The blind spot in the human eye: causes, symptoms & test

The human eye is a true marvel – but in the middle of this highly complex sensory organ there is a spot that actually sees nothing: the blind spot. It is completely normal, is part of the basic anatomical equipment of every eye and usually remains completely hidden from us in everyday life. In this article, you will find out what the blind spot is all about, where it is located, how you can test it with a simple self-experiment – and when it is no longer a harmless phenomenon, but a pathological loss of vision. An exciting look at what we can’t see – and why this is the case.

The blind spot in the human eye – what’s behind it?

Everyone has it – and hardly anyone notices it: the blind spot in the human eye. It is a completely normal structure that is caused by the anatomical structure of the eye. The nerve fibers of the optic nerve emerge from the eye at a certain point on the retina, more precisely at the so-called optic disc (also known as Mariotte’s spot). However, there are no light-sensitive sensory cells at this exit point of the optic nerve – i.e. no photoreceptors that could pick up light signals. This is why the eye cannot perceive an image at this point.

The blind spot is located slightly temporally (i.e. towards the temple) in everyone’s field of vision – to the right in the right eye and to the left in the left eye. But although information is missing here, we don’t notice the gap in everyday life. Why? Because the brain fills in the missing information by processing the data from the other eye or “filling in” the image optically. This creates a complete visual impression, even though the image is actually interrupted at one point.

Why it is completely normal:

The blind spot is not a defect – but a logical consequence of the way our eye is constructed. As only the optic nerve, blood vessels and connective tissue are located at the optic disc, the photoreceptors that convert light into electrical signals are missing there. This is why we speak of a functional gap in the retina – one that occurs symmetrically in both eyes and is completely normal.

The brain “calculates” this missing information – so well that the blind spot is practically unnoticeable in everyday life. It can only be made visible through a targeted test or self-experiment.

How the blind spot develops: a simple explanation

Imagine the eye like a camera. The retina is the light-sensitive layer on which the incoming light is imaged. Its central region – the macula – contains a particularly large number of photoreceptors. However, this light-sensitive structure is missing at a small point, the optic disc. This is where the optic nerve exits and transmits all the optical signals received to the brain.

As neither rods nor cones are present at this exit point, no light stimulus can be processed – the result is a small area in the field of vision where nothing can be seen. This area is usually about six degrees away from the center and lies slightly nasal on the retina, which appears temporal in the visual field.

In medical parlance, this gap is also known as a physiological scotoma – and it is one of the few blind spots that occur completely without disease.

Do you notice the blind spot in everyday life?

The short answer: No, you don’t normally notice the blind spot. Although everyone has it – and it is located in exactly the same place on the retina in every eye – it plays no noticeable role in everyday life. This is mainly due to the amazing performance of our brain and the interaction of both eyes.

The visual system compensates for the missing information by:

• the overlapping visual fields of the left and right eye – the other eye sees what is missing in the blind spot of the first eye.
• the brain’s ability to automatically fill in missing image information, e.g. using colors, lines or structures in the environment.
• the constant movements of the eyeball, which prevent an image from “remaining” permanently on the blind spot.

Are there any symptoms?

As the blind spot is normal and functionally conditioned, it does not cause any symptoms in the classic sense. It usually goes unnoticed as long as:

• both eyes are open,
• there are no additional disorders such as a pathological scotoma or retinal disease.

Self-test: discover the blind spot yourself

As invisible as the blind spot is in everyday life, it can be made visible with a simple self-experiment. All you need is a sheet of white paper, a pen and your eyes. This allows you to experience first-hand how your eye is blind at a certain point in the field of vision – without any illness.

This is how the test works:

1. Draw on a sheet of paper:
   • a small black dot on the left
   • a small cross or square about 15 cm to the right of it
2. Hold the sheet about 40 cm in front of you.
3. Cover your left eye and look directly at the cross with your right eye.
4. Now move the sheet slowly back and forth without taking your eyes off the cross.

➡ At a certain point, the black dot suddenly disappears – it falls exactly on the exit point of the optic nerve, i.e. the blind spot of the right eye.

Differentiation from pathological visual field defects

The blind spot is a completely normal, physiological structure in the human eye – it causes no discomfort and goes unnoticed in everyday life. The situation is quite different with pathological visual field defects, which are also referred to as scotomas, but indicate a disorder or disease. These can seriously impair vision and require clarification by an ophthalmologist.

Important differences at a glance:

Causes of pathological visual field defects:

• Damage to the optic nerve (e.g. due to glaucoma)
• Diseases of the retina (e.g. retinopathies)
• Circulatory disorders in the eye
• Inflammations or infections
• Tumors or lesions along the visual pathway in the brain