Setting up a participant (SOP1)

Owned by Daniel Baker
Last updated: 05 Oct, 2014
4 min read

First obtain informed consent, and talk the participant through the procedure so that they know what to expect. Ensure that there are no contra-indications, such as epilepsy, which would preclude them participating.

To determine the participant’s cap size, measure the head circumference in cm by placing a tape measure around the widest part of the head (usually across the forehead just above the eyebrows, above the ears and across the widest part of the back of the skull).  Use the participant’s head circumference to select the appropriate WaveGuard cap using the chart below.

WaveGuard Cap sizes table:

To place the FPz electrode, measure the distance in cm from the participant’s nasion (indentation where the top of the nose meets the forehead) to inion (indentation where the back of the skull meets the top of the neck), and then mark 10% of that value above the nasion on the forehead. The distance from nasion to inion is usually around 35-40 cm, so FPz will usually be placed around 3.5-4 cm above the nasion. The FPz electrode should be placed in the center of the forehead, directly in line with the nose.

Ask the participant to hold the cap in place with the FPZ electrode.  While they are holding this in place, pull the rest of the cap back so that the rest of the scalp is covered and the ears fit through the holes to the sides. Fasten the chin strap so that it is firm, but not uncomfortable. If necessary adjust the position of the cap so that the midline electrodes FPZ, FZ, CZ etc are all in a straight line down the middle of the scalp.  Also ensure that the front of the cap is sitting around 1cm above the eyebrow line and that there is not too much loose material at the top.

Connect the cap to the amplifier using the two cables. The cable labelled 1 should attach to the upper connector, and the cable labelled 2 to the lower connector. If you wish to record blink activity, you now need to attach a bipolar electrode cable to channel 66. The preferred method of attaching this to a participant is to use pre-gelled self-adhesive electrodes, which are available in the B114 lab. It is also possible to attach the bipolar electrode using tape and gel, though this is messier and less stable. The black terminal should sit above the eyebrow, and the red terminal between the cheek and eyelid, though precise placement is not critical. It is also possible to measure horizontal eye movements by attaching a second bipolar cable between the temples and the outer corners of the eyes, though this is not routinely done at York.

Next, you need to inject gel into each electrode site using a syringe with a blunt tip. See the Obtaining consumables section for gel and syringe recommendations. The aim of this procedure is to achieve good electrical contact between the participant’s scalp and the electrode inside the cap. You can obtain online measures of electrode impedance using the ASAlab software, as detailed on the Running an experiment page, which is highly recommended for efficient cap set up.

Fill the syringe with gel, and insert the blunt tip into the hole in the GND (ground) electrode just above the forehead. Move the tip from side to side or in a circular motion, applying gentle pressure. The aim here is to move hair out of the way, and gently remove any dead skin cells. Then depress the syringe plunger very gently, whilst lifting the syringe out of the electrode site. Imagine that you are creating a tunnel of hair and then filling this with gel. With practise this can be done quickly and efficiently, without overspill of gel outside of the electrode terminal.

Repeat this procedure with all the remaining electrode sites. The impedance monitor screen in ASAlab will allow you to target a particular level of impedance. Expert users typically aim for impedances below 5kOhms, though the tolerance will depend on the experimental paradigm you are using. If all electrodes have poor impedance this may mean that the ground electrode is not well connected and needs further attention.

Once all electrodes are gelled, check that the EEG traces look sensible. Have the subject blink, and perhaps grind their teeth and observe the changes in the trace. You are now ready to begin the experiment. Once you have finished data collection, there are 2 showers located in the Psychology Department where participants can wash their hair after an experiment. The first is located on the first floor of A-block and the second on the top floor of C-block. Towels are available in all labs, and it is the responsibility of lab users to ensure that these are washed after use (a washing machine is available in the kitchen of the top floor of C-block).