Ординатура / Офтальмология / Английские материалы / The Neuropsychology of Vision_Fahle, Greenlee_2003

The Neuropsychology

of Vision

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Foreword

Glyn Humphreys,

University of Birmingham.

Intuitively, vision seems to be a simple process, occurring automatically and with such little effort that it is difficult not to recognise objects even when it would pay us not to! Yet, like many apparently simple processes, it turns out that vision is extremely com- plicated—so much so that, after nearly thirty years of intensive study and investment, we still do not have artificial vision systems that can approximate anything like the human ability to recognise objects and to use vision to negotiate the world. Some important insights into the complexity of vision can be gained by studying its break down after damage to our visual system, and hence the study of the neuropsychology of vision, directed at understanding such break downs, plays an important role in our scientific endeavours. In this excellent book, Fahle and Greenlee bring together state- of-the-art summaries of basic research into vision, providing a background in basic visual physiology to clinical studies of patients with disorders of object and colour processing, to studies of neuropsychological recovery and rehabilitation. The work covers studies at the single cell level to studies of sub-system behaviour measured through brain imaging. It reviews studies that use fine-grained time-based analyses of vision (electroand magnetoencephalography) through to long-term recovery from brain lesion. This coverage is important for grounding our knowledge of why particular forms of neuropsychological deficit occur, and is vital if we are to develop practical applications – for patient recovery, but also for useful, artificial vision systems. By bringing together this breadth of work, this book provides one useful step along this path.

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Preface

Understanding symptoms and predicting outcomes in visual neuropsychology

What always fascinated me about neuropsychology is that the subject has such far reaching implications for so many different areas of science, medicine, and, as I believe, even for the social sciences.

For medical implications, consider, for example, the large number of patients who, after cortical stroke, trauma, or tumours, suffer from symptoms that are highly disquieting and strange for the patients themselves, their friends and/or families, or even the patients’ physicians. This fact alone certainly provides a strong motivation for continued, interdisciplinary research and for teaching in neuropsychology and its related fields, aimed at improving both diagnosis and treatment of these patients.

Most general practitioners and, more often than not, most ophthalmologists do not have an intimate knowledge of neuropsychological symptomology. An important aim of this book is to provide knowledge about visual perception symptoms emanating from damage to the ‘visual’ brain and thus to explain these symptoms based on up-to- date knowledge about the normal function of this part of the (human) brain.

The better we understand the function of the intact brain, the better we will be able to diagnose defects, relate such defects to functional units within the brain, and eventually find ‘workarounds’ for these defects. At the very least, we will be able to reassure patients that they are not ‘mad’ but that the hallucinations or other strange perceptual phenomena they are experiencing are caused by ‘anomalies’ or ‘not their fault’ defects in certain parts of their brains.

Moreover, we will be better able to predict the outcome or prognosis, at least in terms of symptomology, and its effects. For example, by being able to more correctly classify varying symptoms as belonging to a specific entity or syndrome, we will be better equipped to foresee how the symptoms will develop over time based on the experience gained from other patients suffering from the same syndrome. Often, there is a good chance that 1) the symptoms may spontaneously disappear or at least alleviate in months to come or 2) the syndrome can be treated effectively via syndrome-specific methods (see, for example, Chapter 11).

Knowledge of brain physiology helps to classify symptoms

A great challenge in neuropsychology is the variability of symptoms in general and that a patient’s apparent presentments usually are influenced, especially some time after the defect or insult has occurred, by his or her way of dealing with the problem. Patients try to at least partly compensate for the symptom-generated disability, a compensatory plasticity similar to that found in perceptual learning (see Fahle and Poggio 2002).

viii PREFACE

Needless to say that in real life, as opposed to ‘textbook life’, many patients present with not classical textbook symptoms that often differ among patients suffering from the same syndrome but that indicate a similar future course and final outcome. (For example, the size and extent of cortical lesions differ considerably—bleedings and damage in general tend to differ among patients. So it is little surprise that symptoms differ more strongly among neuropsychological patients than they do among patients suffering from most other ‘illnesses’ such as diabetes or common cold, and even patients with these diseases show marked inter-patient symptom differences.)

Knowledge about brain physiology can greatly help in understanding symptoms deviating from the classical syndromes. I firmly believe that knowing the general rules of visual information processing in the visual brain allows the therapist to better understand the root, cause, and etiology of a patient’s symptoms, especially when said symptoms are atypical for a certain syndrome. Hence mere memorizing of symptoms without any knowledge about the underlying neuronal processes is a severe handicap in dealing with neuropsychological patients.

To bring the reader up-to-date with present knowledge about the normal functioning of the visual cortex, the opening chapters by Rainer and Logothetis and by Bullier provide insights into the basic aspects of the neuronal processes underlying visual perception, e.g., the general anatomy and physiology of the normal visual system in humans and primates.

Modern methods of visualizing brain function

There are several highly elucidating methods for studying the cortex, including recording from single nerve cells or small groups of neurons. But these methods simply cannot be used in humans except under very restricted conditions. Thus, a large part of present knowledge about the neuronal processes underlying visual perception comes from our nearest relatives in the animal kingdom, the (primate) monkeys.

Fortunately, the advent of modern imaging techniques such as Computed Tomography (CT) and (functional) Magnetic Resonance Imaging (MRI) now allow us to precisely visualize a patient’s structural cortical defects (which earlier could be determined, if at all, only by autopsy after the patient’s death). These methods moreover allow us to more closely evaluate the changes in cortical activity resulting from different stimuli and to visualize the neuronal activity associated with the visual brain when dealing with different perceptual tasks.

These exciting new methods open up greater possibilities for the researcher to visualize the processes taking place during visual perception in an almost non-invasive way and with high spatial resolution that seemed completely unthinkable twenty years ago. Electrophysiological methods in addition offer analysis of electrical activity produced by the cortex with temporal resolution in the millisecond range by means of recordings through the intact skull. The basics of these methods are presented in Chapter 3 by Munthe and Heinze, while those of functional MRI (fMRI) are explained in Chapter 4 by Greenlee.

PREFACE ix

These methods, which can be applied to both man and monkey, allow better correlation of the results obtained in single neuron studies in monkeys with the results from the more traditional neuropsychological investigations in human patients. The information about the neuronal basis of visual perception and the methods employed to study perception supplied by the first four chapters, allows the reader to better categorize the symptoms of a patient presenting with unusual combinations of symptoms and, thus, to understand and discern, rather than to merely memorize, at least some of the symptoms caused by lesions in different parts of the brain.

Lesion studies in animals and humans: experimental neuropsychology

The chapter by Merigan and Pasternak then continues by elegantly bridging the gap between basic science and patient studies, describing the deficits of visual perception caused by lesions in primate visual cortex, e.g., lesions caused by ablation experiments in monkeys and by relating these symptoms caused by the lesions to the physiology of the visual system. Ellison and co-workers, in Chapter 6, subsequently demonstrate that short lasting, reversible functional lesions can be safely generated in healthy humans, too, by means of transcranial magnetic stimulation (TMS). Hence, TMS constitutes a novel method to correlate animal studies with those on patients and to create a new discipline, experimental neuropsychology, in humans.

Brain function based on cortical specialization and parallel processing

A generally accepted view of specialization in the visual brain is emerging, albeit slowly. Neuropsychological studies in patients and animals, single neuron recordings in animals, and human and animal fMRI studies have made it increasingly clear that there is not just one visual cortex or even five of them devoted to different aspects of visual perception (see Cowey 1994, for review), but that there are more than 40 discernible cortical areas in the occipital, parietal, and temporal lobes involved with different aspects of visual perception (see van Essen, Andersen, & Felleman 1992). These areas together seem to use up almost one-third of the cortical machinery of our brains, thus indicating that humans are most certainly visual animals.

Most of these visual areas seem to represent the entire visual field, and several of their representations are ordered in a topological fashion (e.g. V1, V2, V3). That is, neighbouring points in the outer world are represented by neighbouring neurons in the cortex. This duplication, or rather multiplication, of the representation of the visual surroundings suggests that different cortical areas analyse different aspects of visual scenes, leading to a functional specialization as opposed to a duplication of function.

Vision, it appears, therefore is not just one holistic unit, but quite to the contrary, visual perception is the ‘holistic’ subjective correlate of neuronal activity in different