Researchers have tried to generate nods during speaking from speech and natural language.
they used several acoustic features, such as sound pressure and prosody, for generating nods (Yehia et al., 2002;KG et al., 2004;Graf et al., 2002;Busso et al., 2007;Beskow et al., 2006;Iwano et al., 1996;Ishi et al., 2010).