Of Blind People Who Can Ride a Bike
What Perception Disorders Reveal About Consciousness – Part 2
Some patients who are blind due to damage to their primary visual cortex can dodge objects in their way without even knowing that they are dodging them. This phenomenon, known as blindsight, takes Riddoch’s “ghostly shadows” one step further. Here, patients see nothing at all, yet their brain still guides their behaviour without them knowing. How can they see without knowing that they see?
A quick note before we start. This article is a bit more abstract. Here, we start with patients’ experiences of conscious and unconscious vision, then we look at their brain activity to narrow down on where consciousness sits. And finally, we extrapolate some abstract claims about what and where consciousness is. If you get lost, just keep reading and the pieces should fall into place by the end.
Ready? Here we go.
Catching up from Part 1: Last time, we looked at Riddoch syndrome, a disorder where patients who are blind from damage to the main visual cortex (V1) can still be consciously aware of something moving in their blind field, although it looks ghostly and shapeless. Back in 1917, V1 was considered the sole seat of conscious vision, but neuroimaging studies confirmed that alternate pathways bypass V1 and send coarse information directly to V5, the brain’s motion processing area. That is how Riddoch patients see moving shadows without a primary visual cortex.
You should know by now that I chose my words very carefully, yet I just said “Consciously aware”. This may sound redundant, but you will soon see it is not.
Roll cameras.
What if I told you that there are blind patients who can act as if they can see? They can navigate around obstacles, even squeeze through narrow spaces, and even ride a bike safely!
The Blindsight story
Patient DB was one of the first blindsight patients (Weiskrantz et al., 1974). Surgeons removed an abnormal tangle of blood vessels in his right occipital lobe, which left him blind in his left visual field.
He insisted that he could not see, but when he was pressed to “guess” the direction of a line, point at an object in his blind field, or guess the direction of something moving, he could do that with surprising accuracy.
What looked like lucky guesses was actual proof that the brain can process visual information, and even guide behaviour, while the patient remains unaware.
The test
Blindsight is studied using a forced-choice paradigm. This simply means that the patient is required to give an answer, even when they insist that they cannot see. If they perform at chance (around 50% correct), they are just guessing. But if their performance is consistently above that, then it cannot be explained by chance, and therefore, they “know” … Even if they do not know that they know. In other words, they are aware of the visual stimulus, just not consciously aware. – See what I did there? Brought it right back.
Blindsight in action: Watch a blindsight patient navigate a busy hallway.
Patient TN was a doctor who suffered two consecutive strokes one month apart, each damaging one side of his visual cortex, leaving him blind over his entire visual field.
“He walked like a blind man, using his stick to track obstacles and requiring guidance by another person when walking around the various laboratory buildings during testing” (de Gelder et al., 2008).
To test his blindsight navigation skills, de Gelder and colleagues asked him to walk to the other side of a cluttered hallway, without assistance and without his cane. Remarkably, TN walked smoothly around obstacles of different sizes and heights. At some point, he even turned sideways to squeeze between a box and a wall. “He walked much faster than we had expected, without hesitation or any kind of exploration,” said de Gelder (Robson, 2008). When asked, TN said he had not seen anything and was not aware of avoiding any objects.
This video (de Gelder, et al., 2008) is one of the most compelling demonstrations of the brain guiding movement in the absence of visual awareness. (The person following TN closely was Dr Lawrence Weiskrantz, who also studied DB).
Something to think about:
If patients can respond to unconscious input without realising it, could we too be reacting to cues we do not notice? Can you think of the last time you had a hunch that proved right, or sensed something was “off” before you knew why? I would love to hear about it.
Blindsight vs. Riddoch
But what do these syndromes have in common, and how do they differ? They both involve visual information getting through in their blind field. In Riddoch syndrome, patients are consciously aware of motion, even though they cannot see the moving object itself and are entirely blind to anything stationary. In blindsight, patients report no awareness at all but behave as if they can unconsciously discriminate objects around them. In other words:
Riddoch → conscious motion stripped of form and colour
Blindsight → behaviour guided without conscious awareness.
The most famous blindsight patient
We met patient GY in Part 1, as his case was key in showing V5’s role in conscious motion perception (Barbur et al., 1993).
He was only 8 years old when a car accident damaged his occipital lobe, and he lost vision in his right visual field. For decades, he insisted he could see nothing at all. But he became the most studied blindsight patient, undergoing PET, fMRI, MEG and psychophysics tests.
GY’s case was unusual: he responded differently depending on the stimulus. Sometimes, he reported seeing ghostly shadows (conscious awareness, classic Riddoch syndrome), and sometimes he performed well above chance while insisting he saw nothing (classic blindsight).
This offered Semir Zeki and Dominic Ffytche (1998) a rare opportunity to ask: Does the brain look different when vision is conscious compared to when it is unconscious? They used different stimuli to trigger each response and imaged GY’s brain during both responses. Since one response involved conscious awareness and the other one unconscious awareness, looking at brain activity could tell us something about the neural bases for consciousness.
Brain activity during conscious and unconscious vision
Zeki and Ffytche found that different brain regions were active depending on whether GY was conscious of movement or not. They could classify GY’s state of awareness just by looking at the brain activation.
Conscious awareness (the classic Riddoch syndrome): There was activity in V5, showing yet again that V5 can support conscious motion awareness even when V1 is damaged.
Discrimination without awareness (blindsight): There was no activation in V5, even though there was activity in V3 and other neighbouring areas, showing that the brain was receiving the information.
Conscious awareness → activation in V5
Unconscious discrimination → no V5 activation (but activity in V3, etc)
How the researchers explained it
Zeki & Ffytche showed that conscious and unconscious vision can be anatomically dissociated.
Long story short, they concluded that blindsight is only one manifestation within the broader Riddoch syndrome.
Long story long, they proposed that damage to V1 uncouples two processes that normally run together: the brain’s ability to discriminate a stimulus and our awareness of perceiving it. In everyday vision, these two are fused, so we experience it as the same thing. But the researchers propose that with V1 gone, the two can drift apart.
The difference may seem subtle, but it is one thing for the brain to notice something (discrimination) and even act on it, and another to consciously know that we are perceiving it (conscious awareness). Zeki and Ffytche offer us a visual aid (the red writing is mine):
Hallucination and consciousness
More recently, in 2023, Beyh and colleagues (including both Zeki and Ffytche) revisited the question of V5’s role in conscious vision with a more fine-grained analysis. Their patient, ST, was a man in his early 50s with partial blindness from a V1 lesion. He could see motion in his blind field and fit the classic description of Riddoch syndrome.
By carefully changing the stimulus, the researchers could elicit different perceptual responses, and then link each to distinct brain activity patterns:
When ST consciously saw motion, decodable patterns of activity appeared in V5
When ST reported seeing nothing (while being presented with motion): early visual areas were active but V5 showed no activity.
In other words, they could tell from the brain scans alone whether ST was aware or not. They could see that visual information was getting through to the brain because they found activation in early visual areas. But there was no conscious vision if V5 was not active. As the authors put it:
“Moving stimuli may give rise to neural activity in medial visual areas, but unless this is associated with V5 activity, they remain unseen” (Beyh & Zeki).
But the study did not stop there
By changing the contrast and speed of the stimulus, the researchers could study two more perceptual responses:
False confidence: In some trials, ST reported seeing motion with high confidence, yet his answers were no better than chance. Sometimes he misjudged the direction of movement, other times the movement itself. Brain activity during these trials pointed to the inferior frontal gyrus, an area linked to conflict monitoring and decision-making under uncertainty.
Hallucinatory motion: In blank trials with no movement at all, ST sometimes reported seeing motion. This correlated with activity in the hippocampus, a region important for memory and spatial navigation. The authors suggested these were internally generated perceptions, possibly triggered by the brain’s predictive machinery.
But was V5 active?
There was activity in V5 in three of the four conditions:
When ST could see motion
False confidence
Hallucinatory movement
The only time when there was no activity in V5 was when ST reported not seeing the stimulus, even though moving stimuli were present and other early visual areas in his brain were active.
This suggests that V5 is not just a motion detector. Its activity seems tied to whether motion reaches conscious awareness at all.
And motion is not unique in this respect: V4 plays a similar role for colour. When V4 is damaged, patients lose conscious colour perception, even though their eyes still register wavelengths. They can describe shapes and motion, but the world appears in greyscale, a condition known as cerebral achromatopsia.
Asking the right questions
There is clear value in these approaches of teasing out different degrees of awareness, as shown by Zeki & Ffytche and by Beyh and colleagues. There is not always a clear divide between seeing and not seeing. In the classic blindsight studies by Weiskrantz and colleagues, patients could only answer yes or no when asked, “Did you see anything?” A patient with a vague perception might well say “no,” and still perform above chance. In 2008, Overgaard and colleagues introduced the Perceptual Awareness Scale, where patients could respond with clear image, almost clear image, weak glimpse, or not seen. Their behavioural results tracked with what they reported. What looks like blindsight could in fact be degraded but genuine conscious vision.
What / where is Consciousness?
The implication of this is that conscious vision is not a single, all-or-nothing event but that it might be modular, relying on several micro-consciousness areas distributed across the brain (Zeki, 2003).
“Consciousness is not a unity, and there are instead many consciousnesses that are distributed in time and space” (Zeki, 2003).
Zeki’s “theory of multiple consciousnesses” is based on the idea that consciousness builds on distinct specialised areas, each geographically and functionally separate. Each mini-consciousness area contributes its own slice to our awareness, and the brain binds them together into the illusion of a unified flow of experience, what Zeki calls global consciousness.
Damage to any of those slices results in an alternative visual reality. Word choice again: it is an alternative visual reality but not a false one, since all perception is itself a constructed representation.
And a bonus one for the road
If Zeki’s theory shows us how consciousness is about integrating different pieces, here comes one more case that shows us how the brain can reassemble the puzzle in surprising new ways.
MB was a 21-year-old male patient whose occipital lobes were damaged around the time he was born. Since childhood, he “behaved as profoundly visually impaired,” yet he could guess the direction of fast-moving objects with 100% accuracy. Remarkably, he could ride a bike safely.
“He participated in various ball games and running events. He rode a bicycle and safely avoided people and parked and moving cars. He also played video games that were based on movement” (Giaschi et al., 2003).
An fMRI of MB’s brain showed no activation in V1 (as expected) but also no activity in V5 either. And yet, he could still see motion, consciously. What part of his brain was processing this motion? The activity appeared in the right premotor cortex, the precuneus (linked to memory and self-awareness) and the posterior superior temporal sulcus.
Because MB’s damage occurred so early in life, his brain was still in a critical period of development. The young brain is extraordinarily plastic, and so other regions could take over the task of processing motion.
Consciousness may be more like a sense-making computer, developing networks wherever it can to interpret the world by whichever method is available. The brain finds a way to get the job done, to help you navigate the world and keep you safe. It is not about elegance; it is about existing.
Consciousness is whatever the brain does to make sense of the world.
Final reflections
If conscious visual experience, which feels so direct and unified to us, can be supported by all these varied visual pathways, what does this say about the fundamental nature of consciousness itself? Is it tied rigidly to specific structures, or is it something that emerges from complex integrated activity, wherever it happens in the brain?
Conclusion: what perception disorders tell us about consciousness
The cases of Riddoch syndrome and blindsight challenge our everyday assumption that seeing is knowing. Vision is not a singular faculty neatly housed in one brain area. And neither is consciousness. They emerge from a network of regions, each contributing fragments of awareness. Even when the usual pathways are gone, the brain uses what remains to keep us connected to the world.
Perception disorders reveal something profound: consciousness is not a fixed essence but a state of sense-making, a biological necessity for survival. The brain assembles whatever pieces it can into a workable version of reality, enabling conscious perception when necessary and guiding behaviour even behind our awareness.
This all tells us that awareness goes far beyond just seeing. Your brain knows more than it tells you. Consciousness does not mean having access to everything but knowing enough. The line between conscious and unconscious, seen and unseen, is a bit blurred as the brain tirelessly attempts to interpret, predict, and give you enough to navigate the world in real time.
Acknowledgement:
A warm thank you to Dr. Ahmad Beyh for taking the time to read this article (Parts 1 and 2) and share thoughtful feedback.
It takes a lot of work and time to write these articles. If you find value in what you just read and want to support my work, you can buy me a coffee.
In any case, if you got this far, please like and restack, and feel free to drop any questions in the comments.
REFERENCES
Barbur, J. L., Watson, J. D., Frackowiak, R. S., & Zeki, S. (1993). Conscious visual perception without V1. Brain, 116(6), 1293-1302.
Beyh, A., Rasche, S. E., Leff, A., Ffytche, D., & Zeki, S. (2023). Neural patterns of conscious visual awareness in the Riddoch syndrome. Journal of Neurology, 270(11), 5360-5371.
De Gelder, B., Tamietto, M., van Boxtel, G., Goebel, R., Sahraie, A., Van den Stock, J., ... & Pegna, A. (2008). Intact navigation skills after bilateral loss of striate cortex. Current biology, 18(24), R1128-R1129.
Giaschi, D., Jan, J. E., Bjornson, B., Au, S., Lyons, C. J., & KH, P. (2003). Conscious visual abilities in a patient with early bilateral occipital damage. Developmental Medicine & Child Neurology, 45(11), 772-781.
Overgaard, M., Fehl, K., Mouridsen, K., Bergholt, B., & Cleeremans, A. (2008). Seeing without seeing? Degraded conscious vision in a blindsight patient. PloS one, 3(8), e3028.
Robson, D. (2008, December 22). Blind man ‘sees’ his way past obstacles. New Scientist. https://www.newscientist.com/article/dn16324-blind-man-sees-his-way-past-obstacles/
Weiskrantz, L., Warrington, E. K., Sanders, M. D., & Marshall, J. (1974). Visual capacity in the hemianopic field following a restricted occipital ablation. Brain, 97(1), 709-728.
Zeki, S. (2003). The disunity of consciousness. Trends in cognitive sciences, 7(5), 214-218.
Zeki, S., & Ffytche, D. H. (1998). The Riddoch syndrome: insights into the neurobiology of conscious vision. Brain: a journal of neurology, 121(1), 25-45.
Absolutely fascinating, thank you
Excelente y muy claro