Synthetic speech can be used to express uncertainty in dialogue

systems by means of hesitation. If a phrase like “Next

to the green tree” is uttered in a hesitant way, that is, containing

lengthening, silences, and fillers, the listener can infer that

the speaker is not certain about the concepts referred to. However,

we do not know anything about the referential domain of

the uncertainty; if only a particular word in this sentence would

be uttered hesitantly, e.g. “the greee:n tree”, the listener could

infer that the uncertainty refers to the color in the statement,

but not to the object. In this study, we show that the domain

of the uncertainty is controllable. We conducted an experiment

in which color words in sentences like “search for the green

tree” were lengthened in two different positions: word onsets or

final consonants, and participants were asked to rate the uncertainty

regarding color and object. The results show that initial

lengthening is predominantly associated with uncertainty about

the word itself, whereas final lengthening is primarily associated

with the following object. These findings enable dialogue

system developers to finely control the attitudinal display of uncertainty,

adding nuances beyond the lexical content to message

delivery.