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Brain imaging and hypnosis

Brain imaging and hypnosis

Brain imaging research has given a stimulus to many areas of investigation, and one of these is the phenomenon of hypnosis. Although it is well accepted that hypnosis can influence the mind and behaviour, the precise mechanism is not understood. However, brain imaging research has shed some light on this topic. In coming to an understanding of this area of research by neuroscientists, it is first useful to explain what neuroscientists now believe occurs when processing information – whether this be motor or sensory information or simply that of suggestion (a particular form of sound input).


Neural processing of sensory information

Sensory information enters the body via the sensory organs – eyes, ears, nose mouth and skin. This sensory information is passed up the nervous system to the appropriate primary sensory regions of the brain. For instance, sensory information coming in through the eyes (e.g. your partner’s face) is directed through the optic nerve to the visual cortex of the brain. At this stage there is only a basic process of electrical stimuli. In order for this stimuli to get translated into your partner’s face the information must go to a higher region in order for it to be recognised as your partner’s face and not that of another person – or for that matter a flower! In other words, the incoming information is compared with stored information present in the brain. This higher region is the executive centre, which lies in the frontal lobe of the brain.


The process described so far is referred to as bottom-up or feedforward. The stimulus comes up from the sensory receptors and enters the brain where it is decoded. How you respond, e.g. what you think you see,  depends on what information flows down. This feedback mechanism (as distinct from feedforward) is also called top-down. The information comes down from the brain where it is acted upon. Each sensory input has its own set of nerve fibres, but when decoding occurs this information must be combined before it can be decoded and a composite response initiated. What neuroscientists now know is that the nerves relaying information from top-down are larger in number than those involved in bottom-up. It has been estimated that nerves involved in feedback (top-down) information is ten times that of feedforward (bottom-up). The important point is that what you believe you see, hear feel and believe is based more on top-down feedback processing. This means what you see, feel, etc is not simply based on incoming information, but more on how this information is decoded based on your past experience and how the decoding takes place in the executive higher centres of the brain.


How we respond, therefore, depends as much (if not more) on top-down processing. Here is the crux for hypnosis. If the top-down processing can be convinced of a particular reality (regardless of what that reality is in terms of bottom-up processing), then the person will respond to that top-down reality that is constructed in the brain. So the stage volunteer who eats an onion and responds to the suggestion it is an apple, responds to the apple if the suggestion creates an alternative reality that over-rules the bottom-up information. The degree of hypnotic suggestibility from this perspective depends on the extent to which top-down processing can over-rule bottom-up processing. Focused relaxation (one interpretation of the hypnotic state) can also be viewed as the suitable conditions for top-down feedback processing to over-rule bottom-up feedforward processing. It also explains why children are much more susceptible to hypnosis (and suggestion in general). In the case of children, top-down nerves are not fully matured and so the realities are those more from the suggestion.


Additional studies of brain imaging reveal that there is a part of the brain called the anterior cingulate cortex that checks imagination against reality. Brain scans show that during hypnosis this part of the brain is altered. The anterior cingulate cortex suppresses parts of the brain that are not of current interest. During hypnosis attention is narrowed as the anterior cingulate cortex suppresses other parts of the brain and so there is less checking of suggestions against reality. In other words, ambiguities between the suggestion and reality are ignored.


Neuroimaging designs in testing hypnosis

Although neuroimaging is used to investigate hypnosis there are different ways to do this. Three are most common.


  1. Take an image of the brain in normal waking state and another when hypnotised then compare the two.
  2. Take an image of the brain in a normal waking state while processing a particular suggestion and another when hypnotised while processing the same suggestion and compare the two.
  3. Take an image of the brain in a normal waking state when performing a particular task and another when hypnotised and performing the same task and compare the two.

In each case the difference is attributed to hypnosis. Using the first method it has been usual to compare low and high suggestible hypnotic participants. Although the brain scans show a difference, the problem with this approach is that there is no control over what the participants are thinking about, and it may be this that is creating the difference. Procedures two and three are attempts to control for this. The most common is the third test procedure and a popular task used is that involving the Stroop task. (See Stroop test for a more detailed account of this popular task used in many psychological experiments.)


Briefly, participants are shown a set of colour words in black and asked to state the words as quickly as possible. Then they are presented with the same set of colour words, but now the words are coloured but the colour is different from the word, e.g. the word may be “BLUE” but is coloured red. In this second condition they are to state the colour the word is and not the word itself. Since language is so imprinted, when seeing the word “BLUE” but having it in the colour red, the participant takes longer to process the conflicting information in order to say the colour and ignore the word they are seeing. The Stroop effect denotes the delay involved when processing words from a conflicting colour. The Stroop effect can be quite accurately measured.


What experiments with hypnosis do is to give suggestions under hypnosis that participants will attend only to the colours, i.e. they are given a posthypnotic suggestion. What has been found is that the Stroop effect is much shorter for such hypnotised participants compared to participants who are not hypnotised (the control group). This is consistent with the research on brain scans that show the activity of the anterior cingulate cortex that suppresses checking of ambiguities between suggestions and reality during hypnosis.


What such experiments illustrate is that hypnotic suggestions can over-rule the conflict that takes place in the executive higher centres of the brain so speeding up top-down processing, as shown by brain images. What such experiments are demonstrating is that hypnosis does influence the brain and can influence behaviour by suggesting an alternative reality that over-rules not only sensory information, but also belief systems that the person may hold. They also provide some insight into why hypnosis works – along with a number of other talk therapies and meditation. It also explains the importance of belief and expectation. In particular, what you expect to see (a top-down process) is what you do see. Normally, bottom-up and top-down processes are not in conflict, but when they are (and because the nerves in top-down processing out-number those involved in bottom-up), then it is likely that the top-down process will win out when the conditions are right. Part of the job of the hypnotherapist is to create those suitable conditions, but ultimately only the person being hypnotised can do this (which is why some believe all hypnosis is self-hypnosis).


There is one important caveat, however. The nervous system is not a fixed network of nerves. Nerves can be created (neurogenesis), but more importantly there is a degree of plasticity in the brain occurring from the generation of new nerve connections, e.g. new dendrites that create new connections. Creating a trance state is a learning process, and such learning creates new neural pathways. It is this aspect that is important for self-hypnosis.


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