This protocol provides an effective way to dissociate the confounding factors of unexpectedness in intuitive difficulty in anomalous sentences to determine the real contributor of N400 effect in language studies. Unlike the traditional static approach of manipulating expectancy through corpus frequency or cloze probability. This protocol proposed a dynamic mission to achieve this goal by multiple repetitions.
With this protocol, we can also examine how other ERP components, such as P600, may respond to multiple repetitions. It may help to reveal the underlying mechanism of P600 as well. Begin by escorting the patient to the testing room and have them sit comfortably in a chair.
Then prep the participant's skin with facial scrub and a cotton-swab on the forehead, under the left eye, near the outer canthus of the right eye, and around the left and right mastoid bone. Place the elastic cap with electrodes on the participant's head. Ensure that the CZ electrode on the cap is located at the vertex of the head and the cap has a left-right symmetry with midline electrodes placed over the midline of the head.
Next, stick the adhesive tape to the electrodes that record eye movements and fill them with conductive gel. Fix the one electrode at the outer canthus of the right eye and the other below the left eye. Then fasten the strap under the chin to prevent the adjacent electrodes from moving during the experiment and connect the cap electrodes to the recording system.
Finally, to reduce the impedance of all electrodes below five kilohms or 10 kilohms, switch the recording software to the impedance monitoring interface. Then starting with the ground and reference electrodes, past the blunt tip needle of the syringe containing conductive gel through the eyelets of the electrodes and hairs until it reaches the skin of the scalp. Push the syringe plunger to inject a small dose of the conducting gel onto to the skin until the gel connects the metal circle on the electrode and the skin.
Finally, check the real time impedance level displayed in different colors on the monitor until the impedance decreases to below the threshold value. Check all electrodes. Begin by asking the participant to relax and try to avoid excessive eye blinks and body movements during the experiment.
Inform the participant about the tasks and the repetition part in the tasks in the main sentence part. Ensure the participant understands that an incorrect sentence will be followed by a repetition part and a correct sentence by another complete sentence, which may be correct or incorrect. Then start the stimuli presentation program, and let the participant practice with the practicing section.
For the repetition part, present the extracted core structure and its correct filler expression together seven times in a random order. For the semantic re-initiating complete sentence, present the complete anomalous sentence right after its corresponding repetition part. After the practice session is complete, begin the formal experimental session and record the electric physiological signals.
Finally, after the experiment is complete, save the recorded electric physiological data and help the participant take off the cap. Instruct the participant to wash away the conductive gel on the skin and hair and provide them with a reward for their participation. The 11 related potentials were ERPs in the first time, the fourth time, and the seventh time presentation of the anomalous core structures revealed that the N400 effect in the 300-500 millisecond time window disappeared gradually.
ERPs in the unrepeated anomalous sentence group were more negative than those in the control group. ERPs in the repeated group were more positive than those in the unrepeated group. Remember to place all electrodes at the right location impedance of all electrodes below five kilohm or 10 kilohm before the experiment.
This protocol effectively limits the time of repeating sentences for multiple trials. Multiple repetitions of sentences may help to reveal the real memory in sentence comprehension.
