One mask protrudes from the black surface like an ordinary face, and the other is indented into the surface. When you close one eye and view the two masks, they both look like they are protruding, and when you move sideways, the indented mask seems to turn to follow your movement!
- Two identical white face masks (available at craft stores; Halloween masks also work well but need to be painted white on both sides)
- Black poster board, large enough to fit the masks side by side and leave a 2 1/2 -inch (6-cm) border around each one
- Utility knife or sharp scissors
- Hot-glue gun and glue sticks
- Movable light source such as an adjustable desk lamp
- Optional: white paint (if your mask isn’t white) and masking tape (only necessary if your mask has eye and mouth holes that need to be covered)
- If your masks have eye, nose, or mouth holes or are not already white on both sides, cover the holes with tape and paint the mask. Be sure to put the tape on the “back” side of the mask, opposite the side you will be looking at. Masks with holes and masks with colored outside surfaces will both work, but may detract from the effectiveness of the Snack.
- Use the pencil and ruler to draw a vertical line dividing the poster board in half. Center one of the masks on the left half of the poster board, and carefully trace its outline with the pencil.
- Using the scissors or utility knife, carefully cut along the traced outline to make a hole in the poster board that is exactly the shape of the mask outline.
- Place the mask face up on a stable surface. Lift the poster board over the mask and carefully fit the hole you cut over the mask. Press down until the poster board is flat on the table, with the mask protruding upward through the hole. If you have drawn and cut accurately, the edge of the hole should fit snugly around the edge of the mask.
- With the mask still facing up, run a small bead of hot glue completely around the edge of the mask to secure it to the poster board.
- Once the glue is dry, turn the poster board over, so the mask is now on the right side and concave (inside out).
- Center the second mask face up in the middle of what is now the empty left side of the poster board, and run a small bead of hot glue completely around the edge of the mask to secure it to the poster board. You should now have a convex mask protruding from the left side of the poster board and a concave mask indented into the right side.
Prop the mask board against a box, a stack of books, or anything convenient so it is as upright as possible. Place the light source a little less than 3 feet (1 meter) from the concave mask and a little off to one side.
Standing about 3 feet (1 meter) away from the masks, close one eye, and look at the masks. Move your head to the left and right or walk left and right in front of the board. Both masks should look like they are protruding out from the poster board—even though the one on the right is hollow, or concave. Additionally, the hollow mask should seem to follow you as you move.
Keeping one eye closed, try moving your head up or down while looking at the hollow mask. It should still follow you.
Try moving the light to illuminate the masks from different angles, and try adjusting the intensity of the light.
It’s hard to judge depth with only one eye. When you see the masks with one eye closed and from some distance away, your brain assumes that both masks protrude outward, as ordinary faces do.
But why does the concave mask seem to follow as you move? This illusion is your brain’s attempt to make sense of two conflicting sources of information. First, your visual system notices that the nose of the concave mask moves less than the rest of the face when you move your head. This information suggests that the nose is the most distant feature on the face (and the face is therefore concave), because to a moving observer, distant objects appear to move less than nearby objects. (Think of how highway signs whiz by when you are in car, while the distant scenery seems to barely move at all). But based on its experience in the world, your brain can’t accept the existence of a concave face. So, it concludes that the hollow mask is instead an ordinary protruding face that turns to watch you walk past, its nose following your gaze and therefore having little apparent motion.
The intensity and angle of illumination can influence the effectiveness of the illusion. Areas of shadow and bright reflection can either enhance or detract from the illusion, depending on their location.
Disneyland’s Haunted Mansion uses hollow masks to create eerie illusions of faces that seem to rotate and follow you as you move. The placement of the masks and the lighting are carefully adjusted to maximize the illusion.
Researchers in visual perception have traditionally concluded that, in the absence of other cues, we normally assume objects are illuminated from above (based on the fact that the sun is such a basic source of illumination). We therefore perceive objects with shadows at the bottom as convex and those with shadows at the top as concave. Nonetheless, we tend to perceive all human faces as convex—even hollow masks that have shadows at the top. It would seem that our perception of faces is an exception to our usual interpretation of shape based on light and shadow, perhaps because of the special importance faces hold for us. However, neuropsychologist V. S. Ramachandran has found that this propensity to view hollow masks as projecting outward actually extends to many complex inward-curving objects. His work generalizes this tendency, rather than attributing it to a unique perception of the human face by the brain.
Try painting the features on the inside of a mask to create your own realistic haunted mansion-type mask. Or, just try looking at the inside of a Halloween mask with a reasonably bright light source behind it, so the painted features become visible through the mask.
Richard Gregory, Eye and Brain: The Psychology of Seeing, 4th ed. (Princeton: Princeton University Press, 1990). See pages 190–195 for a discussion of the role of shading and shadow in the perception of concavity and convexity, including the egg-crate illusion.
Richard Gregory, The Intelligent Eye. (New York: McGraw-Hill, 1970). A discussion of the hollow mask illusion appears on pages 126–131.