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V.M. Zemon, J. Gordon, M. Atzram, G. Borukhov, A. Dovgach, B. Solomon, D. Vargas; Spatiotemporal Dependence of Contrast Response Functions: Visual Evoked Potentials (VEPs) and a Nonlinear Model . Invest. Ophthalmol. Vis. Sci. 2006;47(13):5372.
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© ARVO (1962-2015); The Authors (2016-present)
To investigate the spatiotemporal character of luminance contrast mechanisms in ON and OFF pathways of the human visual system.
In swept–parameter contrast runs (depth of modulation ranged from 1–32%), luminance of isolated–check patterns (parametric in check size, 4.7–37.5 arc min) were modulated sinusoidally in time (temporal frequencies ranged from ∼ 4–15 Hz) under appearance–disappearance contrast conditions. Arrays of bright or dark checks were used with each condition to elicit responses from respective ON and OFF pathways. Each step of the sweep was ∼ 1 s in duration and each type of sweep stimulus was presented 10 times to an observer. Synchronized EEG data were processed and filtered in 1–s epochs by Fourier analysis. A multivariate statistical measure was applied to the fundamental frequency component in each set of epochs to derive noise estimates. Amplitude and phase were plotted vs. depth of modulation and the least–squares best fit of a nonlinear model yielded estimates of initial conductance, initial phase, contrast threshold, and the strength of shunting inhibition in the system. The system’s integrative time constant as a function of depth of modulation was also estimated.
Under most spatiotemporal conditions, the model yielded phase advance and compressive amplitude functions with increases in DOM that matched the VEP data closely. Given particular spatiotemporal conditions, however, these nonlinear characteristics were reduced or absent. Under these conditions, the model yielded little shunting inhibition and longer time constant values. Initial phase vs. temporal frequency functions were approximately linear and yielded estimates of transmission delay in the system. There were individual differences in dependence of the parameters on stimulus conditions. In general, the strength of shunting inhibition depended critically on spatial condition at ∼ 10 Hz: shunting was greatest at the lowest spatial frequency and decreased monotonically with increases in spatial frequency.
The model captured the data accurately. It appears that amplitude tuning of responses to luminance contrast depends on an interplay of initial conductance and shunting inhibition.
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