Neurophysiological and Neuropsychological Correlates of Subconscious Auditory Processing During Anesthesia and Their...

Like any other unusual state of consciousness, the condition of anesthesia remains a mystery, especially regarding the information processing events of the brain. Evoked potentials are the only known way to understand the neurophysiological events of brain in this condition. Auditory evoked potentials (AEPs) have been used as a measure of the depth of anesthesia during the intra-operative process. AEPs have been classically divided, on the basis of their latency, into first, fast, middle, slow, and late components. Auditory evoked potential has been advocated for the assessment of intra-operative awareness (IOA) but has not been considered seriously enough to be universalized. It is because we have not explored enough the impact of auditory perception and auditory information processing on the IOA phenomena as well as on the subsequent psychological impact of IOA on the patient. This limitation is because we have poor understanding of the subconscious auditory processing itself. This perspective is especially important because more of the IOA phenomena exist in the subconscious domain than they do in the conscious domain of explicit recall. Two important forms of these subconscious manifestations of IOA are the implicit recall phenomena and post-operative dreams related to the operation. Here we present a review of the neurophysiological and neuropsychological correlates of auditory processing during anesthesia. We start with a brief description of auditory awareness and the factors affecting it. Further, we proceed to the evaluation of conscious and subconscious information processing by auditory modality and how they interact during and after intra-operative period. Further, we show that both conscious and subconscious auditory processing affect the IOA experience and both have serious psychological implications on the patient subsequently. These effects could be prevented using auditory evoked potential during monitoring of anesthesia, especially the midlatency auditory evoked potentials. To conclude, we propose that the use of Auditory evoked potential should be universal with general anesthesia use in order to prevent the occurrences of distressing outcomes resulting from both conscious and subconscious auditory processing during anesthesia.