Project & Data Description

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Project Overview

 

• In this preregistered study, we investigated the effects of tDCS (transcranial direct current stimulation) on performance monitoring in patients with obsessive-compulsive disorder (OCD) and healthy individuals. Cathodal and sham tDCS was applied over the presupplementary motor area (pre-SMA) in two separate sessions, each followed by EEG recording during performance of a flanker task.

 

Procedure

 

• The study employed a randomized, double-blind, sham-controlled, crossover design. Patients with OCD and matched healthy control (HC) participants were invited for two experimental sessions and received one session of cathodal and one of sham tDCS.
  • The cathodal electrode was positioned over the pre-SMA (electrode site FCz) and the anodal reference electrode was placed on the right cheek.
  • In the active tDCS condition, a direct current of 1.5 mA was administered for 20 min. In the sham condition, the current was applied for only 40 s at the beginning of the 20 min period.

 

• Following the stimulation, participants performed an arrow version of the flanker task while EEG was recorded.
  • Stimuli consisted of five vertically arranged arrows pointing to the left or right.
  • Participants were instructed to indicate the direction of the central target arrow as quickly and accurately as possible by button press.
  • The central target arrow was flanked by arrows pointing in the same direction (50% congruent trials) or by arrows pointing in the opposite direction (50% incongruent trials).

 

Hypotheses

 

• Our primary hypothesis was that compared to sham tDCS, cathodal tDCS would reduce the error-related negativity (ERN) amplitude across healthy individuals and patients with OCD.  

• In addition, we expected that if inhibitory pre-SMA stimulation by cathodal tDCS attenuates performance monitoring processes, error rates would be increased and behavioral adaptation after error commission, that is, post-error slowing (PES), would be reduced.  

• Moreover, we predicted that cathodal tDCS would increase the amplitude of the error positivity (Pe).  

• To explore effects of cathodal tDCS on performance monitoring during correct responses, we additionally analyzed the correct-response negativity (CRN).  

• Regarding group differences, we predicted increased ERN amplitudes in patients with OCD compared to healthy participants in the sham condition.  

• We expected the tDCS-induced ERN attenuation to be more pronounced in the patient group compared to the control group.

 

Preregistration

 

• Hypotheses, experimental design, sample size, and analysis plan of this study were preregistered on the Open Science Framework (https://osf.io/7z8hj/).

 

Data Description

 

• Data set: ‘Single_Trial_Data.rda’
• 53760 observations (56 participants, 2 sessions per participant, 480 trials per session), 13 variables

 

VARIABLE	DESCRIPTION	VALUES
participant_id	Participant identifier	C_01 to C_30 and P_01 to P_30
group	Participant group	HC = healthy control participant OCD = patient with OCD
session	Experimental session	T1 = first experimental session T2 = second experimental session
stimulation	Stimulation condition	verum = cathodal tDCS sham = placebo tDCS
trial	Trial number within the flanker task	1 to 480 per participant and session
stimulus_type	Stimulus type in the flanker task	congruent = congruent stimulus incongruent = incongruent stimulus
response_type	Response type in the flanker task	correct = correct response incorrect = incorrect response miss = missing response
rt	RT in the flanker task	RT in milliseconds NAs for trials in which no response was made (miss)
rt_log	Log-transformed RT in the flanker task	log(RT) in milliseconds NAs for trials in which no response was made (miss)
rt_invalid	Indication whether RT in the flanker task was < 100 ms or > 800 ms	TRUE FALSE NAs for trials in which no response was made (miss)
MFN_0_100_FCz	Medial-frontal negativity (MFN); response-locked ERP from 0 to 100 ms at electrode FCz	Mean amplitude in microvolts NAs for trials with EEG artifact
Pe_200_400_Pz	Error positivity (Pe); response-locked ERP from 200 to 400 ms at electrode Pz	Mean amplitude in microvolts NAs for trials with EEG artifact
P3_300_500_CPz	P300; stimulus-locked ERP from 300 to 500 ms at electrode CPz	Mean amplitude in microvolts NAs for trials with EEG artifact

 

Session Info

sessionInfo()

## R version 3.6.1 (2019-07-05)
## Platform: x86_64-w64-mingw32/x64 (64-bit)
## Running under: Windows 10 x64 (build 19044)
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## [3] LC_MONETARY=German_Germany.1252 LC_NUMERIC=C                   
## [5] LC_TIME=German_Germany.1252    
## 
## attached base packages:
## [1] stats     graphics  grDevices datasets  utils     methods   base     
## 
## loaded via a namespace (and not attached):
##  [1] compiler_3.6.1  magrittr_2.0.1  htmltools_0.5.0 tools_3.6.1    
##  [5] yaml_2.2.1      stringi_1.5.3   rmarkdown_2.5   knitr_1.30     
##  [9] stringr_1.4.0   xfun_0.19       digest_0.6.27   rlang_0.4.11   
## [13] renv_0.12.0     evaluate_0.14