Your Brain Sees Things That You Don’t

Apr 7, 2016

Photo credit: Beawiharta Beawiharta/Reuters

By Michael Graziano

In the 1950s, an 8-year-old boy suffered a head injury in a road accident. The back left part of his brain was damaged, specifically the primary visual cortex. As a consequence, he went blind in a large part of his right visual field. Years after the injury, his neurologists uncovered something strange. He could still see on the right side, even though he didn’t entirely know it.

The neurologists told him to face a screen and look at a small cross at the center to stabilize his eyes. A single dot appeared in his blind area, and he was told to point to the dot. In frustration, he insists there is no dot. But then, he takes a wild guess, and points right at it. Try after try, as the dot is flashed in different locations in the blind area, he points accurately most of the time.

This bizarre phenomenon is called blindsight. It was discovered in the 1970s by British researchers Larry Weiskrantz, Nicholas Humphrey, and others. It’s caused by damage to the primary visual cortex, one part of a vast network of brain areas that process vision. Without that part, some aspects of vision are still possible but the conscious visual experience disappears.

Blindsight offers a tantalizing hint about human consciousness. It demonstrates the difference between merely processing visual information in the brain, like in a computer, versus having a reportable conscious experience of it.

But this hint from blindsight proved hard to interpret. Does the primary visual cortex somehow generate awareness? If so, what exactly is being generated and how does it get from the back of the brain into our speech circuitry so that we can say that we have it? Maybe the primary visual cortex doesn’t create awareness itself, but instead sends visual information to a different system in the brain that is more closely related to consciousness. But if that’s so, what is this brain system that causes consciousness, and how does it work?

For a while, it seemed as though blindsight would remain only a tantalizing hint, but in 1999, Robert Kentridge, Charles Heywood, and Larry Weiskrantz stumbled on a new quirk of blindsight. It’s easy to mistake their discovery for a minor detail, but it turned out to be one of the most important insights into consciousness in decades.

Imagine you’re looking at a screen. A distracting dot flashes on the right side. A fraction of a second later, a number appears at exactly the same location. Your job is to report the number as quickly as possible. Your response time is probably pretty fast because the initial dot automatically drew your attention to that location. On the other hand, suppose the dot flashes on the left side of the screen. A fraction of a second later, the number appears on the right side. Now you’re probably slower to read the number. The dot automatically drew your attention to the wrong side and it takes a moment for your attention to readjust. This simple experiment allows researchers to measure how much attention was snagged by that initial dot.

It turns out that in people with blindsight, the dot can snag attention even when it doesn’t snag conscious experience. Bizarrely, attention and awareness can be separated.

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19 comments on “Your Brain Sees Things That You Don’t

  • My dad suffered from blindsightedness in his eighties; when he looked to his right before crossing the road there was a blanc black area starting a few hundred yards away, which blocked out a long stretch of road, so he couldn’t tell if there was a vehicle approaching other than by listening. Terrifying! He got round the problem by turning around and looking to left with his good eye. Having remembered that, suddenly, I miss the old sod!

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  • Brilliant. The beginnings of unravelling the meaning and utility of consciousness. When the first, nasty, slimy bug wallowing about on the bottom of the ocean, developed a light sensitive cell, which helped it survive, did it merely develop an accompanying neurological system which reacted to the light stimulus, by causing the creature to do something, for instance get out of the way ( ie attention), or did it begin to work out that something ‘orrible was about to eat it and it had better scarper (ie consciousness)?

    It seems most probable that consciousness developed much later, as an additional control system, and that early possessors of proto eyes, and even developed eyes, might have had to make do with blindsight.

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  • The difference between attention and awareness has been mooted for a while (in fact). It is a delight to see these elegant experiments tease them apart. We know that much to do with vision and its management is below a threshold of awareness from managing pupil dilation to the visual processing necessitated by continual eye saccades. Clearly attention processing is most likely the processing that directs these saccades. Low level heuristics that are drawn to movement or something nipple-like or lunch-like etc.. Not mentioned here (because they are cautious scientists and I am not) is the process of possible-salience detection that levers our energy-costly slower cortical inference generator into action to better test our sub conscious hunches. Awareness seems to relate at least to the input of this process of salience evaluation. We are aware of that which is judged potentially salient and that which is resolved, finally, to be salient.

    The output of our eye saccades and internal visual field scanning are subject to a Bayesian “trained/developed” set of tests, where simple features and transitions have been found to be the “strong Indicators” of potential salience and worthy of promotion to awareness.

    What is interesting is that this aware level of salience determination consists often of applying new learned types of test or comparison with learned attributes of things or processes. These might have the broad description of being environmental/educational or cultural/educational. The inferences that you make, that unfold before your awareness (it is lasagne. It is her lasagne) are learned processes more than anything. They are Apps to which we can now attend (!).

    I propose awareness is at least facilitated by culture.

    Certainly, I think eejit has it exactly right.

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  • Hi Crooked! #6

    Nice to see you pass through. Hope your are well. Hope you tarry a while.

    I’m not sure if this very early evolution would have much immediately to do with this, though with the way stuff gets later co-opted by evolution into the massively cross-coupled connectome who can say.

    Wakefulness is a state shared by both attention and awareness and as such might be no more significant than an on off switch. BUT reading how RAS can go wrong in a number of pathologies altering arousal and attentional activity, one could well imagine that awareness processes could be variously under utilised or badly overloaded. Its interesting to note both depression and attention deficit disorder indicated under pathologies.

    I wonder if nuancing awareness from attention definitively at last, might lead to re-naming of ADD? It is perhaps more awareness curtailment following attention lability…

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  • Wouldn’t the system of identifying and getting away from something, or eating it, already be present in a creature without site even at that early stage? The eye gave distance. The message gets to that system but not the visual cortex?

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  • Olgun.

    Indeed before brains proper existed light sensors were linked to muscles. A fast shadow might then trigger rapid hiding muscle action (burying in sand) or complete muscle inaction. (Muscles can have complex actions, but with a limited repertoire, without brains, like hearts and intestines.)

    Their are broadly two optical pathways for primates (mammals?), early and late evolved. The early one is the one evolved from these pre-brain arrangements and is related to our automatic behaviours (elevated heartrate at the sight of snakes or orange haired demagogues, etc.)

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  • Within the last year I watched a nice documentary that demonstrated this phenomenon (blindsight), to the viewer, with a simple experiment that revealed that the brain is stimulated in ways . . . one of which was the ability to ‘process’ the emotional state (happy, sad, angry, etc.) of a person’s face, ‘seen’ by the blind eye. Down the road, if I can find the show, I will post a note about it in this thread.

    I would be curious to experiment with this mechanism, test: subject blind since birth vs. control: subject blinded after puberty, to see if this emotional ‘process’ is learned or a primitive reflex, nature or nurture. The nature of blindness would require that the visual system function, less the back of the brain’s visual cortex.

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  • Jeremy (#15); also worth considering (in an experiment) is where the visual impairment is based (eye, optic nerve, brain…) and HOW the impairment happened (genetic anomaly, early injury, late injury…) as these may be important factors.

    The reason I think this is: I have a physical disability; a birth accident damaged my left arm. So I am used to it’s limitations, and conscious of them when I do stuff. But in my dreams (and so, I guess, in my subconscious) I am always fully able-bodied.

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  • ShadowMind,

    Thanks for the interesting reply. I like your dream comment.

    “The nature of blindness would require that the visual system function, less the back of the brain’s visual cortex.” The quoted statement was my vague attempt to address the concern you envision, as to the ‘how’ of the injury.

    On second thought I’m not so sure the experiment will work, too many possible confounding factors. The test group’s functioning blindsight might associate the intrinsic feeling of a good mood with the novel visual information that ‘blindsight sees’ such as happy smiling faces, like a baby being tickled by a loving parent who is smiling. If the vision anatomy of the pre-cortex has its own memory, I would think the brain would associate a smile with a hug, being tickled, smelling food/hormones, sounds of laughter, etc.

    The blindsight documentary that I watched had some nice and simple examples to help the viewer to better understand the brain’s visual anatomy, etc. One picture was a cut away of the anatomy of the brain, looking down upon it, similar to a flying bird looking down upon a person. The brain’s visual anatomy was highlighted, noting the eyes, optic nerve, and the visual cortex. The show seemed to point out that somewhere in the optic nerve there are several areas that preprocess visual information detected by the eye, including the real time awareness that can interpret the emotional state of a face being seen by the blindsight.

    Data results of the supposed experiment might look the same between the test and control, but, if not, the data would help researchers to better understand sensed emotional memory development, communication, and consciousness with a relatively simple experiment. If the data results are the same, those of use who can consciously process visual information, in the visual cortex, might make the extra effort to look a blind person in their eyes so their blindsight can see and understand emotional facial expressions. Perhaps some folks who can see just fine, or so it is believed, might have social problems interpreting the emotional state of a friend due to the impairment of a pathway, or area, processing visual information associated with the visual memory of a smile or frown?

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  • That’s something I hadn’t considered: blindsight is visual processing BEFORE the visual cortex. Perhaps, as the optic nerve basically passes through the brain, the signal can be “tapped” by neuron connections that only form in some people (ie: an anomaly).

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  • The ‘blind sight’ documentary that I watched featured a scene that examined the phenomena of visual processing that can interpret emotional content of a persons face. A man was tested with a special machine that sent two different images to each of his eyes, to the one that worked and to the other that was blind. The guy had perfect sight until a midlife injury ruined the visual cortex of just one of his eyes, so the eyes and their optic nerve anatomy were functional and healthy. The blind eye was exposed to various people’s faces, each expressing emotional content, and the guy was able to correctly identify the emotional content with some credibility.

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