A Reader in Visual Agnosia

Glyn Humphreys, Jane Riddoch

• Publisher: Routledge
• Year: 2016
• ISBN:
  9781317361121
• Edition: 1
• Topic: Psychologie
• Subtopic: Histoire et théorie en psychologie

The case study of John has provided a unique insight into the nature of visual agnosia and more broadly into the underlying processes which support human vision. After suffering a stroke, John had problems in recognizing common objects, faces, seeing colours, reading and finding his way around his environment. A Reader in Visual Agnosia brings together the primary scientific papers describing the detailed investigations for each visual problem which the authors carried out with John, known as patient HJA. This work was summarised initially in To See But Not To See (1987), and 26 years later in A Case Study in Visual Agnosia Revisited (2013).

The chapters are divided into 6 parts corresponding to the key areas of investigation:

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• Integrative visual agnosia

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• Perception of global form

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• Face perception

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• Colour perception

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• Word recognition

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• Changes over time

Each part contains a short introduction, written by the two leading researchers who worked with John, which highlights the relations between the papers and demonstrates the pathway of the case analysis. The book will be invaluable to students and researchers in visual cognition, cognitive neuropsychology and vision neuroscience.

Part I

Integrative visual agnosia

In this section we present some of the chapters that resulted in classifying John as having an integrative visual agnosia – that is, a deficit in which he was impaired at integrating parts of objects to form representations that would support accurate object identification. We have argued that this reflected a selective problem in ‘intermediate’ vision – the processes that structure and organise our perceptual world into coherent objects. John’s problems in intermediate perceptual organisation occurred despite his being able to process and even link together local elements themselves. For example, he produced detailed copies of objects that he could not recognise and he could detect simple patterns formed by the local alignment of elements. However, his visual processing broke down when these elements had to be assembled into more complex shapes with multiple parts, when there could be alternative ways in which the elements might be organised.

Chapter 1: Riddoch, M.J. & Humphreys, G.W. (1987) A case of integrative visual agnosia. Brain, 110, 1431–1462.

This is the chapter that provides the evidence defining the disorder of integrative visual agnosia. We demonstrate that John has a deficit in recognising different objects, not just in finding the name for objects. This contrasts with his ability to draw the objects he could not recognise, and also with his stored knowledge about objects. Traditionally this contrast had been taken to indicate that perception was intact but that such patients had a problem in associating their perceptual representations with their stored knowledge. However we show that John has perceptual problems when he has to process visual elements rapidly and when he has to segment apart overlapping images. We argue that the perceptual problems disrupt the integration of parts into object representations, particularly when alternative organisations of the parts are possible (e.g., when shapes are overlapping). The ability to copy objects should not be taken as the defining index of normal perception.

Chapter 2: Giersch, A., Humphreys, G.W., Boucart, M. & Kovács, I. (2000) The computation of occluded contours in visual agnosia: Evidence for early computation prior to shape binding and figure-ground coding. Cognitive Neuropsychology, 17, 731–759.

Chapter 3: Giersch, A., Humphreys, G.W., Barthaud, J.C. & Landmann, C. (2006) A two-stage account of computing and binding occluded and visible contours: Evidence from visual agnosia and effects of lorazepam. Cognitive Neuropsychology, 23, 261–277.

These two chapters investigate in more detail John’s ability to integrate local elements into object representations. In Chapter 2 (Giersch et al., 2000) we report that John performed at a normal level when he had to detect elements that were aligned, presented against a background of randomly oriented elements. Thus a process of local integration of shape elements was preserved. He was also shown to be able to compute elements that were occluded. However, there was a major impairment when he had to integrate elements and organise them into complex, multi-part objects. The results indicate that a first, local stage of integration can be separated from subsequent processes of organising grouped elements into parts of objects. This second stage of integration was impaired in John’s case.

In Chapter 3 (Giersch et al., 2006) we used a task in which participants had to respond to an occluded line and we assessed whether judgements of that line were affected by it being grouped with the non-occluded external edges that signalled the occluded line in the first place. We show that, while healthy observers were affected by grouping between the occluded and non-occluded edges, John was not. Healthy observers showed a similar pattern to John when the stimuli were briefly presented and masked. These results indicate again that there was coding of the relations between line elements (to compute the occluded edge), but a problem in then grouping the edges. The first but not the second stage could be computed in normal participants with brief stimulus exposures. In John this was a chronic problem.

Chapter 4: Lestou, V., Lam, J., Humphreys, K.L., Kourtzi, Z. & Humphreys, G.W. (2014) A dorsal visual route necessary for global form perception: Evidence from neuropsychological fMRI. Journal of Cognitive Neuroscience, 26, 621–634.

The neural basis of grouping simple visual elements was reported in this article. Participants were presented with ‘Glass’ figures in which a global shape can emerge from grouping between simple dot elements. John showed normal behavioural and neural responses to these figures, in relatively dorsal brain regions (e.g., the intra-parietal sulcus) that were unaffected by his brain lesion. The data indicate that the grouping of simple elements can take place outside the intermediate visual regions damaged by John’s stroke, that this process operates relatively normally in his case.

Chapter 5: Humphreys, G.W., Riddoch, M.J., Quinlan, P.T., Price, C.J. & Donnelly, N. (1992) Parallel pattern processing and visual agnosia. Canadian Journal of Psychology, 46, 377–416.

There has been a long tradition within experimental psychology of using visual search to study how we process complex visual displays. When the target that you are looking for has multiple elements that are shared with distractor stimuli, search is slow and typically affected by the number of stimuli present – there is serial processing of each object to assess if it is the target. However, if the target has very different features to the distractors then it can ‘pop out’, and search is the little affected by the number of distractors present. In other cases the multi-element distractors may share some common property which enables them to be grouped and rejected together, and search can then be efficient even if targets and distractors have features in common. Search efficiency can give an index of whether elements group. In Chapter 5 we report the results from visual search experiments with John and control participants under these different display conditions. John showed efficient search when the features of targets and distractors differed. He showed a normal pattern of serial search (with a normal search rate) when targets and distractors shared features but distractors did not group. However he showed impaired search when multi-element targets and distractors shared features and the distractors grouped – he selectively failed to benefit from grouping together the multiple elements. The results highlight a breakdown in John’s ability to group complex, multi-element forms, which stands in contrast to his ability to group simple local elements, shown in Chapters 2–4.

1

A Case of Integrative Visual Agnosia

M. Jane Riddoch and Glyn W. Humphreys

Summary

A single case study of a patient with visual agnosia is presented. The patient had a marked impairment in visual object recognition along with good tactile object identification and a preserved ability to copy. Detailed investigations demonstrated impaired perceptual processes, with the patient’s identification strongly affected by duration of stimulus exposure and by using overlapping figures. However, his stored knowledge of objects was shown to be intact. The results demonstrate that agnosia may be determined by a specific deficit in integrating form information; and that the input description for visual object recognition, disrupted in this patient, is functionally separate from stored object descriptions, which are intact. The implications of the results for understanding visual agnosia and for theories of normal visual object recognition are discussed.

Introduction

Visual agnosia is a severe modality-specific deficit in the recognition of visually presented objects. It is a recognition, rather than a naming deficit, since visual agnosic patients are unable to gesture the use or to show any recognition of the objects they fail to name. Typically, discussions of visual agnosia centre around Lissauer’s (1890) distinction between apperceptive and associative agnosia. Lissauer argued that visual object recognition was composed of two primary independent stages; apperception, the process of constructing a perceptual representation from vision; and association, the process of mapping a perceptual representation onto stored knowledge of the object’s functions and associations. Lissauer proposed that, following brain damage, patients may be impaired in either the apperception or the association process, with both impairments giving rise to a deficit in visual object recognition.

There have been several cases documented where visual object recognition deficit...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Table of Contents
• Acknowledgements
• Introduction
• Part I Integrative visual agnosia
• Part II Seeing the whole
• Part III What’s in a face?
• Part IV Colour, movement, action!
• Part V The written word
• Part VI Living with agnosia
• Plates