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L. Garcia–Quispe, J. Gordon, V.M. Zemon; The Role of Inhibition in the Development of Contrast Response Functions: AVisual Evoked Potential (VEP) Study . Invest. Ophthalmol. Vis. Sci. 2005;46(13):5677.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: To describe the development of cortical responses to gratings of various contrasts and to characterize the neural mechanisms that contribute to these responses in human infants and adults by means of a nonlinear model. Methods: Square–wave horizontal grating patterns (0.75 c/d and 1.5 c/d) were contrast–reversed at a temporal frequency of ∼ 7.5 Hz with luminance contrast varied in a swept–parameter paradigm (depth of modulation 1–64%). Space–average luminance of the display was 80 cd/m2. The observers viewed the stimulus monocularly at 57 cm. The stimulus duration was 8.5 s, and each stimulus was presented 10 times per eye. Twenty–four infants between 16 and 28 weeks of age were tested. Some adults were tested for comparison with the infant data. Synchronized EEG data were processed and filtered in 1–s epochs by Fourier analysis. A multivariate statistical measure was applied to the second harmonic component 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:Infant contrast response functions demonstrate a nearly linear relation between second harmonic amplitude and contrast as well as invariant phase with contrast, which is reflected by an approximately zero value for the coefficient of shunting inhibition in the model fits. On the other hand, adults show compressive amplitude response functions and phase advance with increases in contrast, which reflect a strong coefficient of shunting inhibition. Responses from left and right eyes within individuals are very similar in both adults and infants. Conclusions: Results are consistent with greater nonlinear (shunting) inhibitory activity, presumably cortical in origin, in adults than in infants.
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