Guest blog: Phenomenological control: Response to imaginative suggestion predicts measures of mirror touch synaesthesia, vicarious pain, and the rubber hand illusion


The Rubber Hand Illusion.  Credit: 30 Second Brain (Ivy Press). Edited by Anil Seth

This is a Guest Blog written by Peter Lush, postdoctoral research at the Sackler Centre for Consciousness Science, and lead author on this new study.  It’s all about our new preprint.

A key challenge for psychological research is how to measure subjective experience. One domain in which this is particularly relevant is for experiences of ‘embodiment’. These experiences show widespread individual variation and can be surprisingly easy to manipulate. The rubber hand illusion, for example, is a famous effect in which a simple procedure generates experiences of ownership over a fake hand. Experience of the illusion can be measured either directly, through subjective reports of illusion experience, or indirectly, by changes in the felt position of the participant’s own hand. Scientists consider these measures to provide insight into the processes by which conscious experiences of embodiment come about.

However, such interpretations overlook the role of trait (i.e., stable individual) differences in the ability to generate experience to meet expectancies, which we call ‘phenomenological control’. If measures of the rubber hand illusion reflect the active generation of expected experience, then existing accounts of this and related effects will be incomplete or incorrect. Our new preprint, on PsyArXiv, reports the results of three large scale studies (more than 1000 participants in total) investigating the relationship between the ability to change experience to fit situational demands (phenomenological control) and established measures of embodiment. These results have implications not only for interpretation of embodiment measures, but also for any research employing measures taken to reflect subjective experience.

Here are the theoretical motivations:

  • Many people are able to generate compelling experiences in response to expectancies arising from imaginative suggestion presented within the context of ‘hypnosis’. Hypnotic responding is voluntary (nobody can be forced to respond) but is experienced as involuntary. A wide range of experiences can be generated. Examples include visual, auditory or gustatory hallucinations, vivid dreams and apparently involuntary movements.
  • The extent to which individuals can control their phenomenology in response to imaginative suggestion is a normally distributed stable trait, with good test-retest reliability over a 25 year period. Only a relatively small number of people (10-15%) are unable to successfully respond to imaginative suggestion. Therefore, the majority of experimental participants in any scientific experiment are likely to have at least some phenomenological control abilities.
  • We know that the hypnotic context (e.g., the presence of a hypnotist or the use of induction procedures) is not required for response to imaginative suggestion.
  • The context of a scientific experiment (e.g., the presence of a scientist and the expectancies generated by participants’ preconceptions of science) may, like the hypnotic context, cause participants to engage in the control of phenomenology to meet their interpretations of the response expected by the experimenter or arising from the experimental procedure (for example, the synchronous brushing which is used to induce the rubber hand illusion may act as an implicit imaginative suggestion).
  • Such responding will be experienced as involuntary by the participant, and will generate convincing reports of changes in subjective experience.
  • Any test procedure in which the expectations of the experimenter are discernible to the participant may therefore reflect phenomenological control rather than the stated theoretical targets of interest.

Note that this proposal differs from common understanding of demand characteristics and experimenter effects, which are generally considered to lead to merely behavioural effects (e.g., social compliance). Subjects engaging in phenomenological control will report genuine experiences.

Hypnosis researchers employ standardised scales to measure response to imaginative suggestion within a hypnotic context. A high score on a hypnotisability scale shows that a participant has the ability to generate and control their phenomenology to meet the expectancies communicated by the ‘hypnotist’ through direct suggestion. Here we employed our Sussex Waterloo Scale of Hypnotisability (SWASH), which consists of ten imaginative suggestions for particular experiences (for example, the touch of a mosquito, a sweet or sour taste, hearing music, and involuntary movement). The most parsimonious theories of hypnotic responding argue that response to hypnotic suggestion involves a voluntary mental or physical act which is experienced as involuntary (e.g., Hilgard. 1977; Spanos, 1986; Dienes & Perner, 2007). For example, a successful response to a suggestion that one’s arm will move of its own accord involves generating the inaccurate phenomenology that a voluntary action is involuntary. Similarly, a suggested experience of hearing music would involve an intentional act of imagination which, again, is experienced as unintentional.

We tested our predictions on three embodiment measures. These effects were chosen because they involve striking changes in experience and therefore have much surface similarity with imaginative suggestion effects.

The rubber hand illusion


Figure 1. A large-scale rubber hand illusion study. We tested 353 participants in total, over the course of one week.    

The rubber hand illusion is perhaps the most well-known of all embodiment effects. To induce the illusion, a visible fake hand and the participant’s concealed real hand are stroked in synchrony, so that the felt touch of the brush on the real hand and the seen touch on the fake hand are closely matched. The level of agreement or disagreement with statements describing illusion experience is taken on a scale from -3 (indicating strong disagreement to +3 (indicating strong agreement). Expected effects in the rubber hand illusion may be easy to discern from the induction procedure alone, even if given no verbal instructions; for example, it may be clear to participants that they are expected to feel the touch of the brushing on their own hand located on the fake hand positioned in front of them.

We tested 353 participants, measuring both their SWASH hypnotisability score and their performance in the rubber hand illusion (Figure 1). Consistent with our predictions, hypnotisability scores predicted subjective report scores and also proprioceptive drift (a measure of changes in the felt position of the participant’s hand). Figure 2 shows that, on average, experience of both felt touch and ownership in the rubber hand illusion requires the ability to control phenomenology to meet expectancies. The 353 participants have been divided here into four groups by their hypnotisability score (error bars show 95% CIs). The figure shows individual illusion agreement scores for standard illusion statements (used in Botvinick & Cohen,1998 and for many subsequent studies). Statement S1 (“It seemed as if I were feeling the touch of the paintbrush in the location where I saw the rubber hand touched”) and S2 (“It seemed as though the touch I felt was caused by the paintbrush touching the rubber hand”) describe experiences of felt touch, while statement S3 (I felt as if the rubber hand were my hand”) an experience of ownership.  The least hypnotisable quarter of participants did not on average agree with statements S2 and S3, but this group did agree with statement S1. This is probably attributable to ambiguous phrasing, as the statement can be interpreted as asking for participants’ mundane experience of touch on their own hand (see Botvinick and Cohen, 1998, in which all participants reported maximum agreement with this statement). In any case, factor analysis suggests that agreement with this statement does not reflect experience of embodiment (Longo et al, 2007).


Figure 2. Mean subjective report scores in participants grouped by hypnotisability score (lowest scores on the left side of the chart).

In summary, common direct and indirect measures of the rubber hand illusion are substantially related to hypnotisability and on average the illusion does not occur in people unable to respond to hypnotic suggestion.

It’s worth noting that the rubber hand illusion has also been associated with common physiological measures like skin conductance response (SCR), histamine reactivity and body temperature. One might think these measures would be immune to phenomenological control.  However, these physiological properties are known to be susceptible to imaginative suggestion (SCR; histamine reactivity; temperature). We therefore predict similar relationships between hypnotisability and these measures.

Mirror touch synaesthesia and vicarious pain

Mirror touch and vicarious pain are experiences of pain or touch in response to the witnessed pain of another. In a research setting, these effects can be studied through the use of videos showing painful stimuli, or of touch to humans and inanimate objects. The primary measure is the proportion of videos which generate a felt touch or an experience of pain in response to visual stimuli.

Again, as predicted, hypnotisability score predicted both vicarious pain and mirror touch response. Figure 3 shows the mean number of vicarious pain experiences reported for videos showing a range of apparently painful events (e.g., injections and sporting injuries) in 404 participants. A clear relationship between hypnotisability and vicarious pain response can be seen.


Figure 3.  Mean number of vicarious pain responses in participants grouped by hypnotisability score.

Figure 4 shows the results for mirror touch synaesthesia. Here, a sample of 154 participants were tested. Mirror touch synaesthetes (defined by response to 9 or more videos) were, on average, highly hypnotisable, with a mean score equal to the cut-off for the top 13% of SWASH scores.


Figure 4. Mean hypnotisability scores in participants grouped by the number of reports of mirror touch experience to video stimuli (no response, 1-8 videos and 9-16 videos).


Measures of three prominent embodiment effects – the rubber hand illusion, mirror touch synaesthesia, and vicarious pain – reflect the ability to generate compelling phenomenology in response to imaginative suggestion. At this stage, we do not know to what extent these effects are attributable to phenomenological control. Further work will be necessary to establish whether or not there are, for example, rubber hand illusion effects which do not require phenomenological control abilities. Note also that, if mirror experiences in everyday life are driven by phenomenological control, experimenter-derived expectancies may have a relatively minimal effect on measures of these experiences in the lab (because participants may respond this way to any similar visual stimulus, away from the scientific context).

Note that, because imaginative suggestion can produce changes in brain activity consistent with the suggestion given (e.g., activity in visual brain areas for suggested visual hallucination), phenomenological control may also account for the results of neuroimaging studies of these embodiment effects.

Our results demonstrate that the engagement of phenomenological control abilities to fulfil expectancies can occur within a scientific context and that such abilities may account for these a range of subjective embodiment effects. Response to imaginative suggestion does not require a hypnotic induction, or even any hypnotic context. All that is required is the ability to control phenomenology and a context in which phenomenological control can be (unconsciously) interpreted as appropriate. Despite this, the majority of research into imaginative suggestion has been conducted within a hypnotic context, and as a result the possibility that scientific experiments present another such context in which phenomenological control abilities are engaged has been overlooked.

We are now developing a phenomenological control scale with which to investigate phenomenological control in many other effects across psychological science which could be influenced by the participant’s subjective experience. The results we present in this paper, therefore, may indicate that the reappraisal of empirical results in behavioural science will be necessary for a broad range of fields.

These studies focus on the role of phenomenological control in existing effects. However, phenomenological control should not be seen merely as confounding existing theories and presenting problems for psychological science. Trait differences in the ability to influence perception by top-down influences are a valuable target for scientific investigations of conscious experience in their own right.


Lush, P*., Botan, V., Scott, R. B., Seth, A.K., Ward, J., & Dienes, Z. (2019, April 16). Phenomenological control: response to imaginative suggestion predicts measures of mirror touch synaesthesia, vicarious pain and the rubber hand illusion.

This research was supported by the Dr Mortimer and Theresa Sackler Foundation, and the Canadian Institute for Advanced Research (CIFAR) Azrieli Programme on Brain, Mind, and Consciousness.

*Corresponding author, and author of this guest blog.



Training synaesthesia: How to see things differently in half-an-hour a day

Image courtesy of Phil Wheeler Illustrations

Can you learn to see the world differently? Some people already do. People with synaesthesia experience the world very differently indeed, in a way that seems linked to creativity, and which can shed light on some of the deepest mysteries of consciousness. In a paper published in Scientific Reports, we describe new evidence suggesting that non-synaesthetes can be trained to experience the world much like natural synaesthetes. Our results have important implications for understanding individual differences in conscious experiences, and they extend what we know about the flexibility (‘plasticity’) of perception.

Synaesthesia means that an experience of one kind (like seeing a letter) consistently and automatically evokes an experience of another kind (like seeing a colour), when the normal kind of sensory stimulation for the additional experience (the colour) isn’t there. This example describes grapheme-colour synaesthesia, but this is just one among many fascinating varieties. Other synaesthetes experience numbers as having particular spatial relationships (spatial form synaesthesia, probably the most common of all). And there are other more unusual varieties like mirror-touch synaesthesia, where people experience touch on their own bodies when they see someone else being touched, and taste-shape synaesthesia, where triangles might taste sharp, and ellipses bitter.

The richly associative nature of synaesthesia, and the biographies of famous case studies like Vladimir Nabokov and Wassily Kandinsky (or, as the Daily Wail preferred: Lady Gaga and Pharrell Williams), has fuelled its association with creativity and intelligence. Yet the condition is remarkably common, with recent estimates suggesting about 1 in 23 people have some form of synaesthesia. But how does it come about? Is it in your genes, or is it something you can learn?

It is widely believed that Kandinsky was synaesthetic. For instance he said: “Colour is the keyboard, the eyes are the harmonies, the soul is the piano with many strings. The artist is the hand that plays, touching one key or another, to cause vibrations in the soul”

As with most biological traits the truth is: a bit of both. But this still begs the question of whether being synaesthetic is something that can be learnt, even as an adult.

There is a rather long history of attempts to train people to be synaesthetic. Perhaps the earliest example was by E.L. Kelly who in 1934 published a paper with the title: An experimental attempt to produce artificial chromaesthesia by the technique of the conditioned response. While this attempt failed (the paper says it is “a report of purely negative experimental findings”) things have now moved on.

More recent attempts, for instance the excellent work of Olympia Colizoli and colleagues in Amsterdam, have tried to mimic (grapheme-colour) synaesthesia by having people read books in which some of the letters are always coloured in with particular colours. They found that it was possible to train people to display some of the characteristics of synaesthesia, like being slower to name coloured letters when they were presented in a colour conflicting with the training (the ‘synaesthetic Stroop’ effect). But crucially, until now no study has found that training could lead to people actually reporting synaesthesia-like conscious experiences.

An extract from the ‘coloured reading’ training material, used in our study, and similar to the material used by Colizoli and colleagues. The text is from James Joyce. Later in training we replaced some of the letters with (appropriately) coloured blocks to make the task even harder.

Our approach was based on brute force. We decided to dramatically increase the length and rigour of the training procedure that our (initially non-synaesthetic) volunteers undertook. Each of them (14 in all) came in to the lab for half-an-hour each day, five days a week, for nine weeks! On each visit they completed a selection of training exercises designed to cement specific associations between letters and colours. Crucially, we adapted the difficulty of the tasks to each volunteer and each training session, and we also gave them financial rewards for good performance. Over the nine-week regime, some of the easier tasks were dropped entirely, and other more difficult tasks were introduced. Our volunteers also had homework to do, like reading the coloured books. Our idea was that the training must always be challenging, in order to have a chance of working.

The results were striking. At the end of the nine-week exercise, our dedicated volunteers were tested for behavioural signs of synaesthesia, and – crucially – were also asked about their experiences, both inside and outside the lab. Behaviourally they all showed strong similarities with natural-born synaesthetes. This was most striking in measures of ‘consistency’, a test which requires repeated selection of the colour associated with a particular letter, from a palette of millions.

The consistency test for synaesthesia. This example from David Eagleman’s popular ‘synaesthesia battery’.

Natural-born synaesthetes show very high consistency: the colours they pick (for a given letter) are very close to each other in colour space, across repeated selections. This is important because consistency is very hard to fake. The idea is that synaesthetes can simply match a colour to their experienced ‘concurrent’, whereas non-synaesthetes have to rely on less reliable visual memory, or other strategies.

Our trained quasi-synaesthetes passed the consistency test with flying colours (so to speak). They also performed much like natural synaesthetes on a whole range of other behavioural tests, including synaesthetic stroop, and a ‘synaesthetic conditioning’ task which shows that trained colours can elicit automatic physiological responses, like increases in skin conductance. Most importantly, most (8/14) of our volunteers described colour experiences much like those of natural synaesthetes (only 2 reported no colour phenomenology at all). Strikingly, some of these experience took place even outside the lab:

“When I was walking into campus I glanced at the University of Sussex sign and the letters were coloured” [according to their trained associations]

Like natural synaesthetes, some of our volunteers seemed to experience the concurrent colour ‘out in the world’ while others experienced the colours more ‘in the head’:

“When I am looking at a letter I see them in the trained colours”

“When I look at the letter ‘p’ … its like the inside of my head is pink”

For grapheme colour synaesthetes, letters evoke specific colour experiences. Most of our trained quasi-synaesthetes reported similar experiences. This image is however quite misleading. Synaesthetes (natural born or not) also see the letters in their actual colour, and they typically know that the synaesthetic colour is not ‘real’. But that’s another story.

These results are very exciting, suggesting for the first time that with sufficient training, people can actually learn to see the world differently. Of course, since they are based on subjective reports about conscious experiences, they are also the hardest to independently verify. There is always the slight worry that our volunteers said what they thought we wanted to hear. Against this worry, we were careful to ensure that none of our volunteers knew the study was about synaesthesia (and on debrief, none of them did!). Also, similar ‘demand characteristic’ concerns could have affected other synaesthesia training studies, yet none of these led to descriptions of synaesthesia-like experiences.

Our results weren’t just about synaesthesia. A fascinating side effect was that our volunteers registered a dramatic increase in IQ, gaining an average of about 12 IQ points (compared to a control group which didn’t undergo training). We don’t yet know whether this increase was due to the specifically synaesthetic aspects of our regime, or just intensive cognitive training in general. Either way, our findings provide support for the idea that carefully designed cognitive training could enhance normal cognition, or even help remedy cognitive deficits or decline. More research is needed on these important questions.

What happened in the brain as a result of our training? The short answer is: we don’t know, yet. While in this study we didn’t look at the brain, other studies have found changes in the brain after similar kinds of training. This makes sense: changes in behaviour or in perception should be accompanied by neural changes of some kind. At the same time, natural-born synaesthetes appear to have differences both in the structure of their brains, and in their activity patterns. We are now eager to see what kind of neural signatures underlie the outcome of our training paradigm. The hope is, that because our study showed actual changes in perceptual experience, analysis of these signatures will shed new light on the brain basis of consciousness itself.

So, yes, you can learn to see the world differently. To me, the most important aspect of this work is that it emphasizes that each of us inhabits our own distinctive conscious world. It may be tempting to think that while different people – maybe other cultures – have different beliefs and ways of thinking, still we all see the same external reality. But synaesthesia, along with other emerging theories of ‘predictive processing’ – shows that the differences go much deeper. We each inhabit our own personalised universe, albeit one which is partly defined and shaped by other people. So next time you think someone is off in their own little world: they are.

The work described here was led by Daniel Bor and Nicolas Rothen, and is just one part of an energetic inquiry into synaesthesia taking place at Sussex University and the Sackler Centre for Consciousness Science. With Jamie Ward and (recently) Julia Simner also working here, we have a uniquely concentrated expertise in this fascinating area. In other related work I have been interested in why synaesthetic experiences lack a sense of reality and how this give an important clue about the nature of ‘perceptual presence’. I’ve also been working on the phenomenology of spatial form synaesthesia, and whether synaesthetic experiences can be induced through hypnosis. And an exciting brain imaging study of natural synaesthetes will shortly hit the press! Nicolas Rothen is an authority on the relationship between synaesthesia and memory, and Jamie Ward and Julia Simner have way too many accomplishments in this field to mention. (OK, Jamie has written the most influential review paper in the area – featuring a lot of his own work – and Julia (with Ed Hubbard) has written the leading textbook. That’s not bad to start with.)

Our paper, Adults can be Trained to Acquire Synesthetic Experiences (sorry for US spelling) is published (open access, free!) in Scientific Reports, part of the Nature family. The authors were Daniel Bor, Nicolas Rothen, David Schwartzman, Stephanie Clayton, and Anil K. Seth. There has been quite a lot of media coverage of this work, for instance in the New Scientist and the Daily Fail. Other coverage is summarized here.

Predictive processing, sensorimotor theory, and perceptual presence


I have a new ‘Discussion’ paper just out in the journal Cognitive Neuroscience. Right now there is just the target paper – eventually it will appear with published commentaries and my response.  The basic idea is to bring together, in a formal theoretical framework, ideas from Bayesian predictive processing and ‘enactive’ sensorimotor theory.  The new theory explains ‘perceptual presence’ in terms of the counterfactual richness of predictive representations, and it can also explain the absence of such presence in important cases like synaesthesia.

A predictive processing theory of sensorimotor contingencies: Explaining the puzzle of perceptual presence and its absence in synaesthesia

(A pre-copy-edit version can be obtained here)

ABSTRACT: Normal perception involves experiencing objects within perceptual scenes as real, as existing in the world. This property of “perceptual presence” has motivated “sensorimotor theories” which understand perception to involve the mastery of sensorimotor contingencies. However, the mechanistic basis of sensorimotor contingencies and their mastery has remained unclear. Sensorimotor theory also struggles to explain instances of perception, such as synaesthesia, that appear to lack perceptual presence and for which relevant sensorimotor contingencies are difficult to identify. On alternative “predictive processing” theories, perceptual content emerges from probabilistic inference on the external causes of sensory signals, however this view has addressed neither the problem of perceptual presence nor synaesthesia. Here, I describe a theory of predictive perception of sensorimotor contingencies which (i) accounts for perceptual presence in normal perception, as well as its absence in synaesthesia, and (ii) operationalizes the notion of sensorimotor contingencies and their mastery. The core idea is that generative models underlying perception incorporate explicitly counterfactual elements related to how sensory inputs would change on the basis of a broad repertoire of possible actions, even if those actions are not performed. These “counterfactually-rich” generative models encode sensorimotor contingencies related to repertoires of sensorimotor dependencies, with counterfactual richness determining the degree of perceptual presence associated with a stimulus. While the generative models underlying normal perception are typically counterfactually rich (reflecting a large repertoire of possible sensorimotor dependencies), those underlying synaesthetic concurrents are hypothesized to be counterfactually poor. In addition to accounting for the phenomenology of synaesthesia, the theory naturally accommodates phenomenological differences between a range of experiential states including dreaming, hallucination, and the like. It may also lead to a new view of the (in)determinacy of normal perception.