Eyes with a mind of their own: Blindsight

“I have three major questions myself that regard consciousness,” says Petra Stoerig, a psychologist at the University of Duesseldorf. “The first one is ‘who has it, meaning what kind of animal’, the second is ‘how is it produced or generated by the brain,’ and the third one is ‘what is it good for.’ And I think blindsight is a phenomenon that allows me to study all three of them; that is why I think it is fascinating.”

The term blindsight first appeared in print in 1974, the year, Larry Weiskrantz and his colleagues at the University of Oxford published their first report on a man who would become the phenomenon's poster child. The patient, known as DB, had surgery to remove a brain tumor. Unfortunately the surgery also destroyed an area on the right side of his brain known as the primary visual cortex.

DB claimed he was unable to see anything in his damaged field of view. If Weiskrantz put a shape in front of DB's left eye and asked whether it was a circle, square or triangle, DB would insist he had no clue. Yet when asked to pick an answer anyway, he did remarkably well. “I’m just guessing,” the patient insisted. “I didn’t see a darn thing.” Weiskrantz recalls it was an extraordinary moment. “We were astonished,” he says. “And he was astonished.”

Over the next 10 years, Weiskrantz and his colleague Elizabeth Warrington studied DB intensely, devising clever tests to address the concerns of skeptics. Gradually, increasing numbers of scientists came to accept blindsight, the paradoxical phenomenon of seeing without awareness.

What stymied scientists at first was the assumption that there was only one major visual pathway -- a single stream of information from the eyes to whatever part of the brain made sense of it. “After all, we have only one pair of eyes and only one indivisible experience of the world,” wrote neuropsychologists Melvyn Goodale and David Milner in a 2006 article. “But of course what seems obvious is not always correct.” In the 1990s, Goodale and Milner showed that vision is processed by two separate pathways, one of which operates at a subconscious level.

The key to this realization came from a woman known as DF, whose vision became impaired after inhaling toxic fumes from a broken gas heater. DF had some awareness of objects in her damaged visual field, but was completely unaware of their orientation, shape, and size. Yet when asked to grab an object placed in her damaged field, say a pencil, she would reach out and perfectly tailor the distance between her finger and thumb and rotate her wrist to match the pencil’s orientation. In other words, her arm and hand seemed to “know” what her conscious mind did not.

Using imaging techniques to monitor the activity in DF’s brain and compare it to that of normal volunteers, Goodale and Milner discovered that the visual information collected by our eyes diverges into two separate streams, known as ventral and dorsal, when it reaches the brain’s cortex. The ventral stream, which was damaged in DF, deals with visual perception—it handles the information we need to recognize objects, scenes and individuals. The dorsal stream deals with action, handling the visual information we need to interact with the world. Whereas the ventral stream is inextricably linked to consciousness, the dorsal stream works subconsciously, allowing DF to grab a pencil without being aware of its shape or form.

All of this brings us back to Petra Stoerig's three big questions. Let's start with the question of how consciousness is produced by the brain. Blindsight shows that damaging or destroying certain areas of the brain can wipe out aspects of consciousness, though corresponding unconscious activity remains. Without the primary visual cortex (V1), a person doesn't know that she's seeing, though her brain may unconsciously process some of the visual information. It's clear that V1 isn't the source of all consciousness, but it does seem to be crucial for bringing vision to consciousness. So blindsight helps to confirm that in the still-mysterious process of creating consciousness, all parts of the brain are not created equal.

Secondly, Stoerig's question "who has consciousness?" If blindsight teaches us anything, it's that this might be a trick question. Consciousness seems to have many parts, and at least some of the parts seem to be optional: you wouldn't say that DB and DF are unconscious just because they lack visual awareness. Which brings us to the zombies. For decades, philosophers have pondered whether there could be organisms who are physically identical to us and who behave just like us, but who are devoid of consciousness. These hypothetical beings, called zombies, have proved useful for conducting several kinds of thought experiments including those that address what consciousness is and what it’s good for. In a limited, but more tangible sense, blindsighted patients have provided a similar device.

Stoerig also says that blindsight helps her think about what consciousness is good for. In some tests of visual discernment, DB's guesses about what he couldn't see were actually more accurate than his answers about what he could see. Does conciousness just get in the way? No, says Stoerig, consciousness is an indispensable part of our human experience. She points out that, when compared to normal sight, blindsight is limited in several ways. For example, blindsighted patients are unable to normally distinguish different colors in their damaged fields of view. Stoerig can’t say whether consciousness helps put color into seeing, but it might. More importantly, she believes that consciousness puts the spice in life. Why do we spend a fortune eating at a gourmet restaurant, or risk our lives climbing a treacherous mountain? These activites don’t help us survive or reproduce, she says, they’re merely pleasurable. And according to Stoerig, that is because they are associated with a conscious experience. “I’m absolutely convinced that conscious experience makes all the difference,” she says, “I think it’s really what motivates us.”