Influencing non-noxious perception by means of expectation.Behavioural and neurophysiological correlates of placebo-like procedures

Sensory perception can be influenced by cognitive functions like attention and expectation. An emblematic case of this is the placebo effect, where a reduction in pain perception can be obtained by inducing expectation of benefit following a treatment. These studies assessed the behavioural and brain activity correlates of a placebo procedure inducing an enhancement of non-noxious somatic sensation. In the first study an experimental group was verbally suggested and surreptitiously conditioned about the effect of an inert cream in enhancing tactile perception, while a control group was informed about the actual inefficacy of the cream. Both groups received non-noxious electric shocks activating A-Beta fibres on the right index finger, before and after application of the cream in the same site. The behavioural and neurophysiological effects of this procedure were measured by a numerical rating scale of subjective perception and by recording cortical and subcortical somatosensory evoked potentials (SEPs). Although the intensity of stimulation was physically identical in the two sessions, the experimental group reported stronger tactile sensation after cream treatment than before. In parallel, the experimental group showed enhanced somatosensory cortical responses (N140, P200) after treatment, whereas subcortical and early-cortical SEP components did not change. We suggest that these findings reflect top-down modulation on tactile perception probably due to an interplay between expectation and attention and might rely on interactions between prefrontal and parietal brain regions. In the second study we tried to induce a placebo-like response in the tactile modality with subjects being only verbally suggested that a treatment (cream) has the power to enhance non-noxious perception. We assessed the same behavioural (NRS) and neurophysiological parameters (late SEP component) Results showed that perceptual judgements were similar before and after the treatment in the experimental and control group, suggesting that verbal suggestion alone was probably not powerful enough in influencing subjects’ tactile perception. Even the neurophysiological parameters (amplitude and latency) of the late SEPs (N140 e P200) did not significantly change in the two groups between the two recording sessions. Three main considerations can be put forward in order to explain the lack of a placebo response in the current study: i) the lack of a conditioning procedure prevented subjects to experience the meaning of stronger tactile feeling per se; ii) verbal information could have induced a neutral emotional valence, thus preventing a strong reaction (positive or negative) to the treatment; iii) the route of treatment administration (local cream) might have been too weak in inducing expectation of enhanced tactile sensation.