How Can Blindsight Happen?

Your conscious experience of the world around you, of the choices and decisions you make, and of the emotions and attitudes that motivate you are not the totality of your mental activity or of your brain’s processing of information. Many, perhaps most, psychologists believe that consciousness is only a small part of your total cognitive activity.^[1]

A person is considered to be blind if they have no conscious experience of the visual world. This conscious experience is based on the flow of information from the eyes through the thalamus in the middle of the brain to the primary visual cortex in the occipital lobe at the back of the brain. If the primary visual cortex is damaged or fails to receive input due to the disruption of visual pathways, then the person will not “see” the objects and events that we normally associate with vision. Remarkably, some people who are blind can still respond to visual stimuli—a phenomena known as blindsight.

Blindsight occurs because the visual system has a primary pathway (retina to thalamus to primary visual cortex), but it also has secondary pathways (retina to thalamus to other brain areas). These “other brain areas” include parts of the frontal lobe that guide eye movements, parts of the midbrain that help guide visual attention, and parts of the occipital lobe that process features of visual perception, including shape, movement, and color.^[2]

The existence of visual processing areas for isolated features of vision and the fact that these areas get some direct visual information (i.e, input that does not first go to the primary visual cortex) means that it is possible for a person to respond accurately to questions about color or motion or shape without consciously “seeing” the objects that have color or shape or are moving.

It is important to remember that everyone has these same “unconscious” pathways in their visual system, not just those with visual impairments. That means your conscious experience of the visual world may not include all of the visual information you are processing. In other words, you may “know” more than you “see.”

Blindsight is not the only condition that involves unconscious or low-consciousness processing. Other neurological syndromes that have an unconscious element include amnesia, hemispatial neglect, dyslexia, aphasia, and various agnosias.^[3]

Creating Blindsight in the Laboratory

Wouldn’t it be great if we could produce blindsight in the laboratory, in order to better understand visual processing and conscious experience?

It turns out, researchers have already done it. Using precisely aimed magnetic pulses, researchers can temporarily disrupt specific areas of the primary visual cortex—the area responsible for conscious vision—without injury. This “blindness” lasts only a fraction of a second, after which vision returns to normal. Would you volunteer to be a participant?

Let’s look at how this works.

TMS: Transcranial Magnetic Stimulation

Transcranial magnetic stimulation (TMS) is a procedure used to stimulate neurons in the brain. A device referred to as a “wand” contains an electric coil that generates a magnetic field that in turn creates a small electric current in the brain.^[4] The electric current induces neurons (brain cells) to produce neural signals called action potentials. When action potentials are produced in normal brain processes, they allow neurons to communicate with one another. However, when action potentials are induced by an outside force—here by the TMS wand—they are meaningless and temporarily interfere with normal communication between neurons. If only a single pulse of electromagnetic energy is produced, then the disruption of the neurons in the targeted region lasts only a fraction of a second. Multiple pulses, called repetitive TMS (rTMS), can produce longer-lasting effects. In fact, rTMS is now used by therapists as a treatment for depression and neuropathic pain.

The TMS pulse can be aimed very precisely at a small area of the brain. When the target is the primary visual cortex in the occipital lobe, the TMS pulse can be focused to interfere with neural communication in a tiny region of the visual field—occurring for such a short time and in such a small area that you would not even notice. But however brief the duration or tiny the affected area, the person receiving the TMS pulse is temporarily blind in a small part of the visual field.

Conclusions from the Research

Experimenters have succeeded in producing the experience of blindness using the TMS apparatus, and they have also succeeded in producing evidence for the unconscious processing of features of the visual experience in normal (college student) volunteers. These results, when put together with the experiences of people with neurological damage, strengthen the case for the theory that some of our visual perception of the world takes place outside of our awareness. Researchers and their volunteer participants helped show that this unconscious processing is not the result of brain damage, but is instead part of our normal perception of the world.

Some Final Thoughts

This module is about consciousness. It is common to assume that everything we know about the world around us and about our own thoughts and internal experiences must go through the doorway of our conscious mind. Evidence from blindsight is one of several lines of research that shows we process more information than we are consciously aware of. Learning just how much this unconscious information can influence our thoughts, actions, preferences, and beliefs, is an important challenge for the next generation of rising scientists.

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