This illustrates classic visual search: a pop-out target (one feature) vs a conjunction target (two features).
What to do: Choose a condition from the dropdown, set the number of items, and click Start trial. Decide as quickly as you can whether a red circle is present or absent in the display, then click your answer. Run at least five trials per condition.
Think about: Compare your average response times for pop-out versus conjunction trials. Pop-out search is typically much faster — why? Consider what Feature Integration Theory says about how we combine visual features, and how set size (the number of items) affects each condition differently.