Figure 1. A model for countermanding .

When the target appears, it triggers a rise of activity in a Go decision unit that would normally initiate a response on reaching threshold. However, if a stop signal is presented (after the stop‐signal delay), it triggers a Stop decision unit that cancels the Go unit if it reaches its threshold first. Because of variability in the rates of rise of the decision signals in different trials, sometimes cancellation may be successful and sometimes not.