You don’t need nodding or really any conscious reaction I think. It should be possible to have some camera directed at face hooked up to another AI that catches slight changes in pupil dilation or other changes imperceptible to naked eye and registers when something looks interesting to the user. You can then quickly show a stream of variations and pick the tagged ones and use them to improve the guesses. I imagine something like this might one day become a preferred way of interacting with computers/AI.