Invisible Stimuli Research Articles

User Verification Using Evoked EEG by Invisible Visual Stimulation

Person authentication using biometric information has recently become popular among researchers. User management based on biometrics is more reliable than that using conventional methods. To secure private information, it is necessary to build continuous authentication-based user management systems. Brain waves are suitable biometric modalities for continuous authentication. This study is based on biometric authentication using brain waves evoked by invisible visual stimuli. Invisible visual stimulation is considered over visual stimulation to overcome the obstacles faced by a user when using a system. Invisible stimuli are confirmed by changing the intensity of the image and presenting high-speed stimulation. To ensure invisibility, stimuli of different intensities were tested, and the stimuli with an intensity of 5 % was confirmed to be invisible. To improve the verification performance, a continuous wavelet transform was introduced over the Fourier transform because it extracts both time and frequency information from the brain wave. The scalogram obtained by the wavelet transform was used as an individual feature and for synchronizing the template and test data. Furthermore, to improve the synchronization performance, the waveband was split based on the power distribution of the scalogram. A performance evaluation using 20 subjects showed an equal error rate of 3.8 %.

The role of awareness in shaping responses in human visual cortex

The visual cortex contains information about stimuli even when they are not consciously perceived. However, it remains unknown whether the visual system integrates local features into global objects without awareness. Here, we tested this by measuring brain activity in human observers viewing fragmented shapes that were either visible or rendered invisible by fast counterphase flicker. We then projected measured neural responses to these stimuli back into visual space. Visible stimuli caused robust responses reflecting the positions of their component fragments. Their neural representations also strongly resembled one another regardless of local features. By contrast, representations of invisible stimuli differed from one another and, crucially, also from visible stimuli. Our results demonstrate that even the early visual cortex encodes unconscious visual information differently from conscious information, presumably by only encoding local features. This could explain previous conflicting behavioural findings on unconscious visual processing.

Afterimage duration depends on how deeply invisible stimuli were suppressed.

Quantifying visual responses to stimuli that are outside of awareness is a critical task for the study of visual consciousness. The current study psychophysically investigated whether afterimages reflect visual responses to stimuli that are not consciously visible throughout adaptation due to interocular suppression. A Gabor adaptor was presented to one eye of the observer, and a counterphase-flickering Gabor suppressor was presented to the other eye, thereby rendering the adaptor invisible during adaptation. To manipulate the depth of the suppression of the invisible adaptor, we varied the orientation difference between the adaptor and suppressor. We found that, even though the adaptor was not visible during adaptation, the afterimage duration varied depending on the orientation selectivity of interocular suppression. The duration was the shortest when the orientations of the adaptor and suppressor were identical and lengthened when the orientation differences increased. This finding could not be explained by confounding factors such as potential changes in contrast sensitivity that were caused by the suppressor. Our findings suggest that the magnitude of visual responses to stimuli suppressed below the threshold of awareness can be measured using the afterimage duration. Afterimages could be an effective tool for quantifying visual responses, irrespective of observers' conscious awareness of a presented stimulus.

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.

Transcranial magnetic stimulation (TMS)-induced Blindsight of Orientation is Degraded Conscious Vision

Patients with blindsight are blind due to an early visual cortical lesion, but they can discriminate stimuli presented to the blind visual field better than chance. Studies using transcranial magnetic stimulation (TMS) of early visual cortex have tried to induce blindsight-like behaviour in neurologically healthy individuals, but the studies have yielded varied results. We hypothesized that previous demonstrations of TMS-induced blindsight may result from degraded awareness of the stimuli due to the use of dichotomous visibility scales in measuring awareness. In the present study, TMS was applied to early visual cortex during an orientation discrimination task and the subjective scale measuring awareness was manipulated: The participants reported their conscious perception either using a dichotomous scale or a 4-point Perceptual Awareness Scale. Although the results with the dichotomous scale replicated previous reports of blindsight-like behaviour, there was no evidence of TMS-induced blindsight for orientation when the participants used the lowest rating of the 4-point graded scale to indicate that they were not aware of the presence of the stimulus. Moreover, signal detection analyses indicated that across participants, the individual’s sensitivity to consciously discriminate orientation predicted behaviour on reportedly unconscious trials. These results suggest that blindsight-like discrimination of orientation in neurologically healthy individuals does not occur for completely invisible stimuli, that is, when the observers do not report any kind of consciousness of the stimulus. TMS-induced blindsight for orientation is likely degraded conscious vision.

The human visual system differentially represents subjectively and objectively invisible stimuli.

The study of unconscious processing requires a measure of conscious awareness. Awareness measures can be either subjective (based on participant's report) or objective (based on perceptual performance). The preferred awareness measure depends on the theoretical position about consciousness and may influence conclusions about the extent of unconscious processing and about the neural correlates of consciousness. We obtained functional magnetic resonance imaging (fMRI) measurements from 43 subjects while they viewed masked faces and houses that were either subjectively or objectively invisible. Even for objectively invisible (perceptually indiscriminable) stimuli, we found significant category information in both early, lower-level visual areas and in higher-level visual cortex, although representations in anterior, category-selective ventrotemporal areas were less robust. For subjectively invisible stimuli, similar to visible stimuli, there was a clear posterior-to-anterior gradient in visual cortex, with stronger category information in ventrotemporal cortex than in early visual cortex. For objectively invisible stimuli, however, category information remained virtually unchanged from early visual cortex to object- and category-selective visual areas. These results demonstrate that although both objectively and subjectively invisible stimuli are represented in visual cortex, the extent of unconscious information processing is influenced by the measurement approach. Furthermore, our data show that subjective and objective approaches are associated with different neural correlates of consciousness and thus have implications for neural theories of consciousness.

“A Christy Minstrel, a Harlequin, or an Ancient Persian”?: Opera, Hindustani Classical Music, and the Origins of the Popular South Asian “Musical”

The story of South Asian colonial modernity and music offers up a multidirectional and polymorphous conceptual terrain featuring, among many agents, Hindustani royalty, touring minstrel and burlesque troupes, Jesuit missionaries and orientalists, and not least, social reformists. Nevertheless, scholarship on the history of Hindustani music consistently traces its development through classicization against the rise of Hindu nationalism while overlooking other palpable clues in the colonial past. This article argues for a substantial reevaluation of colonial South Asian music by positing an alternative and hitherto invisible auditory stimulus in colonial Asia's aural landscape: opera. Janaki Bakhle contends that “as a musical form, opera put down even fewer roots than did orchestral, instrumental Western classical music,” even though she subsequently states that “Western orchestration did become part of modern ceremonial activities, and it moved into film music even as it was played by ersatz marching bands.” Bakhle further argues that Hindustani music underwent processes of sanitization and systematization within a Hindu nation-making project, a view that has been complicated by historians such as Tejaswini Niranjana. Niranjana describes how scholarship that focuses exclusively on the codification or nationalization of Hindustani music through the interpellation of a Hindu public neglects “sedimented forms of musical persistence.” Not dissimilarly, Richard David Williams highlights how the singular emphasis on the movement of Hindustani music reform risks reducing the heterogeneous and complex musicological traditions in the colonial period to the output of a single, monolithic, middle-class “new elite.” Previous scholarship, he argues, concentrates on “one player in a larger ‘economy’ of musical consumption.” Following these calls for more textured perspectives on South Asian musical cultures, I suggest a somewhat heretical thesis: that opera functioned as a common mediating stimulus for both the colonial reinscription of Hindustani music as classical as well as the emergence of popular pan-Asian musical genres such as “Bollywood” music.

Limitations of concurrently representing objects within view and in visual working memory

Representing visibly present stimuli is as limited in capacity as representing invisible stimuli in visual working memory (WM). In this study, we explored whether concurrently representing stimuli within view affects representing objects in visual WM, and if so, whether this effect is modulated by the storage states (active and silent state) of memory contents? In experiment 1, participants were asked to perform the change-detect task in a simultaneous-representing condition in which WM content and the continuously-visible stimuli in view were simultaneously represented, as well as a baseline condition in which only the representations of visual WM content were maintained. The results showed that the representations in visual WM would be impaired when the continuously-visible stimuli in view were concurrently represented, revealed by the reduced CDA amplitude and the lower behavior performance. In experiment 2, a dual-serial retro-cue paradigm was adopted to guide participants to maintain memory items in two different storage states, and the results revealed that simultaneously representing the continuously-visible stimuli and the WM content would only impair the WM representations in the active state. These evidences demonstrated that only the visual WM representations that were maintained in the active state would definitely share the limited resources with the representations of continuously-visible information, and further supported the dissociation between the active state and silent state of visual WM storage.

The relation of discrete stimuli can be integrated despite the failure of conscious identification

ABSTRACTThe integration of the sameness or difference of two unconscious stimuli occurs and affects behaviour without subjective visual consciousness. Yet, intermittent stimulus awareness or a process simpler than integration could also account for these results. In a masked go/no-go task, participants provided or withheld a response based on the same/different relation of two stimuli. Post-trial performance on a discrimination task was used to classify invisible stimuli, providing an objective criterion of awareness. Stimulus pairs with a No-go-associated relation showed reduced and slower responding even when those stimuli were incorrectly discriminated and presumably unconscious. Due to the large set of stimuli, this effect is unlikely to be due to response mapping or action triggers. The integration of the relation of discrete stimuli is a cognitive function that is not dependent upon visual awareness, in the form of conscious identification, to occur.

Spatial Attention Enhances the Neural Representation of Invisible Signals Embedded in Noise.

Recent evidence suggests that voluntary spatial attention can affect neural processing of visual stimuli that do not enter conscious awareness (i.e., invisible stimuli), supporting the notion that attention and awareness are dissociable processes [Wyart, V., Dehaene, S., & Tallon-Baudry, C. Early dissociation between neural signatures of endogenous spatial attention and perceptual awareness during visual masking. Frontiers in Human Neuroscience, 6, 1-14, 2012; Watanabe, M., Cheng, K., Murayama, Y., Ueno, K., Asamizuya, T., Tanaka, K., et al. Attention but not awareness modulates the BOLD signal in the human V1 during binocular suppression. Science, 334, 829-831, 2011]. To date, however, no study has demonstrated that these effects reflect enhancement of the neural representation of invisible stimuli per se, as opposed to other neural processes not specifically tied to the stimulus in question. In addition, it remains unclear whether spatial attention can modulate neural representations of invisible stimuli in direct competition with highly salient and visible stimuli. Here we developed a novel EEG frequency-tagging paradigm to obtain a continuous readout of human brain activity associated with visible and invisible signals embedded in dynamic noise. Participants ( n = 23) detected occasional contrast changes in one of two flickering image streams on either side of fixation. Each image stream contained a visible or invisible signal embedded in every second noise image, the visibility of which was titrated and checked using a two-interval forced-choice detection task. Steady-state visual-evoked potentials were computed from EEG data at the signal and noise frequencies of interest. Cluster-based permutation analyses revealed significant neural responses to both visible and invisible signals across posterior scalp electrodes. Control analyses revealed that these responses did not reflect a subharmonic response to noise stimuli. In line with previous findings, spatial attention increased the neural representation of visible signals. Crucially, spatial attention also increased the neural representation of invisible signals. As such, the present results replicate and extend previous studies by demonstrating that attention can modulate the neural representation of invisible signals that are in direct competition with highly salient masking stimuli.

Continuous Flash Suppression: Stimulus Fractionation rather than Integration

Recent studies using continuous flash suppression suggest that invisible stimuli are processed as integrated, semantic entities. We challenge the viability of this account, given recent findings on the neural basis of interocular suppression and replication failures of high-profile CFS studies. We conclude that CFS reveals stimulus fractionation in visual cortex.

When Perception Trumps Reality

Is semantic priming driven by the objective or perceived meaning of the priming stimulus? This question is relevant given that many studies suggest that the objective meaning of invisible stimuli can influence cognitive processes and behavior. In an experiment involving 66 participants, we tested how the perceived meaning of misperceived stimuli influenced response times. Stroop priming (i.e., longer response times for incongruent than for congruent prime-target pairs) was observed in trials in which the prime was correctly identified. However, reversed Stroop priming was observed when the prime stimulus was incorrectly identified. Even in trials in which participants reported no perception of the prime and identified the primes at close to chance level (i.e., trials that meet both subjective and objective definitions of being subliminal), Stroop priming corresponded to perceived congruency, not objective congruency. This result suggests that occasional weak percepts and mispercepts are intermixed with no percepts in conditions traditionally claimed to be subliminal, casting doubt on claims of subliminal priming made in previous reports.

Unconscious processing of invisible visual stimuli.

Unconscious processing of subliminal visual information, as illustrated by the above-chance accuracy in discriminating invisible visual stimuli, is evident in both blindsight patients and healthy human observers. However, the dependence of such unconscious processing on stimulus properties remains unclear. Here we studied the impact of stimulus luminance and stimulus complexity on the extent of unconscious processing. A testing stimulus presented to one eye was rendered invisible by a masking stimulus presented to the other eye, and healthy human participants made a forced-choice discrimination of the stimulus identity followed by a report of the perceptual awareness. Without awareness of the stimulus existence, participants could nevertheless reach above-chance accuracy in discriminating the stimulus identity. Importantly, the discrimination accuracy for invisible stimuli increased with the stimulus luminance and decreased with the stimulus complexity. These findings suggested that the input signal strength and the input signal complexity can affect the extent of unconscious processing without altering the subjective awareness.

Visible and invisible stimulus parts integrate into global object representations as revealed by combining monocular and binocular rivalry.

Our visual system faces the challenging task to construct integrated visual representations from the visual input projected on our retinae. Previous research has provided mixed evidence as to whether visual awareness of the stimulus parts is required for such integration to occur. Here, we address this issue by taking a novel approach in which we combine a monocular rivalry stimulus (i.e., a bistable rotating cylinder) with binocular rivalry. The results of Experiment 1 show that in a rivalry condition, where one half of the cylinder is perceptually suppressed, significantly more perceptual switches occur that are consistent with visual integration of the whole cylinder than occur in a control condition, where only half of the cylinder is presented at a time and the presentation of the two images is physically alternated. In Experiment 2, stimulation in the observer's dominant eye was kept dominant by presenting the half cylinder in this eye at higher contrast and by surrounding it with a flickering context. Results show that the strong convexity bias that was found in a control condition, where no stimulus was presented in the suppressed eye, almost completely disappears when the unseen half is presented in the suppressed eye, indicating that both halves visually integrate and, subsequently, compete for convexity. These findings provide evidence that unseen visual information is biased towards a representation that is congruent with the current visible representation and, hence, that principles of perceptual organization also apply to parts of the visual input that remain unseen by the observer.

What Type of Awareness Does Binocular Rivalry Assess?

Recent experiments demonstrate that invisible stimulus features can induce binocular rivalry, indicating the phenomenon may be caused by differences in perceptual signal strength rather than conscious selection processes. Here, we clarify binocular rivalry's role in consciousness research by highlighting a critical difference between two distinct types of visual awareness.

Less Is More: Semantic Information Survives Interocular Suppression When Attention Is Diverted.

The extent of unconscious semantic processing has been debated. It is well established that semantic information is registered in the absence of awareness induced by inattention. However, it has been debated whether semantic information of invisible stimuli is processed during interocular suppression, a procedure that renders one eye's view invisible by presenting a dissimilar stimulus to the other eye. Inspired by recent evidence demonstrating that reduced attention attenuates interocular suppression, we tested a counterintuitive hypothesis that attention withdrawn from the suppressed target location facilitates semantic processing in the absence of awareness induced by interocular suppression. We obtained an electrophysiological marker of semantic processing (N400 component) while human participants' spatial attention was being manipulated with a cueing paradigm during interocular suppression. We found that N400 modulation was absent when participants' attention was directed to the target location, but present when diverted elsewhere. In addition, the correlation analysis across participants indicated that the N400 amplitude was reduced with more attention being directed to the target location. Together, these results indicate that inattention attenuates interocular suppression and thereby makes semantic processing available unconsciously, reconciling conflicting evidence in the literature. We discuss a tight link among interocular suppression, attention, and conscious awareness. Interocular suppression offers a powerful means of studying the extent of unconscious processing by rendering a salient stimulus presented to one eye invisible. Here, we provide evidence that attention is a determining factor for unconscious semantic processing. An electrophysiological marker for semantic processing (N400 component) was present when attention was diverted away from the suppressed stimulus but absent when attention was directed to that stimulus, indicating that inattention facilitates unconscious semantic processing during the interocular suppression. Although contrary to the common sense assumption that attention facilitates information processing, this result is in accordance with recent studies showing that attention modulates interocular suppression but is not necessary for semantic processing. Our finding reconciles the conflicting evidence and advances theories of consciousness.

Low-level awareness accompanies "unconscious" high-level processing during continuous flash suppression.

The scope and limits of unconscious processing are a matter of ongoing debate. Lately, continuous flash suppression (CFS), a technique for suppressing visual stimuli, has been widely used to demonstrate surprisingly high-level processing of invisible stimuli. Yet, recent studies showed that CFS might actually allow low-level features of the stimulus to escape suppression and be consciously perceived. The influence of such low-level awareness on high-level processing might easily go unnoticed, as studies usually only probe the visibility of the feature of interest, and not that of lower-level features. For instance, face identity is held to be processed unconsciously since subjects who fail to judge the identity of suppressed faces still show identity priming effects. Here we challenge these results, showing that such high-level priming effects are indeed induced by faces whose identity is invisible, but critically, only when a lower-level feature, such as color or location, is visible. No evidence for identity processing was found when subjects had no conscious access to any feature of the suppressed face. These results suggest that high-level processing of an image might be enabled by-or co-occur with-conscious access to some of its low-level features, even when these features are not relevant to the processed dimension. Accordingly, they call for further investigation of lower-level awareness during CFS, and reevaluation of other unconscious high-level processing findings.

Intrasaccadic perception triggers pupillary constriction.

It is commonly believed that vision is impaired during saccadic eye movements. However, here we report that some visual stimuli are clearly visible during saccades, and trigger a constriction of the eye’s pupil. Participants viewed sinusoid gratings that changed polarity 150 times per second (every 6.67 ms). At this rate of flicker, the gratings were perceived as homogeneous surfaces while participants fixated. However, the flickering gratings contained ambiguous motion: rightward and leftward motion for vertical gratings; upward and downward motion for horizontal gratings. When participants made a saccade perpendicular to the gratings’ orientation (e.g., a leftward saccade for a vertical grating), the eye’s peak velocity matched the gratings’ motion. As a result, the retinal image was approximately stable for a brief moment during the saccade, and this gave rise to an intrasaccadic percept: A normally invisible stimulus became visible when eye velocity was maximal. Our results confirm and extend previous studies by demonstrating intrasaccadic perception using a reflexive measure (pupillometry) that does not rely on subjective report. Our results further show that intrasaccadic perception affects all stages of visual processing, from the pupillary response to visual awareness.

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.

Dissociation Between Attention and Consciousness During a Novel Task: An ERP Study

While consciousness and attention seem to be tightly connected, recent evidence has suggested that one of these processes can be present in the absence of the other. Recent researches showed that observers can pay attention to an invisible (unconscious) stimulus, and that a stimulus can be clearly perceived (seen) in the absence of attention. We have proposed a novel psychophysical task to explore neural correlates of top-down attention and consciousness. The task is meant to confirm that these two processes can occur independently of each other. EEG was recorded during realizations of the task, and target-locked event-related potentials (ERPs) for masked and unmasked conditions were constructed. Time features corresponding to the P100, N150, and P300 components were extracted for each channel separately. Utilizing these features, we employed some common classifiers for classification of the fourfold state. Our task could separate attention and consciousness successfully through their neural correlates. The results indicate that some of the mentioned components changed when attention or consciousness occurs. By comparing difference waves in each condition separately, our results introduce new ERP correlates of attention and consciousness. We also revealed that parieto-occipital areas are the most relevant areas for dissociation between attention and consciousness. To our knowledge, this is the first time that these correlates are introduced in a separable mode, and the classification accuracies are reported for this purpose.