Slow potential changes and choice reaction time as a function of interstimulus interval

Slow potential changes were recorded over the vertex (C_z) during a choice reaction task. The constant interstimulus interval (ISI) between the visual warning (S₁) and the visual imperative signal (S₂) was either 200, 400, 1000 or 2000 msec. The contingent negative variation (CNV) was not only measured between S₁ and S₂ (CNV₂), but also before S₁ (CNV₁).The main results were: (1) The CNV₂ amplitude showed significant variation as a function of ISI. It reached its maximum with an ISI of 1000 msec. (2) CNV₁ developed only before the short ISIs (200 and 400 msec). (3) When CNV₁ and CNV₂ were summed the differences in CNV amplitudes and durations between different ISIs became smaller. (4) The peak-to-peak amplitude P₁-N₁ of the potential evoked by S₁ was enhanced with short ISIs. (5) The correlations between mean CNV and median reaction time (RT) were low but significant for ISIs of 400, 1000 and 2000 msec. When, however, the effect of subjects was partialized out these correlations were drastically reduced, whereas the partialization of session and block effects had no noticeable influence on these correlations. (6) The correlations between single RT and single CNV (measured for the ISI of 1000 msec, individually for two subjects) were weak or even completely lacking.The main conclusion was that CNV coincides with preparedness to react to a stimulus in a choice RT-task, but its amplitude at the moment of onset of the imperative stimulus does not reflect, or reflects weakly, the degree of preparedness (as indicated by RTs) at that moment.