Hypnotic suggestions cognitively penetrate tactile perception through top-down modulation of semantic contents

Abstract

Perception is subject to ongoing alterations by learning and top-down influences. Although abundant studies have shown modulation of perception by attention, motivation, content and context, there is an unresolved controversy whether these examples provide true evidence that perception is penetrable by cognition. Here we show that tactile perception assessed as spatial discrimination can be instantaneously and systematically altered merely by the semantic content during hypnotic suggestions. To study neurophysiological correlates, we recorded EEG and SEPs. We found that the suggestion "your index finger becomes bigger" led to improved tactile discrimination, while the suggestion "your index finger becomes smaller" led to impaired discrimination. A hypnosis without semantic suggestions had no effect but caused a reduction of phase-locking synchronization of the beta frequency band between medial frontal cortex and the finger representation in somatosensory cortex. Late SEP components (P80-N140 complex) implicated in attentional processes were altered by the semantic contents, but processing of afferent inputs in SI remained unaltered. These data provide evidence that the psychophysically observed modifiability of tactile perception by semantic contents is not simply due to altered perception-based judgments, but instead is a consequence of modified perceptual processes which change the perceptual experience.

Figure 1

The two-point discrimination device used in the study. Seven different inter-pin distances (0.7–2.5 mm) and 1 control pin (0 mm) are presented to the index finger in random order.

Figure 2

Effect of different hypnotic conditions on tactile discrimination threshold. Mean two-point discrimination thresholds ± SEM are shown in (A) for BASELINE and hypnosis without suggestion (SUGG_0), and in (B) for the conditions SUGG_0 (hypnosis without suggestion), hypnosis with the suggestion of a bigger index finger (SUGG_B), and hypnosis with the suggestion of a smaller index finger (SUGG_S). (C) Average psychometric curves ± SEM obtained from all participating subjects for the conditions hypnosis without suggestion (SUGG_0), hypnosis with the suggestion of a bigger index finger (SUGG_B), and hypnosis with the suggestion of a smaller index finger (SUGG_S).

Figure 3

Average two-point discrimination thresholds ± SEM for responders (A) and non-responders (B). Responders showed decreased thresholds in SUGG_B and increased thresholds in SUGG_S, compared to SUGG_0. SUGG_0 = hypnosis without suggestion, SUGG_B = hypnosis with the suggestion of a bigger index finger, SUGG_S = hypnosis with the suggestion of a smaller index finger.

Figure 4

Effect of different hypnotic conditions on paired-pulse ratios. (A) Mean paired-pulse ratios are plotted for hypnosis without suggestion (SUGG_0), hypnosis with the suggestion of a bigger index finger (SUGG_B), and hypnosis with the suggestion of a smaller index finger (SUGG_S). (B) Individual paired-pulse ratios.

Figure 5

Correlation between changes of tactile discrimination and changes of paired-pulse ratios. Pearson correlation analysis between the differences in tactile acuity evoked by the semantic content of the hypnotic suggestion and associated differences of paired-pulse suppression. R² = coefficient of determination.

Figure 6

Grand-average waveforms. Recordings of the late SEPs in the Baseline and SUGG_0 condition (A) and in the three hypnotic conditions (B). Late SEP response peaks P45, N60, P80 and N140 are marked.

Figure 7

P80–N140 peak-to-peak amplitudes for different hypnotic suggestions. (A) Mean P80–N140 peak-to-peak amplitudes ± SEM are plotted for hypnosis without suggestion (SUGG_0), hypnosis with the suggestion of a bigger index finger (SUGG_B), and hypnosis with the suggestion of a smaller index finger (SUGG_S). RmANOVA revealed a significant effect of suggestions compared to hypnosis alone. (B) Individual P80–N140 amplitudes.

Figure 8

Resting-state functional connectivity for different hypnotic conditions. Phase-locking synchronization (PLV) ratios of the EEG beta (13–30 Hz) frequency band between left and right hemispheric medial frontal cortex (MFC) and the finger representation (CP3, CP4) in somatosensory cortex (SI). Mean synchronization after hypnosis without suggestion (SUGG_0) and baseline (A), and suggestions SUGG_0, SUGG_B and SUGG_S (B). (C, D) Individual changes of the PLV values.

Figure 9

Study schedule. Experiments always started with the baseline condition, always followed by the SUGG_0 condition: hypnosis without suggestion. Afterwards, the SUGG_B and SUGG_S conditions were performed in a pseudo-randomized order. Also, all experimental tests (2PD, EEG, SEP) were performed in pseudo-randomized order in all conditions. 2PD Two-point discrimination, SEP Somatosensory evoked potentials, EEG Resting-state electroencephalography.

Figure 10

Representative paired-pulse somatosensory evoked potential of one subject. Somatosensory evoked potentials were measured over cortical CP3 or CP4 after single (continuous black trace) and paired-pulse stimulation with an interstimulus interval of 30 ms (continuous grey trace). The dotted black trace results by subtracting the single-pulse trace from the paired-pulse trace. The analyzed amplitudes of the first response (A1) and second response (A2) after paired-pulse stimulation are marked by vertical bars; amplitudes of the second response after subtracting the response to a single pulse are denoted as A2s. Onsets of the applied electrical stimuli are marked by arrowheads.