Late components of lambda responses in cognitive tasks

The effects of stimulus probability were investigated on the late components of saccade-related brain potentials (lambda responses) in two paradigms, 'counting' and 'guessing.' Subjects performed saccadic eye movements from a middle light to the target area where one of three kinds of target light stimuli appeared in a randomized sequence, each with a different probability (0.17, 0.33, and 0.50). Brain activity was recorded from six scalp areas. In the counting task subjects were instructed to count the prespecified low-probability stimuli. In the guessing experiment they had to make a guess before each trial as to which kind of target stimulus would appear. In both experiments the late positive components showed significantly greater amplitude in the lambda responses associated with infrequent stimuli than in those elicited by frequent stimuli. Principal component-Varimax analysis of the lambda responses revealed factors corresponding to the P300 and the slow wave components of traditional evoked potential studies.