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

x PREFACE

cortical areas that analyse different aspects of visual stimuli partly independently from each other in parallel (see Chapters 7 and 10). And even sub-functions of visual perception such as colour or motion perception seem to be divided into still further subunits such as detection of colour borders, achievement of colour constancy, and form identification based on colour to name a few (see Chapter 8).

Functional specialization of cortex leads to specificity of symptoms

Let us consider the consequences of functional specialization of cortical areas for neuropsychology. Clearly, processing of moving stimuli relies at least partly on specific areas that differ from those more specialized for the processing of colour. While certain cortical areas seem to be more important than others for certain sub-aspects of vision, such as V4 for colour and V5 for motion, it would certainly be too simplistic to assume that each sub-function of vision is subserved by one and only one cortical area.

In conclusion, we are left with a highly segregated visual cortex with many sub-areas subserving, probably in well-defined cooperation between areas, the different computations that together constitute the astonishing precision and speed of human object recognition. The most important aspect of this insight for neuropsychology is that a defect in each of these areas will produce a deficiency of visual perception, some of which are profound and debilitating for the patient while others are so subtle that they escape our present clinical tests. The different sub-aspects of perceptual deficits relating to, for example, colour (Chapter 8) or motion (e.g. Chapters 5 and 7) are described in detail in specialized chapters herein.

Learning from neuropsychological patients

Unfortunately, we do not know exactly which of the specialized cortical areas subserve which sub-functions of visual perception. This lack of knowledge is partly due to the fact that cortical physiology is still incompletely known. We still do not know exactly the elements or building blocks of visual perception. As a consequence, we still do not fully understand how the brain analyses or processes visual scenes with such astonishing success (outperforming even the most advanced computers).

A large portion of our current knowledge is based on the insights gained from the testing of neuropsychological patients in combination with the knowledge gained from the studies of animals and normal subjects. These tests have become far more detailed and sophisticated than they were in the past. The improvement is due both to a better (if incomplete) understanding of cortical physiology and to the availability of personal computers with tremendous graphical capabilities that allow performing, on a single device, a large number of sophisticated tests each of which would have required an often voluminous opto-mechanical instrument in the old days.

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Probably the best-known example of how patients suffering from a (limited) disorder of vision helped expand an understanding of the basic principles of visual perception is the study of patients presenting with colour deficiencies. Patients with such symptoms provided clues, including the inability to discriminate between certain hues while clearly discriminating others, which were absolutely crucial for the early (here used historically) development of theories on colour perception.

Investigating the perceptual deficits of neuropsychological patients will undoubtedly improve knowledge about the function of the intact human brain. It is reassuring that many neuropsychological patients have a quite positive attitude toward the testing of their deficits; they often better accept their handicaps when knowing that the testing and exploration thereof somehow contributes to the improvement of knowledge about the functioning of the brain and its disorders.

So the transfer of knowledge works both ways: Basic science improves understanding of neuropsychological symptoms, and testing neuropsychological patients leads to insights about brain physiology, especially for ‘higher level’ disorders of visual perception, such as agnosias (see below).

A hierarchy of processing levels and of associated visual disorders

The emerging picture on cortical physiology is consistent with the assumption that different aspects of visual scenes are computed in parallel by a number of cortical pathways or channels as indicated above. Within each of these channels, processing aims to reduce redundancy of the input signal by extracting important features and eliminating all signals less crucial for adequate analysis of the outer world. More specifically, borders or contours are extracted while areas of constant appearance are largely neglected, as both single cell recordings and psychophysical experiments in normal observers indicate. Disturbances on this level prevent patients from detecting contours based on say, colour, or direction of motion, and I would like to propose the term ‘visual indiscriminations’ for this class of defects.

As a next step, figures or objects are separated from their surroundings by combining the contours belonging to the same object through a process of binding. Defects on this level may lead to apperceptive agnosias.

Thirdly, this representation of an object is categorized via comparison with stored representations. A disorder on this level seems to underlie at least some forms of associative agnosias.

Lastly, semantic knowledge or ‘comprehension’ is bound to this representation via a word or phrase, and a deficit on this level leads to optic aphasia. Deficits may occur on each of these sequential steps, leading to sometimes dramatic symptoms that differ largely from patient to patient depending upon the processing level involved. Hence, symptoms range from complete blindness to agnosia or aphasia.

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Several chapters deal with the symptoms of these more ‘cognitive’ forms of visual defects and categorize the symptoms within the framework of the normal function of the visual system (see Chapters 7 through 10), proceeding from the basis of the knowledge on ‘early’ perceptual processes and their deficits, as outlined in the earlier chapters.

Visual perception as interplay between image analysis and synthesis

This process of sequential analysis is not fully determined by the input, but in a usually iterative process with strong feedback, a representation is synthesized rather than determined by a pure analysis of the retinal image. Agnosias represent, according to this view, defects on different levels of both analysis of objects and synthesis of object representations. Patients suffering from so-called apperceptive agnosias are able to react to visual stimuli but cannot bind together the contours they perceive to coherent objects and, as a result, cannot copy even simple line drawings (cf. Chapter 10). Patients with associative agnosias, on the other hand, are able to copy line drawings in a line-by-line way but are unable to describe—verbally or otherwise—what object they just copied. But even defects restricted to the ‘higher’ processing levels should, according to this new view, deteriorate processing on the lower levels due to lack of top-down influences, hence associative agnosias should impair binding of contours under certain conditions.

Blindsight and neglect: seeing without perceiving?

Finally, two phenomena in neuropsychological patients are presented and discussed that are generally considered to be related to still more cognitive aspects of visual perception. The first, blindsight, reveals that at least a limited analysis of visual signals continues in some patients even in those parts of their visual field for which they are subjectively blind (see Chapter 9). The second phenomenon, neglect, demonstrates the overwhelming importance of attentional processes for conscious visual processing. Patients will not consciously perceive, and thus not react to, stimuli in the contralesional half of the world, in spite of subconsciously analysing at least certain aspects of these stimuli. (See Chapter 7 which also supplies a general overview of all types of patient studies.)

Aims of this book

The initial chapters reviewing the insights gained by basic science on the structure and function of the visual system, as contributed by physiology, anatomy, and imaging studies together with the subsequent chapters on more classical neuropsychological patient studies, provide a clear picture of our present knowledge about the cortical processing of visual signals (mundanely called ‘seeing’) and about its possible disorders. Together,

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the information presented in these and subsequent chapters should make it evident that not only can therapists treating patients benefit from a deeper and more complete knowledge of the neuronal mechanisms underlying normal visual perception but that also researchers in basic science can gain important insights on the functioning of the normal visual system from the symptoms presented by neuropsychological patients.

An outlook

In conclusion, I would like to draw the reader’s attention to the fact that neuropsychology, more than other disciplines, forces us to realize that the brain is the substance, or organ, underlying our subjective experience of the outer world as well as the inner ‘world’ of ourselves. In this respect, neuropsychology has implications for philosophical and theological questions central for the understanding of consciousness and of our own ‘nature’. These metaphysical questions, however, have not been dealt with in the present book. Instead, this book attempts to bring closer together the patientand science-based approaches to the function of the (human) brain. (Perhaps we will add a chapter on the philosophical, theological, and, if you will, metaphysical issues in a later edition.) It is hoped that the present text will prove a useful tool for all those involved with patients suffering from visual disorders caused by cerebral deficits, and for most of those studying brain function.

References

Cowey, A. (1994). Cortical Visual Areas and the Neurobiology of Higher Visual Processes. In The Neuropsychology of High-Level Vision (eds. M.J. Farah and G. Ratcliff). Lawrence Erlbaum, Hillsdale, New Jersey.

Fahle, M. and Poggio, I. (2002). Perceptual Learning. MIT Press.

van Essen, D.C., Anderson, C.H., and Felleman, D.J. (1992). Information processing in the primate visual system: an integrated systems perspectives. Science. 255(5043), 419–23.

Manfred Fahle

2003

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Contents

Contributors xvii

Part 1 Physiology and anatomy of the visual system: single cells

1Vision, behaviour, and the single neuron 3 Gregor Rainer and Nikos K. Logothetis

2Cortical connections and functional interactions between visual cortical areas 23

Jean Bullier

Part 2 Sum potentials in humans: electroencephalography and magnetoencephalography

3Electroand magneto-encephalographic and event-related potential studies of visual processing in normals and neurological patients 67 Thomas F. Münte and Hans-Jochen Heinze

Part 3 Imaging studies: functional magnetic resonance imaging and positron emission tomography

4Functional magnetic resonance imaging and positron emission tomography studies of motion perception, eye movements, and reading 93 Mark W. Greenlee

Part 4 Lesion studies in trained monkeys and humans

(transcranial magnetic stimulation)

5Lesions in primate visual cortex leading to deficits of visual perception 121

William H. Merigan and Tatiana Pasternak

6Magnetic stimulation in studies of vision and attention 163 Amanda Ellison, Lauren Stewart, Alan Cowey, and Vincent Walsh

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Part 5 Psychophysics: patient studies

7Failures of visual analysis: scotoma, agnosia, and neglect 179 Manfred Fahle

8Colour vision and its disturbances after cortical lesions 259 C.A. Heywood and A. Cowey

9Unconscious perception: blindsight 283 L. Weiskrantz

10Perception, memory, and agnosia 307 Martha J. Farah

Part 6 Rehabilitation and recovery

11Recovery and rehabilitation of cerebral visual disorders 319 Josef Zihl

Index 339

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Part 1

Physiology and anatomy of the visual system: single cells