Their study, mostly conducted in the homes of cats so as not to unduly upset or worry the felines, determined just that.
The researchers played recordings of strangers, as well as of the cats' owners, to the felines. The cats could not see the speakers.
The cats responded to human voices, not by communicative behavior- such as by vocalizing or moving their tails -- but by orienting behavior. In this case, "orienting" meant that the cats moved their ears and heads toward the source of each voice.
The felines also, at times, displayed pupil dilation, which can be a sign of powerful emotions, such as arousal and excitement. Other studies have found that natural pupil dilation can be directly tied to brain activity, revealing mental reactions to emotional stimuli.
All of these reactions happened more often when cats heard their owners, and particularly after they had become habituated to, or familiar with, the strangers' voices.
The feline reactions are therefore very subtle, but cats have evolved not to be very demonstrative.