This experiment is concerned with the issue of functional equivalence of emitted (in response to ahsence of a stimulus) and evoked (in response to presentation of a stimulus) P300s. Subjects attempted to estimate a 600-msec time interval hy the method of reproduction. Subjects were informed whether the time interval was under- or overestimated and whether the estimation error magnitude was within a limit (“correct” estimate, positive feedback) or exceeded the limit (“incorrect,” negative feedback). This feedback was presented by means of either a single event or a combination of two events. Each event consisted of a pair of stimuli.
Emitted, as well as evoked, early fronto-central and late parietal P3OOs were elicited by the feedback event regardless of whether positive or negative feedback was delivered, and regardless of whether feedback was delivered by a single event or by the combination of two events. In addition, what appears to be a positive Slow Wave was found following the first stimulus ofthe feedback event when this stimulus was low intensity indicating an underestimation. The other condition (high intensity, overestimation) was followed by a CNV. Since these slow wave findings were serendipitous, counterbalancing was not available to determine whether intensity (low vs high) or information (underestimation vs overestimation) was responsible for the difference.