The bundle of ganglion cells leaving the eye at the optic disk form the optic nerve (click here  to see a cross section of the eye).  This portion of the retina has no photoreceptors, leaving a blind spot in the visual field where no information is picked up by the retina.  If your camera worked like this, there would be a hole in the picture where no image was registered.  We don't typically notice the blind spot because it falls in the periphery, an area with low acuity (resolving power).  Also, with normal binocular vision (i.e., two eyes), the information that is missing from one eye is filled in by the other (because the blind spot falls in different parts of the visual field for the two retina).  Even so, if you shut one eye you don't see a gaping hole in the visual field (though one really does exists), because low level perceptual processes fill in the missing information.  The following demonstration examines the blind spot and how these low level perceptual processes fill in the missing detail.

Shut your right eye and stare at the red fixation cross with your left eye.  Start about 2 feet from the computer display and move your head slowly towards the monitor.  At a critical distance the circle on the left should fall on your blind spot and completely disappear.  If you move closer or farther away the red circle will reappear.  What happens when the red circle disappears?
 
 

Notice that you don't see a black hole.  Instead the region is filled in with the same color as the white background.  Low-level perceptual processes fill in the missing information.  Now try the same procedure on the figure below?  What happens to the red circle?  Do the perceptual processes fill in the missing information in a similar manner?
 
 

You should have noticed that the filled in region actually matched the background pattern.  The low-level perceptual processes attempt to fill in the missing information based on what is surrounding the blind spot.  Now try the demonstration below.  Notice that a vertical black bar runs through the blind spot.  What do you see, two vertical bars with a gap in between, or does perception fill in the missing information so that it appears as a continuous bar?
 
 

In this case, many people see the upper and lower portions as a continuous vertical bar -- even though there is no information connecting them on the computer display.  Now try the demonstration below.  Notice that a vertical black bar and a vertical white bar meet in the blind spot.  What do you see, two vertical bars with a gap in between, or does perception fill in the missing information so that it appears as a continuous bar that changes color?
 
 

Again, many people see a continuous vertical bar that changes color at the blind spot.  The visual system reaches a compromise by smearing the images in the blind spot as it moves from black to white.  Perception can also fill in missing portions of regular objects.  In the figure below, a complete square is often seen even though one corner of the square falls on the blind spot.  This filling property is an example of the Gestalt law of closure.
 
 

Perception can also fill in more complex, but regular patterns such as the figure below.  This should appear as a radiating star even though the center portion falls in the blind spot.

However, when the visual image becomes more complex, the lower-level perceptual processes have difficulty filling in the missing information. In the figure below, what happens when my son's head falls in your blind spot?  Does perception fill in the missing information or is there a void in the picture?