Cognitive auditory evoked potentials in coma: can you hear me?

Abstract

This scientific commentary refers to ‘Neural detection of complex sound sequences in the absence of consciousness’, by Tzovara et al. (doi:10.1093/brain/awv041). Event related potentials (ERPs), classically measured by EEG, are the electrophysiological brain responses to a stimulus. Somatosensory and auditory ERPs have been used as a non-invasive tool for assessing brain functions and predicting outcomes in disorders of consciousness and coma (for review see Bruno et al. , 2011). While early ERPs (such as the absence of cortical responses on somatosensory-evoked potentials) predict poor outcomes, cognitive ERPs may be indicative of recovery of consciousness after coma (Vanhaudenhuyse et al. , 2008). Auditory cognitive ERPs permit assessment of residual higher-order processing, such as echoic memory (e.g. using mismatch negativity potentials), acoustic and semantic discrimination (e.g. P3 or P300 evoked potentials), and incongruent language detection (e.g. N400 potentials). Classically, these ERP paradigms are based on the detection of a stimulus in violation of an auditory regularity, composed of two neural events, each characterized at the EEG level by a stereotypical morphology and latency: a mismatch negativity followed by a later complex named P300 further divided into an early (P300a) and a late component (P300b). The mismatch negativity is thought to reflect unconscious processing (Daltrozzo et al. , 2009), whereas the P300b has been linked to consciousness (Bekinschtein et al. , 2009). Neither of these ERP components are specific to auditory stimuli and both can be elicited by other sensory modalities. However, auditory cognitive ERPs have been used most extensively in the field of coma because they are easy to deliver and—in contrast to visual ERPs—can be easily acquired in eyes closed conditions. Given the …