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The methods of constant stimuli, limits, and adjustment can be used to test the participants, but method of limits is most commonly used in research conducted using the rod and frame task.<ref name=Wenderoth>Wenderoth, 1977</ref> When using the method of limits the experimenter sets the orientation of the rod and frame separately and then the participant is asked to adjust the rod orientation until they perceive it to be vertical. Deviation from true vertical can then be determined. Based on which way the frame is tilted, the rod can be viewed as either being tilted in the same direction as the frame (direct effect), or in the opposite direction of the frame (indirect effect).
==Evidence==
[[File:Example of Perceived Vertical.jpg|thumb|An image showing a 10 degree frame angle and a 7 degree rod angle. Note how the rod appears vertically oriented.]]
The frame of reference with respect to studies of the [[visual system]] refers to perceived reference axes. In the rod and frame illusion there are a number of things that can influence one’s frame of reference. Past research has found that one reason people experience the rod and frame illusion is due to visual-vestibular interactions.<ref name=Spinelli>Spinelli, Antonicci, Daini, and Zoccolotti, 1995</ref> For instance, when a participant is viewing the rod and frame task while physically tilted, the participant acts as though they are tilted opposite of the orientation of the frame. This suggests that the illusion, in part, is due to the person compensating for their perceived vertical in the direction that is opposite of the frame. Other evidence proposed by researchers that is consistent with this is that when participants are put on their sides to view the rod and frame task, they rely on their vision when their [[vestibular system|vestibular]] and [[proprioceptive]] senses are incongruent with those of their visual senses.<ref name=Corbett/> These finding suggest that the rod and frame illusion is processed in a type of hierarchy, where visual input is at the top, then vestibular cues, and finally proprioceptive cues.
In 2010, Lipshits found that along with this hierarchy of processing, that proprioceptive information, as opposed to gravity, is used by the body to determine which way is vertical.<ref name=Lipshits>Lipshits, 2010</ref> The researcher mentions that when we are not able to use vision to determine which way is vertical, people use other cues based on the axis of their head and body.
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