In our laboratory we are studying the structure and timing of human cognitive processes (i.e. mental chronometry) by examining changes in ERP activity and response latency elicited by variations in the properties of visual stimulus processing demands. Departures from normal ERP patterns have been observed in several psychiatric and neurological disorders including traumatic brain injury. Historically, the analysis of ERPs was limited to an examination of the average response to a given stimulus. In several laboratories, the analysis has been extended to include examination of single-trial responses. In our program, single-trial analysis includes synchronization measures, analysis of dynamical transitions and investigations of inter-regional causal relationships. These procedures are introduced in the summary of the work done by the Dynamical Analysis Group.
We are currently investigating ERPs elicited in the supraliminal flanker arrow task. In this task we can manipulate the difficulty of both stimulus identification and response selection. By combining both experimental manipulations in a single task, we are able to study the interaction of these cognitive processes. In this paradigm there are two stimuli, a cue stimulus, which is followed by a blank screen, and the imperative stimulus. A combination of information in both the cue and imperative stimuli determines the subject's correct response. There are two possible cue stimuli, the word "SAME" (the compatible cue) and the word "OPPOSITE" (the incompatible cue). They appear with equal probability. The imperative stimulus is a horizontal arrow pointing to the left or to the right. It is the central element of a horizontal row that includes flanking stimuli that occur with equal probabilities. The flankers can be blanks (neutral flankers), arrows pointing in the same direction as the central arrow (congruent flankers, for example <- <- <- <- <- ) or flanking arrows pointing in the opposite direction (incongruent flankers, for example <- <- -> <- <-). The subject response is a left or right button press. The appropriate choice is determined by a combination of the cue stimulus word and the direction of the center arrow imperative stimulus. The directions of the flanker arrows accompanying the imperative stimulus are irrelevant to the specification of the correct response, but they will have an effect on reaction time and on electrical activity. An example of the ERPs recorded from a single subject is shown in the figure.
Average ERPs from a single control subject are displayed. The responses were produced by the cue word SAME (red) or OPPOSITE (blue) followed by a congruent array where the flanking arrows and the central arrow pointed in the same direction. The three sets of diagrams were recorded at scalp sites Fz (top), Cz (middle) and Pz (bottom).