Orientation sensitivity of the N1 evoked by letters and digits.

Orientation sensitivity of the amplitude and latency of the P1 and the N1 was investigated while participants performed letter-digit category judgments and red-blue color judgments. These two tasks were used in order to ascertain whether the orientation effects reflect access to object identity, which would be necessary for category, but not color, judgments. Character misorientation significantly affected both the latency and the amplitude of the N1, but not the P1, component. The N1 amplitude increased gradually up to 90 degrees , then leveled off up to 150 degrees , and dipped somewhat for 180 degrees . The effect of orientation on N1 latency differed between the hemispheres, with a quadratic function characterizing the effect of orientation on the left, and linear and quadratic trends characterizing the effect on the right. The effects of orientation were attributed to perceptual learning rather than object recognition, and the hemispheric differences in N1 latency suggest feature-based processing in the left hemisphere and holistic processing in the right.