“…Roughly, the purpose is to search for a curve passing through the middle of the observations, as illustrated in Figure 1. Principal curves have a broad range of applications in many different areas, such as physics (Hastie and Stuetzle [26], Friedsam and Oren [23]), character and speech recognition (Kégl and Krzyżak [29], Reinhard and Niranjan [39]), mapping and geology (Brunsdon [10], Stanford and Raftery [43], Banfield and Raftery [4], Einbeck, Tutz and Evers [20,21]), natural sciences (De'ath [14], Corkeron, Anthony and Martin [13], Einbeck, Tutz and Evers [20]) and medicine (Wong and Chung [46], Caffo, Crainiceanu, Deng and Hendrix [11]). …”