In psychology lab, tetiana is conducting an experiment on depth perception using the howard-dolman box. inside the box are two vertical rods and a horizontal ruler. the participant manipulates the rods until they appear to be aligned at the same distance away, then the experimenter measures how far out of alignment they are. there are three conditions: left eye only, right eye only, and both eyes. tetiana is using a repeated-measures design. what is the independent variable in this experiment?