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V. M. Zemon, J. Gordon, N. Baranchuk, L. Baronoff, E. Borukhov, M. Legatt, S. Pena, E. Santos; Differences in ON- and OFF-Cell Circuitry in Humans Assessed With Visual Evoked Potentials (VEPs). Invest. Ophthalmol. Vis. Sci. 2008;49(13):4502.
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© ARVO (1962-2015); The Authors (2016-present)
To investigate the spatial properties of ON- and OFF-cell circuitry in the human visual system through the analysis of luminance contrast response functions.
In swept-parameter runs, luminance contrast of hexagonal arrays of dots (horizontally aligned) were modulated sinusoidally in time (temporal frequency of ~ 9 Hz) under appearance-disappearance conditions (depth of modulation ranged from 0.5-16%). At a viewing distance of 114 cm, field size was 10 deg, dot diameter was 23.4 minarc and spatial separation of adjacent elements was varied parametrically from 7 to 47 minarc. Arrays of bright or dark elements 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 binocularly 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.
In most data sets, contrast response functions exhibited the nonlinear phenomenon of contrast gain control (phase advance and compressive amplitudes with increases in DOM), and the model based on shunting inhibition provided good fits to the data. Time constant values decreased over a 20-fold range with increasing DOM. The largest responses were obtained under the dark condition with the smallest interdot spacing. There were distinct differences in the initial conductance parameter (contrast gain) between bright and dark conditions as a function of interdot spacing for each observer. The inhibitory effects were strong under the bright condition but weak under the dark condition for the smallest interdot spacing.
Greater excitatory effects in the OFF pathway and greater inhibitory effects in the ON pathway with stimulus elements closely packed imply finer sampling of the visual field with OFF cells and a different pattern of inhibitory interactions within each of these subsystems.
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