Fridlund & Cacioppo (1986)
Guidelines for Human Electromyographic Research
Psychophysiology, 23, 567-589
Based on a survey from over 100 psychophysiologists experienced in EMG.
P568 ‘ The EMG signal is a quasi random train of motor unit action potentials discharged by the contraction of striate ( KRO check) muscle tissue. This signal train is characterized by a frequency range from several Hz to over 2 kHZ and by amplitudes ranging from fractions of a millivolt to several hundred microvolt’ p574 ‘ the raw or filtered EMG signal is a stochastic train of motor unit action potentials’ p576 ‘ the motor unit action potential is bipolar and asymmetrical about electrical zero’
Technical considerations
1 Noise
Video/computer/TV other lab or home based sources of interference
2. Electrodes
Ground at 1 point only
Advantage of surface ( Ag-AgCl0 electrodes – broad ( non-selective) detection of firings of aggregates of motor units that correlate well with the overall level of contraction of the muscle groups but can’t always attribute activation to a particular muscle. 0.25 cm with 1 cm between is recommended for facial muscles
Advantage of fine wire- ‘They do not obstruct small movements as much as surface electrodes. Many electrodes can be used simultaneously without hiding facial areas from camera view’
Electrode Placement
- Avoid arrangements that straddle the motor endplate
- Try to identify anatomical landmarks that are uniform between participants
- Ease of attachment ( eg not too many skin folds)
Davis (1952) manual a good resource used by most researchers to date
Diagrams page 571
Orbicularis oculi inferior orbital portion (constricts the eye fissure ). The first electrode is attached 1 cms inferior to the exacanthion (outer commisure of the eye fissure). The second electrode is placed 1 cm medial to, and slightly inferior to, the first, so that the electrode pair runs parallel to the lower lip border’. ( ad hoc)
Zygomotor major ( pulls the lip corner up and back). One electrode is placed midway along an imaginary line joining the chellion and the preauricular depression ( the bony dimple above the posterior edge of the zygomatic arch) and the second electrode is placed 1 cm inferior and medial to the first ( i.e. toward the mouth) along the same imaginary line. ( based on available information)
Ground midline approx 3-4 cms superior to the upper borders of the inner brow.
Preparation
JFET amplifiers reduce the need for electrode site preparation
Measure resistance with voltmeter.
Relative advantage & disadvatage of monopolar versus bipolar recording
3. Filters
Primary energy 10-200 Hz, p573 ‘between 10-30 Hz power due in great part to the firing of motor units; beyond 30 Hz the shapes of the motor units action potentials are more important’ note p574 ‘preamplifier noise is most intrusive below 10 Hz and above 500-1000 Hz.’
P574 ‘ there may be implications for pass band selection for discriminating ‘fast’ versus ‘slow’ motor units, and for detecting ‘proximal’ versus ‘distal’ motor units
4. Recording & Presenting data
A/D conversion rate ( for my purposes need to be able to detect on and off)
Storage for repeated analysis
5. measurement
3 things to note
do not assume amplifier gain accuracy, check it directly
need to ensure that the measurement scale for each channel is equivalent
p579 ‘ recording EMG signals with AC-coupled amplifiers ( or DC amplifiers with zero offsets) ensures that the average value will be zero. Simple averaging of the raw EMG signal is thus uninformative’
Validity checks very important especially construct validity
Psychological consideration
1 social context
p580 ‘psychophysiological assessment is mostly non-invasive but can be psychologically intrusive’
hint
use neutral rather than technical vocabulary
observation may inhibit facial expression
facility to beckon experimenter at will
2 experimenter bias
p583 ‘ especially important when tasks involve emotion and social interaction’
