Orientation-selective summing mechanisms revealed in visual search

We investigated properties of the neural mechanisms that mediate detection of complex grating targets in an orientation-based visual search task. Targets and distractors were composed of small patches of compound sinusoidal gratings. Components were chosen to differ enough in spatial frequency to stimulate separate and independent mechanisms at the primary cortical layer of processing. The orientations of the components were both vertical in distractor patches. In the uncrossed condition, both components of the target tilted either 3 degrees left or right. In the crossed condition, one component of the target tilted left and the other tilted right. Search was faster and more accurate in the uncrossed condition, ruling out mediation either by V1-like tuned mechanisms or by a higher-level mechanism that signals differences in orientation. Results were consistent with two classes of mid-level summing mechanisms. We argue that mid-level mechanisms such as these may be the neural substrate for conceptual orientation feature maps.