Event-related potentials in a two-choice task involving within-form comparisons of pictures and words

Behavioral as well as electrophysiological evidence suggests that words are processed differently than pictures in a variety of tasks. In this study fifteen adult subjects were tested on a speeded "same-different" judgment task between printed names and drawings of common objects. In one condition, subjects decided on the identity between their internal image of the object that was named by S1 (word) and a subsequently presented drawing of an object (Word-Picture condition). In a second condition, comparisons were made on the basis of the name of the depicted object (Picture-Word trials). Event-related potentials (ERPs) were recorded from 12 scalp locations in response to the second item in each pair. No-match waveforms were characterized by larger N350 and P500 deflections compared to Match ERPs. The two responses could be distinguished on the basis of their lateral and anterior-posterior scalp distribution. Within-form comparisons involving pictures produced increased positivity between 150 and 600 ms poststimulus onset at posterior recording sites, whereas the opposite effect was noted at anterior sites during the early portion of the ERP. Decisions on word stimuli were associated with prolonged reaction time and longer N350 peak latency compared to decisions on pictures. These results demonstrate the existence of independent sources of ERP variability, each possibly reflecting a different aspect of cognitive comparisons. Latency and reaction time data provided valuable information regarding differences in the course of the comparison process when linguistic, as opposed to pictorial, stimuli/representations were involved.