“…In the available literature, most efforts have been focused on the development of parametric models. For instance, Presnell, Morrison, and Littel (1998) and the references therein dealt with a circular response and linear covariates; SenGupta and Ugwuowo (2006) proposed some asymmetric models accounting for the circular nature of the covariate and Downs and Mardia (2002) and Kato, Shimizu, and Shieh (2008), among others, addressed the regression with circular response and covariates. Regression estimation avoiding the assumption of a specific parametric shape for the regression curve was studied by Marzio, Panzera, and Taylor (2009) who extended the least squares local polynomial to the case of d-dimensional circular predictors and real-valued responses; Qin, Zhang, and Yan (2011a,b) extended nonparametric models to the case when there is one circular predictor and one or more linear predictors and the response is real-valued, and more recently Marzio, Panzera, and Taylor (2012) proposed a nonparametric estimator for the regression function when the response is circular and the covariate is circular or linear.…”