  Conversational agents (CAs) have the great potential in mitigating the
clinicians' burden in screening for neurocognitive disorders among older
adults. It is important, therefore, to develop CAs that can be engaging, to
elicit conversational speech input from older adult participants for supporting
assessment of cognitive abilities. As an initial step, this paper presents
research in developing the backchanneling ability in CAs in the form of a
verbal response to engage the speaker. We analyzed 246 conversations of
cognitive assessments between older adults and human assessors, and derived the
categories of reactive backchannels (e.g. "hmm") and proactive backchannels
(e.g. "please keep going"). This is used in the development of TalkTive, a CA
which can predict both timing and form of backchanneling during cognitive
assessments. The study then invited 36 older adult participants to evaluate the
backchanneling feature. Results show that proactive backchanneling is more
appreciated by participants than reactive backchanneling.
