Expressions which are so brief that they are barely perceptible to the untrained observer. Micro displays may be fragments of squelched, neutralized, or masked display. Micro displays may also show full muscular movements associated with a macro affect display, by may be greatly reduced in time. Such micro displays when shown in slow motion do convey emotional information to observers, and that expert clinical observers can see micro displays and read the emotional information without the benefit of slow motion projection.
Ekman, P., & Friesen, W. V. (1969). Nonverbal leakage and clues to deception. Psychiatry, 32(1), 88-106. doi: 10.1521/00332747.1969.11023575