Surface electromyography (EMG) proposes to record the spatiotemporal summation of all muscular fiber potentials recruited at the skin level. These action potentials occur at random intervals. So at any one moment, the EMG signal may be either positive or negative voltage. Surface electromyography detects firings of aggregates of motor units that correlate well with the overall level of contraction of muscles underlying and near the electrodes. However, the poor selectivity of surface electrodes can make it difficult to pinpoint exactly which muscle/fiber is contracting (see also cross-talk phenomenon). The amplitude of electromyograpical recordings of skeletal muscles can vary from fraction of microVolts to hundreds of microVolts, depending on the number of motor unit action potentials recruited, the size of the muscle and many other different noise-related factors.
Recording methods and tips (source: Biopac)
- Fridlund, A. J., & Cacioppo, J. T. (1986). Guidelines for human electromyographic research. Psychophysiology, 23(5), 567-589.
The startle reflex is an automatic defensive response to a sudden, intense, and unexpected stimulus (i.e., acoustic startle probe).
The startle eyeblink reflex is specifically potentiated in response to unpleasant stimuli and attenuated in response to pleasant stimuli.
- Blumenthal, T. D., Cuthbert, B. N., Filion, D. L., Hackley, S., Lipp, O. V., & Van Boxtel, A. (2005). Committee report: Guidelines for human startle eyeblink electromyographic studies. Psychophysiology, 42(1), 1-15.
Post Auricular reflex
The Post Auricular Reflex is a vestigial muscle microreflex in humans that serves to pull the ear backward and upward. As for the eyeblink reflex, the PAR can be elicited with an acoustic startle probe. the PAR magnitude is potentiated during presentation of pleasant stimuli relative to neutral or unpleasant stimuli.
- Stussi, Y., Delplanque, S., Coraj, S., Pourtois, G., & Sander, D. (2018). Measuring Pavlovian appetitive conditioning in humans with the postauricular reflex. Psychophysiology.