To investigate the opponent structure of complex (second-order) channels we ran texture segregation experiments. A complex channel is assumed to have a linear-nonlinear-linear form: linear first-stage receptive fields followed by a pointwise nonlinearity followed by linear second-stage receptive fields.

We consider three structures:

(i) SIGN-but-not-orientation-opponent, (ii) ORIENTATION-but-not-sign-opponent, and (iii) BOTH-orientation-and-sign-opponent.

“Orientation-opponent” means that the small first-stage receptive fields of the complex channel are orthogonal to the large second-stage receptive fields. (“Not-orientation-opponent” means that the receptive fields are of the same orientation.) “Sign-opponent” means that the centers of the second-stage receptive fields are of the opposite sign from the surrounds. (“Not-sign-opponent” means that the sign is the same.)

Our experiments used a forced-choice region-segregation task with element-arrangement patterns composed of vertical and horizontal Gabor patches arranged in checkered or striped patterns.

The results are best fit by the SIGN-but-not-orientation-opponent structure.