Conscious Detection Research Articles

Single-trial decoding of auditory novelty responses facilitates the detection of residual consciousness

Detecting residual consciousness in unresponsive patients is a major clinical concern and a challenge for theoretical neuroscience. To tackle this issue, we recently designed a paradigm that dissociates two electro-encephalographic (EEG) responses to auditory novelty. Whereas a local change in pitch automatically elicits a mismatch negativity (MMN), a change in global sound sequence leads to a late P300b response. The latter component is thought to be present only when subjects consciously perceive the global novelty. Unfortunately, it can be difficult to detect because individual variability is high, especially in clinical recordings. Here, we show that multivariate pattern classifiers can extract subject-specific EEG patterns and predict single-trial local or global novelty responses. We first validate our method with 38 high-density EEG, MEG and intracranial EEG recordings. We empirically demonstrate that our approach circumvents the issues associated with multiple comparisons and individual variability while improving the statistics. Moreover, we confirm in control subjects that local responses are robust to distraction whereas global responses depend on attention. We then investigate 104 vegetative state (VS), minimally conscious state (MCS) and conscious state (CS) patients recorded with high-density EEG. For the local response, the proportion of significant decoding scores (M=60%) does not vary with the state of consciousness. By contrast, for the global response, only 14% of the VS patients' EEG recordings presented a significant effect, compared to 31% in MCS patients' and 52% in CS patients'. In conclusion, single-trial multivariate decoding of novelty responses provides valuable information in non-communicating patients and paves the way towards real-time monitoring of the state of consciousness.

Segregating the Neural Correlates of Physical and Perceived Change in Auditory Input using the Change Deafness Effect

Psychophysical experiments show that auditory change detection can be disturbed in situations in which listeners have to monitor complex auditory input. We made use of this change deafness effect to segregate the neural correlates of physical change in auditory input from brain responses related to conscious change perception in an fMRI experiment. Participants listened to two successively presented complex auditory scenes, which consisted of six auditory streams, and had to decide whether scenes were identical or whether the frequency of one stream was changed between presentations. Our results show that physical changes in auditory input, independent of successful change detection, are represented at the level of auditory cortex. Activations related to conscious change perception, independent of physical change, were found in the insula and the ACC. Moreover, our data provide evidence for significant effective connectivity between auditory cortex and the insula in the case of correctly detected auditory changes, but not for missed changes. This underlines the importance of the insula/anterior cingulate network for conscious change detection.

Altered cortical communication in amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a disorder associated primarily with the degeneration of the motor system. More recently, functional connectivity studies have demonstrated potentially adaptive changes in ALS brain organization, but disease-related changes in cortical communication remain unknown. We recruited individuals with ALS and age-matched controls to operate a brain–computer interface while electroencephalography was recorded over three sessions. Using normalized symbolic transfer entropy, we measured directed functional connectivity from frontal to parietal (feedback connectivity) and parietal to frontal (feedforward connectivity) regions. Feedback connectivity was not significantly different between groups, but feedforward connectivity was significantly higher in individuals with ALS. This result was consistent across a broad electroencephalographic spectrum (4–35Hz), and in theta, alpha and beta frequency bands. Feedback connectivity has been associated with conscious state and was found to be independent of ALS symptom severity in this study, which may have significant implications for the detection of consciousness in individuals with advanced ALS. We suggest that increases in feedforward connectivity represent a compensatory response to the ALS-related loss of input such that sensory stimuli have sufficient strength to cross the threshold necessary for conscious processing in the global neuronal workspace.

Dynamic attentional modulation of vision across space and time after right hemisphere stroke and in ageing

IntroductionAttention modulates the availability of sensory information to conscious perception. In particular, there is evidence of pathological, spatial constriction of the effective field of vision in patients with right hemisphere damage when a central task exhausts available attentional capacity. In the current study we first examined whether this constriction might be modulated across both space and time in right hemisphere stroke patients without neglect. Then we tested healthy elderly people to determine whether non-pathological ageing also leads to spatiotemporal impairments of vision under conditions of high attention load. MethodsRight hemisphere stroke patients completed a task at fixation while attempting to discriminate letters appearing in the periphery. Attentional load of the central task was modulated by increasing task difficulty. Peripheral letters appeared simultaneously with the central task or at different times (stimulus onset asynchronies, SOAs) after it. In a second study healthy elderly volunteers were tested with a modified version of this paradigm. ResultsUnder conditions of high attention load right hemisphere stroke patients have a reduced effective visual field, over a significantly extended ‘attentional blink’, worse for items presented to their left. In the second study, older participants were unable to discriminate otherwise salient items across the visual field (left or right) when their attention capacity was loaded on the central task. This deficit extended temporally, with peripheral discrimination ability not returning to normal for up to 450 msec. ConclusionsDynamically tying up attention resources on a task at fixation can have profound effects in patient populations and in normal ageing. These results demonstrate that items can escape conscious detection across space and time, and can thereby impact significantly on visual perception in these groups.

The effect of stimulus duration and motor response in hemispatial neglect during a visual search task.

Patients with hemispatial neglect exhibit a myriad of profound deficits. A hallmark of this syndrome is the patients' absence of awareness of items located in their contralesional space. Many studies, however, have demonstrated that neglect patients exhibit some level of processing of these neglected items. It has been suggested that unconscious processing of neglected information may manifest as a fast denial. This theory of fast denial proposes that neglected stimuli are detected in the same way as non-neglected stimuli, but without overt awareness. We evaluated the fast denial theory by conducting two separate visual search task experiments, each differing by the duration of stimulus presentation. Specifically, in Experiment 1 each stimulus remained in the participants' visual field until a response was made. In Experiment 2 each stimulus was presented for only a brief duration. We further evaluated the fast denial theory by comparing verbal to motor task responses in each experiment. Overall, our results from both experiments and tasks showed no evidence for the presence of implicit knowledge of neglected stimuli. Instead, patients with neglect responded the same when they neglected stimuli as when they correctly reported stimulus absence. These findings thus cast doubt on the concept of the fast denial theory and its consequent implications for non-conscious processing. Importantly, our study demonstrated that the only behavior affected was during conscious detection of ipsilesional stimuli. Specifically, patients were slower to detect stimuli in Experiment 1 compared to Experiment 2, suggesting a duration effect occurred during conscious processing of information. Additionally, reaction time and accuracy were similar when reporting verbally versus motorically. These results provide new insights into the perceptual deficits associated with neglect and further support other work that falsifies the fast denial account of non-conscious processing in hemispatial visual neglect.

The influence of distracter and target features on distracter induced blindness.

The inhibitory effect of the processing of target-like distracters has already been shown to affect the conscious detection of simple motion and simple orientation stimuli in a random dot kinematogram. In two experiments we examined the effects of single-feature motion distracters, single-feature orientation distracters, and combined-feature distracters containing both motion and orientation information. The target was specified as a coherent motion episode (Experiment 1) or as a combined-feature episode where the coherent motion was accompanied by an abrupt change in line orientation (Experiment 2). Results showed that (a) the respective feature-specific inhibitory processes operate separately even when the distracter features are presented simultaneously and (b) both inhibitory processes contribute to the blindness effect when the conjunction of two features is defined as the target. Again, this inhibitory-process is feature-specific: Only features that are defined in the task are represented in the inhibitory task set. In case of combined- feature task-sets, these representations remain separate, so that combined-feature distracters as well as single-feature distracters are able to induce blindness effects.

Early dissociation between neural signatures of endogenous spatial attention and perceptual awareness during visual masking

The relationship between spatial attention and conscious access has often been pictured as a single causal link: spatial attention would provide conscious access to weak stimuli by increasing their effective contrast during early visual processing. To test this hypothesis, we assessed whether the early attentional amplification of visual responses, around 100 ms following stimulus onset, had a decisive impact on conscious detection. We recorded magnetoencephalographic (MEG) signals while participants focused their attention toward or away from masked stimuli which were physically identical but consciously detected half of the time. Spatial attention increased the amplitude of early occipital responses identically for both detected and missed stimuli around 100 ms, and therefore, did not control conscious access. Accordingly, spatial attention did not increase the proportion of detected stimuli. The earliest neuromagnetic correlate of conscious detection, around 120 ms over the contralateral temporal cortex, was independent from the locus of attention. This early activation combined objective information about stimulus presence and subjective information about stimulus visibility, and was followed by a late correlate of conscious reportability, from 220 ms over temporal and frontal cortex, which correlated exclusively with stimulus visibility. This widespread activation coincided in time with the reorienting of attention triggered by masks presented at the uncued location. This reorienting was stronger and occurred earlier when the masked stimulus was detected, suggesting that the conscious detection of a masked stimulus at an unexpected location captures spatial attention. Altogether, these results support a double dissociation between the neural signatures of endogenous spatial attention and perceptual awareness.

Attentional Selection Dilates Perceived Duration

How do observers judge the passage of time at a short time-scale? Humans are not equipped with a dedicated sensory system for perceiving durations in the same way as they are equipped with systems for perceiving light and sound. Thus, subjective duration depends on the sensory and cognitive processes triggered by sensory input, eg visual or auditory stimuli. Previous studies have demonstrated that the dynamics of this sensory input (eg the rate of stimulus presentation) affect duration judgments. However, it is yet unclear whether automatic or attentive processing of such dynamics accounts for their effect on subjective duration. Automatic and attentive stimulus processing can be distinguished when stimuli are presented in a rapid serial visual presentation (RSVP) paradigm. The second of two targets embedded in an RSVP stream often fails to attract participants' attention and escapes conscious detection, in spite of being automatically processed at a perceptual level. In the present study, we presented RSVP streams and combined a target detection task with a prospective duration judgment task. We demonstrate in three experiments that the number of subjectively perceived target stimuli (and not the number of objectively presented targets) determines subjective duration of the entire RSVP sequence. Target stimuli which escape attentional selection did not affect perceived duration. This finding indicates that attentive rather than automatic processing of stimulus dynamics leads to the subjective time dilation of dynamic stimuli.

Anxiety, conscious awareness and change detection

Attentional scanning was studied in anxious and non-anxious participants, using a modified change detection paradigm. Participants detected changes in pairs of emotional scenes separated by two task irrelevant slides, which contained an emotionally valenced scene (the 'distractor scene') and a visual mask. In agreement with attentional control theory, change detection latencies were slower overall for anxious participants. Change detection in anxious, but not non-anxious, participants was influenced by the emotional valence and exposure duration of distractor scenes. When negative distractor scenes were presented at subliminal exposure durations, anxious participants detected changes more rapidly than when supraliminal negative scenes or subliminal positive scenes were presented. We propose that for anxious participants, subliminal presentation of emotionally negative distractor scenes stimulated attention into a dynamic state in the absence of attentional engagement. Presentation of the same scenes at longer exposure times was accompanied by conscious awareness, attentional engagement, and slower change detection.

Dogs Cannot Bark: Event-Related Brain Responses to True and False Negated Statements as Indicators of Higher-Order Conscious Processing

The present study investigated event-related brain potentials elicited by true and false negated statements to evaluate if discrimination of the truth value of negated information relies on conscious processing and requires higher-order cognitive processing in healthy subjects across different levels of stimulus complexity. The stimulus material consisted of true and false negated sentences (sentence level) and prime-target expressions (word level). Stimuli were presented acoustically and no overt behavioral response of the participants was required. Event-related brain potentials to target words preceded by true and false negated expressions were analyzed both within group and at the single subject level. Across the different processing conditions (word pairs and sentences), target words elicited a frontal negativity and a late positivity in the time window from 600–1000 msec post target word onset. Amplitudes of both brain potentials varied as a function of the truth value of the negated expressions. Results were confirmed at the single-subject level. In sum, our results support recent suggestions according to which evaluation of the truth value of a negated expression is a time- and cognitively demanding process that cannot be solved automatically, and thus requires conscious processing. Our paradigm provides insight into higher-order processing related to language comprehension and reasoning in healthy subjects. Future studies are needed to evaluate if our paradigm also proves sensitive for the detection of consciousness in non-responsive patients.

I am conscious

I am the only one to know that I am conscious. I can nevertheless interact with others using verbal communication or motor responses to let them know that I am. What would happen if, to some extent, I would know this, but I would be unable to express it? How would my surroundings know? Here is the challenge facing clinicians involved in the care of patients recovering from coma: differentiating reflex from voluntary activity, or, in other words, detecting consciousness in noncommunicative patients. Behavioral assessment is the primary method to detect signs of consciousness and, hence, to determine diagnosis. However, a recent study has shown a misdiagnosis rate of 41% in patients who were clinically diagnosed as being in a vegetative state.1 Detection of consciousness is confounded by numerous factors including fluctuations in arousal level, sensory and motor impairments, medication side effects, and the possibility of aphasia.2 Because misdiagnosis can lead to serious consequences, especially related to pain treatment and end-of-life decisions,3 additional tools have to be used to characterize more completely and accurately remaining brain functioning linked to …

EEG analysis periods of minimum length for detection of consciousness and unconsciousness by approximate and permutation entropy

EEG analysis periods of minimum length for detection of consciousness and unconsciousness by approximate and permutation entropy

Neurolaw and consciousness detection

Neurolaw and consciousness detection

Cascade of Neural Events Leading from Error Commission to Subsequent Awareness Revealed Using EEG Source Imaging

The goal of the present study was to shed light on the respective contributions of three important action monitoring brain regions (i.e. cingulate cortex, insula, and orbitofrontal cortex) during the conscious detection of response errors. To this end, fourteen healthy adults performed a speeded Go/Nogo task comprising Nogo trials of varying levels of difficulty, designed to elicit aware and unaware errors. Error awareness was indicated by participants with a second key press after the target key press. Meanwhile, electromyogram (EMG) from the response hand was recorded in addition to high-density scalp electroencephalogram (EEG). In the EMG-locked grand averages, aware errors clearly elicited an error-related negativity (ERN) reflecting error detection, and a later error positivity (Pe) reflecting conscious error awareness. However, no Pe was recorded after unaware errors or hits. These results are in line with previous studies suggesting that error awareness is associated with generation of the Pe. Source localisation results confirmed that the posterior cingulate motor area was the main generator of the ERN. However, inverse solution results also point to the involvement of the left posterior insula during the time interval of the Pe, and hence error awareness. Moreover, consecutive to this insular activity, the right orbitofrontal cortex (OFC) was activated in response to aware and unaware errors but not in response to hits, consistent with the implication of this area in the evaluation of the value of an error. These results reveal a precise sequence of activations in these three non-overlapping brain regions following error commission, enabling a progressive differentiation between aware and unaware errors as a function of time elapsed, thanks to the involvement first of interoceptive or proprioceptive processes (left insula), later leading to the detection of a breach in the prepotent response mode (right OFC).

Bead-Based Fluid Array Detection of Pentaerythritol Tetranitrate: Comparison of Monoclonal vs. Llama Polyclonal Antibodies

In today's security conscious environment rapid detection of explosives, such as pentaerythritol tetranitrate (PETN), the high energy material common in plastic explosives, is of critical importance. We evaluated two monoclonal antibodies and a rabbit polyclonal antibody, developed for the detection of PETN. Although these antibodies showed binding to bioconjugates of PETN as well as the hapten, PETN-succinate, we did not observe binding by free PETN. For demonstration of a competitive bead based fluid array immunoassay for PETN detection, we developed llama polyclonal antibody for the detection of free PETN. Our ultimate goal is developing anti-PETN single domain antibodies.

COMT Val108/158Met polymorphism effects on emotional brain function and negativity bias

Biases toward processing negative versus positive information vary as a function of level of awareness, and are modulated by monoamines. Excessive biases are associated with individual differences in mood and emotional stability, and emotional disorder. Here, we examined the impact of the catechol-O-methyltransferase (COMT) Val108/158Met polymorphism, involved in dopamine and norepinephrine catabolism, on both emotional brain function and self-reported negativity bias. COMT genotyping and self-reported level of negativity bias were completed for 46 healthy participants taking part in the Brain Resource International Database. Functional MRI was undertaken during perception of facial expressions of fear and happiness presented under unmasked (consciously identified) and masked (to prevent conscious detection) conditions. Structural MR images were also acquired. A greater number of COMT Met alleles predicted increased activation in brainstem, amygdala, basal ganglia and medial prefrontal regions for conscious fear, but decreased activation for conscious happiness. This pattern was also apparent for brainstem activation for the masked condition. Effects were most apparent for females. These differences could not be explained by gray matter variations. The Met-related profile of activation, particularly prefrontally, predicted greater negativity bias associated with risk for emotional disorder. The findings suggest that the COMT Met allele modulates neural substrates of negative versus positive emotion processing. This effect may contribute to negativity biases, which confer susceptibility for emotional disorders.

Surprise-induced blindness: A stimulus-driven attentional limit to conscious perception.

The cost of attending to a visual event can be the failure to consciously detect other events. This processing limitation is well illustrated by the attentional blink paradigm, in which searching for and attending to a target presented in a rapid serial visual presentation stream of distractors can impair one's ability to detect a second target presented soon thereafter. The attentional blink critically depends on 'top-down' attentional settings, for it does not occur if participants are asked to ignore the first target. Here we show that 'bottom-up' attention can also lead to a profound but ephemeral deficit in conscious perception: Presentation of a novel, unexpected, and task-irrelevant stimulus virtually abolishes conscious detection of a target presented within half a second after the 'Surprise' stimulus, but only for its earliest occurrences (generally 1 to 2 presentations). This powerful but short-lived deficit contrasts with a milder but more enduring form of attentional capture that accompanies singleton presentations in rapid serial visual presentations. We conclude that the capture of stimulus-driven attention alone can limit explicit perception.

How Ongoing Fluctuations in Human Visual Cortex Predict Perceptual Awareness: Baseline Shift versus Decision Bias

Visual perception fluctuates across repeated presentations of the same near-threshold stimulus. These perceptual fluctuations have often been attributed to baseline shifts--i.e., ongoing modulations of neuronal activity in visual areas--driven by top-down attention. Using magnetoencephalography, we directly tested whether ongoing attentional modulations could fully account for the perceptual impact of prestimulus activity on a subsequent seen-unseen decision. We found that prestimulus gamma-band fluctuations in lateral occipital areas (LO) predicted visual awareness, but did not reflect the focus of spatial attention. Moreover, these prestimulus signals influenced the decision outcome independently from the strength of the following visual response, suggesting that baseline shifts alone could not explain their perceptual impact. Using a straightforward decision-making model based on the accumulation of sensory evidence over time, we show that prestimulus gamma-band fluctuations in LO behave as a decision bias at stimulus onset, irrespectively of subsequent stimulus processing. In contrast, spatial attention suppressed prestimulus alpha-band signals in the same region, and produced a sustained baseline shift that also predicted the outcome of the seen-unseen decision. Together, our results indicate that prestimulus fluctuations in visual areas can influence the conscious detection of an upcoming stimulus via two distinct mechanisms: an attention-driven baseline shift in the alpha range, and a decision bias in the gamma range.

The Narcotrend Index Indicates Age-Related Changes During Propofol Induction in Children

The Narcotrend electroencephalogram monitor is designed to measure hypnotic state during anesthesia. We performed this study to evaluate the effectiveness and reliability of the Narcotrend monitor in assessing hypnotic state and loss of consciousness (LOC) during propofol anesthesia induction in children. Sixty-two children, aged 1-5 (n = 17), 6-12 (n = 23), and 13-16 (n = 21) yr, scheduled for elective surgery were studied. The patients were premedicated with oral midazolam 0.5 mg/kg. After IV access, propofol target controlled infusion (TCI) was started with 0.5 microg/mL and increased by 0.5 microg/mL increments every 2 min until the child did not respond to any verbal command or physical stimuli. A manual scheme was used for children weighing <15 kg. Hypnotic state was measured every minute from the start of the propofol infusion using the University of Michigan Sedation Scale (UMSS). LOC was defined as a transition of UMSS scale value 2 to 3. The Narcotrend index (NI) was recorded before the start of induction and during the whole study period. NI values were noted simultaneously, yet independently of the sedation measurements. Prediction probability (PK) was used to assess the correspondence between NI and UMSS. Sensitivity and specificity of NI for differentiating between consciousness and unconsciousness were calculated. NI values at specific UMSS levels were compared between the different age groups and the relationships between TCI propofol concentrations and sedation levels were assessed using correlation analysis. A PK-value of 0.84 (95% CI [0.80-0.88]) of NI was calculated from the data for the detection of LOC. Similarly, a PK value of 0.82 (95% CI [0.78-0.86]) indicated agreement between NI and UMSS values. The average NI values differed between successive UMSS sedation levels 0 and 1 and levels 1 and 2 (P < 0.01). In the youngest age group, the NI discriminated between UMSS levels 2 and 3, in the second age group between levels 1 and 2 and 2 and 3, and in the oldest age group between 0 and 1. Furthermore, the NI values differed significantly between age groups at UMSS levels 1-4 (P < 0.005), with the NI values being higher in younger compared with older children. The average NI value at LOC was 68. For the detection of consciousness, a sensitivity of 0.67 and specificity of 0.79 were achieved. Spearman correlation coefficients indicated higher association between TCI propofol concentrations and UMSS (0.96) than between NI and UMSS (-0.68). During propofol induction in children, the Narcotrend electroencephalogram monitor was capable of following changes in the sedation level of children to some extent, but also had a relatively high probability (0.18) of incorrectly predicting changes in conscious state. Therefore, the monitor should not solely be used to guide sedation and anesthesia. NI was age-dependent and younger children had higher NI-values than older children at the same level of sedation.

Facilitation of visual processing by masked presentation of a conditioned facial stimulus

We tested whether a conditioned emotional stimulus could facilitate visual processing when presented subliminally. Participants received Pavlovian conditioning in which a fearful expression (conditioned stimulus) was paired with electric finger shock (unconditioned stimulus). In a subsequent session, they were subject to a simple perceptual task involving subliminal presentation of facial expressions followed by a simple color patch. Although none of the participants reported conscious detection of the facial stimuli, early posterior negativity was significantly enhanced to the conditioned fear face and the neutral face of the same identity. Paralleling the brain activity, reaction times to the conditioned stimulus faces were also facilitated. These results suggest that conditioned threat stimuli can facilitate perceptual processing even when they are processed unconsciously.