Effects of eccentricity on the attention-related N2pc component of the event-related potential waveform

The N2pc ERP component has been widely used as a measure of lateralized visual attention. It is characterized by a 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 the receptive fields in these areas often extend a few degrees into the ipsilateral hemifield, we might expect that near-midline stimuli would be processed by both 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 with 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, including when the inner edge was at the midline, except that N2pc amplitude was reduced by more than 50% at the greatest eccentricity (8°). 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 use near-midline stimuli without compromising the N2pc amplitude, but should avoid large eccentricities. Implications about the neural generators of the N2pc are also discussed.