Late positive ERP peaks observed in stimulus-response compatibility tasks tested under speed-accuracy instructions.

Speed-accuracy instructions were manipulated in two symbolic stimulus-response compatibility tasks in an event-related potential study with young adults. For both tasks, the stimuli were the words left and right written in upper and lower case. Three late positive peaks (early P3, P3b, and P4) were seen in one task (with compatibility type blocked) with an additional peak (P3c) in the task that required identifying two stimulus attributes (compatibility type signaled by case). In both tasks, stimulus-response incompatibility and an emphasis on accuracy increased reaction time, made the P4 more prominent and increased its latency. With very rapid responding, P4 was not prominent because of overlap with the preceding peak. Functional characteristics of the peaks are discussed as well as potential sources of latency and amplitude measurement error when P3b is measured in a temporal window broad enough to include the P4.