title: Visual pattern adaptation in subjects with photoparoxysmal EEG response: evidence for increased visual cortical excitability.
abstract: OBJECTIVE
  Photosensitivity, or photoparoxysmal response (PPR), is an abnormal EEG reaction to intermittent photic stimulation (IPS), consisting of spikes, spike-waves, and intermittent slow waves. Depending on the spread of the abnormal activity, PPR subgroups have been defined as having either propagating PPR or localized, occipital spikes (OS) only. Recent research suggests an enhanced excitability in the occipital cortex may underlie this reaction during IPS, but it remains unclear whether changes in excitability affect the function of the occipital cortex to other, less provocative visual stimuli. In this study, cortical function in photosensitivity was assessed using two visual aftereffects that occur after prolonged adaptation.
  
  
  METHODS
  Motion and tilt aftereffects were compared in healthy subjects with (n = 14, seven with propagating PPR, seven with OS) or without (n = 14) PPR.
  
  
  RESULTS
  The duration of the motion aftereffect was shorter in the PPR group than in the control group. The size of the tilt aftereffect did not differ between the groups. Thirteen from each group had participated in an earlier study in which occipital transcranial magnetic stimulation (TMS) was used to elicit phosphenes and to suppress the perception of briefly presented letters. The TMS intensity required to elicit phosphenes correlated with the size of the tilt aftereffect (TAE) in the PPR group only.
  
  
  CONCLUSIONS
  This study provides further evidence of enhanced cortical excitability in subjects with photosensitivity, which is likely to reflect changes in excitatory neurotransmission.
authors.affiliation: School of Psychology, Birkbeck College, University of London, London, United Kingdom. a.shepherd@bbk.ac.uk
authors.forename: Alex J
authors.initials: AJ
authors.lastname: Shepherd
authors.forename: Michael
authors.initials: M
authors.lastname: Siniatchkin
mesh_terms: D000221:Adaptation, Ocular / Q000502:physiology*; D000293:Adolescent; D000328:Adult; D000360:Afterimage / Q000502:physiology; D002648:Child; D004569:Electroencephalography; D019054:Evoked Potentials, Motor / Q000502:physiology; D005074:Evoked Potentials, Visual / Q000502:physiology; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D009039:Motion Perception; D015835:Ocular Motility Disorders / Q000503:physiopathology*; D010364:Pattern Recognition, Visual / Q000502:physiology*; D010775:Photic Stimulation / Q000009:adverse effects*; D050781:Transcranial Magnetic Stimulation; D014793:Visual Cortex / Q000503:physiopathology*; D014796:Visual Perception / Q000502:physiology*