Virtual lesions : examining cortical function with reversible deactivation

Virtual lesions: Examining cortical function with reversible deactivation analyses the use of various transient cortical inactivation techniques to explore functional localisation and connectivity within the brain. The editors have assembled a cohesive series of relevant chapters, each authored by neuroscientists who have published extensively in this area. Reversible cortical deactivation is presented as an emerging and viable means by which to characterise regional brain function and organisation. The numerous specialised techniques available to accomplish this are described. This book furnishes the reader with ample theoretical background and practical detail for each experimental approach, whether it is used in animals or humans. Pharmacological, cooling and transcranial magnetic stimulation (TMS) techniques are described, and their utility in elucidating brain function, ranging from single cell recording to neuropsychological evaluation, is supported. This publication does not attempt to downgrade or render obsolete the traditional techniques of permanent brain lesioning, which have helped both clinicians and neurobiologists understand the workings of this complex organ. It does, however, provide compelling data to support the more widespread use of reversible lesions, particularly when the details of more complex neural circuits are being sought. Virtual lesions: Examining cortical function with reversible deactivation is divided into three sections. Part 1 (Exploring Neural Circuits) is primarily concerned with the use of pharmacological deactivation techniques with the goal of further defining neuronal interactions in the visual and auditory systems. Chapter 1 describes the use of GABA-iontophoresis for rapidly reversible inactivation restricted to a region of cortex 300–400 lm in diameter. The authors use this technique, as well as neuroanatomical tracing methods, to elucidate the neural basis of orientation and direction selectivity in the cat’s primary visual cortex. In Chapter 2, the selective blockade of neurotransmitter systems is used to examine the role of local intracortical networks and ascending thalamic connections in the modulation of neuronal responses in the feline primary visual cortex. The author of Chapter 3, Casanova, moves away from the primary visual cortex and uses both focal cortical cooling and chemical inactivation to study the extrageniculate corticothalamic loops in visual processing. This retinocollicular pathway relays in the pulvinar complex, en-route to secondary visual cortices. These techniques enabled the author to characterise the complicated corticothalamic networks involved. Chapter 4’s author, Villa, uses focal cortical cooling to examine the nature of thalamic networks in the processing of auditory information, and how this is modulated by the primary auditory cortex. Chapter 5 focuses on cortico-cortical connections, employing cooling techniques to study feedback and feedforward pathways between primary and secondary visual cortices in the monkey. The basis of neural plasticity is investigated in Chapter 6, using chronic pharmacological manipulation of the kitten visual cortex during monocular deprivation. Part 2 (Investigating Behaviour in Animals) utilises both cooling and pharmacological inactivation techniques to study the localisation of motor and cognitive function, as well as to explore the neural basis of cortical plasticity in the mature and immature brain. The authors of Chapter 7 demonstrated the effects of cooling deactivation on cortical reorganisation in the developing and mature brain. By studying the effects of inactivation of the primary visual cortex in the cat, they have clearly shown that the neural basis for plasticity in these 2 age groups is different, with more extensive reorganisation occurring in the immature cerebrum. Chapters 8 and 9 use reversible inactivation to characterise the disposition of cortical and subcortical neural circuits in somatic motor and eye movements. Part 3 (Probing the Human Brain) moves away from animal models and uses humans to study the effects of disruption of cortical elements of the neural circuitry by TMS. Chapter 11 provides a detailed description of the technique of TMS in human subjects, including its theoretical basis, methodological and technical considerations. It also describes the use of TMS in normal subjects and patients with neurological deficits, and pays particular attention to the role of the right parietal cortex in the phenomena of visual extinction and neglect. In Chapter 12, TMS is used to elucidate the neural basis of cross-modal plasticity. This is illustrated by the activation (as seen with functional neuroimaging) of the visual cortex by tactile stimulation in blind subjects, and the impairment of Braille reading in such subjects by the application of TMS to their occipital poles. Chapters 13 and 14 utilise TMS to produce transient functional lesions in the primary visual and motor cortices, respectively. Not only do they enhance our understanding of the functional connectivity of these important regions, but they may also shed light on the changes which may occur during disease processes, such as with the visual cortex and migraines. Virtual lesions: Examining cortical function with reversible deactivation is of interest to basic scientists and clinicians alike. It comprises fourteen well-written, succinct, and adequately illustrated chapters, which acquaint the reader with the techniques and potential applications of reversible cortical deactivation.