title: The measurement of pupil cycling time.
abstract: OBJECTIVE
  To assess the influence of pupil size on pupil cycling time (PCT), a measure which may be elongated in cases of neurological disease.
  
  
  METHODS
  Clinically, pupil "cycling" is produced by focusing a slit-lamp beam on the pupil margin. A rhythmic contraction and dilation of the pupil is produced, as changes in pupil size alternately prevent and allow the light beam to reach the retina. In this study, however, the light beam was controlled electronically so that cycling could be produced around different pupil sizes. Measurements of the variation of PCT with pupil size were taken from 22 young normal subjects.
  
  
  RESULTS
  PCT was seen to depend upon pupil size, increasing monotonically but non-linearly as size increased. The wave-form of pupil cycling is typically sawtooth, contraction being much faster than dilation. There was considerable variation amongst subjects in the range of pupil sizes where pupil cycling could be elicited.
  
  
  CONCLUSIONS
  The results point to the need for a standardised procedure for the measurement of cycling time, because differences in clinical conditions will themselves influence pupil size, as will other external variables such as ambient light level. The results also bring into question neurological explanations for increased PCT in those diseases where pupil size is affected, because normal subjects have PCTs well beyond the accepted limits of normality when their pupils are enlarged.
authors: Department of Human Sciences, Loughborough University, UK. p.a.howarth@lboro.ac.uk
authors: P A
authors: PA
authors: Howarth
authors: G
authors: G
authors: Heron
authors: L
authors: L
authors: Whittaker
mesh_terms: D000293:Adolescent; D000328:Adult; D003941:Diagnostic Techniques, Ophthalmological; D005260:Female; D006801:Humans; D008027:Light; D008297:Male; D011680:Pupil / Q000502:physiology*; D012025:Reflex, Pupillary / Q000502:physiology*; D013997:Time Factors