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Category: Procedure

Description: (modified from Fleming et al. 2015) Parameters might vary after pilot study. Participants 16 to 25 healthy volunteers with normal or corrected to normal vision and no history of neurological or psychiatric disorders will participate in the study. We will exclude participants whose percentage of responses doesn't fit the requirements (20% missed responses maximum). Equipment Participants will perform the experiment in a dark and sound-shielded Faraday cage and will seat in a comfortable chair 100 cm in front of a 15 inch CRT monitor with a refresh rate of 60 Hz. Buttons will be fixed on the top of two plastic cylinders (3 cm in diameter, 7.5 cm in height) separated by a distance of 20 cm. The thumb press will be measured as a force signal and will be digitized on line (A/D rate 2 kHz) allowing us to record the force applied by the participant and trigger a response signal when a force threshold of 60g is exceeded. Stimuli and procedure Grating contrast will be individually adjusted to threshold performance using a staircase procedure(70% correct performance) before the main task. The experiment will consist of a minimum of 700 trials splits into blocks of 70 trials. There will be a self-paced resting period in between blocks.A gabor patch is centrally presented of a short time (33 ms), and the subjects have to indicate, as quickly as possible, whether it had been horizontally or vertically oriented by pressing the appropriate response key with the right or the left thumb. The association between the stimulus (vertical vs. horizontal) and the response (left vs. right thumb) will be balanced across subjects. Finally, responses longer than 1500ms will be disregarded, and a feedback will be provided to the subject signalling that his answer was too slow. Following their response, a screen confirming their answer is displayed for 500 ms. The subjects then have to rate out loud their confidence on a scale from 1 (low confidence) to 4 (high confidence). Participants will be encouraged to use the full range of the confidence scale. The responses will be recorded each time and written down by the experimenter, who is outside the room where the participant sits. Along with EMG recordings of the flexor pollicis brevis activity of each hand, the thumb press force will be recorded for every trial. The total time of each trial will be 4,333 ms. Electrophysiological recording Electromyographic (EMG) activities were recorded continuously from preamplified Ag/AgCl electrodes (Biosemi® Active-Two electrodes®, Amsterdam). The signal was filtered and digitized on-line (bandwidth: 0–268 Hz, 3 dB/octave, sampling rate: 1024 Hz). Electrodes were pasted onto the skin of the thenar eminence over the flexor pollicis brevis of each thumb, about two centimeters apart. The EMG signal will be continuously monitored by the experimenter in order to avoid any background activity to facilitate the EMG onset detection. If the signal becomes noisy, the experimenter will ask the subject to relax his (her) muscles. Data analysis The EMG activities recorded during each trial will be displayed on a computer screen aligned to the onset of the imperative stimulus and the onsets of the changes in activity will determined visually and marked with the computer mouse.Trials will be classified as “partial error” if there is an activation of muscles of the non-responding hand. Reaction time (RT) will be measured between the onset of the stimuli and the mechanical response. Effects of partial errors, accuracy, RTs, response force and their interactions on confidence ratings will be assessed via hierarchical linear mixed-effects models.