This protocol enables researchers to dynamically establish an indifference point for each participant, effectively eliminating the influence of any biased indifference points on the assessment of inter-temporal choices. The event related potential or ERP techniques, high-temporal resolution enables the differentiation of the temporal sequence of different cognitive processes. Additionally, ERP components can serve as indicators of specific cognitive processes.
Begin by making the participant perform the delayed discounting task or DDT as the first stage task to measure the participant's indifference points. To do so, give the participant two paper cards to choose from, one of which indicates a fixed amount of 10 or 20 China Yuan that the participant will obtain immediately. And the other card shows a variable amount of money that will be obtained after one month.
Let the participant continue to the next trial if he or she chooses the immediate reward. After each trial, change the variable amount card to set the next pair of cards before asking the participant to make a new choice. Starting from 10 or 20 China Yuan, increase the variable amount by one China Yuan for each trial, up to 25 or 35 China Yuan, respectively.
End the experiment when the participant chooses a delayed reward card. Consider that card amount as the participant's indifference point. After the first stage task, proceed to make the participant perform a gambling task in the second stage of the event related potential task.
Begin the task by displaying the fixation at the center of the screen for a random duration of 400 to 600 milliseconds. Then display the opponent's name for 1000 milliseconds before presenting two cards symmetrically on both sides of the fixation at the center of the cue interface screen. Ask the player to choose either the left card by pressing one on an external numeric keypad or the right card by pressing three on the keypad.
Ensure the selected card is highlighted with a red rim for 1000 milliseconds after the participant decides. Display the outcomes of both the participant and the opponent for 1000 milliseconds. Finally, display a blank screen for 500 milliseconds.
Ask only one from the participant pair to make a response to the task and wear the electrode cap. Instruct participants to enter the experimental room and sit in a comfortable chair about one meter from the screen. Next, have them perform the DDT task as demonstrated previously to record the indifference points of the participant.
After the DDT task, use the alcohol and facial scrub one after the other to gently scrub the skin on the participant's face. To position the electrode cap with the 64 silver silver chloride electrodes on the participant's head, first, place the CZ electrode at the top of the head and then confirm the position of the cap. Then, place the corresponding electrodes on the tip of the participant's nose.
Place the reference electrodes about 10 millimeters above and below the left eye, near the other canthus of both sides, and on the left and right mastoids. After filling the conductive gel, tighten the external electrodes with an adhesive bandage. Moderately tighten the electrode cap strap on the chin to prevent the electrodes from shifting during the experiment.
Next, using a blunt tipped syringe, fill the internal cylindrical space of all electrodes in the cap with conductive gel. Observe the real-time impedance value on the display until the impedance falls below the desired threshold of 10 kilo ohms. Prior to the beginning of the experiment, display a guide on the screen to familiarize the participants with the procedures and appropriate responses.
Implement the formal experiment session using 480 trials and record the corresponding electroencephalogram or EEG signals of the participant. Divide the trials into six blocks, allowing the participants to take a two-minute break between blocks. After saving the recorded EEG data, assist the participants with removing the electrode cap and help them wash the conductive gel residue from their hair or skin.
A two by four set of gambles were designed based on time delay and social comparison, which was further factored with two social distance parameters. The waveform diagram and topographic map of the brain for the N100 component showed significant main effects of social distance and time delay. Friends caused more negative N100 amplitude than strangers and immediate outcome conditions caused more negative N100 amplitude than delayed outcome conditions.
The waveform diagram and topographic map of the P300 components showed the main effects of social comparison. The results of post pairwise comparison revealed that the better outcome of the positive unfair, and equal outcome of the large fair were more positive than the worse outcome of negative unfair. The social comparison coupled with time delay interaction was significant at the 0.1 level.
When the result was a worse outcome of the negative unfair condition, the P300 caused by the immediate gain was greater than the delayed gain. When the outcome was delayed, the P300 components of small fair condition, positive unfair condition, and large fair condition, although not significantly different, were more positive than the worse outcome of negative unfair. The assessment of the indifference point for the participant is crucial, and the value must be recorded accurately and applied in the subsequent experimental steps.
This protocol can be adopted to further explore the functional integration and separation involving different times and different brain regions in the field of inter-temporal decision making by means of FMRI. This protocol not only investigates the joint effect of social comparison and social distance, but also studies the influence of other external factors on the evaluation of inter-temporal choice outcomes.
