What makes us who we are? Where is our personal history recorded, or our hopes? What explains autism or schiziphrenia or remarkable genius? Sebastian Seung argues that it’s all in the connections our neurons make. In his new book, Connectome , he argues that technology has now reached a point where it is conceivable to start mapping at least portions of the connectome. It’s a daunting task, he says, but without it, neuroscience will be stuck. He answered questions from Mind Matters editor Gareth Cook.
Cook: You argue in your book that neuroscience has a fundamental problem. What is the problem?
Seung: Most people are familiar with the regional approach to neuroscience: divide the brain into regions such as the "left brain" and "frontal lobe," and figure out what each region does. This approach has helped physicians interpret the symptoms of brain injuries, but at the same time has frustrating limitations. How do regions carry out their functions? Why do they malfunction in mental disorders? What happens to regions when we learn? We can never obtain satisfying answers to these questions if we consider regions as the elementary, indivisible units of the brain.
An obvious solution is to understand a region by subdividing it into neurons, and figure out how the neurons work together to perform the region's function. This neuronal approach has the potential to answer the big questions above, but so far has not succeeded. In fact, those who study regions sometimes criticize those who study neurons as too focused on minutiae.
Cook: What made you think that there is another way?
Seung: The neuronal approach is finally gathering steam because of technological innovations, especially in genetics and imaging. The nervous systems of animals can now be genetically engineered, allowing researchers to carry out much more precise and conclusive experiments. And there are powerful new methods of looking into the brain to see how neurons signal each other and how they are connected into networks. These developments make neuroscientists optimistic that we are finally going to understand the brain as a network of neurons.
Cook: What do you mean by the connectome?
Seung: A connectome is a map of a neural network. It is like one of those route maps you find in the back of airline magazines. Just replace each city with a neuron, and each route between cities by a connection between neurons. Keep in mind, though, that your brain contains about 100 billion neurons, so your connectome would never fit in the pages of a magazine.
Cook: Are there particular diseases which this research might help understand?
Seung: In brain diseases like Alzheimer's and Parkinson's, neurons degenerate and die. Autopsy reveals that something is visibly wrong with the brain. Yet for many mental disorders, such as autism and schizophrenia, a clear and consistent pathology of the brain has not been found. Why? Researchers have conjectured that the individual neurons are healthy, but they are connected with each other in an abnormal pattern. Unfortunately, such "miswirings" or "connectopathies" have remained merely hypothetical, because our technologies for mapping neural connections have been too primitive. Imagine what it was like to study infectious diseases before the microscope was invented. You could observe symptoms, but not the microbes that caused disease. Similarly, most mental disorders are still defined only by their symptoms. We need to uncover their causes in the brain, and the new field of connectomics will be important for that.