Active-controlled, brief body-scan meditation improves somatic signal discrimination.

Here in the science blog-o-sphere we often like to run to the presses whenever a laughably bad study comes along, pointing out all the incredible feats of ignorance and sloth. However, this can lead to science-sucks cynicism syndrome (a common ailment amongst graduate students), where one begins to feel a bit like all the literature is rubbish and it just isn’t worth your time to try and do something truly proper and interesting. If you are lucky, it is at this moment that a truly excellent paper will come along at the just right time to pick up your spirits and re-invigorate your work. Today I found myself at one such low-point, struggling to figure out why my data suck, when just such a beauty of a paper appeared in my RSS reader.

data_sensing (1)The paper, “Brief body-scan meditation practice improves somatosensory perceptual decision making”, appeared in this month’s issue of Consciousness and Cognition. Laura Mirams et al set out to answer a very simple question regarding the impact of meditation training (MT) on a “somatic signal detection task” (SSDT). The study is well designed; after randomization, both groups received audio CDs with 15 minutes of daily body-scan meditation or excerpts from The Lord of The Rings. For the SSD task, participants simply report when they felt a vibration stimulus on the finger, where the baseline vibration intensity is first individually calibrated to a 50% detection rate. The authors then apply a signal-detection analysis framework to discern the sensitivity or d’ and decision criteria c.

Mirams et al found that, even when controlling for a host of baseline factors including trait mindfulness and baseline somatic attention, MT led to a greater increase in d’ driven by significantly reduced false-alarms. Although many theorists and practitioners of MT suggest a key role for interoceptive & somatic attention in related alterations of health, brain, and behavior, there exists almost no data addressing this prediction, making these findings extremely interesting. The idea that MT should impact interoception and somatosensation is very sensible- in most (novice) meditation practices it is common to focus attention to bodily sensations of, for example, the breath entering the nostril. Further, MT involves a particular kind of open, non-judgemental awareness of bodily sensations, and in general is often described to novice students as strengthening the relationship between the mind and sensations of the body. However, most existing studies on MT investigate traditional exteroceptive, top-down elements of attention such as conflict resolution and the ability to maintain attention fixation for long periods of time.

While MT certainly does involve these features, it is arguable that the interoceptive elements are more specific to the precise mechanisms of interest (they are what you actually train), whereas the attentional benefits may be more of a kind of side effect, reflecting an early emphasis in MT on establishing attention. Thus in a traditional meditation class, you might first learn some techniques to fixate your attention, and then later learn to deploy your attention to specific bodily targets (i.e. the breath) in a particular way (non-judgmentally). The goal is not necessarily to develop a super-human ability to filter distractions, but rather to change the way in which interoceptive responses to the world (i.e. emotional reactions) are perceived and responded to. This hypothesis is well reflected in the elegant study by Mirams et al; they postulate specifically that MT will lead to greater sensitivity (d’), driven by reduced false alarms rather than an increased hit-rate, reflecting a greater ability to discriminate the nature of an interoceptive signal from noise (note: see comments for clarification on this point by Steve Fleming – there is some ambiguity in interpreting the informational role of HR and FA in d’). This hypothesis not only reflects the theoretically specific contribution of MT (beyond attention training, which might be better trained by video games for example), but also postulates a mechanistically specific hypothesis to test this idea, namely that MT leads to a shift specifically in the quality of interoceptive signal processing, rather than raw attentional control.

At this point, you might ask if everyone is so sure that MT involves training interoception, why is there so little data on the topic? The authors do a great job reviewing findings (even including currently in-press papers) on interoception and MT. Currently there is one major null finding using the canonical heartbeat detection task, where advanced practitioners self-reported improved heart beat detection but in reality performed at chance. Those authors speculated that the heartbeat task might not accurately reflect the modality of interoception engaged in by practitioners. In addition a recent study investigated somatic discrimination thresholds in a cross-section of advanced practitioners and found that the ability to make meta-cognitive assessments of ones’ threshold sensitivity correlated with years of practice. A third recent study showed greater tactile sensation acuity in practitioners of Tai Chi.  One longitudinal study [PDF], a wait-list controlled fMRI investigation by Farb et al, found that a mindfulness-based stress reduction course altered BOLD responses during an attention-to-breath paradigm. Collectively these studies do suggest a role of MT in training interoception. However, as I have complained of endlessly, cross-sections cannot tell us anything about the underlying causality of the observed effects, and longitudinal studies must be active-controlled (not waitlisted) to discern mechanisms of action. Thus active-controlled longitudinal designs are desperately needed, both to determine the causality of a treatment on some observed effect, and to rule out confounds associated with motivation, demand-characteristic, and expectation. Without such a design, it is very difficult to conclude anything about the mechanisms of interest in an MT intervention.

In this regard, Mirams went above and beyond the call of duty as defined by the average paper. The choice of delivering the intervention via CD is excellent, as we can rule out instructor enthusiasm/ability confounds. Further the intervention chosen is extremely simple and well described; it is just a basic body-scan meditation without additional fluff or fanfare, lending to mechanistic specificity. Both groups were even instructed to close their eyes and sit when listening, balancing these often overlooked structural factors. In this sense, Mirams et al have controlled for instruction, motivation, intervention context, baseline trait mindfulness, and even isolated the variable of interest- only the MT group worked with interoception, though both exerted a prolonged period of sustained attention. Armed with these controls we can actually say that MT led to an alteration in interoceptive d’, through a mechanism dependent upon on the specific kind of interoceptive awareness trained in the intervention.

It is here that I have one minor nit-pick of the paper. Although the use of Lord of the Rings audiotapes is with precedent, and likely a great control for attention and motivation, you could be slightly worried that reading about Elves and Orcs is not an ideal control for listening to hours of tapes instructing you to focus on your bodily sensations, if the measure of interest involves fixating on the body. A pure active control might have been a book describing anatomy or body parts; then we could exhaustively conclude that not only is it interoception driving the findings, but the particular form of interoceptive attention deployed by meditation training. As it is, a conservative person might speculate that the observed differences reflect demand characteristics- MT participants deploy more attention to the body due to a kind of priming mechanism in the teaching. However this is an extreme nitpick and does not detract from the fact that Mirams and co-authors have made an extremely useful contribution to the literature. In the future it would be interesting to repeat the paradigm with a more body-oriented control, and perhaps also in advanced practitioners before and after an intensive retreat to see if the effect holds at later stages of training. Of course, given my interest in applying signal-detection theory to interoceptive meta-cognition, I also cannot help but wonder what the authors might have found if they’d applied a Fleming-style meta-d’ analysis to this study.

All in all, a clear study with tight methods, addressing a desperately under-developed research question, in an elegant fashion. The perfect motivation to return to my own mangled data ☺

6 thoughts on “Active-controlled, brief body-scan meditation improves somatic signal discrimination.

  1. I noticed that people who practice mindfulness tend to be less susceptible to the rubber hand illusion (it never works for me for example). I speculate that this might be due to higher resolution of internal body model in those people. I wish I could see some research on that.

    • Hi Alek-

      That is pretty interesting. I wonder if anyone has examined this. In a similar vein I would like to see a Haggard-style Sense of Agency investigation in MT. All of this could point to an alteration of somatic predictions in the insula-acc network (following Seth and Critchley’s model). Thanks for your comment!

  2. Great post Micah, and very interesting study!
    I just wanted to clarify something you mention about the SDT analysis. You say that “MT leads to a shift in the relative quality of signal processing rather than an absolute performance increase.” because the false alarm rate (FA) changes but not hits (H). But the definition of performance is d’, and d’ goes up after MT. So they do show a basic increase in sensory ability.

    I think the confusion comes from there being at least two ways to change performance. d’ is proportional to (H – FA). So you can either increase H, or decrease FA, or both, to get a performance change. There are ways of linking each change to differences in the noise characteristics of the input stage (e.g. but I don’t think that one can make this claim here. Instead the important point is that they find a d’ difference but not a criterion difference (which would just be a baseline change in the tendency to say “yes”). So there is a true performance increase caused by MT, which in this case is driven by lower FA.

    I agree that studying metacognitive changes within-subject with meditation seems like a next natural step (although the Christoff paper goes some way towards establishing that link).

    • Hey Steve- Thanks!

      And thanks also for the useful clarification. I will update the body of the post now to reflect this detail – I think I took my interpretation direct from the authors, who tried to distinguish absolute performance (hits) from “discerning the signal from the noise, d'”. But of course d’ depends upon both measures, and “discerning signal from noise” is what we would usually describe as perceptual performance (we don’t want to include decision bias in our estimate of performance, obviously). Would it still be accurate to say that the d’ effect being driven by FA (and not hits, or both) may reflect a specific feature of stimulus processing?

      On the other note, we’ve discussed in detail applying meta d’ to MT here in Aarhus. Actually it is kind of funny because Antoine and I met at the ASSC the year you won an award for that study and were very close to implementing your paradigm in my longitudinal study. In the end we went with an error-awareness paradigm, a decision I have actually come to regret due to how finicky the aware vs unaware comparison is.


      • Aha yes the aware vs. unaware error stuff I find very interesting, but I’ve also heard it can be quite paradigm-specific.

        Would it still be accurate to say that the d’ effect being driven by FA (and not hits, or both) may reflect a specific feature of stimulus processing?

        Yes this seems reasonable… for instance it could be due to a change in noise variance, or some other distributional effect.

  3. Great post! I wonder whether the interoception finding depends crucially on the type of meditation you’re doing. For example, MBSR with its bodyscan, and vipassana with its focusing on the sensations of the breath as it enters and leave the body may do much more for interoception than the practices in the Tibetan tradition I am used to which mostly ignore the feelings in the body–the body is only referenced in terms of the posture.

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