The physiological blind spot, also known as a scotoma, is a small area in the visual field where objects or light cannot be perceived. This phenomenon is a normal, inherent feature of the human eye’s anatomy. It represents a physical gap in the visual input received by the brain, yet most people remain entirely unaware of its existence in daily life.
The Anatomical Cause
The blind spot exists because of the design where the optic nerve exits the back of the eye. This exit point, called the optic disc or optic nerve head, is where approximately one million nerve fibers gather to form the optic nerve, transmitting visual signals to the brain. To leave the eyeball, these nerve fibers must pass through the retina, the light-sensitive tissue lining the back of the eye.
This specific location on the retina is devoid of photoreceptors, the specialized cells that convert light into electrical signals. These cells include rods for low light and cones for color vision. Since no light-detecting cells exist at the optic disc, any light that lands on this area cannot be registered by the eye. This effectively creates a small, permanent “hole” in the initial visual data sent to the brain. The blind spot is positioned about 12 to 15 degrees temporally, toward the side of the head, from the center of the visual field.
Perceptual Completion and Why We Don’t Notice It
Despite the physical absence of photoreceptors, we rarely perceive a dark gap because the brain actively compensates for the missing information. This process is known as perceptual completion or filling-in. The brain extrapolates information from the surrounding visual scene to create a continuous, seamless image.
A primary reason we do not notice the blind spot is binocular vision, meaning we use both eyes simultaneously. The visual field of one eye overlaps with the blind spot of the other, ensuring a complete image is always available from at least one eye. Since the blind spots of the two eyes are physically separated and do not align, this allows for natural compensation.
Even when one eye is closed, the brain works to fill the gap. It analyzes the patterns, colors, and textures immediately surrounding the blind spot and predicts what should be in the missing area. If the visual field around the blind spot is a solid color, the brain fills the gap with that same color, making the area visually uniform. Rapid, constant eye movements, called saccades, also prevent an image from being fixed on the blind spot for any noticeable period.
Locating the Physiological Blind Spot
It is possible to isolate and locate this normal blind spot using a simple visual test. This experiment requires focusing on a target with only one eye open, forcing the image to fall directly onto the optic disc. To begin, draw a distinct small mark, such as an ‘X’ and a dot, on a piece of paper.
To find the blind spot of the right eye, close the left eye and focus the right eye solely on the ‘X’. Hold the paper at arm’s length and slowly move it closer while maintaining focus on the ‘X’. The dot will disappear and then reappear as its image lands precisely on the optic disc, proving the absence of photoreceptors at that spot.