Language: The N400 in Semantic Incongruity

1. Participant recruitment

1. Recruit 20 participants for the experiment.
2. Make sure that the participants have been fully informed of the research procedures and have signed all the appropriate consent forms.

2. Data collection

Figure 1: Electrode placement. Placement of the face electrodes to detect EOG artifacts and record mastoid movement (A). Diagram of measurement from directly between the eyebrows to just under the bump in the back of the head. 10% of this measurement is measured above the mid-eye mark, and this is where the FPZ electrode of the cap is placed (B).

1. EEG preparation (Note: These steps are for use with the Neuroscan 4.3 system with Synamps 2 amplifier and a 64-channel quick cap.)
   1. Participants in an EEG study should not have any hair products (e.g., gel, mouse, or leave-in conditioner) in their hair prior to their participation.
   2. Fill 2-4, 10-ml syringes with conductive electrode-gel (i.e., Quick-gel). It is suggested to stir the gel before using it to release air bubbles.
   3. Brush hair and scalp thoroughly (about 5 min).
   4. Clean head with alcohol and cotton gauze. Also clean the skin for placement of electrodes: two mastoids (behind each ear), below and above the left eye VEO (vertical electro-ocular), and the far sides of each eye HEO (horizontal electro-ocular; Figure 1, left).
   5. Using two-sided adhesive disks, place the electrodes.
   6. Measure the head from the front (directly between the eyebrows, mid-eye) to the inion (below the bump of the head in the back). This distance will determine the size of the cap (small, medium, or large). To place the cap, mark the 10% of the measured distance on the forehead and make sure that the mid-frontal electrode (FPz) is placed on this marked point.
   7. Attach the face electrodes to their respective cords on the cap.
   8. Start filling the electrodes with gel, using the blunt needle tip to scrape the hair aside underneath the electrode, so the electrode is in direct contact with the scalp. Be mindful not to injure the skin.
      1. Lifting up the electrode a bit makes it easier to insert the gel. In most cases, there will be hair underneath the electrode. Moving it out of the way will allow for better impedance.
   9. Take the participant to the soundproof room and plug in the cap and individual electrodes.
   10. Check the impedance of the electrode-scalp connection to keep it under 10 KΩ. If the impedance is high make sure the electrode has conductive gel and is in touch with the scalp.
       1. Impedance is the tendency to impede the flow of an alternating current. High impedance may increase noise in the data and should be minimized before the study begins.
       2. In most cases, the hair is in the way of the electrode. Moving it out of the way should get better impedance.
   11. Once the impedance is acceptable for all electrodes and EEG traces are void of noise, data collection can begin.
2. EEG data collection
   1. Prepare the participant to do the task.
      1. Place the participant in a chair 75 cm from the computer screen, in a sound and light-attenuated room (acoustically and electrically shielded).
      2. Tell the participant that he/she will be reading seven-word sentences, presented one word at a time on the screen in front of them. They are to read each sentence in order to answer questions about the contents of the sentences at the end of the experiment.
         1. Each of the seven words of the sentence is shown individually for 100 ms, with a 1000 ms interstimulus interval between each word.
            1. Words are presented one at a time in the center of the screen to minimize eye movements during the experiment.
         2. The participant first sees 10 practice sentences.
         3. Show the 240 stimuli in a random order, in two presentations of 120 sentences. One-third of the sentences are normal sentences, where the last word is semantically congruent with the rest of the sentence. One-third of the sentences are incongruent sentences, where the last word is semantically incongruent with the rest of the sentence. One-third of the sentences are normal sentences where the last word is presented in a larger font than the rest of the sentence.
3. Start the system, and have continuous recording of EEG throughout the presentation of the functional task.
4. EEG is amplified by amplifiers with a gain of 1024 and a band-pass of 0.01-100 Hz.
5. Trials contaminated by eye-blinks and artifact rejection (approximately 15% of trials) will be eliminated off-line.

3. Data analysis

1. Offline, reference data to averaged mastoids.
2. Epochs are baseline corrected using the epoch 200 ms before the onset of the stimulus.
3. To correct for motion artifacts, exclude epochs with a signal change exceeding 150 µV at any EEG electrode in the average.
4. Digitally filter the data offline (bandpass 0.05-20 Hz).
5. Use the ERP averages that are displayed from the Pz recording sites for the time course of each sentence (Figure 2).
   1. The peak (amplitude and latency) of the parietal N400 is automatically obtained from all electrodes.

Figure 2: Results of the semantic incongruity task. The participant is presented with a sentence, one word at a time. Each word appears for 100 ms, followed by 1 s of a blank screen. Participants see congruous sentences (red), incongruous sentences (blue), and sentences where the last word is presented in a larger size (green). Only the incongruous sentences produce the N400 response when the last word is presented. When the last word is congruous but larger in size, there is a later P560 response.

6. Statistical analysis
   1. Plot ERP averages from the parietal Pz electrodes for congruent, incongruent, and deviant size conditions.
   2. For peak amplitude and latencies, use F-tests for each latency range to determine whether there is a difference between target and control stimuli.