The Great Brain Debate: Nature or Nurture?
John E. Dowling. Washington, DC: Joseph Henry Press, 2004. $24.95.
Is our vision determined by our experience or by our genes? Questions about the relative influence of nature versus nurture have been widely discussed and debated in a broad range of fields for centuries. Some of the fiercest debates relate to human intelligence, development, and aging, but the question also applies to topics that are fundamental to optometry, including refractive error, amblyopia, and age-related maculopathy. For anyone interested in these issues, Dowling's new book, The Great Brain Debate: Nature or Nurture, is must reading.
Dowling, the Llura and Gordon Gund Professor of Neuroscience, is a preeminent vision scientist who has received many prestigious awards for his work on the vertebrate retina. In the Great Brain Debate, Dowling uses his considerable knowledge and intellect to lay out, in a thoughtful way, much of what modern neurobiology has taught us about the developing, adult, and aging brain. Although Dowling has aimed the book not at the neurobiologist, but at “non-experts and non-scientists interested in the issues and how they are approached,” the book is not for everyone; in my view, it is aimed at fairly sophisticated readers with some knowledge of basic biology (e.g., optometrists and other health professionals). Rather than the hard-line view on nature or nurture with which many authors have approached this topic, Dowling's approach is fair, balanced, and informed.
The book, which is illustrated with sparse but effective figures, is laid out in three distinct parts: The Developing Brain, The Adult Brain, and the Aging Brain. The first section begins with a description of how the brain is formed and how this formation is largely dependent on genetically specified molecular mechanisms. It then goes on to discuss the maturation of the brain, its circuitry and behavior, and the critical role that experience plays in this process. Because so much of what neurobiology has taught us about development and maturation has accrued through studies of the visual pathways, these chapters will be of special interest to optometrists. Here for instance, Dowling draws on the pioneering work of Hubel and Wiesel to discuss the effects of visual deprivation, critical periods, plasticity, and whether enriched environments can enhance brain circuitry. All of these topics will appeal and inform the optometrist interested in development, binocular vision, and pediatrics.
The second part (The Adult Brain) takes on the widely held view that once we reach adulthood, the brain becomes “hard-wired” and cannot be modified. This is a fascinating section that deals with a number of important issues related to cortical plasticity in the adult brain as it pertains to vision, touch, memory, and learning. Dowling aptly names one of the chapters “Teaching Older Dogs New Tricks.” He also takes on a number of controversial issues related to the complex interplay of genes, environment, and behavior—an area that Dowling's own work has contributed to enormously.
In many instances, Dowling successfully points to the relationship between behavior and biology; however, more than once he points to an unexplained puzzle: how is it that the brains of cold-blooded vertebrates, which are highly plastic in terms of neuronal replacement and repair, seem to lack the perceptual plasticity of mammalian (and more specifically human) brains? Specifically, if a frog's optic nerves are severed, the eyes rotated by 180°, and then the nerves reattached, the optic nerves regenerate, and the frog can see, but their visual world is upside down and inverted. In contrast, the mammalian nervous system is not able to regenerate, but early studies in which humans wore inverting prisms have been widely interpreted by others to indicate that the subjects adapt to their upside-down world, i.e., that they learned to “see upside down.” What puzzles Dowling is that based on this evidence, it seems incongruous that mammalian brains are less plastic in terms of neuronal regeneration and repair, but appear to be more plastic in terms of visual perception.
In fact, the late Gordon Walls provides a wonderful resolution to this apparent paradox. Walls was something of an iconoclast—a renaissance man who initially trained as an engineer, took a Master's degree in zoology, was an accomplished histologist and illustrator. and taught visual physiology at UC Berkeley, School of Optometry. In a wonderfully entertaining three-part article. “The Problem of Visual Direction” published in this journal more than 50 years ago,1–3 Walls tackled many of the same issues that are addressed in the Great Brain Debate. It is interesting to reread Walls, who was an outright nativist, in light of what we have learned in the last 50 years. Stratton's prism experiments4 posed a serious challenge to Walls' views and he undertook a very detailed analysis of Stratton's papers and of his diary. Walls concludes that Stratton in fact never actually learned to see upside down, but rather that he inverted his body image with respect to his real body. As he quotes from Stratton's writings, “whatever there was of abnormality seemed to lie in myself, as if head and shoulders were inverted and I were viewing objects from that position, as boys sometimes do from between their legs.”
The last section deals with the Aging Brain—a topic made all the more important by the graying of the U.S. population. Dowling asks the controversial question of whether the aging of the brain is a disease (like Alzheimer and Parkinson or age-related macular degeneration [ARM]). To illustrate the causes and potential therapies of age-related changes, Dowling uses retinitis pigmentosa (RP) as a model, because, it might be, as he puts it, “the best-understood neurodegenerative disease at the moment, and therapies for its cure, at least in animals, look promising.” This is a fascinating section—and if you have not kept up with the fast moving developments, it is a “must read.”
Finally, Dowling has an excellent concluding chapter that provides his own speculations about the relative importance of nature and nurture and about the future. So is it nature or nurture? Like any good mystery story—the reviewer should never give away the ending. You will have to read it yourself!
Dennis M. Levi
University of California at Berkeley
School of Optometry