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High-density ERPs evoked by rotated alphanumeric characters were examined to determine how neural processing is affected by stimulus orientation during letter/digit classifications and during mirror/normal discriminations. The former task typically produces response times that are unaffected by stimulus orientation while the latter is thought to require mental rotation. Sensitivity to orientation was first observed around 100-140 ms and this effect was attributed to differences in low-level features between vertical and oblique orientations. Subsequently, character misorientation amplified the N170, a neural marker of object classification, between 160 and 220 ms. Top-down processing is reflected in the ERPs beginning at 280-320 ms and this time range may reflect binding of ventral and dorsal stream information. In the case of mirror-normal discrimination these top-down processes can lead to mental rotation between 340 and 700 ms. Therefore, although neural processing reflects object orientation, these effects do not translate into increases in reaction-times or impaired accuracy for categorisation, and precede those that do in the mental-rotation task. (Contains 3 tables and 3 figures.)

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.