Spatial Dependence of Contrast Response Functions: Visual Evoked Potentials (VEPs) Elicited by Arrays of Dots

Abstract

Purpose:: To investigate the spatial character of luminance contrast mechanisms in ON and OFF pathways of the human visual system.

Methods:: In swept-parameter contrast runs (depth of modulation ranged from 1-32%), luminance of isolated elements (rectilinear arrays of checks or dots, or hexagonal arrays of dots) were modulated sinusoidally in time (temporal frequency of 11.2 Hz) under appearance-disappearance contrast conditions. Arrays of bright or dark elements were used with each condition to elicit responses from respective ON and OFF pathways. In each condition, the width of an element was 16 pixels on a display that spanned 256x256 pixels. Three viewing distances were used (57, 114, 228 cm) with corresponding field sizes of 20, 10, and 5 degrees. 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.

Results:: Under most conditions, responses exhibited phase advance and compressive amplitudes with increases in DOM, and the model provided good fits to the data. Time constant values increased with viewing distance and typically they were shorter under dark than bright conditions. Initial conductance (a contrast gain parameter) and shunting inhibition (a contrast gain control parameter) decreased with viewing distance and were generally greater under dark than bright conditions. Arrays of dark hexagonal dots yielded the greatest initial conductance at each viewing condition and usually produced more shunting inhibition.

Conclusions:: OFF cells, which are smaller than ON cells at a given retinal eccentricity, exhibit greater contrast gain and contrast gain control when stimulated with hexagonal arrays of dots that are spaced equally from all neighboring dots (spatial frequency power distributed over a wide range of orientations).

Keywords: pattern vision • electrophysiology: non-clinical • visual cortex