Hearing in a closed loop
The team recently published a detailed study protocol to evaluate the effects of auditory stimulation—specifically a technique known as closed-loop auditory stimulation—on patients undergoing elective surgery.
The goal is to determine whether this technique can effectively enhance delta-wave activity under anesthesia, thereby stabilizing the patient’s state and avoiding the use of higher doses of anesthetic drugs.
Real-time brainwave monitoring using electroencephalography (EEG) is required to precisely time and deliver auditory stimulation in the form of brief bursts of sound resembling gentle clicks, transmitted through specialized earbuds worn by the patient.
“We use pink noise, which sounds a bit like a waterfall but lasts only 50 milliseconds,” explained Duclos. “Unlike white noise, which has equal volume across all pitches, pink noise is more intense in the lower frequencies while still spanning the entire sound spectrum.”
Duclos stressed that timing is everything: sleep research showed that synchronizing the sound with the peaks of delta waves increases their amplitude and strengthens them, while delivering the sound at the troughs of waves could disrupt brain activity.
Promising early results
While tests have so far been conducted on only a small number of patients, preliminary results are promising. They show that under anesthesia, stimulating the brain just before the troughs of delta waves may be the most effective way to amplify them.
“This is an unexpected result, as it differs from what is observed during sleep,” said Duclos. Her team is now continuing their investigations to confirm this finding, uncover the underlying mechanisms and determine optimal conditions for clinical use.
Duclos also believes this technique could help solve a major surgical challenge: managing “unconscious pain.” Even when patients are fully knocked out, their nervous system still registers the physical trauma of surgery.
This hidden reaction, known as nociception, triggers harmful neurological stress, including sudden spikes in brain activity that disrupt the depth of anesthesia and reduce its effectiveness.
Duclos believes that auditory stimulation could counteract these disruptions by helping to maintain strong delta waves despite the influx of subconscious pain signals triggered by surgery.
