The physiological blind spot is a normal, unseeing region in everyone’s field of vision where light cannot be detected. This natural feature of the human eye’s anatomy is not a medical condition.
The Anatomy of the Blind Spot
The blind spot originates at the optic disc on the retina. This is where the optic nerve exits the back of the eye to transmit visual information to the brain. The optic disc lacks photoreceptor cells (rods and cones) that detect light. Therefore, light falling on this region cannot be converted into visual signals, creating an area of no vision.
A Simple Test to Find Your Blind Spot
You can easily locate your own blind spot with a simple test using a piece of paper and a pen. First, draw a small “X” on the left side of a piece of paper. Then, measure about 5.5 to 8 centimeters horizontally to the right of the “X” and draw a distinct circle. Hold the paper at arm’s length directly in front of you.
Next, cover your left eye and focus your right eye solely on the “X” you drew, without looking directly at the circle. Slowly move the paper closer to your face while keeping your gaze fixed on the “X”. At a certain distance, typically between 10 to 14 inches (about 25-35 cm), the circle will disappear from your peripheral vision.
This point marks your blind spot for that eye; moving the paper closer or farther will cause the circle to reappear. Repeat the process, covering your right eye and focusing your left eye on the circle until the “X” disappears, to find the blind spot in your other eye.
How Your Brain Hides the Blind Spot
Despite having a blind spot in each eye, people rarely notice this gap in their vision during daily activities. This is due to two mechanisms employed by the brain.
First, our binocular vision, meaning we use both eyes, ensures that the blind spot of one eye is covered by the overlapping field of view from the other eye. Since the blind spots are not in the same corresponding location for both eyes, one eye’s view compensates for the other’s missing information.
Additionally, the brain actively “fills in” the missing information through a process called interpolation or “filling-in”. Even when viewing with a single eye, the brain uses surrounding visual cues, such as color, brightness, and texture, to construct a plausible image that seamlessly bridges the gap. This means that instead of perceiving a black hole, the brain essentially invents what it expects to be there, creating a continuous and complete visual perception of the world.