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V. M. Zemon, J. Gordon, V. Dolzhanskaya, T. McCloskey-Chillemi, E. Marcus, V. Terentii, E. Pisakhova, P. Weinger; Transient Visual Evoked Potentials (tVEPs) to Pattern Onset/Offset: A Frequency Domain Analysis. Invest. Ophthalmol. Vis. Sci. 2010;51(13):1505.
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© ARVO (1962-2015); The Authors (2016-present)
To characterize the tVEP waveform elicited by onset and offset of a checkerboard pattern in comparison to that elicited by contrast-reversal, and identify mechanisms in the frequency domain.
In 40 runs, checkerboard patterns (field size =10 deg, 18.75’ per check) were presented in on/off mode (contrast of 100%) with a 1 Hz square-wave signal. This extensive testing of individuals (N = 10) served to decrease greatly the noise levels in the records. Some of the observers were presented with the same stimulus or with the checkerboard contrast-reversed at either 50 or 100% during individual 1-min runs. EEG samples (synchronized to the display’s frame rate) were divided into four 8-s epochs, yielding a total of 160 epochs in the 40-run experiment, and ten 6-s epochs for the 1-min runs. Fourier analysis yielded the frequency content of each epoch and a statistic, magnitude-squared coherence (MSC), was applied to determine relative signal power and significance of each component. Principal component analysis (PCA) was applied to the MSC values to explore the number and character of underlying mechanisms. Even and Odd responses were synthesized from the extracted components, as were time-series wavelets based on this analysis.
MSC analysis revealed that the tVEP to pattern onset/offset is comprised of significant odd- and even-order components extending to high frequency (100 Hz). PCA yielded six frequency bands for even harmonics under both onset/offset and contrast-reversal conditions, but only four prominent frequency bands for odd-order components. In a combined even/odd PCA, some mechanisms were dominated by either odd- or even-order components. Some synthesized wavelets were consistent in temporal range and phase across observers. Typically, the initial negativity (cortical excitation) in the Odd response to pattern onset occurred earlier in time than it did in the corresponding Even response.
The extracted even-order responses to pattern onset/offset may serve the same purpose as recording tVEPs to contrast-reversing patterns, and the odd-order responses may provide critical measures of pattern-specific mechanisms. The presence of several distinct frequency bands in the Fourier decomposition of these tVEPs, which may reflect distinct neurophysiological mechanisms, indicates that this type of analysis might provide selective and sensitive measures of dysfunction in various disease states.
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