strong brain–behavior correlations within the ventral pathway, weaker correlations in the dorsal pathway, and no correlations in early visual cortex (V1–V3). These results suggest that the organization of higher level visual cortex constrains visual awareness and the overall processing capacity of visual cognition.

Preferential awareness of protofacial stimuli in autism

It has been suggested that a subcortically mediated, innate sensitivity to protofacial stimuli leads to specialized face processing and to the development of the social brain. A dysfunction of this face-processing pathway has been associated with atypical social development in individuals with autism spectrum disorder (ASD). This study investigated whether individuals with ASD exhibit primary sensitivity to monochrome protoface stimuli using continuous flash suppression (CFS). Under CFS, visual stimuli are suppressed from awareness, and cortical processing is strongly reduced while subcortical regions continue to respond to invisible stimuli. We found that both adolescents with ASD and typically developing adolescents showed preferential detection of upright protoface stimuli under CFS but not in a non-CFS control condition. These results challenge the notion that a primitive sensitivity to protoface stimuli is essential for typical social development. Rather, our findings suggest such sensitivity is not a sufficient condition for typical social development and that the presence of other complementary factors is necessary for the development of the social brain.

Definitely maybe: can unconscious processes perform the same functions as conscious processes?

Hassin recently proposed the “Yes It Can” (YIC) principle to describe the division of labor between conscious and unconscious processes in human cognition. According to this principle, unconscious processes can carry out every fundamental high-level cognitive function that conscious processes can perform. In our commentary, we argue that the author presents an overly idealized review of the literature in support of the YIC principle. Furthermore, we point out that the dissimilar trends observed in social and cognitive psychology, with respect to published evidence of strong unconscious effects, can better be explained by the way how awareness is defined and measured in both research fields. Finally, we show that the experimental paradigm chosen by Hassin to rule out remaining objections against the YIC principle is unsuited to verify the new default notion that all high-level cognitive functions can unfold unconsciously.