At least two main findings emerge from this study. First, analgesic and motor placebo responses appear to be mediated by verbally induced expectations. Although the subjects' expectations were not directly measured, many studies shows verbal suggestions affect expectations. Opposite verbal suggestions modulated pain in contradictory directions and most important, even after a pharmacological analgesic preconditioning, a significant hyperalgesic effect occurred when suggestion of pain increase was given This indicates that the placebo effect of was caused by expectation of analgesia and not by the pharmacological preconditioning per se. This can also be seen in Parkinsonian patients, whose motor worsening appears to depend on verbally induced expectation; in fact, the opposite verbal suggestion was enough to reverse this effect. The second important finding is that verbally induced expectations of hormonal increase and decrease had no effect on hormonal plasma concentrations. However, placebo administration after sumatriptan preconditioning mimicked the effects of the sumatriptan itself. It is important to point out that these sumatriptan-like effects occurred regardless of whether verbal suggestions were given for GH increase or decrease. Thus verbal manipulations that are likely to affect expectations did not influence hormonal secretion.
“[I]n the present study, conditioning appears to play a crucial role in the placebo responses of human unconscious physiological functions, whereas expectations replace conditioning when conscious perception is involved (e.g., pain and motor performance).” Benedetti et al., 2003The two key mechanisms underpinning placebo effects and nocebo effects are expectations, and conditioning (Pavlovian, Classical Conditioning). The mechanism explaining the placebo or nocebo effect depends on the context. In some cases conditioning and expectations may contribute to the effect, while in others one or the other may be involved. However, some have pointed out the conditioning is a form of expectation. Thus, if this is the case conditioning always involves expectation, whereas the reverse may not be true.