The power of suggestion: placebo, hypnosis, imaginative suggestion and attention

behavior. Proverbs, like “we tend to get what we expect,” and concepts, such as optimistic

thinking or self-fulfilling prophecy, reflect this intuition of an important link between one’s

dispositions and subsequent behavior. In other words, one’s predictions directly or

indirectly cause them to become true. In a similar manner, every culture, country or

religion has their own words for ‘expectation,’ ‘belief,’ ‘disappointment,’ ‘surprise,’ and

generally all have the same meaning: under uncertainty, what one expects or believes is the

most likely to happen. This relation between what caused a reaction in the past will

probably cause it again in the future might not be realistic. If the expected outcome is not

confirmed, it may result in a personal ‘disappointment’, and if the outcome fits no

expectations, it will be a ‘surprise’. Our brain is hardwired with this heuristic capacity of

learning the cause-effect relationship and to project its probability as the basis for much of

our behavior, as well as cognitions. This experience-based expectation is a form of

learning that helps the brain to bypass an exhaustive search in finding a satisfactory

solution. Expectations may thus be considered an innate theory of causality; that is, a set of

factors (causes) generating a given phenomenon (effects) influence the way we treat

incoming information but also the way we retrieve the stored information. These

expectancy templates may well represent one of the basic rules of how the brain processes

information, affecting the way we perceive the world, direct our attention and deal with

conflicting information. In fact, expectations have been shown to influence our judgments

and social interactions, along with our volition to individually decide and commit to a

particular course of action. However, people’s expectations may elicit the anticipation of

their own automatic reactions to various situations and behaviors cues, and can explain that

expecting to feel an increase in alertness after coffee consumption leads to experiencing

the consequent physiologic and behavioral states. We call this behavior-response

expectancy. This non-volitional form of expectation has been shown to influence

cognitions such as memory, pain, visual awareness, implicit learning and attention, through

the mediation of phenomena like placebo effects and hypnotic behaviors. Importantly,when talking about expectations, placebo and hypnosis, it is important to note that we are

also talking about suggestion and its modulating capability. In other words, suggestion has

the power to create response expectancies that activate automatic responses, which will, in

turn, influence cognition and behavior so as to shape them congruently with the expected

outcome. Accordingly, hypnotic inductions are a systematic manipulation of expectancy,

similar to placebo, and therefore they both work in a similar way. Considering such

assumptions, the major question we address in this PhD thesis is to know if these

expectancy-based mechanisms are capable of modulating more high-level information

processing such as cognitive conflict resolution, as is present in the well-known Stroop

task. In fact, in a recent series of studies, reduction or elimination of Stroop congruency

effects was obtained through suggestion and hypnotic induction. In this PhD thesis, it is

asked whether a suggestion reinforced by placebos, operating through response-expectancy

mechanisms, is able to induce a top-down cognitive modulation to overcome cognitive

conflict in the Stroop task, similar to those results found using suggestion and hypnosis

manipulation.