On each trial in an experiment, a researcher exposes participants to either a pleasant or an unpleasant priming picture for several milliseconds. She then shows participants a picture of a person's face and asks the participants to rate the person's likeability. The researcher is MOST likely studying:

subliminal differences

Explanation:

The meaning of subliminal is something that is difficult to feel and explain; but may be remembered due to constant repetition.

Subliminal messages are stimuli that lie below our threshold of conscious awareness. 

Subliminal differences manipulate our minds and modify our behaviors.

Importantly, research on action priming has shown that subliminal stimuli can trigger only actions a receiver of the message plans to perform.

the declaration states, “we hold these truths to be self-evident, that all men are created equal, that they are endowed by their creator with certain unalienable rights, that among these are life, liberty, and the pursuit of happiness….”