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.