Although many studies have claimed the existence of unconscious priming, a valid statistical procedure for substantiating such claims is often lacking. For instance, the absence of prime awareness is often erroneously inferred from non-significant p-values, and priming and awareness are typically tested by comparing the outcomes of two separate t-tests. In a classic study by Vorberg (2003), observers were slower to identify the direction of an arrow mask when it followed an arrow prime pointing in the opposite direction (priming effect), despite being unable to identify the direction of the arrow prime (prime awareness). Using that observation as a starting point, we address several methodological shortcomings that we identified in that study (as in many other studies in the field). To overcome these shortcomings, we implemented three critical changes. First, we employed a Bayesian methodological-statistical framework to test priming effects and prime awareness at the single-subject level over multiple experimental sessions. Second, participants underwent extensive training in the prime awareness task to ensure they understood and executed the task as intended. Third, we directly compared prime awareness and the priming effect. By using this approach, we show unconscious processing at short prime-mask stimulus onset asynchronies (SOAs) in three out of six participants, replicating Vorberg et al. (2003). However, we also show that – unlike in the original study – unconscious priming was not consistently present across all SOAs, and awareness increased with increasing prime-mask SOA. Notably, priming effects were highly consistent across participants, whereas awareness showed substantial variability across participants.

A central question in consciousness research is whether perceptual awareness is primarily reflected in early posterior sensory activity or in later frontal processes associated with access and report. Empirical tests of these alternatives are challenging because most paradigms rely on explicit perceptual reports, introducing motor and decisional confounds. Here, we used an electroencephalography design combining two independent tasks to decode stimulus location across report and no-report conditions and to examine how these task-general stimulus-location signals vary with subjective visibility. Participants viewed lateralized gratings in a backward-masking task and rated their subjective visibility, whereas in a separate no-report task, they viewed unmasked gratings without reporting. Classifiers were trained on stimulus location in the no-report task and tested on masked trials sorted by subjective visibility in the masking task. This cross-decoding approach identifies stimulus-location signals that generalize across tasks and vary with subjective visibility, making them less likely to reflect reporting or decision-related processes. Decoding revealed stronger stimulus-location signals for subjectively visible than invisible stimuli in an early time window from 130 to 170 msec, most prominently over posterior occipital, temporal, and parietal electrode groups. These results indicate that subjective visibility enhances early posterior EEG decoding of stimulus location across report contexts.

Science uses objective measurements to falsify predictions, whereas consciousness is thought to be intrinsically personal and subjective. Consequently, behavioral measures of consciousness are often designated as “objective” or “subjective”. Surprisingly however, the exact meaning of the terms objective and subjective is not clearly defined. Moreover, applying this label to a given behavioral measure is misleading, as the same measure can often be analyzed within a “subjective” or “objective” framework. Thus, the terms objective and subjective are not only relevant to the measure that is used, but to several other dimensions as well. In this chapter we provide a brief overview of dichotomies along which one might conceptualize the objective-subjective distinction, such as the empirico-analytical method (manipulation vs trial-by-trial sorting), the mode of stimulus presentation (forced-choice vs nonforced-choice), the response type (Type 1 vs Type 2), and the adoption of a ground truth when computing the outcome measure (performance vs appearance). Although these dichotomies often overlap, the correspondence between them is only partial. Problems and pitfalls are discussed. Finally, we recommend to always explicitly outline how one’s stimulus presentation, task, response measure, and analysis approach score on these dichotomies, requiring further explicit justification when claiming to capture “consciousness” as a construct.

Perception is increasingly viewed as an inferential process wherein sensory inputs are integrated with prior expectations. We employed time-resolved decoding on electroencephalography (EEG) data (n = 30 male participants) to investigate how expectations modulate sensory processing across varying levels of stimulus complexity and tested the effect of attention and NMDA receptor blockade. We designed a visual stimulus containing features of different complexity whose processing relies on distinct neural mechanisms: local contrast, collinearity, and the Kanizsa illusion, involving primarily feedforward, lateral, and feedback processes, respectively. EEG decoding revealed that expectations modulated lateral and feedback processing (better decoding for unexpected stimuli) but not feedforward processing. These expectation effects were confined to attended (task-relevant) features and were not observed for task-irrelevant features. The NMDA receptor antagonist memantine selectively enhanced decoding of the Kanizsa illusion, implicating NMDA-mediated feedback mechanisms in perceptual inference, but it did not modulate the effects of expectation or attention. These findings highlight the differential impact of expectations across different stages of sensory processing and reveal a distinct role of NMDA receptor-mediated feedback mechanisms.

Abstract A central question in consciousness research is whether perceptual awareness arises predominantly from activity in posterior sensory regions or from later frontal processes associated with access and report. Empirical tests of these alternatives are challenging because most paradigms rely on explicit perceptual reports, introducing motor and decisional confounds. Here, we used an electroencephalography design combining two independent tasks to isolate neural signatures of subjective visibility while minimizing report-related activity. Participants viewed lateralized gratings in a backward-masking task and rated their subjective visibility, while in a separate no-report task they viewed unmasked gratings without reporting. Classifiers were trained on stimulus location in the no-report task and tested on masked trials sorted by subjective visibility in the masking task. This cross-decoding approach isolates stimulus-specific neural representations that generalize across tasks and are therefore independent of reporting and decision processes. Decoding revealed a reliable difference between subjectively visible and invisible stimuli in an early time window from 130 to 170 milliseconds, driven by posterior sensors over occipital, temporal, and parietal regions. No corresponding effects were observed over frontal sensors. These results indicate that subjective awareness is associated with early, posterior neural representations, whereas frontal activity observed in report-based paradigms likely reflects post-perceptual processing rather than awareness per se.

Arousal fluctuations shape behavioural performance, with optimal performance often observed at intermediate arousal levels, but it remains unclear whether these effects primarily arise during stimulus encoding, attentional orienting, decision formation, or motor execution. Here, we asked whether arousal modulates spatial orienting specifically, or instead affects broader components of task performance. Across four exogenous and one endogenous Posner cueing experiments, we tested whether spontaneous fluctuations in tonic arousal, indexed by pre-stimulus pupil size, modulate spatial orienting. Overall response speed followed a robust U-shaped relationship with pupil-linked arousal: responses were fastest at intermediate pupil sizes and slower at both smaller and larger pupil sizes. In contrast, spatial cueing effects were not consistently modulated by pupil-linked arousal. Together, these findings reveal a dissociation between arousal-related changes in response speed and spatial orienting. More broadly, these results suggest that tonic arousal can modulate task performance without necessarily altering selective attention.

Breaking continuous flash suppression (bCFS) is a widely used experimental paradigm that exploits detection tasks to measure the time an invisible stimulus requires to access awareness. Oneunresolved issue is whether differences in detection times reflect unconscious or conscious processing. To answer this question, here we introduce a novel approach (reverse-bCFS [rev-bCFS]) that measures the time an initially visible stimulus requires to be suppressed from awareness. Results from two experiments using face stimuli indicate that rev-bCFS can capture conscious effects, which indicates that contrasting standard bCFS with rev-bCFS can isolate unconscious processing occurring specifically during bCFS. For example, while face inversion impacted both bCFS and rev-bCFS, effects were larger in bCFS, suggesting a distinct contribution of unconscious processing to the advantage of upright over inverted faces in accessing awareness. Combining standard bCFS and rev-bCFS may offer a fruitful approach to disentangle conscious and unconscious effects occurring during interocular suppression.

In Fahrenfort et al., 2025 we show the influence of non-perceptual criterion shifts on neural measures of consciousness. We fully agree (and point out in our article) that it was already known that subjective measures are sensitive to criterion confounds, and we are happy to read that this is acknowledged by Sandberg and Overgaard in their comment (Sandberg and Overgaard, 2025). However, we contest that the main findings of our simulations and empirical studies had already been demonstrated. Several findings from our studies are novel, such as the fact that criterion effects reveal themselves as over- (or under-) estimations of both conscious and unconscious processing in tandem, and that this has tangible implications when analyzing real neural data. We also challenge the suggestion that our experimental manipulations are (too) radical compared to signal-to-noise variations that occur naturally between experiments.

This study investigates failures in conscious access resulting from either weak sensory input (perceptual impairments) or unattended input (attentional impairments). Participants viewed a Kanizsa stimulus with or without an illusory triangle within a rapid serial visual presentation of distractor stimuli. We designed a novel Kanizsa stimulus that contained additional ancillary features of different complexity (local contrast and collinearity) that were independently manipulated. Perceptual performance on the Kanizsa stimulus (presence vs. absence of an illusion) was equated between the perceptual (masking) and attentional (attentional blink) manipulation to circumvent common confounds related to conditional differences in task performance. We trained and tested classifiers on electroencephalogram (EEG) data to reflect the processing of specific stimulus features, with increasing levels of complexity. We show that late stages of processing (~200–250 ms), reflecting the integration of complex stimulus features (collinearity, illusory triangle), were impaired by masking but spared by the attentional blink. In contrast, decoding of local contrast (the spatial arrangement of stimulus features) was observed early in time (~80 ms) and was left largely unaffected by either manipulation. These results replicate previous work showing that feedforward processing is largely preserved under both perceptual and attentional impairments. Crucially, however, under matched levels of performance, only attentional impairments left the processing of more complex visual features relatively intact, likely related to spared lateral and local feedback processes during inattention. These findings reveal distinct neural mechanisms associated with perceptual and attentional impairments and thus contribute to a comprehensive understanding of distinct neural stages leading to conscious access.

A central question of consciousness research is which cognitive processes can occur unconsciously. To investigate this, researchers typically compare participants' ability to consciously discriminate a stimulus to their unconscious processing of the same stimulus (e.g. measured via reaction time or brain activity). If participants are not significantly different from chance in the awareness (or "direct") measure while nevertheless there is a significant effect in the processing (or "indirect") measure, researchers argue that there is no conscious processing of the stimulus, while the stimulus is nevertheless somehow processed, as indicated by the processing measure. In consequence researchers conclude that the stimulus has been processed unconsciously. Using neuroimaging techniques such as functional magnetic resonance imaging (fMRI), researchers then infer which brain regions are involved in unconscious versus conscious processing. However, this methodology is based on a fundamental statistical fallacy that has likely led to an overestimation of the scope of unconscious processing, regarding both its capacity and the brain areas involved. The key problem is that sensitivities in the two measures are never directly compared. Therefore, it is not appropriate to conclude that the processing measure had higher sensitivity than the awareness measure. We reanalyzed the results from 16 fMRI studies directly comparing the sensitivities of both measures in 80 experimental conditions. Our results show that, using this sensitivity comparison method, only eight experimental conditions provide evidence for unconscious processing. These results question the validity of the interpretations commonly drawn in the field.

Abstract Perceptual inference requires the integration of visual features through recurrent processing, the dynamic exchange of information between higher and lower level cortical regions. While animal research has demonstrated a crucial role of NMDA receptors in recurrent processing, establishing a causal link between NMDA receptors and recurrent processing in humans has remained challenging. Here, we report two pharmacological studies with randomized, double-blind, crossover designs in which we administered the NMDA antagonist memantine, while collecting human electroencephalography (EEG). We trained and tested EEG classifiers to reflect the processing of specific stimulus features with increasing levels of complexity, namely differences in stimulus contrast, collinearity between local line elements, and illusory surfaces of a Kanizsa triangle. In two experiments involving different participants and visual tasks, we found that memantine selectively improved decoding of the Kanizsa illusion, known to depend on recurrent processing, while leaving decoding of contrast and collinearity largely unaffected. Interestingly, the results from an attentional blink (experiment 1) and task-relevance manipulation (experiment 2) showed that memantine was only effective when the stimulus was attended and consciously accessed. These findings suggest that NMDA inhibition through memantine enhances recurrent processing, especially for attended objects, and thereby provide a crucial step toward bridging animal and human research, shedding light on the neural mechanisms underpinning perceptual inference and conscious perception. This research was supported by a grant from the H2020 European Research Council (ERC STG 715605, SVG).

Perceptual inference requires the integration of visual features through recurrent processing, the dynamic exchange of information between higher- and lower-level cortical regions. While animal research has demonstrated a crucial role of NMDA receptors in recurrent processing, establishing a causal link between NMDA receptors and recurrent processing in humans has remained challenging. Here, we report two pharmacological studies with randomized, double-blind, crossover designs in which we administered the NMDA antagonist memantine, while collecting human electroencephalography (EEG). We trained and tested EEG classifiers to reflect the processing of specific stimulus features with increasing levels of complexity, namely differences in stimulus contrast, collinearity between local line elements, and illusory surfaces of a Kanizsa triangle. In two experiments involving different participants and visual tasks, we found that memantine selectively improved decoding of the Kanizsa illusion, known to depend on recurrent processing, while leaving decoding of contrast and collinearity largely unaffected. Interestingly, the results from an attentional blink (experiment 1) and task-relevance manipulation (experiment 2) showed that memantine was only effective when the stimulus was attended and consciously accessed. These findings suggest that NMDA inhibition through memantine enhances recurrent processing, especially for attended objects, and thereby provide a crucial step toward bridging animal and human research, shedding light on the neural mechanisms underpinning perceptual inference and conscious perception.

How consciousness arises from brain activity has been a topic of intense scientific research for decades. But how does one identify the neural basis of something that is intrinsically personal and subjective? A hallmark approach has been to ask human observers to judge stimuli as ‘seen’ (conscious) and ‘unseen’ (unconscious) and use post hoc sorting of neural measurements based these judgments. Unfortunately, cognitive and response biases are known to strongly affect how observers place their criterion for judging stimuli as ‘seen’ versus ‘unseen’, thereby confounding neural measures of consciousness. Surprisingly however, the effect of conservative and liberal criterion placement on neural measures of unconscious and conscious processing has never been explicitly investigated. Here, we use simulations and electrophysiological brain measurements to show that conservative criterion placement has an unintuitive consequence: rather than selectively providing a cautious estimate of conscious processing, it inflates effect sizes in neural measures of both conscious and unconscious processing, while liberal criterion placement does the reverse. After showing this in simulation, we performed decoding analyses on two electroencephalography studies that employ common subjective indicators of conscious awareness, in which we experimentally manipulated the response criterion. The results confirm that the predicted confounding effects of criterion placement on neural measures of unconscious and conscious processing occur in empirical data, while further showing that the most widely used subjective scale, the Perceptual Awareness Scale (PAS), does not guard against criterion confounds. Follow-up simulations explicate how the experimental context determines whether the relative confounding effect of criterion placement is larger in neural measures of either conscious or unconscious processing. We conclude that criterion placement threatens the construct validity of neural measures of conscious and unconscious processing.

One widely used scientific approach to studying consciousness involves contrasting conscious operations with unconscious ones. However, challenges in establishing the absence of conscious awareness have led to debates about the extent and existence of unconscious processes. We collected experimental data on unconscious semantic priming, manipulating prime presentation duration to highlight the critical role of the analysis approach in attributing priming effects to unconscious processing. We demonstrate that common practices like post-hoc data selection, low statistical power, and frequentist statistical testing can erroneously support claims of unconscious priming. Conversely, adopting best practices like direct performance-awareness contrasts, Bayesian tests, and increased statistical power can prevent such erroneous conclusions. Many past experiments, including our own, fail to meet these standards, casting doubt on previous claims about unconscious processing. Implementing these robust practices will enhance our understanding of unconscious processing and shed light on the functions and neural mechanisms of consciousness.

Visual perception involves binding of distinct features into a unified percept. Although traditional theories link feature binding to time-consuming recurrent processes, Holcombe and Cavanagh (2001) demonstrated ultrafast, early binding of features that belong to the same object. The task required binding of orientation and luminance within an exceptionally short presentation time. However, because visual stimuli were presented over multiple presentation cycles, their findings can alternatively be explained by temporal integration over the extended stimulus sequence. Here, we conducted three experiments manipulating the number of presentation cycles. If early binding occurs, one extremely short cycle should be sufficient for feature integration. Conversely, late binding theories predict that successful binding requires substantial time and improves with additional presentation cycles. Our findings indicate that task-relevant binding of features from the same object occurs slowly, supporting late binding theories.

Background Altered affective state recognition is assumed to be a root cause of aggressive behavior, a hallmark of psychopathologies such as psychopathy and antisocial personality disorder. However, the two most influential models make markedly different predictions regarding the underlying mechanism. According to the integrated emotion system theory (IES), aggression reflects impaired processing of social distress cues such as fearful faces. In contrast, the hostile attribution bias (HAB) model explains aggression with a bias to interpret ambiguous expressions as angry.Methods In a set of four experiments, we measured processing of fearful and angry facial expressions (compared to neutral and other expressions) in a sample of 65 male imprisoned violent offenders rated using the Hare Psychopathy Checklist-Revised (PCL-R, Hare, R. D. (1991). The psychopathy checklist-revised. Toronto, ON: Multi-Health Systems) and in 60 age-matched control participants.Results There was no evidence for a fear deficit in violent offenders or for an association of psychopathy or aggression with impaired processing of fearful faces. Similarly, there was no evidence for a perceptual bias for angry faces linked to psychopathy or aggression. However, using highly ambiguous stimuli and requiring explicit labeling of emotions, violent offenders showed a categorization bias for anger and this anger bias correlated with self-reported trait aggression (but not with psychopathy).Conclusions These results add to a growing literature casting doubt on the notion that fear processing is impaired in aggressive individuals and in psychopathy and provide support for the idea that aggression is related to a hostile attribution bias that emerges from later cognitive, post-perceptual processing stages.

AbstractPerceptual inference requires the integration of visual features through recurrent processing, the dynamic exchange of information between higher and lower level cortical regions. While animal research has demonstrated a crucial role of NMDA receptors in recurrent processing, establishing a causal link between NMDA-mediated recurrent processing and human perception has remained challenging. Here, we report two pharmacological studies with randomized, double-blind, crossover designs in which we administered the NMDA antagonist memantine, while collecting human electroencephalography (EEG). We trained and tested EEG classifiers to reflect the processing of specific stimulus features with increasing levels of complexity, namely differences in stimulus contrast, collinearity between local line elements, and illusory surfaces of a Kanizsa triangle. In two experiments involving different participants and visual tasks, we found that memantine selectively affected decoding of the Kanizsa illusion, known to depend on recurrent processing, while leaving decoding of contrast and collinearity largely unaffected. Interestingly, the results from an attentional blink (experiment 1) and task-relevance manipulation (experiment 2) showed that memantine was only effective when the stimulus was attended and consciously accessed. These findings demonstrate that NMDA inhibition selectively affects recurrent processing, especially for attended objects, and thereby provide a crucial step toward bridging animal and human research, shedding light on the neural mechanisms underpinning perceptual inference and conscious perception.

A target question for the scientific study of consciousness is how dimensions of consciousness, such as the ability to feel pain and pleasure or reflect on one's own experience, vary in different states and animal species. Considering the tight link between consciousness and moral status, answers to these questions have implications for law and ethics. Here we point out that given this link, the scientific community studying consciousness may face implicit pressure to carry out certain research programs or interpret results in ways that justify current norms rather than challenge them. We show that because consciousness largely determines moral status, the use of nonhuman animals in the scientific study of consciousness introduces a direct conflict between scientific relevance and ethics-the more scientifically valuable an animal model is for studying consciousness, the more difficult it becomes to ethically justify compromises to its well-being for consciousness research. Finally, in light of these considerations, we call for a discussion of the immediate ethical corollaries of the body of knowledge that has accumulated and for a more explicit consideration of the role of ideology and ethics in the scientific study of consciousness.