The supplementary motor area in motor and sensory timing: evidence from slow brain potential changes

The present study investigated the processing of durations on the order of seconds with slow cortical potential changes. The question is whether trial -to-trial fluctuations in temporal productions or judgments correspond to v ariations in the amplitude of surface Laplacians computed over particular s calp regions. Topographical analyses were done using the source derivation method. Subjects performed three successive tasks: (1) time production, in which they produced a 2.5-s interval separated by two brief trigger presses ; (2) time discrimination, in which they detected small differences in inte rvals delimited by two brief clicks in comparison with a memorized standard interval; and (3) intensity discrimination (control task, devoid of time j udgments), in which they detected small differences between the intensity o f clicks, in comparison with standard clicks initially memorized. In order to focus on subjective differences, in the two discrimination tasks most co mparison stimuli were identical to the standard, without the subjects being aware of it. At FCz, reflecting activity from the mesial frontocentral cor tex that mainly includes the supplementary motor area (SMA), larger negativ ities were found during the longer target intervals, whether these were pro duced (task 1) or judged so (task 2). Those performance-dependent trends we re restricted to the target intervals of the temporal tasks; they appeared neither during the 2 s preceding the target, nor during the control task. T he data therefore suggest that the SMA subserves important functions in tim ing both sensory and motor tasks. We propose that the SMA either provides t he "pulse accumulation" process commonly postulated in models of time proce ssing or that it receives output from this process through striatal efferen t pathways.