The relationship between difficulty of discrimination and speed of stimulus evaluation can be examined in visual tasks using N2, a negative component that occurs at approximately 200 msecs after a stimulus. One study (Porjesz et al. 1987) used the line target discrimination task described above, with frequently occurring vertical nontargets interspersed with both easy and difficult rare targets. Subjects were instructed to press a button as quickly as possible to all nonvertical stimuli.
