This term refers to distressed vocal and other behavior that is less intense than crying and often considered to be either an intermediate state or a transition between crying and settled behavior (awake or asleep). Vocalizations made on expiration, often with a glottal stop. Unlike crying, eyes usually open, often intermittently, when fussing. Attempts to distinguish fussing from crying on the basis of parameters obtained from sound recordings (e.g., fundamental frequency F₀) have proved to be rather fruitless. Thus, if the distinction is to be made, it relies on subjective impressions of the quality of sounds, movements and postures.
See Colic, Crying, Crying peak