title: Steady-state contrast response functions provide a sensitive and objective index of amblyopic deficits.
abstract: OBJECTIVE
  Visual deficits in amblyopia are neural in origin, yet are difficult to characterize with functional magnetic resonance imagery (fMRI). Our aim was to develop an objective electroencephalography (EEG) paradigm that can be used to provide a clinically useful index of amblyopic deficits.
  
  
  METHODS
  We used steady-state visual evoked potentials (SSVEPs) to measure full contrast response functions in both amblyopic (n = 10, strabismic or mixed amblyopia, mean age: 44 years) and control (n = 5, mean age: 31 years) observers, both with and without a dichoptic mask.
  
  
  RESULTS
  At the highest target contrast, the ratio of amplitudes across the weaker and stronger eyes was highly correlated (r = 0.76) with the acuity ratio between the eyes. We also found that the contrast response function in the amblyopic eye had both a greatly reduced amplitude and a shallower slope, but that surprisingly dichoptic masking was weaker than in controls. The results were compared with the predictions of a computational model of amblyopia and suggest a modification to the model whereby excitatory (but not suppressive) signals are attenuated in the amblyopic eye.
  
  
  CONCLUSIONS
  We suggest that SSVEPs offer a sensitive and objective measure of the ocular imbalance in amblyopia and could be used to assess the efficacy of amblyopia therapies currently under development.
authors: Department of Psychology, University of York, Heslington, York, United Kingdom.
authors: Daniel H
authors: DH
authors: Baker
authors: Department of Psychology, Université du Québec à Montréal, Montréal, Canada.
authors: Mathieu
authors: M
authors: Simard
authors: Department of Psychology, Université du Québec à Montréal, Montréal, Canada.
authors: Dave
authors: D
authors: Saint-Amour
authors: McGill Vision Research, Department of Ophthalmology, McGill University, Montréal, Canada.
authors: Robert F
authors: RF
authors: Hess
mesh_terms: D000328:Adult; D000550:Amblyopia / Q000503:physiopathology*; D015350:Contrast Sensitivity / Q000502:physiology*; D004569:Electroencephalography; D005074:Evoked Potentials, Visual / Q000502:physiology*; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D010775:Photic Stimulation; D012683:Sensory Deprivation / Q000502:physiology*; D055815:Young Adult
keywords: amblyopia; contrast; dichoptic masking; steady-state EEG
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