Human discrimination of the implicit orientation of simple symmetrical patterns

Thresholds for detecting the angle of rotation of vertical symmetrical patterns containing few or no explicit vertical or horizontal contours were found to be almost as low as those for an actual vertical line extending approximately the same range. This hyperacuity performance, which we refer to as implicit orientation discrimination, shares most of its properties with the orientation discrimination of explicit lines, suggesting a category of orientation processing whose neural mechanisms are related to those involved in the processing of straight contours and those underlying the detection of axes of symmetry. Strong binding effects between the components of the figures were demonstrated and their temporal interactions were also investigated. Our results have implications for possible neural interactions early in the cortical visual stream.