What our paper on hypnosis following brain injury does not show

We just published a paper in Brain entitled “Improving working memory performance in brain-injured patients using hypnotic suggestion.” We argue that for patients with acquired brain injury, hypnotic suggestion which asks the patient to recover an ability to concentrate and remember improves working memory performance. I also reported the study and the results at the Oxford University Press blog, including an estimate of how skeptical you should be. We expect quite some publicity - and perhaps even hype - around the results of this paper.

Hype and overinterpretation work for advertising but not for science. In this post, I will try to counter some potential misunderstandings. We are incredibly excited about our findings. We just want to make sure that the public is excited for the right reasons and at the right level. As I argued on the OUP blog post, the appropriate state of belief is probably that of a “pessimist turned hopeful” who awaits further data to decide whether to leave the pessimistic stance or keep it. Given that brain injury constitutes the second- and third-largest health-related cost in the world, even a small hope is quite substantial.

Before I continue, I want to highlight that this research was a joint effort by Professor Morten Overgaard, hypnotist Rikke Overgaard, and me. The project was funded by the European Research Council and the Karen Elise Jensen Foundation and organized under Cognitive Neuroscience Research Unit at Center for Functionally Integrative Neuroscience and Aalborg University. The following reflect the views of all authors.

This study does not show that subjects were “cured”

The participants in our study improved to the population average in measures of working memory, which is an important but not all-encompassing cognitive function. We did not assess other domains such as psychological trauma, physical impairment, etc. because we did not target the hypnosis at these domains and therefore it was not part of the research question. Therefore, we cannot make any data-based claims about the effect of hypnosis on those domains.

Hypnosis does not involve anything “mystic” or otherwise “out of this world”

On the gray market of hypnotists, it is all too common to see the same persons offering regression to previous lives, contact with the dead, healing auras, etc. As a scientist, I reject this. In our experiment, the hypnotist sat in a chair and read aloud from a piece of paper. The patient sat in another chair and listened. There’s only a vibrating vocal chord, sound moving through the air, hitting the eardrums of the patient, and then sensory transduction to action potentials in the brain. From there, we strongly expect that the mechanism underlying the effect will eventually be identified among the on-shelf phenomena in psychology and neuroscience, e.g. neural plasticity working bottom-up, unlearning learned non-use, using a different strategy/realization of the same surface behavior, etc. The only novelness of our findings, if they replicate, would then be that these mechanisms apply to brain injury with a large effect size. There’s a long and serious research history on hypnosis in the top neuroscience journals by top researchers. See e.g. this 2013-paper in Nature Reviews Neuroscience by Oakley & Halligan. This is the line of research we are continuing, and it is very different from the current public perception of what hypnosis is.

The cognitive consequences following acquired brain injury are not unreal or hypocrisy

Some may say “if it’s that easy to recover, then it wasn’t a real problem” and perhaps even put the patient at fault. I hope you would agree with me that such a claim would be straight up ridiculous. First, such a claim would entail a psychology-only understanding of brain injury, and since we do now know the mechanism of why hypnotic suggestion works, this remains an open question. Second, even if it turns out that the cognitive impairment following acquired brain injury somehow can be described in purely psychological terms, this does not change the seriousness of the condition. PTSD is not less severe or hypocrisy because it has a psychogenic origin.

No direct evidence of clinical effectiveness (yet)

Neuropsychological test have surprisingly low predictive value for the ability to participate in society, e.g. returning to work, living independently, etc. which is often the primary criterion of clinical success in rehabilitation (sorry, psychologists!) We did not assess the latter directly, but we plan to this in an in-progress follow-up experiment. However, we do have anecdotal evidence that targeted hypnotic suggestion could be effective for participation. Many of the patients returned to us with reports of getting jobs, beginning or finishing education, not getting exhausted from grocery shopping and family parties, etc. This effect was immediate for some and slower for others. I recall one case where the patient only realized that there had been an improvement when the spouse pointed to very tangible evidence such as no more bruises from inattentively walking into stuff. Similarly, an independent psychologist was inspired by our findings and used hypnosis on two brain injured clients with excellent clinical results. But these reports are prone to selection bias (only the positive cases report back) and neglecting base-rate improvements (some would have improved anyway). We need direct evidence on clinical effectiveness, and we don’t have that now.

No direct evidence that hypnosis drives the effect (yet)

In casual talk, it’ll be easy to say that “hypnosis improved working memory.” We even do this ourselves. However, it does not follow directly from our design since both the targeted suggestion procedure and our non-targeted (intended as placebo) suggestion procedure was administered under hypnosis. We did not deliver the same treatment in a non-hypnotic setting (the so-called London-Fuhrer design). So it remains possible that hypnosis is not necessary. From the empirical literature, however, it is generally found that hypnosis boosts the effect of suggestion (though it’s disputed why; some of it is just that the Gandhi-Oakley finding that the word “hypnosis” increases suggestibility in and of itself). My best guess is that hypnosis adds considerably to the effect given that, if not, the effect of suggestion on working memory performance would have been discovered a long time ago in studies of psychotherapeutic treatment following acquired brain injury.


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