title: Two visual mechanisms of photosensitivity.
abstract: OBJECTIVE
  Photosensitive epilepsy is the most common of the "reflex" epilepsies. Precipitated by television viewing, flickering light, or specific visual patterns, it is the cause of seizures in 10% of young people with epilepsy. Photosensitivity is associated with two types of EEG abnormalities: photoparoxysmal responses (PPRs) and occipital spikes (OSs). It is unclear whether these abnormalities are mediated by different mechanisms, and furthermore, the clinical significance of OS is unknown.
  
  
  METHODS
  By using our previously established population of patients with photosensitive epilepsy, all showing EEG abnormalities on intermittent photic stimulation or pattern stimulation, we examined the effects of pattern contrast, spatial and counterphase temporal frequency, and colour on these abnormalities.
  
  
  RESULTS
  PPRs and not OSs show linear contrast dependency and are elicited by stationary stimuli and by non-colour-opponent isoluminant stimuli.
  
  
  CONCLUSIONS
  PPRs and OSs are generated independently by the parvocellular and magnocellular visual systems, respectively. The results add support to the hypothesis that only PPRs and not OSs are clinically significant.
authors.affiliation: Clinical Neurophysiology Unit, Aston University, Birmingham, England, UK. G.F.A.Harding@aston.ac.uk
authors.forename: G F
authors.initials: GF
authors.lastname: Harding
authors.forename: F
authors.initials: F
authors.lastname: Fylan
mesh_terms: D002540:Cerebral Cortex / Q000502:physiology / Q000503:physiopathology; D003118:Color Perception / Q000502:physiology; D015350:Contrast Sensitivity / Q000502:physiology; D004569:Electroencephalography / Q000706:statistics & numerical data*; D004827:Epilepsy / Q000175:diagnosis* / Q000209:etiology* / Q000503:physiopathology; D005556:Form Perception / Q000502:physiology; D006801:Humans; D009778:Occipital Lobe / Q000502:physiology / Q000503:physiopathology; D010775:Photic Stimulation / Q000009:adverse effects*; D013028:Space Perception / Q000502:physiology; D014795:Visual Pathways / Q000502:physiology* / Q000503:physiopathology; D014796:Visual Perception / Q000502:physiology*