DIY: Vestibular illusions

Illusion 1 – Sensing Yaw Motion

What to Do
The volunteer sits on the chair with head upright and fists on his or her thighs in the “two thumbs up” position. Tell them to rotate their wrists so that the thumbs point in the direction of movement. If the movement changes to a different direction, the wrists should be rotated so that the thumbs point in that direction. If the volunteer does not perceive any motion, the thumbs should be pointed upwards. Cover the volunteer’s eyes with the blindfold and touching only the seatback of the chair, give the chair a spin. Push the chair hard enough to rotate it eight to ten times. If necessary, give the chair an additional gentle push to keep it rotating. Gripping the chair back, slow the chair to a rapid but smooth stop. Wait a few moments to observe thumb movements and then remove the blindfold. Tell the volunteer to stare at a fixed point on the wall.

What Happens
At first, the volunteer will point thumbs in the same direction the chair is rotating. After stopping the chair, the volunteer will reverse the direction of the thumbs, indicating a feeling of movement in the opposite direction. Upon opening his or her eyes, the volunteer will experience rapid side-to-side flicking motions of the eyes that can be observed by staring directly at the volunteer’s face.

Why The rotation of the chair causes the endolymph within the yaw axis semicircular canal to begin moving. At first, the inertia of the fluid causes it to lag behind the motion of the subject’s body. This causes the cupula and its hair cells to bend. Now stimulated, the hair cells send signals to the brain telling it that motion has been initiated and in what speed and direction. When the chair is stopped, the momentum of the now moving endolymph causes it to continue moving even though the volunteer’s head and semicircular canals have stopped. The hair cells are now bent in the exact opposite direction as before. This sends a false signal to the brain that the direction of motion has reversed. Nystagmus, an involuntary flicking eye movement, shows the link between the vestibular and visual systems. This reflex occurs when the brain mistakenly believes the body is still moving in this Illusion and instructs the eyes to “look ahead”. The eyes track objects that the brain believes are coming into the field of vision even though the view isn’t changing.

Above: The test subject indicates the perceived direction of movement by pointing his or her thumbs.

Safety Precautions

• The Barany Chair is not an amusement ride. Please follow the directions and exercise caution when it is being used.
• Use the safety lap belt and a spotter at all times.

• Assist students in getting in and out of the chair. A small step stool may be helpful.

• Following demonstrations, allow students to sit in a non-rotating chair until any dizziness wears off.

• Perform only one illusion at a time. Allow a few minutes for the effects of the first illusion to wear off before beginning another.

• Screen candidates for motion sickness, but keep a plastic bag or container nearby in the event of illness.

Vestibular Illusion 2 – Failure to Sense Motion

What to Do
Follow the same set-up used for Illusion 1. Put a dark blindfold on the volunteer and provide ear protection to diminish auditory clues. Rotate the chair as before and have the volunteer identify the direction of motion with their thumbs. Keep the chair spinning 10 or 15 times before very gently stopping it. As with the first illusion, the volunteer should point his or her thumbs in the direction of perceived movement or upward if the volunteer perceives that motion has stopped.

What Happens
The volunteer will perceive the start of motion by pointing his or her thumbs in the direction of rotation. After a number of rotations, the
volunteer will point the thumbs upward even though the chair is still rotating. Finally, the volunteer will point thumbs the opposite direction
from the first movement to indicate counter rotation.

Why
As with the first illusion, endolymph in the yaw semicircular canal will lag behind the initial motion. Signals sent to the brain will be interpreted as bodily rotation in a particular direction. Gradually, the endolymph in the yaw semicircular canal will catch up with the motion, and stimulation of the hair cells in this canal will stop. The brain will falsely interpret the lack of hair cell stimulation to mean that the chair has come to rest. Later, when the chair slows down or stops, the momentum of endolymph will cause it to continue to flow through the yaw canal. Stimulation in the opposite direction will be falsely interpreted as movement in the opposite direction.

Vestibular Illusion 3 – Sensing Roll Motion

What to Do
Have the volunteer grip the arm rests with both hands. After putting the blindfold in place, instruct the volunteer to drop his or her chin to the chest and close the eyes. Spin the chair at least ten times then bring it to a smooth stop. Tell the
volunteer to sit up straight and open their eyes. Safety Reminder: Be sure to use a spotter when performing this illusion.

What Happens
The volunteer will experience a powerful cartwheeling sensation to the left or right (depending upon the spin direction) upon opening his or her eyes. The volunteer will find it difficult to remain sitting straight up and will tend to lean aggressively to one side or the other.

Why
By tilting the head forward, the roll axis
semicircular canal will be brought into the same plane of rotation as the Barany Chair. By stopping the chair and tilting the head back to the vertical position, the roll axis will be repositioned while the endolymph fluid is still moving in the roll axis canal. This will cause a strong sensation of cartwheeling movement. The volunteer will try to lean in the opposite direction to compensate for the effect.

Vestibular Illusion 4 – Sensing Pitch Motion

What to Do
Have the volunteer grip the arm rests with both hands. Instruct the subject to close their eyes, lean forward slightly, and turn their head as far to one side as possible. Spin the chair at least eight times in the direction the volunteer is facing, then bring it to a smooth stop. Tell the volunteer to sit back and raise his or her head to the upright position and open their eyes. Safety Reminder: Do not use a blindfold or ear covers when performing Illusion 4. Be sure to use a spotter when performing this illusion.

What Happens
The volunteer will sense that he or she is tumbling backwards and may have a difficult time sitting up.

Why
By leaning forward and tilting the head to the side, the pitch axis semicircular canal will be brought into the same plane of rotation as the motion of the Barany Chair. After stopping and returning to the upright position, endolymph fluid will continue to move in the pitch axis canal. This will cause a strong sensation of tumbling. The volunteer will readjust his or her body position in order to counteract the perceived movement.

Important Safety Note: While it is possible to simultaneously stimulate all three semicircular canals with the Barany Chair, it is NOT recommended. Simultaneous stimulation of the three canals can lead to total spatial disorientation sensation and illness.

Other Uses for the Barany Chair

The classroom version of the Barany Chair is ideal for a variety of other demonstrations of physics and technological challenges.

Conserving Angular Momentum – Hand the volunteer small barbells to extend at arm’s length during the initial rotation. By bringing the barbells in toward the chest, the rotation rate will increase. The rotation rate increases because the barbells are traveling in a smaller circle than before. To conserve their angular momentum, the rotation rate has to increase. Extending the barbells back outward slows the rotation rate, but angular momentum is still conserved. This demonstration gives the illusion of getting something for nothing.

Newton’s Laws of Motion – Hand the volunteer an electric leaf blower. While preventing the cord from wrapping too tightly around the pedestal, have the student turn on the blower and direct the exhaust at right angles. The chair will begin to accelerate. After a few rotations, the exhaust should be directed the other way so that the chair decelerates. The rotational movement of the chair demonstrates Newton’s First and Third Laws of Motion. The rate at which the chair accelerates or decelerates demonstrates the Second Law of Motion.

Working In Space – Firmly hold a threaded pipe joint over the head of the volunteer. Have the volunteer screw a pipe nipple tightly into the joint. The chair simulates microgravity and Newton’s Third Law of Motion comes into play. Without a fixed anchor point, the astronaut rotates in the opposite direction from the turning motion. This demonstration illustrates why space-walking astronauts require foot restraints as they work in space.

NASA learning resources