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Prolegomena to the use of the tachistoscope in neuropsychological research
BRAIN
AND
COGNITION
5,
127-130 (1986)
Prolegomena to the Use of the Tachistoscope in Neuropsychological Research JUSTINESERGENT Montreal
Neurological
Instituie
The lateral tachistoscopic technique of stimulus presentation probably offers one of the most democratic approaches to studying the functional asymmetry of the cerebral hemispheres. It is quite undemanding in terms of equipment and subjects, and the crossed conformation of the visual pathways provides a straightforward means of initially segregating the input to one side or/and the other of the brain. The general validity and reliability of this technique has been suggested by its capacity to produce results in broad agreement with findings from unilaterally brain-damaged patients as well as to discriminate group performance of left- and righthanded subjects who typically ditfer in their respective functional cerebral organization. The relative ease with which tachistoscopic experiments can be implemented, along with the view that knowledge becomes more secure through the convergence of widely differing lines of evidence, have thus contributed to making visual laterality studies an important source of information about functional cerebral asymmetry. The widespread adoption of the lateral tachistoscopic technique in neuropsychological research has not, however, been consistently accompanied by the methodological rigor that one would expect of a scientific endeavor, and democracy, despite its virtues, may swing toward anarchy unless agreed upon rules and conventions are established and followed. Any behavior is the result of a multitude of factors interacting in complex ways, and those interested in understanding the underlying processes leading to a particular behavior, and espeially the respective participation of the cerebral hemispheres in these processes, are faced with the considerable task of disentangling all the factors involved from the initiation of such a behavior to its realization. These factors may not Send requests for reprints to Justine Sergent, Department of Neurology and Neurosurgery, Montreal Neurological Institute, 3801 University Street, Montreal, Quebec H3A 2B4, Canada. 127 0278-2626186$3.00 Copyright 0 1986 by Academic Press. Inc. All tights of reproduction in any form reserved.
128
JUSTINE SERGENT
contribute equally to the resulting behavior, but this cannot be establtshed a priori, and, as noted by Zaidel (1983a), “it is simple, common sense and basic experimental psychology that in order to understand the phenomenon of hemispheric specialization we must first systematically vary each experimental variable” (p. 525). While experimental variables are ultimately trivial and irrelevant to the description of the functions of the cerebral hemispheres, they are inescapable in the experimental process itself and constitute an integral part of the neuropsychological investigation. Unless one uses techniques allowing the isolation of the particular stage in processing at which a given variable exerts its effect (e.g., Hellige, 1980), there is no way of knowing the particular determinants of a pattern of results but methodically to manipulate experimental variables and to identify the nature, the extent, and the locus of their influence on performance as a function of the requirements imposed on the processing organism. Every technique devised for the study of functional cerebral asymmetry in normal subjects brings with it a series of unique characteristics that make its originality as well as its liability. These characteristics are mainly peripheral to the core of the problem under study, in that they do not directly impinge on the very processing structures one seeks to understand. In this sense, any effect that could result from the use of one such technique, and that would be unique to it, could be regarded as a slight nuisance to the understanding of the problem itself. Indeed, experimental factors associated with a particular technique, that is methodological variables, can be viewed as no more than the necessary conditions under which an investigation must be carried out, and, as such, of no critical importance. There is, however, another sense in which methodological variables may prove to be of substantial relevance for the comprehension of cerebral processes. As an interface between a stimulus and the processing organism, the tachistoscopic technique, by the unusual conditions under which the brain is stimulated, carries with it a series of implications for the understanding of performance achieved in such artificial circumstances. Whereas Gibson denied any use of the tachistoscopic approach to the study of visual perception, it is nonetheless the case that the brain reacts to, and processes, tachistoscopicahy presented information, and therefore displays a sample of its competence which lends itself to analysis and inference. Understanding how the brain operates, however, cannot be achieved independently of the conditions under which it has to operate, and the tachistoscopic mode of stimulus presentation imposes on the brain specific constraints to which it adapts in particular ways that may partly depart from its functioning in other, more natural, circumstances. Methodological procedures, in this sense, are transactions with the processing organism, and are, therefore, inherently related to substantive
PROLEGOMENA
129
issues. What may be required, then, is to identify the numerous variables involved in tachistoscopic experiments, especially those unique to this mode of investigation, to specify their particular effects on the processing organism, and to consider the theoretical implications of their observed influence on performance. The present issue is thus concerned with outlining and discussing some of the methodological problems that are inescapable in the course of a lateral tachistoscopic experiment, and it deals with practical and theoretical questions of relevance to the interpretation of visual-field asymmetries. Kitterle presents a review of psychophysical studies aimed at examining the effects of stimulating the retinal periphery, and offers several suggestions regarding the importance of various stimulus parameters and the limitations resulting from projecting information on retinal areas that have not evolved for object perception and pattern recognition. One important issue in the understanding of hemisphere specialization, and particularly in the context of tachistoscopic studies, concerns the spreading of information through the brain and across the hemispheres after unilateral stimulation, and whether an observed laterality pattern is best accounted for by direct-access or callosal-relay models in normal subjects. Zaidel (1983b) has presented a thorough discussion of this issue which will therefore not be repeated here. On the other hand, knowledge of anatomical organization of the visual structures and of their interhemispheric connections may be useful to specify the means by which, and the level at which, exchange of information across the hemispheres can be achieved. Marzi provides a description of these structures and pathways, and suggests a model accounting for visual laterality effects, based on anatomical and physiological evidence. Empirical evidence of the role of input and task factors in visual-field asymmetries is reviewed and critically examined by Hellige and Sergent in two separate papers. In the first one, the influence of the input is considered in terms of spatial-frequency contents determined by stimulus composition and methodological manipulations, and the theoretical underpinning and implications of this approach, along with its strengths and weaknesses, are outlined. The second paper discusses the necessity to sort out the effects of procedural variables from those directly associated with hemisphere specialization, and illustrates how variations in task requirements modify the relative contribution of the cerebral hemispheres to performance. Hardyck draws on his own work and discusses the progression of his research aimed at disclosing the unsuspected influence of some experimental variables in tachistoscopic studies. In doing so, he reminds us that nothing in the implementation of an experiment and in the interpretation of the results can be taken for granted. Corballis will conclude this issue by offering a “meta-methodological” perspective on the previous papers.
130
JUSTINE SERGENT
As this brief outline suggests, the present issue is restricted in its scope and addresses only a small portion of the problems inherent in visual laterality studies, let alone the question of cerebral lateralization of functions. It is probable, and even desirable, that concern with purely methodological issues will progressively dissipate as the nature and implications of our investigative techniques come to be better understood and integrated to our comprehension of brain functioning. In the meantime, and before we are ready to wean ourselves from such ultimately trivial considerations, we are compelled to grasp the essence of our methods and to examine whether bullying the brain into processing lateral tachistoscopic information allows direct inference about normal hemispheric competence. REFERENCES Hellige, J. B. 1980.Effect of perceptual quality and visual fields of probe stimulus presentation on memory search for letters. Journal of Experimental Psychology: Human Perception and Pe$ormance,
6, 639-65 1.
Zaidel, E. 1983a. Advances and retreats in laterality research. The Behavioral
and Brain
Sciences, 6, 523-528.
Zaidel, E. 1983b. Disconnection syndrome as a model for laterality effects in the normal brain. In J. B. Hellige (Ed.), Cerebral Hemisphere Asymmetry: Method, Theory, and Application. New York: Praeger.
AND
COGNITION
5,
127-130 (1986)
Prolegomena to the Use of the Tachistoscope in Neuropsychological Research JUSTINESERGENT Montreal
Neurological
Instituie
The lateral tachistoscopic technique of stimulus presentation probably offers one of the most democratic approaches to studying the functional asymmetry of the cerebral hemispheres. It is quite undemanding in terms of equipment and subjects, and the crossed conformation of the visual pathways provides a straightforward means of initially segregating the input to one side or/and the other of the brain. The general validity and reliability of this technique has been suggested by its capacity to produce results in broad agreement with findings from unilaterally brain-damaged patients as well as to discriminate group performance of left- and righthanded subjects who typically ditfer in their respective functional cerebral organization. The relative ease with which tachistoscopic experiments can be implemented, along with the view that knowledge becomes more secure through the convergence of widely differing lines of evidence, have thus contributed to making visual laterality studies an important source of information about functional cerebral asymmetry. The widespread adoption of the lateral tachistoscopic technique in neuropsychological research has not, however, been consistently accompanied by the methodological rigor that one would expect of a scientific endeavor, and democracy, despite its virtues, may swing toward anarchy unless agreed upon rules and conventions are established and followed. Any behavior is the result of a multitude of factors interacting in complex ways, and those interested in understanding the underlying processes leading to a particular behavior, and espeially the respective participation of the cerebral hemispheres in these processes, are faced with the considerable task of disentangling all the factors involved from the initiation of such a behavior to its realization. These factors may not Send requests for reprints to Justine Sergent, Department of Neurology and Neurosurgery, Montreal Neurological Institute, 3801 University Street, Montreal, Quebec H3A 2B4, Canada. 127 0278-2626186$3.00 Copyright 0 1986 by Academic Press. Inc. All tights of reproduction in any form reserved.
128
JUSTINE SERGENT
contribute equally to the resulting behavior, but this cannot be establtshed a priori, and, as noted by Zaidel (1983a), “it is simple, common sense and basic experimental psychology that in order to understand the phenomenon of hemispheric specialization we must first systematically vary each experimental variable” (p. 525). While experimental variables are ultimately trivial and irrelevant to the description of the functions of the cerebral hemispheres, they are inescapable in the experimental process itself and constitute an integral part of the neuropsychological investigation. Unless one uses techniques allowing the isolation of the particular stage in processing at which a given variable exerts its effect (e.g., Hellige, 1980), there is no way of knowing the particular determinants of a pattern of results but methodically to manipulate experimental variables and to identify the nature, the extent, and the locus of their influence on performance as a function of the requirements imposed on the processing organism. Every technique devised for the study of functional cerebral asymmetry in normal subjects brings with it a series of unique characteristics that make its originality as well as its liability. These characteristics are mainly peripheral to the core of the problem under study, in that they do not directly impinge on the very processing structures one seeks to understand. In this sense, any effect that could result from the use of one such technique, and that would be unique to it, could be regarded as a slight nuisance to the understanding of the problem itself. Indeed, experimental factors associated with a particular technique, that is methodological variables, can be viewed as no more than the necessary conditions under which an investigation must be carried out, and, as such, of no critical importance. There is, however, another sense in which methodological variables may prove to be of substantial relevance for the comprehension of cerebral processes. As an interface between a stimulus and the processing organism, the tachistoscopic technique, by the unusual conditions under which the brain is stimulated, carries with it a series of implications for the understanding of performance achieved in such artificial circumstances. Whereas Gibson denied any use of the tachistoscopic approach to the study of visual perception, it is nonetheless the case that the brain reacts to, and processes, tachistoscopicahy presented information, and therefore displays a sample of its competence which lends itself to analysis and inference. Understanding how the brain operates, however, cannot be achieved independently of the conditions under which it has to operate, and the tachistoscopic mode of stimulus presentation imposes on the brain specific constraints to which it adapts in particular ways that may partly depart from its functioning in other, more natural, circumstances. Methodological procedures, in this sense, are transactions with the processing organism, and are, therefore, inherently related to substantive
PROLEGOMENA
129
issues. What may be required, then, is to identify the numerous variables involved in tachistoscopic experiments, especially those unique to this mode of investigation, to specify their particular effects on the processing organism, and to consider the theoretical implications of their observed influence on performance. The present issue is thus concerned with outlining and discussing some of the methodological problems that are inescapable in the course of a lateral tachistoscopic experiment, and it deals with practical and theoretical questions of relevance to the interpretation of visual-field asymmetries. Kitterle presents a review of psychophysical studies aimed at examining the effects of stimulating the retinal periphery, and offers several suggestions regarding the importance of various stimulus parameters and the limitations resulting from projecting information on retinal areas that have not evolved for object perception and pattern recognition. One important issue in the understanding of hemisphere specialization, and particularly in the context of tachistoscopic studies, concerns the spreading of information through the brain and across the hemispheres after unilateral stimulation, and whether an observed laterality pattern is best accounted for by direct-access or callosal-relay models in normal subjects. Zaidel (1983b) has presented a thorough discussion of this issue which will therefore not be repeated here. On the other hand, knowledge of anatomical organization of the visual structures and of their interhemispheric connections may be useful to specify the means by which, and the level at which, exchange of information across the hemispheres can be achieved. Marzi provides a description of these structures and pathways, and suggests a model accounting for visual laterality effects, based on anatomical and physiological evidence. Empirical evidence of the role of input and task factors in visual-field asymmetries is reviewed and critically examined by Hellige and Sergent in two separate papers. In the first one, the influence of the input is considered in terms of spatial-frequency contents determined by stimulus composition and methodological manipulations, and the theoretical underpinning and implications of this approach, along with its strengths and weaknesses, are outlined. The second paper discusses the necessity to sort out the effects of procedural variables from those directly associated with hemisphere specialization, and illustrates how variations in task requirements modify the relative contribution of the cerebral hemispheres to performance. Hardyck draws on his own work and discusses the progression of his research aimed at disclosing the unsuspected influence of some experimental variables in tachistoscopic studies. In doing so, he reminds us that nothing in the implementation of an experiment and in the interpretation of the results can be taken for granted. Corballis will conclude this issue by offering a “meta-methodological” perspective on the previous papers.
130
JUSTINE SERGENT
As this brief outline suggests, the present issue is restricted in its scope and addresses only a small portion of the problems inherent in visual laterality studies, let alone the question of cerebral lateralization of functions. It is probable, and even desirable, that concern with purely methodological issues will progressively dissipate as the nature and implications of our investigative techniques come to be better understood and integrated to our comprehension of brain functioning. In the meantime, and before we are ready to wean ourselves from such ultimately trivial considerations, we are compelled to grasp the essence of our methods and to examine whether bullying the brain into processing lateral tachistoscopic information allows direct inference about normal hemispheric competence. REFERENCES Hellige, J. B. 1980.Effect of perceptual quality and visual fields of probe stimulus presentation on memory search for letters. Journal of Experimental Psychology: Human Perception and Pe$ormance,
6, 639-65 1.
Zaidel, E. 1983a. Advances and retreats in laterality research. The Behavioral
and Brain
Sciences, 6, 523-528.
Zaidel, E. 1983b. Disconnection syndrome as a model for laterality effects in the normal brain. In J. B. Hellige (Ed.), Cerebral Hemisphere Asymmetry: Method, Theory, and Application. New York: Praeger.