Ronald Poppe Khiet P. Truong Dennis Reidsma Dirk Heylen Backchannel Strategies for Artificial Listeners. 146-158 2010 IVA http://dx.doi.org/10.1007/978-3-642-15892-6_16 conf/iva/2010 db/conf/iva/iva2010.html#PoppeTRH10