Our final words to loved ones may not fall on deaf ears. As humans lay dying, new research suggests that one crucial sense is still functioning: The brain still registers the last sounds a person will ever hear, even if the body has become unresponsive.
A study released in June suggests that hearing is one of the last senses to disappear during death. Scientists found that the brains of "actively dying" patients in palliative care (some unresponsive, some still responsive) still registered activity in response to sounds. The patterns of activity were similar to those seen in a sample of healthy controls, suggesting that people still hear as they die.
Importantly there's a difference between hearing something and understanding it. But, what we do know from this work, is that dying loved ones may hear something if we speak to them, explains Lawrence Ward, the study's senior author and a professor at the University of British Columbia.
"It is possible that some of their cognitive processes are still functioning even though they can't respond overtly," Ward tells Inverse. "What we don't know is whether they understand and are comforted by those words."
The study was published on June 25 in Scientific Reports.
How the brain "hears" during death – This study is based on the brain activity seen in 17 healthy control patients, eight responsive hospice patients, and five unresponsive hospice patients.
Each patient was presented with two kinds of 5 note songs. One version was just five repeated notes, whereas others had changes in tones or differing patterns of tones. The healthy and the responsive hospice participants were asked to count the number of songs where those patterns changed.
As participants listened, the scientists looked for patterns of activity linked to those tone changes – which would signal that the brain had picked up on them. The scientists were searching for two very specific types of brain signals, which were actually identified in a 2009 Proceedings of the National Academy of Sciences study on non-responsive patients, and conscious patients.
The 2009 study revealed that when the brain detects a new sound, two things happen: First there's a "mismatch negativity response" (MMN). This manifests as a quick dip in brain activity. Then, there's a followup response they refer to as the P300 complex, which is comprised of two spikes in signaling.
The first is called P3a — which happens between 200 and 300 milliseconds after a tone change — and the second is called P3b — which happens about 300 milliseconds after a tone change. The MMN is thought to represent a non-conscious response (as is P3a), whereas the P3b response has been thought to represent an "index of working memory updating," that study reports.
"I would say it is possible — but certainly not proven — that at least some patients, some of the time, can comprehend what loved ones are saying to them."
Together, all of that signaling represents a "normal" response to tone changes.
The scientists were also interested in seeing how much of that "normal" signaling persists across healthy, responsive, and unresponsive patients.
All of the control patients and most of the responsive hospice patients showed that MMN response and the P3a response. All five unresponsive patients also showed than first-line MMN response, or a P3a, following tonal changes.
That, the authors suggest, is a sign that hearing is happening.
"What we know is that some of these patients' auditory systems are functioning in what seems to be close to a 'normal' manner," Ward says.
That said, they only found scant evidence of the P3b response, suggesting that the question of conscious understanding is still very much up in the air.
Hearing and understanding – Hearing and understanding are not the same. That said, there's a shred of early-stage evidence that there may be comprehension. In two of the unresponsive patients, Ward's team found two instances of a P3b response in response to a change in pattern.
Those responses to pattern changes may indicate that these patients' brains were trying to count rare patterns, even if they weren't actually able to communicate, says Ward. That "could indicate a form of comprehension."
These blips on the brain's radar are far from definitive signs of understanding. Ward notes that the study wasn't designed to detect that higher level of thinking in the first place. What we have right now is the possibility that even if someone isn't responding, those final words are being heard somewhere in the brain.
"So I would say it is possible — but certainly not proven — that at least some patients, some of the time, can comprehend what loved ones are saying to them, or even what is being said around them when they are unresponsive," Ward says.
Abstract: This study attempts to answer the question: “Is hearing the last to go?” We present evidence of hearing among unresponsive actively dying hospice patients. Individual ERP (MMN, P3a, and P3b) responses to deviations in auditory patterns are reported for conscious young, healthy control participants, as well as for hospice patients, both when the latter were conscious, and again when they became unresponsive to their environment. Whereas the MMN (and perhaps too the P3a) is considered an automatic response to auditory irregularities, the P3b is associated with conscious detection of oddball targets. All control participants, and most responsive hospice patients, evidenced a “local” effect (either a MMN, a P3a, or both) and some a “global” effect (P3b) to deviations in tone, or deviations in auditory pattern. Importantly, most unresponsive patients showed evidence of MMN responses to tone changes, and some showed a P3a or P3b response to either tone or pattern changes. Thus, their auditory systems were responding similarly to those of young, healthy controls just hours from end of life. Hearing may indeed be one of the last senses to lose function as humans die.