title: Frequency and metrics of square-wave jerks: influences of task-demand characteristics.
abstract: OBJECTIVE
  Square-wave jerks (SWJs) during visual fixation and pursuit tracking of targets of varying speed and predictability were investigated in the present study.
  
  
  METHODS
  SWJs were measured in 91 subjects as they fixated a target or a remembered target location and tracked targets that varied in velocity and predictability.
  
  
  RESULTS
  Percentages of subjects making SWJ and mean SWJ frequency per minute in the high- and low-predictability conditions were 99% and 9.34 and 91% and 2.78, respectively. SWJ rates were significantly lower when observers fixated remembered target locations rather than visual targets and during tracking of faster-moving and less predictable targets. Differences in task conditions cannot be explained by volitional influences to control the first saccade in the SWJ. There was also no influence of age on SWJ rate.
  
  
  CONCLUSIONS
  Reduced SWJ rates after manipulations that increased task demands on visual pursuit of targets suggest an inverse relationship between current demands imposed by visual tasks and rates of intrusive saccades. These findings suggest that signals from cortical attentional systems may suppress inappropriate saccades that would divert the eyes from objects of interest during conditions imposing high task demands on the visual system.
authors.affiliation: Social and Behavioral Sciences Program, Arizona State University West, Phoenix, Arizona, USA.
authors.forename: Dennis M
authors.initials: DM
authors.lastname: Shaffer
authors.forename: Christine M
authors.initials: CM
authors.lastname: Krisky
authors.forename: John A
authors.initials: JA
authors.lastname: Sweeney
mesh_terms: D000293:Adolescent; D000328:Adult; D005403:Fixation, Ocular / Q000502:physiology*; D006801:Humans; D008875:Middle Aged; D011698:Pursuit, Smooth / Q000502:physiology; D012438:Saccades / Q000502:physiology*; D014796:Visual Perception / Q000502:physiology*