Similar EEG Activity Patterns During Experimentally-Induced Auditory Illusions and Veridical Perceptions

Hallucinations and illusions are two instances of perceptual experiences illustrating how perception might diverge from external sensory stimulations and be generated or altered based on internal brain states. The occurrence of these phenomena is not constrained to patient populations. Similar experiences can be elicited in healthy subjects by means of suitable experimental procedures. Studying the neural mechanisms underlying these experiences not only has the potential to expand our understanding of the brain's perceptual machinery but also of how it might get impaired. In the current study, we employed an auditory signal detection task to induce auditory illusions by presenting speech snippets at near detection threshold intensity embedded in noise. We investigated the neural correlates of auditory false perceptions by examining the EEG activity preceding the responses in speech absent (false alarm, FA) trials and comparing them to speech present (hit) trials. The results of the comparison of event-related potentials (ERPs) in the activation period vs. baseline revealed the presence of an early negativity (EN) and a late positivity (LP) similar in both hits and FAs, which were absent in misses, correct rejections (CR) and control button presses (BPs). We postulate that the EN and the LP might represent the auditory awareness negativity (AAN) and centro-parietal positivity (CPP) or P300, respectively. The event-related spectral perturbations (ERSPs) exhibited a common power enhancement in low frequencies

Late positivity does not meet the criteria to be considered a proper neural correlate of perceptual awareness

Contrastive analysis has been widely employed in the search for the electrophysiological neural correlates of consciousness. However, despite its clear logic, it has been argued that it may not succeed in isolating neural processes solely involved in the emergence of perceptual awareness. In fact, data from contrastive analysis would be contaminated by potential confounding factors reflecting distinct, though related, processes either preceding or following the conscious perception. At present, the ERP components representing the proper correlates of perceptual awareness still remain to be identified among those correlating with awareness (i.e., Visual Awareness Negativity, VAN and Late Positivity, LP). In order to dissociate visual awareness from post-perceptual confounds specifically related to decision making, we manipulated the response criterion, which affects how a percept is translated into a decision. In particular, while performing an orientation discrimination task, participants were asked to shift their response criterion across sessions. As a consequence, the resulting modulation should concern the ERP component(s) not exclusively reflecting mechanisms regulating the subjective conscious experience itself but rather the processes accompanying it. Electrophysiological results showed that N1 and P3 were sensitive to the response criterion adopted by participants. Additionally, the more the participants shifted their response criterion, the bigger the ERP modulation was; this was consequently indicative of the critical role of these components in the decision-making processes regardless of awareness level. When considering data independently from the response criterion, the aware vs. unaware contrast showed that both VAN and LP were significant. Crucially, the LP component was also modulated by the interaction of awareness and response criterion, while VAN results to be unaffected. In agreement with previous literature, these findings provide evidence supporting the hypothesis that VAN tracks the emergence of visual awareness by encoding the conscious percept, whereas LP reflects the contribution from post-perceptual processes related to response requirements. This excludes a direct functional role of this later component in giving rise to perceptual awareness

A Neural Signature Encoding Decisions under Perceptual Ambiguity

People often make perceptual decisions with ambiguous information, but it remains unclear whether the brain has a common neural substrate that encodes various forms of perceptual ambiguity. Here, we used three types of perceptually ambiguous stimuli as well as task instructions to examine the neural basis for both stimulus-driven and task-driven perceptual ambiguity. We identified a neural signature, the late positive potential (LPP), that encoded a general form of stimulus-driven perceptual ambiguity. In addition to stimulus-driven ambiguity, the LPP was also modulated by ambiguity in task instructions. To further specify the functional role of the LPP and elucidate the relationship between stimulus ambiguity, behavioral response, and the LPP, we employed regression models and found that the LPP was specifically associated with response latency and confidence rating, suggesting that the LPP encoded decisions under perceptual ambiguity. Finally, direct behavioral ratings of stimulus and task ambiguity confirmed our neurophysiological findings, which could not be attributed to differences in eye movements either. Together, our findings argue for a common neural signature that encodes decisions under perceptual ambiguity but is subject to the modulation of task ambiguity. Our results represent an essential first step toward a complete neural understanding of human perceptual decision making

A Neural Signature Encoding Decisions under Perceptual Ambiguity

People often make perceptual decisions with ambiguous information, but it remains unclear whether the brain has a common neural substrate that encodes various forms of perceptual ambiguity. Here, we used three types of perceptually ambiguous stimuli as well as task instructions to examine the neural basis for both stimulus-driven and task-driven perceptual ambiguity. We identified a neural signature, the late positive potential (LPP), that encoded a general form of stimulus-driven perceptual ambiguity. In addition to stimulus-driven ambiguity, the LPP was also modulated by ambiguity in task instructions. To further specify the functional role of the LPP and elucidate the relationship between stimulus ambiguity, behavioral response, and the LPP, we employed regression models and found that the LPP was specifically associated with response latency and confidence rating, suggesting that the LPP encoded decisions under perceptual ambiguity. Finally, direct behavioral ratings of stimulus and task ambiguity confirmed our neurophysiological findings, which could not be attributed to differences in eye movements either. Together, our findings argue for a common neural signature that encodes decisions under perceptual ambiguity but is subject to the modulation of task ambiguity. Our results represent an essential first step toward a complete neural understanding of human perceptual decision making

Isolating the proper neural correlates of visual awareness from its neural consequences

One of the major challenges in the neuroscience of consciousness is to disclose the timing and neural mechanisms underlying visual awareness, the subjective experience of seeing. Electroencephalography (EEG) studies investigating the time course of consciousness-related neural processes have found two potential correlates of visual awareness: the N2 and the P3 ERP components. However, recent works have suggested that only N2 correlates to visual awareness, whereas later neural activity expressed by the P3 component reflects post-perceptual processes related to subjects\u2019 report or to accumulation of sensory evidence leading to decision-making. Building upon this observation, the objective of this study was to provide further evidence that the P3 component reflects a post-perceptual process. To this end, we designed two EEG experiments in which we directly manipulated decision making processes and visual awareness while keeping constant the physical property of visual stimuli. This experimental manipulation allowed us to identify the possible influences of post-perceptual processing over the time course of neural responses and determine the temporal window at which such influence occurs. In Experiment 1, we manipulated participant\u2019s decision criterion by inducing, respectively, a liberal and a conservative decision bias in two different experimental sessions. The aim of this first Experiment was to determine whether our manipulation of the decision processes would produce a modulation of P3 ERP component. Experiment 2 was identical to Experiment 1 except that participants were not requested to adjust their decision criterion (own criterion session). The aim of this experiment was to examine whether in a condition in which there was no manipulation of post-perceptual processes, N2 and P3 ERP component would distribute differently. Electrophysiological and Behavioral results of Experiment 2 were then compared with those of Experiment 1. If the amplitude of the P3 reflects post-perceptual processes related to decision making processes, one would expect to find some differences in the topography or in the time-course of the P3 between the condition in which a decision criterion was imposed (Experiment 1) and the condition in which there was not a decision bias induced (Experiment 2). ERP results of Experiment 1 revealed that the amplitude of the N2 and the P3 components were enhanced for those trials were subjects reported to have seen the stimulus as compared to unaware trials. Importantly, the amplitude of the P3 was modulated by the decision criterion: it was enhanced when participants adopted the liberal criterion compared to the conservative criterion, suggesting that P3 reflects brain processes related to decision making that occurs after that awareness has emerged. ERP data of Experiment 2 confirmed that aware responses were associated with enhanced N2 and P3 amplitude than unaware responses. Interestingly, the decision criterion manipulation had an effect on P3 component revealing that the own and the liberal criteria were associated with an increased positivity over central areas if compared to the conservative criterion. In addition, we found that the amplitude of the N2 was enhanced for the own session if compared to conservative and liberal sessions. Overall these results suggest that when sensory information was relevant for the task (own session) a bigger N2 was observed. On the contrary, P3 amplitude was sensitive to the manipulation of the decision criterion, suggesting a critical role of neural activity expressed by the P3 component in decision making processes. These findings support the hypothesis that P3 might reflect post-perceptual processes that occur after that awareness has emerged while the N2 component reflects a proper correlate of visual awareness

A neural decision signal during internal sampling from working memory in humans

How humans transform sensory information into decisions that steer purposeful behaviour is a central question in psychology and neuroscience that is traditionally investigated during the sampling of external, environmental signals. The decision-making framework of gradual information sampling toward a decision has also been proposed to apply when sampling internal sensory evidence from working memory. However, neural evidence for this proposal remains scarce. Here we show (using scalp-EEG in male and female human volunteers) that sampling internal visual representations from working memory elicits a scalp-EEG potential associated with gradual evidence accumulation – the Central Parietal Positivity (CPP). Consistent with an evolving decision process, we show how this signal (i) scales with the time participants require to reach a decision about the cued memory content and (ii) is amplified when having to decide among multiple contents in working memory. These results bring the electrophysiology of decision making into the domain of working memory and suggest that variability in memory-guided behaviour may be driven (at least in part) by variations in the sampling of our inner mental contents

Relation between centro-parietal positivity and diffusion model parameters in both perceptual and memory-based decision making

Several studies have suggested that the centro-parietal positivity (CPP), an EEG potential occurring approximately 500 ms post- stimulus, reflects the accumulation of evidence for making a decision. Yet, most previous studies of the CPP focused exclusively on perceptual decisions with very simple stimuli. In this study, we examined how the dynamics of the CPP depended on the type of decision being made, and whether its slope was related to parameters of an accumulator model of decision making. We show initial evidence that memory- and perceptual decisions about carefully-controlled face stimuli exhibit similar dynamics, but offset by a time difference in decision onset. Importantly, the individual-trial slopes of the CPP are related to the accumulator model's drift parameter. These findings help to further understand the role of the CPP across different kinds of decisions

Metacognitive Decisions on Decision Accuracy: Confidence Judgment and Changes of Mind

Even in the absence of external feedback, humans are capable of subjectively estimating the accuracy of their own decisions, resulting in a sense of confidence that a decision is correct. While decision confidence has been proposed to be closely related to other metacognitive judgments, including error awareness (i.e., awareness that a decisions error has occurred) and changes of mind (i.e., reversal of previously made decisions), their relationships so far remain unclear. The current project aimed to investigate how confidence could be related to metacognitive judgments from two perspectives. First, Studies 1 and 2 investigated how confidence and changes of mind were affected by changes in different stimulus properties, particularly absolute evidence strength. In a brightness judgment task, participants were presented with two flickering, grayscale squares and required to select the square that appeared brighter. After each trial, participants reported their subjective accuracy on a rating scale ranging from “surely incorrect” to “surely correct”. Results showed that with stronger absolute evidence (i.e., increased overall luminance across both squares), confidence was increased and the proportion of changes of mind trials was reduced. These consistent changes support the hypothesis that higher confidence could contribute to less frequent changes of mind. Second, Study 3 investigated the relationships between confidence and the event-related potential (ERP) components of the centro-parietal potential (CPP) and the error positivity (Pe), which have been respectively proposed to be indexes of pre- and post-decisional evidence accumulation processes. In the same brightness judgment task, it was found that the relationships between confidence and these two ERP components depended on decision accuracy: Confidence was positively related to CPP amplitudes in correct trials, but negatively related to Pe amplitudes in error trials. These findings suggest that confidence in correct and error decisions involve different pre- and post- decisional processes. Overall, the current findings suggest that (a) confidence could serve as a basis of changes of mind, and (b), confidence in correct and erroneous decisions was differentially related to pre- and post-decisional ERP indexes of evidence accumulation. Taken together, they suggest that confidence might emerge during decision formation and could, with the contribution from post-decisional processes, serve as a basis of changes of mind

Evidence accumulation under uncertainty - a neural marker of emerging choice and urgency

To interact meaningfully with its environment, an agent must integrate external information with its own internal states. However, information about the environment is often noisy. In this study, we identify a neural correlate that tracks how asymmetries between competing alternatives evolve over the course of a decision. In our task participants had to monitor a stream of discrete visual stimuli over time and decide whether or not to act, on the basis of either strong or ambiguous evidence. We found that the classic P3 event-related potential evoked by sequential evidence items tracked decision-making processes and predicted participants' categorical choices on a single trial level, both when evidence was strong and when it was ambiguous. The P3 amplitudes in response to evidence supporting the eventually selected option increased over trial time as decisions evolved, being maximally different from the P3 amplitudes evoked by competing evidence at the time of decision. Computational modelling showed that both the neural dynamics and behavioural primacy and recency effects can be explained by a combination of (a) competition between mutually inhibiting accumulators for the two categorical choice outcomes, and (b) a context-dependant urgency signal. In conditions where evidence was presented at a low rate, urgency increased faster than in conditions when evidence was very frequent. We also found that the readiness potential, a classic marker of endogenously initiated actions, was observed preceding movements in all conditions - even when those were strongly driven by external evidence