The rod and frame test is a psychophysical method of testing perception. It relies on the use of a rod and frame apparatus which uses a rotating rod set inside an individually rotatable drum, allowing an experimenter to vary the participant's frame of reference and thus test for their perception of vertical. Rod and frame illusion Rod and frame illusion occurs because of the effect of the orientation of the frame on the rod. In the simplest example of the rod and frame illusion, the illusion will cause the participant to perceive the rod to be oriented congruent with the orientation of the frame. When the participant is viewing the rod and frame that are both positioned at 0 degrees (or vertical), he or she perceives the rod as vertical with perfect accuracy. However, when the frame is tilted away from vertical, the participant's perception of vertical is affected. The participant tends to perceive the rod to be tilted in the same direction as the frame is oriented (e.g., if the frame is tilted in the counterclockwise direction, the rod will also be perceived as being tilted counterclockwise). As the tilt of the frame increases, the participants' perceived vertical increasingly deviates from true vertical. Rod and frame test To perform the rod and frame task, an apparatus consisting of a rod in a square frame is used. An example commercial apparatus can be seen in picture 1. When the participant is being tested using the apparatus, their head is fastened firmly in the chin rest to prevent the participant from collecting visual cues from outside of the apparatus. The rod and frame are shown in the center of the far end of the apparatus, which provides a frame of reference to the participant. Both the participant and the experimenter are able to adjust the orientation of the rod, while only the experimenter can adjust the frame orientation by using the appropriate knobs on the apparatus, as seen in picture 2. The experimenter is able to see the exact degree measurement of the rod and frame from vertical, while the participant sees the physical rod and frame inside the apparatus. 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. When using the method of limits, the experimenter sets