title: Factor structure of the human gamma band oscillatory response to visual (contrast) stimulation.
abstract: OBJECTIVE
  Visual contrast stimulation evokes in man an oscillatory mass response at approximately 20.0-35.0 Hz, consistent with stimulus-dependent synchronous oscillations in multiunit animal recordings from visual cortex, but shorter in duration and phase-locked to stimulus. A factor analysis was applied to characterize the signal structure under stimulus conditions inducing an oscillatory response and to identify possible subcomponents in normal volunteers.
  
  
  METHODS
  Contrast stimuli were gratings with a sinusoidal luminance profile (9.0 degrees; 5.0 cycle/degree; 80% contrast; reversal 1.06 Hz). The amplitude spectrum of the signal was computed by Discrete Fourier Transform (DFT) and the oscillatory response was separated from the corresponding visually evoked potential (VEP) by DFT high-pass filter at 19.0 Hz. Nine consecutive waves were identified in all subjects (60 volunteers), with amplitudes/latencies consistent with normative studies. A factor analysis was computed 1- in the frequency domain, on the amplitude values of the signal components (2 Hz resolution), and 2- in the time domain, on the latencies/amplitudes of the averaged VEP and oscillatory responses.
  
  
  RESULTS
  (1) Two non-overlapping factors accounted for the approximately 2-20.0 and approximately 20.0-40.0 Hz signal components, with separation of the approximately 20.0-35.0 Hz oscillatory response from low frequency VEPs. (2) Two factors on latencies and one factor on amplitudes (independent of each other and from those of VEPs) accounted for the average approximately 20.0-35.0 Hz oscillatory response.
  
  
  CONCLUSIONS
  The factor structure further indicates an oscillatory structure and some independence from conventional VEPs of the human oscillatory response to contrast, with a separation between the oscillatory response early and late waves possibly reflecting functional differences.
authors.affiliation: Section and Unit of Neurophysiopathology, Department of Motor Science and Rehabilitation, University Hospital, University of Genoa, Largo R. Benzi 10, I-16132 Genoa, Italy.
authors.forename: Simone
authors.initials: S
authors.lastname: Carozzo
authors.forename: Fabrizio
authors.initials: F
authors.lastname: De Carli
authors.forename: Manolo
authors.initials: M
authors.lastname: Beelke
authors.forename: Moreno
authors.initials: M
authors.lastname: Saturno
authors.forename: Sergio
authors.initials: S
authors.lastname: Garbarino
authors.forename: Cristina
authors.initials: C
authors.lastname: Martello
authors.forename: Walter G
authors.initials: WG
authors.lastname: Sannita
mesh_terms: D000328:Adult; D015350:Contrast Sensitivity / Q000502:physiology*; D004569:Electroencephalography; D005074:Evoked Potentials, Visual; D005163:Factor Analysis, Statistical; D005583:Fourier Analysis; D006801:Humans; D009991:Oscillometry; D010775:Photic Stimulation / Q000379:methods; D011930:Reaction Time; D012016:Reference Values