“Why does the eye see a thing more clearly in dreams than the imagination when awake?” Leonardo da Vinci once asked. While this is an interesting question to ponder, Da Vinci definitely gets one thing wrong. It isn’t actually the “eye” that sees anything. It is the brain.
Mel Goodale —this year’s Zubek Memorial Lecturer — reminded the audience of this fact, which, while potentially mind-blowing in itself, was only the tip of the iceberg in his fascinating presentation.
The annual Zubek lecture, held by the department of psychology, brings in eminent, cutting-edge researchers to talk to us humble Manitobans about some of the latest psychological research. There’s even free wine and snacks afterwards.
Goodale, who authored the recent prize-winning book, Sight Unseen, with colleague David Milner, is one of the first scientists to report a case of “blindsight.”
In the tradition of popular neuroscientists like Oliver Sacks and V.S. Ramachandran, Goodale illustrated his research with relevant case studies.
It all started with a patient nicknamed “D.F.” who, as Goodale puts it, was “like the Rosetta stone of neuropsychology.” This patient had damage to a particular area of the brain known as the ventro-lateral occipital region — a section of brain tissue within the larger area of the brain responsible for vision.
During initial testing, D.F. could not recognize objects (neurologists call this “visual form agnosia,” if you’re interested.) At one point during her neurological testing, David Milner held up a pencil and asked if she could identify it. No way. Suddenly, and much to Milner’s and Goodale’s amazement, she reached out and swiftly grabbed the pencil in order to inspect it more closely. But if she was blind, how was this possible? She was able to accurately pinpoint exactly where the pencil was located in space, and to fluidly grasp and manipulate the pencil, despite having no conscious awareness of seeing it.
This bizarre occurrence led the researchers to investigate this phenomenon more closely, and through these investigations they eventually theorized the existence of two independent visual processing areas in the brain — one for conscious perception, and one for subconscious action. The former they termed the “ventral” or the “what” pathway, which is responsible for determining what you are looking at, as well as our conscious experience of seeing. The second they termed the “dorsal” or the “how” pathway, as it is responsible for the vision needed for action (how to interact with objects).
The “what” pathway allows us to recognize an object as an envelope, for example. We consciously know that we are seeing an envelope, and we also know that when the envelope is rotated (forming a different image on the retina) it is still the same envelope.
The second pathway allows us to pick up the envelope and place it in a mail slot. It guides our fingers so we know how far apart to place them as we reach for the object. To do this, the “how” pathway takes into account the envelope’s orientation, or its different possible retinal images. It does this subconsciously, for the most part.
Perhaps you see where this is going. D.F. had damage to the “what” pathway only, which prevented her from consciously perceiving objects. The “how” pathway, however, remained intact, so she was still able to use the kind of vision that guides action.
Goodale then performed a clever experiment with D.F. to confirm his suspicion. He placed two 3-D rectangles of different sizes next to each other and asked D.F. to tell him which one was the larger of the two. Of course, she couldn’t do this. He then asked her to pick each one up and measured the distance between her fingers as she grasped for each. Her fingers accurately estimated the sizes of each object as she reached for them, opening just enough to grasp each one. This is a task we unconsciously perform every time we pick up a coffee cup, book, guitar, cat or anything. Our fingers automatically open just wide enough to grab the object we desire.
In D.F.’s case, it was as if one part of her brain “knew” which was the bigger rectangle, even though she couldn’t consciously state that knowledge. It is for this reason that V.S. Ramachandran describes this “how” pathway as a “zombie” in the brain.
Some may find it interesting to note that D.F.’s eyes were not damaged at all. Visual information was still being picked up by the eyes and transmitted along the optic nerve; it just wasn’t getting to all of the brain areas it was supposed to.
This comes back to the original point that vision does not arise from the eye. It actually arises in the brain, and D.F.’s case is just one example of many illustrating this point.
Thinking back to Da Vinci’s question in light of this research, we realize that the fact that we dream at all should cause doubt as to whether it is indeed the eye that sees. During dreams our eyes are not receiving any visual input at all, so how could they “see” anything? Dreams are purely the result of brain activation, as are most aspects of vision, as Goodale and Milner’s research confirms.
Goodale described several other, equally fascinating case studies which provide evidence for the existence of these two separate visual systems, as well as recent FMRI images that confirm the physical separation of these two pathways. He even discussed possible evolutionary origins and selection pressures for the two pathways. The ventral pathway may have evolved as a result of selection pressures to plan and communicate about events in one’s environment, while the dorsal pathway may have evolved for humans to develop things like more skilled tool usage.
Goodale also inadvertently created a new version of an old thought experiment. Most of us at some point or another (usually due to the prompting of a friend) have perversely pondered the question “would you rather be deaf or blind?” Now, thanks to Goodale and colleagues, we can wonder if we’d rather have the “how” or the “what” pathway.
