March 5, 2019
Written by: Nitsan Goldstein
If you ask a group of children the question “what part of our body lets us see”, they will likely tell you “our eyes!” They’re not wrong, of course. Our eyes are the gates through which light enters our bodies. However once light enters the eye, it is converted into electrical signals with very minimal processing, and those signals are sent straight to the brain. The brain is what processes information from the eyes and actually constructs the images that make up what we see in the world, in our imaginations, and in our dreams. When there’s damage to the eyes and light can no longer pass through them, the brain never receives the signals it needs to produce sight. What happens, though, when instead of damage to the eyes, there’s damage to a part of the brain that processes vision? In rare cases, this can lead to blindsight, a phenomenon where people are able to respond to visual stimuli that they cannot consciously perceive. In this post we’ll discuss how the healthy visual system processes information, how neuroscientists believe blindsight occurs, and what it can tell us about consciousness.
The Healthy Visual System
How is visual information processed in the healthy brain? When light enters the eye, it is converted into electrical signals by special cells in the retina. Those signals then travel to the brain through the optic nerve. In Figure 1, this information is represented by the blue and red lines. Most of the information is carried by the blue lines, and travels to a region in the brain called the lateral geniculate nucleus, and then to another region called the primary visual cortex, commonly referred to as “V1”. The signal that reaches V1 contains information about the objects we see—their color, edges, and motion. V1 neurons selectively respond to some of these qualities, which allows the brain to start to piece together objects from the information contained in the light that entered the eye. Once this information is processed in V1, the signal travels to many other areas of the brain that are involved in identifying what objects are, where they are in space, and how we should respond to what we are seeing.
You may have noticed, though, that some signal from the retina (represented by the red line in Figure 1) does not travel to the lateral geniculate nucleus, but instead goes to a region called the superior colliculus (green dotted rectangle). The superior colliculus receives input directly from the retina and is very important in generating eye, head, and even full body movements that orient you towards a particular stimulus1. Imagine seeing a bolt of lightning out of the corner of your right eye. Immediately, your eyes and head turn towards to the right, where you saw the light. The superior colliculus is involved in coordinating these rapid motor responses to visual stimuli. These two streams of information, one that goes to V1 and one to the superior colliculus, allow us to process visual information and respond appropriately to our environment.
What is blindsight?
What happens when part of this system is damaged? Damage to the visual system results in partial or total blindness. Most of the time, this damage occurs in the retina. As a result, the light entering the eye is not properly converted to electrical signals, so the brain does not receive the information it needs to produce sight. Sometimes, however, usually due to an injury or stroke, the main area of the brain that processes visual information (V1) is damaged. In this case, the signal reaches the brain, but the region that first processes the signal and sends it to other regions that tell us what we’re seeing is not working. This is called cortical blindness because the damage is in the cortex, not the retina. Remember, however, that another region receives information from the retina: the superior colliculus. When damage is contained in V1, the superior colliculus continues to receive visual information from the retina and processes it normally. This means that the superior colliculus is still able to generate motor responses in the eyes, head, and body in response to visual stimuli, a phenomenon that has been termed blindsight. What makes blindsight so fascinating, however, is that this stream of information that travels directly to the superior colliculus is never consciously sensed. Therefore, people with blindsight are able to respond to stimuli that they have no perception of. Imagine your head turning to the right in response to the lightning strike, but this time, you have no idea why your head turned. You did not consciously perceive the light, but still had the motor response to it. This rare condition has fascinated scientists for decades and has given us unique insight into how our visual systems operate, both consciously and unconsciously.
What does blindsight tell us about healthy sight?
Movie 1. Navigating with blindsight. A stroke to both visual cortices left this man cortically blind. Watch as he is still able to maneuver down a hallway, dodging obstacles he is not aware of. From de Gedler, 2008.
To understand how different areas of the brain might be involved in blindsight, scientists sometimes use animal models. In fact, many of the studies uncovering the role of the superior colliculus in generating eye and head movements were done by stimulating neurons in the superior colliculi of cats and monkeys and observing their responses2,3. To study blindsight in people, however, scientists rely on case studies on patients who have suffered an injury or stroke that permanently damaged their visual cortex. Studying these patients has led scientists to discover that the brain interprets much more visual information outside of conscious perception than expected4. One patient was able to navigate down a hallway, dodging obstacles he had no awareness were in front of him (See Movie 1). Patients with blindsight are also capable of adjusting their grip to properly hold an object that is in their blind field of view5. Incredibly, one patient was even able to discriminate different facial expressions that were presented in his blind field6.
These studies are important in understanding how the brains of patients with blindsight are processing visual information. However, they also identify which parts of visual processing occur at least partially subconsciously. These processes include things like orienting responses, adjusting grip, and even emotional responses. It is interesting to think about similar mechanisms in other sensory systems, like taste or hearing. How much of our responses to our sensory environment could actually occur without conscious perception? Advancements in techniques to study these rare cases will continue to shed light on the answers to such questions.
- Gandhi, N.J. & Katnani, H., A. Motor Functions of the Superior Colliculus. Rev. Neurosci. 34, 205-231 (2011).
- Missal, M., Lefevre, P., Delinte, A., Crommelinck, M., & Roucoux, A. Smooth Eye Movements Evoked by Electrical Stimulation of the Cat’s Superior Colliculus. Brain. Res. 107, 382-90 (1996).
- Kinoshita, M.,et al. Dissecting the circuit for blindsight to reveal the critical role of pulvinar and superior colliculus. Comm. 10, 135 (2019).
- Tamietto, M., et al. Collicular vision guides nonconscious behavior. Cogn. Neurosci.22, 888-902 (2010).
- Whitwell, R., L., Striemer, C.L., Nicolle, D.A., & Goodale, M., A. Grasping the non-conscious: preserved grip scaling to unseen objects for immediate but not delayed grasping following a unilateral lesion to primary visual cortex. Vision Res. 51, 908-24 (2011).
- de Gelder, B., Vroomen, J., Pourtois, G., Weiskrantz, L. Non-conscious recognition of affect in the absence of striate cortex. Neuroreport 10, 3759-63 (1999).
Cover Photo by Laitr Keiows, Wikimedia Commons CC BY-SA 3.0. https://commons.wikimedia.org/wiki/File:Iris_-_left_eye_of_a_girl.jpg
Figure 1 adapted KDS444, Wikimedia Commons Public Domain. https://commons.wikimedia.org/wiki/File:Gray722-svg.svg
Movie 1 from de Gelder et al. “Intact navigation skills after bilateral loss of striate cortex.” Current Biology, Volume 18, Issue 24, 23 December 2008, Pages R1128-R1129.