Specific Psychophysiological Measures
When it comes to trying to access the central nervous system, EEG is one of the easiest measurement tools for game testing and research to turn to. This is because unlike higher resolution PET scans or fMRI measurement, EEG does not require participants to be placed, lying down and still, in large, expensive (and magnetic) machinery.
Rather, EEG operates through the use of electrodes on a player's skull that measure the electrical impulses generated by the brain.
EEG setups range from full electrode caps, which take an hour or more to attach and are capable of measuring specific activations in certain brain regions, to relatively simple headbands which are capable of only general brain wave analysis.
Thankfully, when it comes to game research, this latter less intrusive and expensive form of EEG does provide quite workable measures of engagement and emotion by measuring various different frequencies of brain activity (or brain waves).
In terms of these frequencies, the bands of interest are usually the:
- Alpha band (8-14 hz) that reflects calm, mental work.
- Beta band (14-30 hz) that reflects focused, engaged mental work
- Delta band (1-4 hz) that reflects sleep, relaxation and fatigue
- Theta band (4-8 hz) that reflects emotions and sensations
So if someone is playing a game and EEG records an increase in Beta wave function, then you can assume that the player is actively engaged in some kind of mental work.
The Star Wars Force Trainer relies on reading Beta waves via a simple EEG setup.
However, there are several disadvantages to EEG. The first is that it is relatively expensive compared to other measures, especially if you want to go for the full electrode cap, and is quite time-consuming and invasive to set up and use. For example, with a full electrode cap setup, each electrode must be very specifically placed, with the addition of conducting gel, which is usually applied using a needle to ensure good coverage. While the needle is not used to pierce the skin, it can still be somewhat unpleasant and I have known of participants with aversions to needles to even faint during this process.
Furthermore, as with all of these measures, EEG is somewhat prone to producing artifacts if players move too much or speak (speaking activates areas of the brain, of course). Another commonalty for all of the measures I will mention is that there are considerable individual differences in psychophysiology, which means that baseline measures must always be taken. This is especially important for EEG, since some individuals do not produce any activity in the Alpha band at all (but are otherwise normal).
Finally, EEG can be difficult to interpret. For example, if you detect increased Delta activity, it could be that your game is relaxing and therefore enjoyable. On the other hand, it may be that it is boring and tiring. Similarly increased Beta band activity may indicate your game is engaging, or perhaps that the player is disengaged and thinking about a particularly hard day they had at work.
EMG is all about detecting the activation of muscles through the use of electrodes, which are attached to the relevant muscle (or muscles). So again, like EEG, (and like most of the measures I am mentioning) this method relies on detecting electric current. However, unlike EEG, EMG is a direct indication of activation in the peripheral nervous system.
EMG can be applied to basically any muscle -- for example, the muscles of the upper back could be examined to test tension or stress. But of particular interest in game research and testing is generally facial EMG. This is where electrodes are attached to specific facial muscles that are sad to be related to negative or positive emotional reactions.
Specifically these are muscles in the:
- Brow (Corrugator supercilii) that register negative emotion (unpleasant valence)
- Cheeks (Zygomaticus major) that register positive emotion (pleasant valence)
- Area around the eyes (Orbicularis oculi) that are said to register expressions of enjoyment and "genuine pleasure" (whatever that is)
This makes facial EMG one of the few physiological measures that can actually tap the valence axis of the typical two-axis view of emotions. Furthermore, the sensitivity of facial EMG means that changes in these muscles that could otherwise be missed from direct observation, or facial analysis software, can be detected and used.
Working all three muscles.
However, once again EMG has its disadvantages. First of all, you still have to deal with individual differences, so baselines are required (although this is less of a problem with EMG as it is with other measures). Then there is the intrusive nature of electrodes being on a player's face, combined with wires hanging off them.
This may actually limit natural movement of the face, and since it gives and indication to players that their facial muscles are being recorded, they may themselves produce unnatural responses -- perhaps overemphasizing facial movements in an attempt to assist your data collection (something that may even occur subconsciously).
Finally, and this will become somewhat of a broken record (or a corrupt mp3 player), but there is always the possibility of artifacts in your data caused by electrical interference or non-target events -- excessive body movement for example, or your players talking.
Electrodermal Activity (EDA)
Electrodermal Activity (EDA) is also known as Galvanic Skin Response or Skin Conductance and, as the "electro" prefix hopefully gives away, is related to measuring changes in electric current on the skin. Specifically, changes in electric current caused by the activation of sweat glands.
EDA is typically taken by recorded electrodes to two fingers (or toes) and are probably most famous for their use in "lie detector" tests. Since it does only use two electrodes, this means that EDA is less expensive and somewhat easier to set up than other physiological methods. Although care should be taken that the digits to which the electrodes are attached are not moved much during data recording -- something that can obviously be a problem if controllers have to be manipulated easily.
In terms of what EDA measures, it is seen as reacting to emotional arousal and mental workload, and gives very distinctive "spikes" in response to emotional stimuli and workload. This means that EDA can be handy for at looking at specific events during gameplay -- although it can also be averaged over time and examined.
A single EDA response, showing latency, the response, and the recovery period.
However, as the graph above shows, there can be quite a time between a game event occurring and the EDA response -- usually between one to five seconds. There is also a recovery period in EDA that must pass before any further response can be registered. This is of course a problem if you have lots of events going on in your game, as some may be missed, and due to the time lag it may not be clear exactly what an individual EDA is in response to.
Furthermore, EDA is quite a sensitive and noisy signal, which means that it suffers from specificity problems -- in other words an increase in EDA may be because a player was talking, moving too much, is engaged in your game, or is thinking about how cute you are (or a combination of these factors).