Combining 1-D components to extract pattern information_It is about more than component similarity

Christian Quaia, Lance M. Optican, Bruce G. Cumming (National Eye Institute)

CHALLENGE - At least under some conditions, plaid stimuli are processed by combining information first extracted in orientation and scale-selective channels. The rules that govern this combination across channels are only partially understood. Although the available data suggests that only components having similar spatial frequency and contrast are combined, the extent to which this holds has not been firmly established. To address this question, we measured, in human subjects, the short-latency reflexive vergence eye movements induced by stereo plaids in which spatial frequency and contrast of the components are independently varied.

SOLUTION - We used the eye search coil technique to record binocular horizontal and vertical eye position from three male subjects. Stimuli were unidisparity plaids formed by summing a vertical sinusoidal grating having 6908 of phase disparity and an oblique (6458) sinusoidal grating having zero disparity. All measures reported here are based on vergence velocity, which was obtained by differentiating eye position traces to obtain velocity signals for each eye. Two descriptive models were developed and were fit to the data using a multiobjective genetic algorithm under the optimization software package modeFRONTIER. The model had nine parameters, and nine error measures were defined, which were then consolidated into two objectives to be minimized. We then defined two optimization objectives, Opeak and Obw, as the maximum value within each group of error measures. The algorithm thus sought to minimize simultaneously the worse error within each group. The outcome of the optimization was then a set of designs on the Pareto front.

BENEFITS - We selected as the ‘‘best’’ design the one with the lowest value for the weighted sum of the two objectives with a weight of 0.7 for  O peak and a weight of 0.3 for Obw. We found that the SF and contrast of the two gratings jointly determine the magnitude of the vergence responses in a nonseparable manner.