Boosting the hypnotic experience. Inhibition of the dorsolateral prefrontal cortex alters hypnotizability and sense of agency. A randomized, double-blind and sham-controlled tDCS study

Alteration of hypnotic experience following transcranial electrical stimulation of the left prefrontal cortex

“…Except from Attention, the other dimensions participate in the regression equation used to generate the hypnoidal state score (HSS; Pekala & Kumar, 1984 ) which was increased by 15.4% in the active group. The intensity and the effect size of the increase of hypnotizability further strengthened the results of a previous study ( Perri et al, 2022 ) where a bilateral montage was adopted to stimulate the lDLPFC. However, unlike the previous investigation, the volition and the sense of agency were not affected by tDCS as revealed by the assessments of the hypnotic experience (PCI-HAP) and the response to hypnotic suggestions (SOARS).…”

“…In particular, activity of the supplementary motor area (SMA) was associated with feeling of control and ownership of movements ( Walsh et al, 2015 ), and it is recruited to predict the sensory consequences of an action such as to process agency error during its execution ( Zito et al, 2020 ); the dorsal fronto-median cortex has been associated with explicit assignments of agency ( Spengler et al, 2009 ), and the deactivation of the medial PFC described as a sign of the reduced engagement of the default network (DN), a brain network associated with internal attention and mind wandering reflecting the reduction of automatic thoughts in hypnosis (for a review see Landry et al 2017 ); as for the DLPFC, its activation correlated with the self-rated level of hypnotic “depth” ( Deeley et al, 2012 ), and it was associated with metacognition ( Dienes & Perner, 2007 ), conscious judgments about the self ( Miele et al, 2011 ) and action-selection processes ( Haggard, 2017 ; Perri et al, 2016 , 2017 ). It is probably not a case that the volitional control was reduced in the study where the dorsal-medial PFC was inhibited ( Perri et al, 2022 ), while the self-awareness and the self-rated hypnotic depth are among the dimensions affected by the present stimulation which targeted lateral portions of the PFC. As a further confirmation, neurostimulation studies on agency outside of hypnosis revealed that the tDCS over the pre-SMA affected the implicit measures of agency (for a review see Haggard, 2017 ) while stimulation of the DLPFC altered the feeling of control over voluntary actions ( Perri & Perrotta, 2021 ; Perrotta et al, 2021 ), but only when participants selected between multiple actions (for a meta-analysis see Khalighinejad et al, 2016 ).…”

“…In our opinion, the reason of the unaltered sense of agency may lie in the electric field distribution of the tDCS montage. In fact, while the bilateral stimulation of the lDLPFC spread the current effects to the dorsal-medial cortical areas (for details see Perri et al, 2022 ) with unpredictable consequence on the right hemisphere ( DaSilva et al, 2011 ), the extracephalic montage of the present study targeted a more focused and lateral region probably corresponding to the left middle frontal gyrus (MFG; see Fig. 1 ).…”

“…Direct current was transferred by a saline-soaked pair of surface sponge electrodes (25 cm 2 ) and delivered by a battery-driven constant current stimulator in a randomized, sham-controlled protocol. We decided to adopt the same stimulation parameters of a previous study (see Perri et al, 2022 for more details on this choice) with a different montage in order to check for any differences of the bilateral vs unilateral stimulation of the DLPFC. In particular, we adopted a unilateral extracephalic tDCS montage with the target electrode over the left DLFPC (F3 site of the 10/20 system) and the return electrode over the right deltoid.…”

“…In particular, two studies ( Coltheart et al, 2018 ; Dienes & Hutton, 2013 ) documented transient increases of responsiveness to hypnotic suggestions when disrupting the activity of the right or left dorsolateral prefrontal cortex (DLPFC) through the repetitive transcranial magnetic stimulation (rTMS). The same cortical areas have been stimulated by our group as well ( Perri et al, 2022 ), which however adopted the transcranial direct current stimulation (tDCS). Moreover, unlike previous studies we measured the hypnotic experience through the Phenomenological Consciousness Inventory: the Hypnotic Assessment Procedure (PCI-HAP; Pekala et al, 2010 ) in order to assess the variations in consciousness in addition to the canonical measures of hypnotizability.…”