THE ASYMMETRICAL BRAIN

Extract

THE ASYMMETRICAL BRAIN
 Edited by Kenneth Hugdahl and Richard Davidson
 2003. MIT Press
 Price $90. ISBN 0‐262‐08309‐4

This book provides an update of the scientific research on cerebral asymmetry. In 1995, Brain Asymmetry, from the same editors, appeared. Since then, the field progressed rapidly by the introduction of new neuro‐imaging techniques, which enabled more detailed study of anatomical and functional differences between the hemispheres. This book is not an update but a new book that consists of 21 original chapters from various authors, divided over seven major parts that include: animal models of asymmetry and basic asymmetrical functions, neuro‐imaging studies, visual asymmetry, auditory asymmetry, emotional asymmetry and applications for neurological and psychiatric disorders.

In part one (animal models and basic functions), Güntürkün provides an extensive review of the visual system of birds. Several bird species have a remarkably better pattern recognition with their right eye–left hemisphere than with the left eye–right hemisphere system. The main advantage of studying birds is the ease with which each eye can be tested separately. The avian optic nerves decussate nearly completely (99.9%), so simple eye caps can be used instead of difficult hemifield projection. The anatomy of the avian visual system displays significant morphological asymmetries: the left hemisphere receives both ipsilateral and contralateral afferents from the tegmental areas, while the right hemisphere receives mainly contralateral afferents. Therefore, the left hemisphere is able to integrate information from both sides of the bird, which gives it a head start for spatial orientation. Asymmetry of the visual system in birds is induced by a tight interplay of genetic and epigenetic factors. Embryo’s of all avian species keep the head so that the right eye is exposed to the light shining through the translucent eggshell, while the left eye is occluded by the body of the embryo. This leftward turn of the head during embryogenesis is probably genetically defined, while early light stimulation is the key epigenetic factor leading to higher activity levels and greater neuronal connectivity that trigger the visual system to develop in an asymmetrical fashion. In the next chapter, Tang describes several experiments by his group in which rat pups are treated with ‘neonatal novelty exposure’, which induced a rightward shift in hippocampal volumetric asymmetry and an increase in right hippocampal neuronal plasticity. Berridge, España and Stalnaker continue this part of the book by describing dopaminergic efferents to the prefrontal cortex. In rats, the neurons projecting to the right (but not to the left) prefrontal cortex are found to be essential for anxiety‐regulation and coping behavior.