title: Forward and backward adaptive effects in global flash multifocal electroretinogram stimulation.
abstract: OBJECTIVE
  The present study investigated retinal adaptive responses in concert with the modulation of forward and backward adaptation induced by periodic global flashes using the global flash multifocal electroretinogram (mfERG).
  
  
  METHODS
  Six normal subjects were recruited for global flash mfERG measurements, which consisted of 103 scaled hexagonal elements followed by a global flash frame. In experiments I and II, with constant luminance maintained in both local and global flash frames, the number of dark frames was independently varied and these frames were either inserted prior to or following the global flash frame to investigate the forward or backward adaptive effect of the global flash on the mfERG. In experiment III, the number of dark frames was fixed but the luminance of the global flash frame was varied with constant luminance of the focal flash. This was used to demonstrate that the adaptive effect related not to time but to variation of luminance.
  
  
  RESULTS
  All the central, para-central and peripheral direct component amplitudes were found to be significantly influenced by variation of the number of dark frames (p < 0.01). Reducing the forward adaptive effect of the global flash enhanced the direct component response and it became steady after five dark frames were inserted following the global flash. Reducing the backward adaptive effect of the global flash also enhanced the direct component response but it started reducing after four dark frames were inserted prior to the global flash frame. These changes were different with luminance modulation of the global flash intensity with fixed dark frames, while the direct component amplitude grew approximately linearly with decreasing mean luminance of the global flash stimulation.
  
  
  CONCLUSIONS
  The retina plays a major role in visual adaptation. Both forward and backward adaptive effects of the global flash on the direct component have been illustrated in this study. The results show that the forward and backward adaptive phenomena in the global flash mfERG are different and demonstrate that backward adaptation is found at the retinal level.
authors.affiliation: Laboratory of Experimental Optometry (Neuroscience), School of Optometry, The Hong Kong Polytechnic University, Hong Kong SAR, China. henryhl.chan@polyu.edu.hk
authors.forename: Patrick H W
authors.initials: PH
authors.lastname: Chu
authors.forename: Yiu-Fai
authors.initials: YF
authors.lastname: Ng
authors.forename: Henry H L
authors.initials: HH
authors.lastname: Chan
mesh_terms: D000222:Adaptation, Physiological; D000328:Adult; D004596:Electroretinography / Q000379:methods*; D006801:Humans; D010775:Photic Stimulation; D012160:Retina / Q000502:physiology*