Spatial patterns of placebo-induced sensations.

<p>Sensations elicited by placebo irritant solution (left) and placebo laser (right). The stimulation site is marked with an arrowhead. Only areas that were reported by two or more subjects are shown. Note the referral of sensations to areas remote from the stimulation site as well as to the contralateral side of the body.</p

Spatial patterns of placebo-induced sensations as imagined by subjects to result from laser stimulation.

<p>The color in each point represents the number of subjects that imagined having sensations in that location. Only areas that were reported by two or more subjects are shown. The site of (imagined) stimulation is marked with an arrowhead. The upper image shows the results from subjects that actually experienced sensations during imagination, the lower of those who did not. Note the remarkable amount of overlap of sensations in both groups as well as the divergence of lines into an ulnar and a median line at the wrist.</p

Similarities of placebo-induced sensations and referred pain.

<p>Spatial comparison showing similarities of placebo-induced and imagination-induced sensation patterns (center) with patterns from low-level laser stimulation (left) and referred pain induced by hypertonic saline injection into paravertebral muscles (right) as reported in [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0124808#pone.0124808.ref031" target="_blank">31</a>].</p

Verbal descriptors for sensations.

<p>Incidence of the nine most common verbal descriptors for placebo-induced sensations. Of all 78 possible descriptors, these were the only ones chosen by 20% or more of the subjects. Together, they accounted for 59.9% of all reported sensations, which illustrates the similarity of sensations across subjects. Results from the imagined stimulation (experiment 3) are shown in the small box. In addition to the two most common descriptors from experiments 1 and 2, subjects frequently reported radiating sensations for imagined stimulation.</p

Stimulation loci and placebo interventions.

<p>a) Locations stimulated in the experiment. b) Placebos used in the study. Left side: Placebo irritant solution (water), right side: Placebo laser (switched off). c) Application of the two placebos to the little finger of the hand.</p

Onset of sensations.

<p>Histograms of placebo-induced sensation onset time (defined as the time between the announced placebo stimulation and the onset of the first sensation reported by the subject). Median values were 38s for placebo irritant solution and 42.5s for placebo laser. No difference between distributions was found.</p

Differences in character traits of susceptible and non-susceptible subjects.

<p>Results from a comparison of character traits in subjects with and without placebo-induced sensation during imagined stimulation (experiment 3). Susceptible subjects showed a significantly lower TCI score in the “novelty seeking” category (p = 0.0071) pointing to higher basal dopaminergic activity in these subjects [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0124808#pone.0124808.ref024" target="_blank">24</a>].</p

Cumulative presentation of the selections of the three discriminators found to provide a questionnaire that distinguishes between A_δ and C fiber mediated pain.

<p>Left: presentation separately for the four pain models and standardized at 100% of the ratings for each model. Right: presentation of the selections of the three discriminators, pooled for predominately A_δ fiber mediated pain sensations evoked with von Frey hair puntuate pressure stimuli or with laser heat stimuli and C fiber mediated pain sensations evoked with blunt pressure stimuli or with 5-Hz electrical stimuli.</p

Choices of descriptors of the pain stimuli, summed across 20 subjects (A_δ selectivity more to the left, C fiber selectivity right).

<p>The colored bars indicate the three discriminators found to provide a questionnaire that distinguishes between A_δ and C fiber mediated pain (blue = “pricking”, green = “pressing”, red = “dull”, as specified in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0012944#pone-0012944-g002" target="_blank">Figure 2</a>).</p