Effects of line orientation in visual evoked potentials. Spatial dynamics, and gender differences of neural oblique effect.
The aim of the study was to investigate neural mechanisms of orientation discrimination in humans by recording the 128-channel event-related potentials (ERPs) of the brain. Stimuli were black-and-white gratings having four orientations: horizontal, vertical, and oblique 45 and 135 deg. from vertical. Thirty-five healthy subjects (16 males, 19 females) participating in the experiments were asked to differentiate the orientations and push the button. The results showed that the orientation processing consists of two main stages. In the occipital, parietal and temporal areas the P100 and N150 components reflecting the early perceptual stage of processing showed amplitudes greater for oblique orientations than for cardinal ones - \"inverse\" oblique effect. Further, in the central-parietal and frontal areas the P300 and N400 components reflecting the later cognitive stage of processing showed the \"classic\" oblique effect with greater amplitudes for cardinal orientations. Females demonstrated lower ERP amplitudes and less pronounced differences between responses elicited by cardinal and oblique orientations. The results suggest that in humans the orientation discrimination occurs in a distributed neural network involving the early sensory as well as higher anterior cortical areas. The classic behavioral oblique effect might be controlled by later processing in the anterior cortical areas. The insufficient neural orientation selectivity observed in females may be considered as the biological basis of their poorer visuospatial abilities.