Behavioral and neurophysiological investigation of the influence of verbal suggestion on tactile perception

Recently we demonstrated that it is possible to influence tactile perception by applying a placebo manipulation consisting of verbal suggestion and conditioning and that this influence is associated to changes in the late components (N140 and P200) of somatosensory evoked potentials (SEP) (Fiorio et al., 2012). Due to the powerful effects of words in changing symptoms perception in the clinical domain, aim of this study was to investigate whether even in the tactile modality, perception can be changed by the mere use of persuasive words in a specific context. To this purpose, we adopted the same experimental setting of our previous study, apart from the conditioning procedure. A group of subjects (experimental group) has been verbally suggested about the effect of an inert cream in enhancing tactile perception, while a control group was informed about the inefficacy of the cream. In order to unveil the neurophysiological underpinnings of this effect, we compared the amplitude of late SEPs (P100, N140, P200), before and after treatment. Results showed that the experimental group did not perceive an increase of tactile sensation after the treatment and no modification occurred in the late SEPs. This study proves that verbal suggestion alone is not sufficient to induce enhanced tactile perception (at least with this experimental setting), suggesting that a conditioning procedure may be necessary in the tactile modality. The absence of changes in the late SEP components could reflect the lack of strong expectation following the placebo procedure.