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Blindsight. A case study and implications
Neuropsycholog,a. Vol. 26. No. 5, pp. 783-786. 1988 PergamonPress plc. Prmted 10 Great Britam.
BOOK REVIEWS Blindsight. A Case Study and Implications. Oxford, 1986.
L. WEISKKANTZ. Oxford
Psychology
Series, No. 12, Clarendon
Press,
BLINDSIGHT is the story of a discovery which has renewed our conceptions on the mechanisms of perception and awareness. This is what makes this monograph a valuable and exemplary scientific document. The study of residual visual capacities in man following lesions of visual cortical areas deeps its roots in the now classical anatomical and physiological studies of visual function in destriated monkeys, to which L. Weiskrantz and his group have themselves significantly contributed. These studies (reviewed in Part 1, Ch. 1) have revealed that residual visual function shown by these animals is not as rudimentary as Marquis and Hilgard and Kluver had first suggested. Indeed, they are able to detect density ofcontours, line orientation, motion and spatial location of visual objects. These findings soon called into question the conventional opinion that pathological destruction of visual cortex in man produces total blindness, except for sluggish pupillar responses to light and detection of sudden changes in illumination. Clinical observations suggestive of“relative” blindness within scotomata ofcortical origin had already been mentioned by many authors. However, systematic experimental evidence for preserved visual abilities within such scotomata was first reported by Poeppel er al. in 1973 121. These authors owed their success to the introduction in human testing of a new methodological approach derived from monkey studies. In routine clinical examination, visual functions are assessed by asking the subject whether or not he or she can “see” the stimulus. Patients suffering from a lesion of part of their visual cortex do not report seeing targets within the corresponding area of their visual field, i.e., they lack the common subjective experience of “seeing”. By contrast, in monkeys whose subjective experience is not accessible to the experimenter through language, visual abilities are tested in a different way. Animals are trained to reach for targets or to indicate manually which oftwo patterns they have selected. Poeppel et al. suspected that such behavioural responses could also be useful for testing visual capacities in man. Indeed, their cortically lesioned subjects, who would not see the stimuli that were presented within their scotomata, were able to reach them by turning their eyes. Because the subjects remained unaware of the stimuli, they subjectively experienced “guessing” rather than “seeing”. In other words, demonstration of residual vision could be obtained when subjects were forced to abandon their usual, perceptual, mode of visual detection, and to use an alternative mode based on a visuomotor response. When Weiskrantz and his colleagues first examined their patient DB (operated in 1973 for a right occipital angioma), and were able to replicate the Poeppel et a/.‘~ finding, they became aware of the potential importance of this new paradigm. DB was submitted to extensive testing over more than 10 yr, with a variety of procedures. Much of this work, which had remained unpublished, is now made fully available. The set of results reported in the book (Part 2, Chapters 3 to 17) includes data obtained with DB as a subject in two main experimental situations, the reaching situation (derived from Poeppel et al.), and the detection situation. The latter was probably the most productive, and was used for probing visual acuity, movement thresholds, discrimination of orientation, form perception, and for plotting the optic disc within the scotoma. The detection method was systematically based on forced choice responses. The patient had to choose between “presence” and “absence” of a stimulus, “lines” and “no lines” on a grating, movement and no movement of a spot of light, “up and down” and “across” for vertical or horizontal bars, “X” and “O”, “X” and “triangle”, “curved” and “straight” outlines, “square” and “diamond”. etc, for simple shapes. The outcome of this remarkable study is that DB’s vision cannot be compared to a degraded normal vision, nor to peripheral vision: it is a qualitatively different capacity dominated by a double dissociation between impaired form recognition and relatively well preserved ability to detect. locate and discriminate. Chapter 18 is devoted to comparison with other cases reported in the literature since 1975. A fascinating aspect of blindsight is that Weiskrantr’s observations were soon replicated in several different laboratories in patients with lesions of different age. origin and extent. Indeed, the fact that DB is not unique and that blindsight is a replicable effect with identifiable characteristics opens a new area of research in the neural mechanisms of visual perception. This point is discussed in the first part of the last Chapter (Ch. 19, Status, Issues and Implications). Specifically. the presence of residual vision in cortically blind fields raises critical issues concerning the function and co-operation of the different cortical and subcortical visual pathways. Blindsight was first demonstrated in patients, like DB, with lesions hmited to the striate area, which led to the logical conclusion that residual vision could be mediated by other cortical pathways. Yet. it was later found. with similar characteristics, in patients with complete hemidecortication (I ). which invalidates this conclusion. Interestingly in this case (as it is often true in Neuropsychology). human findIngs raise new questions where all the relevant evidence is necessarily derived from animal research.
783
7x4
Boon R~VIFWS
“The complete positivistic sceptic can lcave us at this stage”. This warning (p. 163) addresses those who are not convinced that phenomenal experience is a legitimate subject for scientific enquiry. What makes blindsight so interesting is precisely that it is a “visual capacity in a field defect in the absence of acknowledged awareness” (p. 166). Although “a large amount of bodily processing, including in sensory channels, proceeds quite detached from any awareness” , “awareness of an event is a form of privileged access that allows further perceptual and cogntive manipulations to occur” (pp. 16X 169). In this context, blindsight acquires a broader status. as a paradigm for understanding the biological value of conscious awareness and for investigating this striking phenomenon of dissociation between performance and awareness. This book makes very enjoyable reading. The style is entertaining. Presentation of the data ia original in that it departs from the usual austere psychological format. Quantilied data are interweaved with other (no less important) aspects of the study: informal attempts, failures, anecdotes and verbal reports from the patient are also included. which gives to the whole hook a nice clinical touch. I recommend Blindsigh/ not only to those interested in the study ofthc mechanisms ofvisual perception. hut also to those interested in the more general issues ofcognitivc processing and modular organization of mental functions. I. PFXENIPZ.M. T. and JFANNEKOI), M. Visual function within the hemianopic field following early cerebral hcmidccortication in man. I. Spatial localization. Nrurops!,~holoyia 16, I 13. 1978. 2. POI:PPEL, E., Hrt I), R. and FKOST. D. Residual visual function after brain wounds involving the central visual pathways in man. :V~lurr,, I,ond. 243, 295 296, 1973. M. JLANNLKOI)
Neuropsychological
Rehabilitation after Brain Injury. Edited by G. P. PKIGATANO. 1986. ISBN O-8018-2644-6
THIS H(X)K clearly and concisely presents the theoretical orientation to ncuropsychological rehabilitation of the Presbyterian Hospital Neuropsychological Group in Oklahoma City under the direction of Georges Prigatano and some results of their clinical research. In the first three chapters the authors brielly review the cognitive disorders. the personality changes, and the psycho-social difliculties of patients after brain Injury. These chapters contain little new information for the clinical neuropsychologist. but the passages on personality and affective disorders are interesting in their own right because of the extent of the data presented and the many insights of the authors. The second part presents the theoretical orientation of Prigatano’s group to cognitive retrainmg (Chapter 4) and psychotherapy (Chapter 5). The theoretical elaboration of cognitive retraining is hriefand general but, the chapter devoted to psychotherapy is the best of the whole book even if the authors adhere to an eclectic and pragmatic thcorctical position: “The point is that it dots not matter what model IS used (for psychotherapy) to help a patlent understand his or her difficulties in coping with a brain injury. provided that model makea sense to the therapist and the patient. given the circumstances of their li\cs and treatment” (p. 77). The interest of this chapter lies thus not in its theoretical originality hut in the considerable amount and diversity of the clinical data prebented. ‘md in the attempt the authors make to take into account ail the variables that inRuence the affcctlve reactions of the patients and their social readjustment. The book ends with three chapters on the rehabilitation program of Prigatano’s team and the results (Chapter 7) ohtamed in a pilot study comparing patients who rccci\ed classic but not detailed therapy with patients who received Prigatano’s rehabilitation program The results are in favor of the systematic therapy. hut. given the multidimensional approach adopted (cognitlbe at different lebels and lndibidual group psgchothcrapy). it 1s virtually impossible to identify the caubal factors responsible for improbcment and thus to ,justify the thcrapq on :I theoretical Icvcl. Smce most of the chapters ha\,c aIrcad> been published as chapters in other hoohs or as articles. the main Interest Hospital of this book is that it brings togcthcr some of the dispersed writings of the Presbyterian Neuropsychological Group. X.4VIf.K SkKOh
Imprinting and Cortical Plasticity. Comparatiw Aspects of Sensitive Periods. Edited b> J. P. RAI,s(.HtcKf P. NAIXK. Wiley Series m Neuroscience. 19X7, 377 paces. f57.50.
K and
THFKF AKI fashions in dress and perhaps 111sc~cnce too. Or at Icast so it occurred to mc when I plcked up”Imprintmg and Cortical Plasticity”. Cortical plasticity as a recurrent theme is \er! much associated 111m> mmd with the 1970‘s and has connotations of hell bottom trouser\ and heroic attempts to unloch developmental secrets of that most by means of sclectibe deprilntion! The title ih certaInI> evocative. cortical evolved piece of neural machlncrq
BOOK REVIEWS Blindsight. A Case Study and Implications. Oxford, 1986.
L. WEISKKANTZ. Oxford
Psychology
Series, No. 12, Clarendon
Press,
BLINDSIGHT is the story of a discovery which has renewed our conceptions on the mechanisms of perception and awareness. This is what makes this monograph a valuable and exemplary scientific document. The study of residual visual capacities in man following lesions of visual cortical areas deeps its roots in the now classical anatomical and physiological studies of visual function in destriated monkeys, to which L. Weiskrantz and his group have themselves significantly contributed. These studies (reviewed in Part 1, Ch. 1) have revealed that residual visual function shown by these animals is not as rudimentary as Marquis and Hilgard and Kluver had first suggested. Indeed, they are able to detect density ofcontours, line orientation, motion and spatial location of visual objects. These findings soon called into question the conventional opinion that pathological destruction of visual cortex in man produces total blindness, except for sluggish pupillar responses to light and detection of sudden changes in illumination. Clinical observations suggestive of“relative” blindness within scotomata ofcortical origin had already been mentioned by many authors. However, systematic experimental evidence for preserved visual abilities within such scotomata was first reported by Poeppel er al. in 1973 121. These authors owed their success to the introduction in human testing of a new methodological approach derived from monkey studies. In routine clinical examination, visual functions are assessed by asking the subject whether or not he or she can “see” the stimulus. Patients suffering from a lesion of part of their visual cortex do not report seeing targets within the corresponding area of their visual field, i.e., they lack the common subjective experience of “seeing”. By contrast, in monkeys whose subjective experience is not accessible to the experimenter through language, visual abilities are tested in a different way. Animals are trained to reach for targets or to indicate manually which oftwo patterns they have selected. Poeppel et al. suspected that such behavioural responses could also be useful for testing visual capacities in man. Indeed, their cortically lesioned subjects, who would not see the stimuli that were presented within their scotomata, were able to reach them by turning their eyes. Because the subjects remained unaware of the stimuli, they subjectively experienced “guessing” rather than “seeing”. In other words, demonstration of residual vision could be obtained when subjects were forced to abandon their usual, perceptual, mode of visual detection, and to use an alternative mode based on a visuomotor response. When Weiskrantz and his colleagues first examined their patient DB (operated in 1973 for a right occipital angioma), and were able to replicate the Poeppel et a/.‘~ finding, they became aware of the potential importance of this new paradigm. DB was submitted to extensive testing over more than 10 yr, with a variety of procedures. Much of this work, which had remained unpublished, is now made fully available. The set of results reported in the book (Part 2, Chapters 3 to 17) includes data obtained with DB as a subject in two main experimental situations, the reaching situation (derived from Poeppel et al.), and the detection situation. The latter was probably the most productive, and was used for probing visual acuity, movement thresholds, discrimination of orientation, form perception, and for plotting the optic disc within the scotoma. The detection method was systematically based on forced choice responses. The patient had to choose between “presence” and “absence” of a stimulus, “lines” and “no lines” on a grating, movement and no movement of a spot of light, “up and down” and “across” for vertical or horizontal bars, “X” and “O”, “X” and “triangle”, “curved” and “straight” outlines, “square” and “diamond”. etc, for simple shapes. The outcome of this remarkable study is that DB’s vision cannot be compared to a degraded normal vision, nor to peripheral vision: it is a qualitatively different capacity dominated by a double dissociation between impaired form recognition and relatively well preserved ability to detect. locate and discriminate. Chapter 18 is devoted to comparison with other cases reported in the literature since 1975. A fascinating aspect of blindsight is that Weiskrantr’s observations were soon replicated in several different laboratories in patients with lesions of different age. origin and extent. Indeed, the fact that DB is not unique and that blindsight is a replicable effect with identifiable characteristics opens a new area of research in the neural mechanisms of visual perception. This point is discussed in the first part of the last Chapter (Ch. 19, Status, Issues and Implications). Specifically. the presence of residual vision in cortically blind fields raises critical issues concerning the function and co-operation of the different cortical and subcortical visual pathways. Blindsight was first demonstrated in patients, like DB, with lesions hmited to the striate area, which led to the logical conclusion that residual vision could be mediated by other cortical pathways. Yet. it was later found. with similar characteristics, in patients with complete hemidecortication (I ). which invalidates this conclusion. Interestingly in this case (as it is often true in Neuropsychology). human findIngs raise new questions where all the relevant evidence is necessarily derived from animal research.
783
7x4
Boon R~VIFWS
“The complete positivistic sceptic can lcave us at this stage”. This warning (p. 163) addresses those who are not convinced that phenomenal experience is a legitimate subject for scientific enquiry. What makes blindsight so interesting is precisely that it is a “visual capacity in a field defect in the absence of acknowledged awareness” (p. 166). Although “a large amount of bodily processing, including in sensory channels, proceeds quite detached from any awareness” , “awareness of an event is a form of privileged access that allows further perceptual and cogntive manipulations to occur” (pp. 16X 169). In this context, blindsight acquires a broader status. as a paradigm for understanding the biological value of conscious awareness and for investigating this striking phenomenon of dissociation between performance and awareness. This book makes very enjoyable reading. The style is entertaining. Presentation of the data ia original in that it departs from the usual austere psychological format. Quantilied data are interweaved with other (no less important) aspects of the study: informal attempts, failures, anecdotes and verbal reports from the patient are also included. which gives to the whole hook a nice clinical touch. I recommend Blindsigh/ not only to those interested in the study ofthc mechanisms ofvisual perception. hut also to those interested in the more general issues ofcognitivc processing and modular organization of mental functions. I. PFXENIPZ.M. T. and JFANNEKOI), M. Visual function within the hemianopic field following early cerebral hcmidccortication in man. I. Spatial localization. Nrurops!,~holoyia 16, I 13. 1978. 2. POI:PPEL, E., Hrt I), R. and FKOST. D. Residual visual function after brain wounds involving the central visual pathways in man. :V~lurr,, I,ond. 243, 295 296, 1973. M. JLANNLKOI)
Neuropsychological
Rehabilitation after Brain Injury. Edited by G. P. PKIGATANO. 1986. ISBN O-8018-2644-6
THIS H(X)K clearly and concisely presents the theoretical orientation to ncuropsychological rehabilitation of the Presbyterian Hospital Neuropsychological Group in Oklahoma City under the direction of Georges Prigatano and some results of their clinical research. In the first three chapters the authors brielly review the cognitive disorders. the personality changes, and the psycho-social difliculties of patients after brain Injury. These chapters contain little new information for the clinical neuropsychologist. but the passages on personality and affective disorders are interesting in their own right because of the extent of the data presented and the many insights of the authors. The second part presents the theoretical orientation of Prigatano’s group to cognitive retrainmg (Chapter 4) and psychotherapy (Chapter 5). The theoretical elaboration of cognitive retraining is hriefand general but, the chapter devoted to psychotherapy is the best of the whole book even if the authors adhere to an eclectic and pragmatic thcorctical position: “The point is that it dots not matter what model IS used (for psychotherapy) to help a patlent understand his or her difficulties in coping with a brain injury. provided that model makea sense to the therapist and the patient. given the circumstances of their li\cs and treatment” (p. 77). The interest of this chapter lies thus not in its theoretical originality hut in the considerable amount and diversity of the clinical data prebented. ‘md in the attempt the authors make to take into account ail the variables that inRuence the affcctlve reactions of the patients and their social readjustment. The book ends with three chapters on the rehabilitation program of Prigatano’s team and the results (Chapter 7) ohtamed in a pilot study comparing patients who rccci\ed classic but not detailed therapy with patients who received Prigatano’s rehabilitation program The results are in favor of the systematic therapy. hut. given the multidimensional approach adopted (cognitlbe at different lebels and lndibidual group psgchothcrapy). it 1s virtually impossible to identify the caubal factors responsible for improbcment and thus to ,justify the thcrapq on :I theoretical Icvcl. Smce most of the chapters ha\,c aIrcad> been published as chapters in other hoohs or as articles. the main Interest Hospital of this book is that it brings togcthcr some of the dispersed writings of the Presbyterian Neuropsychological Group. X.4VIf.K SkKOh
Imprinting and Cortical Plasticity. Comparatiw Aspects of Sensitive Periods. Edited b> J. P. RAI,s(.HtcKf P. NAIXK. Wiley Series m Neuroscience. 19X7, 377 paces. f57.50.
K and
THFKF AKI fashions in dress and perhaps 111sc~cnce too. Or at Icast so it occurred to mc when I plcked up”Imprintmg and Cortical Plasticity”. Cortical plasticity as a recurrent theme is \er! much associated 111m> mmd with the 1970‘s and has connotations of hell bottom trouser\ and heroic attempts to unloch developmental secrets of that most by means of sclectibe deprilntion! The title ih certaInI> evocative. cortical evolved piece of neural machlncrq