LAUSR

Jung et al. found that using a patient’s name during recovery from anaesthesia may be an easy and effective method of facilitating this process [1]. Hearing is often described as the ‘last sensation to go and the first to return’ during induction and emergence from anesthesia, and this is attributed at a neurophysiological level to hyperacusis. However, it seems as if hearing does not actually ‘go’, even during deep anaesthesia [2]. A patient does not need to be aware of sounds for activation of acousticlimbic pathways [3]. Deeprose et al. and Aceto et al. independently found that, under general anesthesia, intraoperative information processing continues to occur in the absence of being fully awake [4, 5]. Knippera et al. investigated neural input from the central auditory system to the amygdala and hippocampus. They showed that the basolateral amygdala (BLA) is connected directly to the auditory thalamus and the auditory cortex. The BLA, in turn, contacts the hippocampus, which has direct contact with the auditory cortex. Distressing information or noise can activate the BLA through the hypothalamicpituitary-adrenal axis. Hyperacusis on induction develops centrally as an increase in signal gain in response to decreased sensory input from auditory nerves, probably in the same manner as tinnitus is produced [6]. The same processes may be responsible for this phenomenon during recovery. Functional MRI techniques have been used to identify hyperacusis networks that exist within the auditory network, including the cerebellum, reticular formation, amygdala, and hippocampus [7]. Patients easily recall distressing, emotionally pertinent information heard during surgery, whereas more banal information is not retained [8]. An enhanced capacity to process information while anesthetised can be shown to be subject to various conditions that alter the brains responsiveness to stimuli independently of levels of anesthetic agents [9]. For example, learning or not learning during anesthesia has been shown to depend on the presence of low levels of adrenaline [10, 11]. Using postoperative hypnosis, the ability to recall intra-operative conversations also depends on how distressing the information was to the patient and the relevance to the patient (name) [12]. There is more evidence of the persistence of auditory neural network activity. Intra-operative auditory distraction techniques have been shown to mitigate the unpleasant experiences of surgery under sedation or local anesthesia [13]. Intra-operative music, and music in combination with therapeutic suggestions, have some beneficial effects on postoperative recovery after hysterectomy [14]. Flashing light in the alpha range, or sound modulated at the same stimulus frequency, significantly reduce the intensity of pain, and thus arousal, induced by laser-heat [15]. As Jung’s paper demonstrates, auditory signals can disturb the response to anaesthetic drugs through the co-activity of the limbic system. I believe that this simple paper indicates we should more thoroughly investigate the role of the limbic system and its connections during general anaesthesia.