Is auditory localization a sign of consciousness? Evidence from neuroimaging and electrophysiology

Background
Auditory localization is often part of the clinical evaluation of patients recovering from coma. There is however no clear consensus whether it should be considered as a reflex or as a conscious behavior. For example, auditory localisation corresponds to the diagnosis of unresponsive wakefulness syndrome (UWS) in the Coma Recovery Scale-Revised, while it is considered a sign of consciousness in other post-coma scales. This study aims to determine if auditory localization reflects conscious processing in patients with disorders of consciousness.
Methods
We first evaluated the proportion of patients with and without auditory localisation in 186 patients with severe brain injury, including 64 UWS, 28 minimally conscious minus (MCS-), 71 minimally conscious plus (MCS+), i.e., language relatively preserved) and 23 who emerged from MCS (EMCS). We then measured brain metabolism using fluorine-18 fluorodeoxyglucose positron emission tomography, functional connectivity using magnetic resonance imaging (MRI) and high-density electroencephalography (EEG) in patients in UWS with and without auditory localization.
Findings
Auditory localization was observed in 12% of patients in UWS, 46% of patients in MCS-, 62% of patients in MCS+ and 78% of patients in EMCS. Brain metabolism of patients in UWS without auditory localization was mostly restricted to primary areas, whereas a more widespread activity, including associative areas, was observed in patients in UWS with auditory localisation. Brain functional connectivity was also higher in patients in UWS with auditory localisation in the frontoparietal fMRI resting state network, along with higher EEG connectivity in alpha frequency band, compared to patients without auditory localization. Finally, differences were also found regarding the outcome, as the survival rate at two years appeared to be significantly higher in UWS patients with auditory localization as compared to those without auditory localization.
Interpretation.
Both clinical data in post-comatose patients and neuroimaging examinations in UWS patients with and without auditory localization support the idea that auditory localization should be considered as a sign of consciousness.