The Effect of Eccenrticity on Electrophysiological Markers of Attention

The N2pc ERP component has been widely used as a measure of lateralized visual attention. It is characterized by a sustained negativity contralateral to the attended location or target, and it is thought to reflect contralaterally enhanced processing of attended information in intermediate-to-high levels of the ventral visual pathway. Given that receptive fields in these areas often extend a few degrees across the midline, it is possible that near-midline stimuli might be processed equally by the contralateral and ipsilateral hemispheres, resulting in a diminished N2pc. However, little is known about the effect of eccentricity on the N2pc component. To address this gap in knowledge, we recorded the EEG while participants performed a discrimination task on stimuli presented at one of five eccentricities (0°, 0.05°, 1°, 2°, 4° and 8° between the inner edge of the stimulus and the midline). We found that the N2pc amplitude remained relatively constant across eccentricities, except for a significantly smaller N2pc at 8°. N2pc onset latency, however, became systematically shorter as the eccentricity increased (up to 4°). We also examined the contralateral positivity that often follows the N2pc. This positivity became progressively larger, and the transition from negative to positive occurred progressively later, as the eccentricity increased. These findings suggest that future experiments looking at the N2pc can be designed with stimuli anywhere within central vision (but not the far periphery) without compromising the N2pc amplitude. Implications about the neural generators of the N2pc are also discussed.