Cardiac activity preparatory to overt and covert behavior.

The heart rates of 17 women subjects were recorded as they prepared to make both overt (key press) and covert (silently thinking the word "stop") responses. A very reliable preparatory cardiac response was obtained regardless of whether the overt or covert response mode was employed. The temporal development of this cardiac response faithfully reflected the speed with which the subjects were asked to respond, suggesting that in the covert condition heart rate could be used to detect the time at which a mental event was being generated.