Event-related Potentials and the Oddball Task

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

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.

Figure 1: Electrode placement. Placement of the face electrodes to detect EOG artifacts (left). 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 (right).

2. EEG data collection
   1. Prepare the participant to do the task.
      1. Place the participant in a chair 75 cm from the 16-in. computer screen, in a sound and light-attenuated room (acoustically and electrically shielded).
      2. Tell the participant that he/she will be seeing colored circles appear on the screen. Every time a green circle is seen, the participant should press a button held in his/her right hand (Figure 2).
         1. Show each stimulus for 1000 ms, with a 1000 ms interstimulus interval between stimulus presentations.
         2. Show the 64 target stimuli, randomly interspersed among 96 presentations of the non-target red circles. Repeat this sequence twice, for a total of 128 target stimulus trials and 192 non-target control trials.
   2. Start the system, and have continuous recording of EEG throughout the presentation of the functional task.
   3. EEG is amplified by amplifiers with a gain of 1024 and a band-pass of 0.01-100 Hz.
   4. Trials contaminated by eye-blinks and artifact rejection (approximately 15% of trials) will be eliminated off-line.

Figure 2: Study design for the oddball task. The participant is presented with either a red circle or a green circle. Each stimulus appears for 1 s, followed by a 1-s blank screen. Each time the participant sees a green circle, he is instructed to press a button held in his right hand.

3. Data analysis

1. Offline, reference data to averaged mastoids.
2. Segment continuous EEG data into epochs, starting 200 ms before and ending 1000 ms after the onset of the stimulus.
3. Epochs are baseline corrected using the epoch 200 ms before the onset of the stimulus.
4. To correct for motion artifacts, epochs with a signal change exceeding 150 microvolt at any EEG electrode were not included in the average.
5. The data are digitally filtered offline (bandpass 0.05-20 Hz).
6. Use the ERP averages that are displayed from the Pz recording sites for target and control stimuli.
   1. The peak (amplitude and latency) of the parietal P300 is automatically obtained at electrode Pz.
7. Statistical analysis
   1. Plot ERP averages from the parietal Pz electrodes.
   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.