• Background
  • Instructions
  • Illustration
  • Quiz


Objects that lie along the horopter are perceived as single unified objects when viewed with both eyes. We can also fuse the images from the left and right eyes for objects that fall inside Panum’s area of fusion. Inside Panum’s area, including the horopter, we see images as singles. Outside Panum’s area, either in front or behind, we see double images. As we move in front of the horopter, we are looking at objects closer and closer to ourselves. At some point, we lose the perception of these objects as being single unified objects and we see instead two images of the same object. Thus, when looking at the lamp 3 m away, the image of your thumb appears as a double. This double vision is known as diplopia.

Use this activity to explore the Panum's area of fusion and and where diplopia occurs.


Full Screen Mode

To see the illustration in full screen, which is recommended, press the Full Screen button, which appears at the top of the page.

Illustration Tab


Below is a list of the ways that you can alter the illustration. The settings include the following:

Disparity Object X Position: Moves the disparity object (the green dot that is not being 'looked at') closer or farther from the eyes.
Disparity Object Y Position: Moves the disparity object up and down on the screen.
You can also move the dot by clicking and dragging on the screen with a mouse or by touching the screen. Fixation Object Position: Moves the fixation object (the red dot that is being 'looked at') closer or farther from the eyes.
Eye Separation: Makes the eyes closer or farther apart. The farther the separation, the greater the vergence change and the more disparity.


Pressing this button restores the settings to their default values.