Divided Brains: The Biology and Behaviour of Brain Asymmetries, written by Lesley J. Rogers, Giorgio Vallortigara, and Richard J. Andrew and published this year by Cambridge University Press, elegantly brings together research, especially from the past two decades, on asymmetry (lateralization) of brain structure and function in a wide range of species. Once thought to be a characteristic of humans alone and one that distinguishes humans from all nonhuman animals (animals), brain lateralization refers to structural and functional differences between the left and right hemispheres of the brain. 

We now know the idea of human uniqueness was incorrect. Indeed, this is yet another example in which human exceptionalism has been shown to be false. In fact, this book traces the evolution of lateralization of brain and behavior from the earliest vertebrates to humans. In a wide number of species, including fish, amphibians, birds, and mammals, there is evidence that the left hemisphere is specialized to categorize objects and other stimuli and to take charge of routine behavior, whereas the right hemisphere is specialized to respond to novel stimuli and predators and to express intense emotions. The authors argue that moderate asymmetries may enhance the efficiency with which a brain operates, and this could play a role in evolution. 

Divided Brains discusses brain asymmetry from four perspectives, namely, evolution, function, development, and causation, in a way that is accessible to a broad audience. Focusing on these four areas of inquiry stems from Nobel-laureate Niko Tinbergen's seminal ideas about the important questions that need to be studied in ethological and other analyses of behavior (for detailed discussion see Tinbergen's classic book, The Study of Instinct). Summarizing the evidence and highlighting research from the last twenty years, this book covers a wide range of animals, including humans, and integrates research from the fields of neuroscience, psychology, behavior, and evolution. 

The final chapter makes it clear why knowledge about brain lateralization is important to those interested in situations that animals find pleasurable or stressful and how various stimuli affect their well being. For example, horses are more likely to attack conspecifics seen in their left visual field (inputs from the left eye are processed by the right hemisphere) and they show stronger fear responses to humans or novel stimuli seen with their left eye than with their right eye. Horses also use their right nostril to sniff unfamiliar stallions. Dogs, too, react more fearfully to stimuli seen on the left and processed in the right hemisphere, and they use the right hemisphere to process disturbing sounds, such as thunder and barks of other dogs that are arousing and distressing. Dogs also use their right nostril to sniff arousing odors, which also means use of the right hemisphere since inputs from each nostril are processed by the hemisphere on the same side as the nostril (i.e. no crossing the midline as for vision). 

We also learn about handedness in various animals, including data that show that male dogs show a clear left paw preference, whereas females show only a trend to prefer their right paw. We also know that dogs with weaker paw preferences are more likely than dogs with strong paw preferences to suffer from noise phobia and show heightened stress to sounds like thunderstorms. In addition, dogs show high amplitude tail wagging to the right side (left brain activation) when seeing stimuli that are expected to elicit an approach, whereas they show tail wagging to the left side (right brain activation) to stimuli that are expected to elicit withdrawal. 

Divided Brains will clearly be of interest to those who wish to understand more about the cognitive and emotional lives of human and nonhuman animals. As psychologist Michael Corballis of New Zealand's University of Auckland writes in his endorsement of the book, “This is the most in-depth analysis to date, by three foremost authorities on animal asymmetries, of a phenomenon that has fascinated scientists and philosophers through the centuries.”

I really enjoyed this extremely stimulating and important book and learned a lot about how brains work and how much remains to be discovered about asymmetries and what they mean in a wide variety of species. 

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