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Blindsight: Definition & Causation

Instructor: Danielle Haak

Danielle has a PhD in Natural Resource Sciences and a MSc in Biological Sciences

Blindsight is a fascinating condition in which a person who is visually blind reacts to some external stimulus like movement or light. It's still being researched, but read this lesson to learn about what we know so far.

What is Blindsight?

Blindsight is a difficult process to explain, and there is still a lot we don't know about it. Consider this question: have you ever been able to recall details about something you have no recollection of seeing? For example, on long drives, have you ever 'zoned out' and suddenly you're all the way home? However, later, when your spouse asks you if Main Street is still under construction, you are able to say yes, even though you spaced out during your ride home? Before researchers began studying blindsight, it was believed that a person had to consciously perceive something in order for it to register in their brain or affect their behavior. But it might be more complicated than that.

People with blindsight are able to react to certain visual stimuli without functionally being able to see them. Two different types of blindsight have been recorded. With the first, the person is completely unable to perceive a stimulus but they can predict certain things about it. For example, Rob is functionally blind, yet he is able to identify the colors of super-balls placed in front of him at a much better rate than by chance alone. With the second type of blindsight, the person has some awareness of the stimulus but they don't visually perceive it.

Causes of Blindsight

To understand the possible mechanisms behind blindsight, first we need to understand the basics about how visual information is processed. When light enters the eye, it is focused on the retina, found at the back of the eyeball. This visual information becomes electrical signals that transmit from the retina through the optic nerve. It is passed to the optical chiasm, the optical tracts, and the optic radiation, before entering the primary visual cortex, also called V1, which is in the occipital lobe of the brain.

The pathway electrical signals travel from the eye to the occipital lobe of the brain.
visual system pathway

The V1 creates a map of spatial information and helps interpret orientation, spatial frequency, and color. Some signals are passed on to the secondary visual cortex, also called V2, where more complex properties exist. Then there are also the third visual cortex (V3), which has numerous connections with other regions of the brain, and the fourth visual cortex (V4), which has similar properties of V1. It sounds confusing, but let's not get bogged down by the details. The important thing to remember is that the occipital lobe contains four sections that have various jobs in interpreting visual signals.

Blindsight occurs when the primary visual cortex (V1) in the occipital lobe has been damaged. It is believed that V1 is responsible for a person's perception of sight and their conscious interpretation of sight, so when V1 is damaged, they become functionally blind and are unable to consciously interpret any information their eyes transmit to the brain.

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