Selective attention is tested when two signals compete and you must focus on only one. You show rows of color words printed in the wrong ink to twenty male and twenty female participants. Each person reads the ink colors out loud while ignoring the word meaning. You time each person with a stopwatch to measure the effort of focusing.

A simple instruction can switch focused attention on or off. You divide participants into two groups and have both watch the same TV show. One group is told to remember as many details as possible while watching. The other group is told to just watch. Afterward, both groups answer questions about what they saw. Comparing the scores shows whether intentional focus actually improves recall.

The ability to suppress one response in favor of another — and how much that ability changes with age — is what a three-part Stroop test measures. Participants first read color names printed in black, then read color names printed in mismatched colors, and finally say the ink color instead of the word. That last task is the hardest: the brain must ignore the written word entirely and focus only on the ink color. Across 60 participants ranging from age 10 to 60, each group gets 30 seconds per test. Younger participants score higher on all three tests, and the biggest gap appears on the hardest test — where the brain's filtering effort is greatest.