Abstract
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 (<4 Hz) in hits and FAs. The low-frequency power enhancement has been frequently shown to be accompanied with P300 as well as separately being a marker of perceptual awareness, referred to as slow cortical potentials (SCP). Furthermore, the comparison of hits vs. FAs showed a significantly higher LP amplitude and low frequency power in hits compared to FAs. Generally, the observed patterns in the present results resembled some of the major neural correlates associated with perceptual awareness in previous studies. Our findings provide evidence that the neural correlates associated with conscious perception, can be elicited in similar ways in both presence and absence of externally presented sensory stimuli. The present findings did not reveal any pre-stimulus alpha and beta modulations distinguishing conscious vs. unconscious perceptions.
Highlights
The brain was viewed as a processing unit which passively translates sensory inputs into perceptions
Utilizing the previously specified criteria to categorize the trials, before removing artifactual trials based on EEG, the average number of hits was equal to 110.80 ± 21.15 (61% ± 12)
The lower amplitude of event-related potentials (ERPs) in false alarms (FA) compared to hits in our results provide evidence supporting that centroparietal positivity (CPP) encodes confidence rather than the decision variable
Summary
The brain was viewed as a processing unit which passively translates sensory inputs into perceptions. In this traditional account, sensory processing and perception were considered as a feature extraction and reconstruction algorithm implemented in hierarchical neural structures (Engel et al, 2001). Perception of sensory inputs is shaped by prior expectations and attentional and working memory states (Summerfield and Egner, 2009; Kok et al, 2013). The balanced interaction of feedforward information transfer from external sensory inputs (bottom-up) and the feedback coming from brain’s predictions (top-down) results in what has been referred to as normal perception in previous studies (Powers et al, 2016). Studying the neural mechanisms underlying these experiences helps us elucidate both basic and pathological mechanisms underlying normal and abnormal perceptions
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