Effects of Methylphenidate on Stimulus Evaluation and Response Processes: Evidence from Performance and Event-Related Potentials


  • This research was supported by NIMH Grants MH32103 and MH38118 to Rafael Klorman. Joan Brumaghim's participation was funded in part by a postdoctoral research fellowship from NIMH Grant MH1704. We gratefully acknowledge the contributions of John Chapman, Gary Dell, Gregory Miller, David Kurtz, James Metz, Lawrence Ota, Joseph Sergeant, and Michael Tanenhaus.

Address requests for reprints to: Rafael Klorman, Psychology Department, River Campus, University of Rochester, Rochester, NY 14627.


This study investigated the effects of methylphenidate in a memory scanning task with two levels of high cognitive load (memory set sizes 2 and 4 presented in displays of size 4) and two response requirements (simple mapping or rotation). Twenty young adults were tested under placebo and methylphenidate (0.3 mg/Kg) in a double-blind protocol. As expected, memory load increased misses, false alarms, confusions, and failures to respond by the deadline. In turn, the rotation requirement increased confusions and nonresponses. Reaction time (RT) was slowed by both factors. P3b latency also was increased by memory load and, to some extent, by the rotation requirement. These results are consistent with the proposition that P3b latency reflects largely evaluation, rather than response processes.

Misses and reaction time were decreased in response to targets presented in the center vs. the periphery of the display. Confusions, however, showed the opposite trend. The display position did not affect P3b latency. These results can be explained by assuming that the subject was focusing on the center of the display and that accuracy diminished when stimuli were presented toward the periphery of the display.

The stimulant challenge speeded up reaction times overall and specifically reduced the slowing effect of rotation. However, P3b latency was not affected by methylphenidate, so that the speeding of reaction time by the stimulant can be attributed to post-evaluation processes.