title: Binocular summation improves performance to defocus-induced blur.
abstract: OBJECTIVE
  To assess whether there are any advantages of binocular over monocular vision under blur conditions.
  
  
  METHODS
  The effect of defocus, induced by positive lenses, was measured on the pattern reversal visual evoked potential (VEP) and on visual acuity (VA). Monocular (dominant eye) and binocular VEPs were recorded from 13 volunteers (average age, 28 ± 5 years; average spherical equivalent, -0.25 ± 0.73 D) for defocus up to 2.00 D using positive powered lenses. VEPs were elicited using reversing 10 arcmin checks (4 reversals/s). The stimulus subtended a circular field of 7° with 100% contrast and mean luminance 30 cd/m(2). VA was measured under the same conditions using ETDRS charts. All measurements were performed at 1 m viewing distance with best spectacle sphero-cylindrical correction and natural pupils.
  
  
  RESULTS
  With binocular stimulation, amplitudes and implicit times of the P100 component of the VEPs were greater and shorter, respectively, in all cases than for monocular stimulation. Mean binocular enhancement ratio in the P100 amplitude was 2.1 in focus, increasing linearly with defocus to be 3.1 at +2.00 D defocus. Mean peak latency was 2.9 ms shorter in focus with binocular than for monocular stimulation, with the difference increasing with defocus to 8.8 ms at +2.00 D. As for the VEP amplitude, VA was always better with binocular than with monocular vision, with the difference being greater for higher retinal blur.
  
  
  CONCLUSIONS
  Both subjective and electrophysiological results show that binocular vision ameliorates the effect of defocus. The increased binocular facilitation observed with retinal blur may be due to the activation of a larger population of neurons at close-to-threshold detection under binocular stimulation.
authors.affiliation: Institute of Vision & Optics, School of Health Sciences, University of Crete, Crete, Greece. plainis@med.uoc.gr
authors.forename: Sotiris
authors.initials: S
authors.lastname: Plainis
authors.forename: Dionysia
authors.initials: D
authors.lastname: Petratou
authors.forename: Trisevgeni
authors.initials: T
authors.lastname: Giannakopoulou
authors.forename: David A
authors.initials: DA
authors.lastname: Atchison
authors.forename: Miltiadis K
authors.initials: MK
authors.lastname: Tsilimbaris
mesh_terms: D000328:Adult; D005074:Evoked Potentials, Visual / Q000502:physiology*; D005260:Female; D006801:Humans; D008297:Male; D010364:Pattern Recognition, Visual / Q000502:physiology; D012030:Refractive Errors / Q000503:physiopathology*; D012160:Retina / Q000503:physiopathology*; D015348:Vision, Binocular / Q000502:physiology*; D015349:Vision, Monocular; D014792:Visual Acuity / Q000502:physiology*; D055815:Young Adult