Tuesday, December 7, 2010

Out of Sight, Out of Mind, but Not Entirely Out of Order: What Cortical Blindness Reveals about Consciousness

Everybody knows that our thoughts are linked to our brains in some way. We say things like "Use your brain!" to encourage people to think things through, or "Can I pick your brain?" in the hopes of gaining thoughtful insight from a friend. But while we take it for granted that our thoughts – our very consciousness – is a product of our brains, centuries of scientific evidence was collected before this idea worked its way into our everyday language. One of the most fascinating demonstrations of the principle that our brains generate our conscious experience comes from the frontiers of neuropsychology, with a curious syndrome called “blindsight.”

Blindsight is a strange phenomenon - an oxymoron! Surely, there can be no such thing as “blind” sight. How could an individual see and be blind at the same time?

Remarkably, such a phenomenon does exist in a population of individuals who are cortically blind. This form of blindness occurs when there is tissue damage to the main visual area of the brain - the primary visual cortex, or “v1.” V1 lies at the very back of the brain and is fundamental for visual experience. While our eyes receive light signals from the world around us, v1 interprets those signals to generate what we experience as “seeing.” Individuals with cortical blindness have properly functioning eyes, but their v1 is damaged.

For at least a century, doctors knew about cortical blindness, and they assumed that it resulted in the same visual impairment as ordinary blindness – the inability to see. But roughly 35 years ago, it was discovered that cortically blind patients were able to do things that ordinarily blind individuals could not. A series of carefully designed studies in the 1970’s revealed that cortically blind patients were able to point to the location of objects and distinguish between various shapes, all the while claiming that they could not see the objects they were locating or the shapes that they were discriminating! At the time, the discovery transformed the way scientists understood vision and generated much speculation about the nature of consciousness.

Why consciousness?

Blindsight tells us something about consciousness because the syndrome revealed that cortically blind patients are not really blind; they are just unaware of what they see.

We now know that vision is the product of the unified effort of many different brain networks, each specializing in a particular aspect of our visual experience. Like a machine, the brain has many different wires traveling back and forth between different locations. These brain wires (i.e., neurons) carry information that gives rise to specific mental states and behaviors. In our visual brain, some of these networks specialize in recognizing faces; others specialize in recognizing colors, and still others are used for interpreting the lines and curves that comprise letters and familiar objects.

At a more basic level, the visual pathways of the brain can be reduced to two main networks: One pathway connects our eyes to v1, and a second pathway connects our eyes to a region of the brain that controls movement. While the first pathway is fundamental for our visual experience, allowing us to see the rich detail in our environment, the second pathway is more primal, allowing us to respond reflexively to sudden movement even before we are fully aware of what we have seen. This second pathway bypasses v1 and serves a motor, rather than a visual, function. It allows you to flinch in response to something flying in your direction, even if you are not aware of the projectile. It also helps your direct your gaze to something important, like a car moving towards you.

In blindsight, the pathway responsible for visual experience is damaged, but the pathway that coordinates movement in response to objects in our environment is preserved. Thus, although individuals with blindsight cannot see, they can respond reflexively to a sudden movement.

Blindsight provides evidence that the brain is responsible for our conscious experience because damage to the brain tissue of v1 diminishes visual awareness. Individuals with blindsight can still process what they see in important ways. Through the non-visual, reflexive pathway, they can identify the location of objects, avoid obstacles in their path, and direct their gaze to moving objects. What they lack is the awareness of the objects they are locating and the obstacles they are avoiding – it is their very visual consciousness that is impaired as a result of damage to v1, and therefore, v1 is crucial for such consciousness.

-Levi Riven


  1. Here's a wild question: Its fairly common that when damage is done to a something being controlled by the cortex is lost, say a finger, the area responsible for controlling whatever was lost is recruited to help do something different. In the case of losing your index finger, the motor cortex that used to control the index finger might now contribute to controlling your middle finger, possibly making that finger more dexterous. Could this phenomenon extend to consciousness? ie, if we lose conscious sight, can some conscious power be recruited to serve another function?

  2. yes... but in this case you've damaged the cortical area responsible for visual perception/consciousness, so I doubt it can be recruited to do anything. It would be more cool if this unperceived visual information suddenly went somewhere else-- somatosensory cortex let's say, and you could feel colour-- wait this sounds like synaesthesia. Someone should write a post on this! Anyway, is there such a thing as super-consciousness?

  3. IS V1 responsible for visual consciousness? As far as I know no one has any idea where the seat of consciousness is or whether or not there is one. That was my point. We have this overarching thing called consciousness that processes experience serially, and V1 apparently feeds into this; V1 itself certainly isn't responsible for consciousness since cortically blind people are still conscious. If there were a physical seat of consciousness that V1 inputs to, and if when V1 is damaged those inputs recede, would the denervated target neurons serve another function? If consciousness is a serial processor that probably isn't the case, and forget about super-consciousness - is there such a thing as consciousness? Fun mental masturbation nonetheless.

  4. Great discussion!
    Typically, only a portion of v1 is damaged in blindsight, and accordingly, only a portion of the visual field is "blindsighted." For most reported case studies, there remains sufficient cortex in v1 to feed higher association vision areas, never quite leaving those neurons free for reassignment. You raise an interesting point nevertheless - it would be fascinating to see what would happen to higher visual areas if they were completely deprived of input from v1. Interestingly, it has been recently observed that blindsight diminishes with time, allowing visual perception to be restored to the area of the visual field that had gone "blind." This suggests that some reorganization is at play, with neighboring v1 cortex compensating for the damaged area.