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Walter G. Sannita, Simone Carozzo, Mauro Fioretto, Sergio Garbarino, Cristina Martinoli; Abnormal Waveform of the Human Pattern VEP: Contribution from Gamma Oscillatory Components. Invest. Ophthalmol. Vis. Sci. 2007;48(10):4534-4541. doi: 10.1167/iovs.07-0234.
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purpose. Stimulus-related oscillatory activity above approximately 20 Hz (gamma band) is observed in the firing rate and membrane or field potentials of neurons in the visual cortex of cat and monkey. Cortical mass responses in the same frequency range are evoked in humans by contrast stimulation and proved (partly) independent of conventional VEPs. Visual evoked responses (pattern VEPs) with abnormal waveform (quasi-sinusoidal or with bifid wave P100) are reportedly common in diseases affecting the visual pathways (e.g., multiple sclerosis). Contributions of gamma activity to the distorted VEP waveform are possible if the mechanisms of generation are differentially affected by brain disorders. This hypothesis was investigated in patients with documented impairment of the visual system.
methods. VEPs and oscillatory responses to contrast stimulation (central 9° of visual field; 1.3 to 5.0 cyc/deg; 80% contrast; reversal at 2.13 Hz) were recorded in 765 patients referred for standard visual testing and frequency analyzed. Controls were 40 healthy volunteers. The stimulus conditions and recording procedures replicated previous normative studies on the human oscillatory response.
results. Conventional VEPs were replaced by oscillatory responses comparable to those of controls in the unfiltered recordings of 9.8% of examined patients (with postchiasmatic disorders in 59.9% of cases). Signal amplitude in the frequency interval of the VEPs (below approximately 19 Hz) was significantly lower in the frequency spectra of patients than of controls but did not differ in the frequency range above 20 Hz, reflecting the oscillatory response.
conclusions. The human gamma oscillatory response mediating in cortical visual information processing further proved independent of the VEPs. It can contribute to VEP waveform distortion, and its observation in substitution of VEPs should be regarded as an indication of impaired visual pathway function.
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