In an artificial continuum between a voiceless and a voiced bilabial plosive, each new step differs from the preceding one in the amount of VOT. The first sound is a pre-voiced , i.e. it has a negative VOT. Then, increasing the VOT, it reaches zero, i.e. the plosive is a plain unaspirated voiceless . Gradually, adding the same amount of VOT at a time, the plosive is eventually a strongly aspirated voiceless bilabial . (Such a continuum was used in an experiment by Lisker and Abramson in 1970. The sounds they used are available online.) In this continuum of, for example, seven sounds, native English listeners will identify the first three sounds as and the last three sounds as with a clear boundary between the two categories. A two-alternative identification (or categorization) test will yield a discontinuous categorization function (see red curve in Figure 4).