Patients with a damaged retina or visual cortex often report hole(s) in their sight. However inconvenient they may seem, these holes in many cases do not bother the patients and are sometimes not noticed at all. How do they block these holes from their awareness? In fact, this is a question that we should all ask ourselves, because we all have natural blind spots in our visual perception. This blind spot is caused by a small region in the back of our eyes that contains no retinal neurons. Instead, this region is dedicated for the retinal output neurons to send signals to the brain. Therefore, we walk around with two holes (one on each side) in our visual field. How do we not notice them, even when we try seeing with only one eye at a time?
Our brain uses visual information surrounding the blind spot to fill in what should be expected there. The brain is not simply ignoring that region of space, as demonstrated by V.S. Ramachandran (yes, he wrote Phantoms in the Brain and other popular neuroscience books) and R.L. Gregory1. The authors found that they could reveal the filling-in phenomenon by letting their human subjects (with normal eyesight) look at a screen displaying a regular pattern (e.g. parallel lines) but with a small patch of irregularity (e.g. a gray color but no lines) on it. When the subjects put the irregular patch in his/her blind spot, the irregular spot disappears and is filled with the same pattern as its surrounding (Figure 1). The surrounding pattern can be one of many things, including parallel lines, dots, white noise, visual illusions, and even letter strings (English, Latin, or gibberish). In the case of letter strings, the subjects could tell that the patch was filled with letters but could not read them. This suggests that our brain has a surprisingly intricate system to guess what should be in the blind spots, so vivid that it tricks most of us right in front of our eyes.
- Ramachandran, V. S. & Gregory, R. L. Perceptual filling in of artificially induced scotomas in human vision. Nature354, 699–702 (1991).