title: The detection of small simulated field defects using multifocal VEPs.
abstract: BACKGROUND
  The multifocal visual-evoked potential (mfVEP) has been widely investigated in the study of diseases of the visual system. However, the sensitivity of the mfVEP in objective detection of field defects has not been determined. This study investigates the variation of the mfVEP responses whilst simulating field defects by using different sizes of mask on the stimulus pattern.
  
  
  METHODS
  Simulated field defects of four different sizes (2, 3, 5, and 7 degrees) at two different eccentricities (10 and 16 degrees) were generated on a standard mfVEP dartboard stimulus using opaque masks. These masks were placed at the centre of each dartboard sector and the modified stimuli were used to elicit mfVEPs from 10 normal subjects. The response densities and latencies of N1, P1 of the mfVEP were compared, without and with small simulated field defects.
  
  
  RESULTS
  The minimum size of simulated field defect causing significant response density reduction in P1 and N1 was 5 degrees at both retinal eccentricities. N1 showed similar reduction in response density at both retinal eccentricities, but P1 showed larger reduction at the 10-degree location than at the 16-degree location. There was no change in latencies with simulated field defect at either location.
  
  
  CONCLUSIONS
  The mfVEP is only sensitive to a simulated field defect equal to or larger than 5 degrees in diameter, and mfVEP has greater sensitivity at 10-degree eccentricity than at 16-degree eccentricity.
authors.affiliation: Department of Optometry and Radiography, The Hong Kong Polytechnic University, Hong Kong SAR, China. orhenry@polyu.edu.hk
authors.forename: H L
authors.initials: HL
authors.lastname: Chan
authors.forename: W K
authors.initials: WK
authors.lastname: Tam
authors.forename: C L
authors.initials: CL
authors.lastname: Chen
authors.forename: N C
authors.initials: NC
authors.lastname: Ng
mesh_terms: D000328:Adult; D005074:Evoked Potentials, Visual; D005260:Female; D006801:Humans; D008297:Male; D010470:Perceptual Masking; D012160:Retina / Q000473:pathology*; D012607:Scotoma / Q000175:diagnosis*; D012680:Sensitivity and Specificity; D014794:Visual Fields; D014796:Visual Perception