Among the more interesting brain responses are the slow potentials or slow waves. They are slow in the sense that they take longer to develop than do the sensory-evoked potentials and motor potentials discussed earlier. Two of these slow potentials are the contingent negative variation (CNV) and the readiness potential (RP), categorized by Vaughan ( 1969) as steady potential shifts. A third type of slow wave includes the components of the ERP that occur between 250 to 900 msec after the stimulus commonly referred to as P300. Some researchers prefer to call these later positive components P3 or P3b, without imposing a particular latency on the responses. The term P300 is still commonly used because the original studies by Sutton and his associates ( Sutton, Braren, & Zubin, 1965) found that the response occurred about 300 msec after some initiating stimulus. All of the slow waves mentioned thus far are endogenous in nature. This means they are produced by internal (endogenous) processes and are less dependent on external (exogenous) stimuli than are the sensory or motor potentials previously described. These internal processes are among the more fascinating ones that can be studied by psychologists and include, among other attributes, intentions to move (readiness potential), expectancies regarding the occurrence of some stimulus (CNV), and decisions (P300). Because of their relation to cognitive events, many cognitive scientists have become interested in these slow waves, especially the P300.
In the now classic paper by Walter, Cooper, Aldridge, McCallum, and Winter ( 1964), CNV was described as a steady, relatively long-lasting, negative shift in brain activity, which developed between the time of a warning signal (S1) and a second stimulus (S2) that demanded a response. Walter and colleagues referred to this slow shift in EEG baseline, in the S1-S2 interval, as a contingent negative variation because its occurrence depends on the contingency between two events and because it shows a negative drift. It was later found that CNV can occur without motor responses to S2. For example, Cohen and Walter ( 1966) obtained CNV in anticipation of pictorial stimuli that were used as S2. No motor response was made by the subjects. On the basis of this kind of result, Cohen and Walter suggested that the CNV reflects a state of expectancy and have used the term E wave as a substitute for CNV. Other investigators proposed that CNV indicates an intention to act ( Low, Borda, Frost, & Kellaway, 1966), subject motivation ( Irwin, Knott, McAdam, & Rebert, 1966), or attention ( McCallum, 1969; Tecce & Scheff, 1969). Whatever the terminology used, it is clear that CNV is related to psychological and performance factors, and some of these are reviewed here.