Phasic or Transient? Comment on the Terminology of the AASM Manual for the Scoring of Sleep and Associated Events

Abstract

In the recently published American Academy of Sleep Medicine (AASM) scoring manual for sleep stages,1 the term phasic was replaced throughout by transient. Recommendations for changes in terminology should be made with great care, especially when terminology is a core topic, as indicated by the subtitle of the AASM manual, “Rules, Terminology, and Technical Specifications.” The decision by the Visual Task Force was substantiated in an accompanying paper.2 The recommendation was based on the following reasoning, “Dictionary definitions of phase do not include short-lived transient activity and the term is used in physics for a particular appearance or state in a regularly cycle of changes.”2p129 This argument is misleading, since the term phasic in the context of rapid eye movement (REM) sleep was derived from physiology not physics. Physiologists discriminate between phasic and tonic muscle fibers according to the discharge pattern (see, e.g.3). This also becomes clear by the following quotation from Hobson and Scheibel.4 “Orlovsky (1970) has shown that, complementing these slowly conducting systems, fast reticulospinal fibers are activated in phase with mainly the flexors of one of the limbs in each step and Shimamura and Kuruge (1977) have demonstrated that the reticulospinal neurons thought to be part of the spinal-bulbar-spinal reflex are also phasically activated. Phasic rhythmic activity may also be recorded in vestibulo-spinal and rubrospinal pathways during locomotion (Orlovsky, 1972a,b).”4p62 In his book Neuronal Substrates of Sleep and Epilepsy, Steriade devoted a whole chapter to phasic events.5p187–98