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V.M. Zemon, J. Gordon, W. Wisotsky, C.L. Hau, K. Kotenko, L. Hexstall, J. Piesco; Orientation and Contrast Response Functions: AVisual Evoked Potential (VEP) Study . Invest. Ophthalmol. Vis. Sci. 2003;44(13):4199.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: To describe cortical responses to gratings of various orientations and to characterize the neural mechanisms that contribute to these responses in humans by means of a nonlinear model. Methods: Square-wave grating patterns were presented in appearance-disappearance mode at a temporal frequency of ~ 6 Hz with luminance contrast varied in a swept-parameter paradigm (depth of modulation 1-32%). Four orientations were used: 0 (horizontal), 45 (left tilt), 90 (vertical), and 135 deg (right tilt). Space-average luminance of the display was 130 nits. Each step of the sweep was ~ 1 s in duration and each type of sweep stimulus was presented 10 times. The observer was optically corrected if necessary and viewed the stimulus monocularly at 57 cm. Synchronized EEG data were processed and filtered in 1-s epochs by Fourier analysis. A multivariate statistical measure was applied to the fundamental and second harmonic components in each set of epochs to derive signal-to-noise ratios (S/N). A nonlinear model based on shunting inhibition (presented at ARVO in 2002 by Zemon & Gordon) was used to quantify both amplitude and phase functions. Results: A distinctive set of contrast response functions was obtained for each individual. And for a given individual, different response functions were associated with each orientation. Surprisingly, more robust responses were found under oblique conditions. Although many functions exhibited strong saturation, little phase shift occurred at the fundamental frequency with changes in contrast. Conclusions: Results are consistent with greater inhibitory activity, presumable cortical in origin, generated in response to vertical and horizontal stimuli.
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