Studying the neural mechanisms underlying these experiences helps us elucidate both basic and pathological mechanisms underlying normal and abnormal perceptions.Įven though experiencing illusions and hallucinations is mostly associated with psychotic disorders and has been prominently studied in the psychiatric patient populations ( Silbersweig et al., 1995 Sritharan et al., 2005 Kindler et al., 2011), it is not restricted to clinically diagnosed patients as evidenced by many reports from otherwise healthy populations ( Sommer et al., 2010 Baumeister et al., 2017). Perturbation in normal balance of the corresponding mechanisms has been hypothesized to give rise to perceptual abnormalities such as hallucinations, i.e., perceptions in the absence of a corresponding external stimulus ( Friston, 2005) and illusions, i.e., perceptions that deviate from the original stimulus ( Langguth 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). In other words, perception of sensory inputs is shaped by prior expectations and attentional and working memory states ( Summerfield and Egner, 2009 Kok et al., 2013). However, further developments in the field have shown that sensory perception is not merely driven by stimulus properties but the brain acts as an active inference device which influences perception based on its existing internal state ( Kersten et al., 2004). 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). unconscious perceptions.Ĭlassically, the brain was viewed as a processing unit which passively translates sensory inputs into perceptions. The present findings did not reveal any pre-stimulus alpha and beta modulations distinguishing conscious vs. 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. Generally, the observed patterns in the present results resembled some of the major neural correlates associated with perceptual awareness in previous studies. FAs showed a significantly higher LP amplitude and low frequency power in hits compared to 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). The event-related spectral perturbations (ERSPs) exhibited a common power enhancement in low frequencies (<4 Hz) in hits and FAs. We postulate that the EN and the LP might represent the auditory awareness negativity (AAN) and centro-parietal positivity (CPP) or P300, respectively. 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). The results of the comparison of event-related potentials (ERPs) in the activation period vs. 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. 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. 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. Similar experiences can be elicited in healthy subjects by means of suitable experimental procedures. The occurrence of these phenomena is not constrained to patient populations. 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.
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