Human memory is associative in nature. In the world wide web, websites on related topics are much more likely to link to each other than those with no affinity: two websites devoted to LOL cats are more likely to link to each other than to websites devoted to learning Esperanto. Similarly, groups of neurons are more likely to be linked or connected if they are activated by stimuli that are somehow associated with each other. Seeing the word 'Africa' activates a group of neurons in your brain, and this activity 'spreads' to neurons that over the course of your lifetime have been associated with this concept. Indeed, if I now ask you to name any animal, you are more likely to think of one from Africa. To a first approximation we can measure the degree to which different concepts are associated with each other by the ability of one concept to 'prime' the other. One way in which this can be done is by quantifying the amount of time (the reaction time) it takes to decide if a string of letters, such as "FISH" is an actual word or not, compared to how long it takes when that same word was primed by a word, such as "TUNA", by presenting it immediately before.

The example above captures the principals behind a semantic priming experiment. However, since the mean reaction times are only based on 5 words the priming effect - in which the reaction time to primed words is shorter than that to nonprimed words - may not be observed.