When you play a game, how do you decide where to go next? Which door to go through, or which staircase to go up. Which path to take out in the open? Here you will study theories of perception and cognition and apply these to designing a game level, or modding one with additional assets which should influence the players' decisions.
Primary research would involve getting a load of folk to play your level(s). You would make a video log of their behaviour and subject this to analysis.