Measuring laterality effects in dichotic listening Bruno H. Repp Haskins Laboratories, 270 Crown Street, New Haven, Connecticut 06511
(Received 3 March 1977; revised5 May 1977)
This paper discusses methodologicalissuesand problemsrelated to measuringlaterality effectsin dichotic listening.SectionI describesthe standarddichotic two-responseparadigm,as well as a number of indices of the degreeof ear advantageproposedin the literature. The numericalrange of most of theseindicesis constrainedby performancelevel; only one particular index avoids theseconstraints.However, this does not make this index necessarilythe optimal one. First, a correction for guessingis proposed.Then, analogiesto signal-detectiontheory are discussed,as well as theoretical and empirical criteria for
choosing the correctindexof laterality.While this indexis still unknown,the indexcalledhere egis proposedas the best solutionat the presentstate of knowledge.SectionII discusses the phenomenonof dichotic fusion and the dichotic single-response paradigm, which offers many methodologicaladvantages over the two-responseparadigm. Section III discussesthe factors of ear dominance and stimulus dominancein the perceptionof fused stimuli. An index of ear dominanceis derived by again taking advantage of analogiesto signal-detectiontheory. In Sec. IV, a number of remaining problems are discussed:stimulus intelligibility, guessingand selectiveattention, blend responses,test reliability, validity, and homogeneity. PACS numbers:43.70.Dn, 43.66.Rq, 43.66.Yw
INTRODUCTION
Since Kimura's (1961) demonstration of an average
right-ear advantage(REA) in the recognitionof dichotic verbal stimuli, many researchers have used dichotic listening tasks to measure hemispheric dominancefor
language. Kimura's hypothesisthat hemispheric dominance for-language is the major factor underlying the ear asymmetries has been almost universally accepted. While some studies have been content with diagnosing the
mere direction of the average ear advantage (left or right) and testing its significance, many recent studies have attempted to compare different individuals, different tests, or different experimental conditions with respect to the observed magnitude of the ear asymmetry. Underlying these attempts has been the belief that cerebal lateralization, like handedness, is a matter of degree and can be measured on a continuous scale (Zangwill, 1960; Shankweiler and Studdert-Kennedy, •07•).
In order to yield meaningfulandreliable measurements, dichotie testing must meet certain formal and methodological requirements which have been given rela-
gation or have not been dealt with at all in the past. Although the discussion is restricted to dichotic listening, many of the issues should also apply to other situations
in which lateral asymmetries are measured (e.g., visual hemifield differences,
binocular rivalry,
or ocu-
lar dominance), and therefore may be of interest to a wider
audience.
One focus of the present discussion is the problem of choosing a numerical index of the ear advantage. This problem is fundamental to the measurement of lateralization; unless it is solved, no meaningful comparisons between subjects, tests, or experimental conditions are possible. Section I--which heavily relies on earlier
discussionsby Halwes (1969) and Marshall, Caplan, and Holmes (1975)--discusses a number of indices that have been proposed and used in the past in conjunctionwith
the dichotic two-response paradigm (which requires the listener to identify both stimuli in a dichotic pair). All but one of these indices
fail
to take into account
the con-
straints imposed by performance level on the range of differences
between
the scores
for the two ears.
In ad-
dition, none of them corrects for guessing, despite the
used.
fact that most dichotic studies use only a few different stimuli, which results in substantial guessing probabilities. An index is described that takes both performance level and guessing into account. However, the correct index must be based on a correct theory and empirical evidence of how scores for the two ears change with performance level and how guessing operates, and this theoretical and empirical basis is not available at present. The index proposed here is merely based on plausible assumptions; the question whether it is the correct index remains open.
The present paper summarizes the issues that must be dealt with in constructing an effective dichotic test for the purpose of measuring hemispheric dominance. The dichotic listening situation is remarkably complex. In the discussion that follows, I provide some suggestions but point out many problems that need further investi-
The second focus of the present paper is finding ways of simplifying dichotic testing and circumventing some of the problems encountered in the standard two-response paradigm. Sections II and III describe an approach to dichotic listening that in many ways seems simpler than the two-response paradigm. This method,
tively little attention in the past. If dichotic listening tasks are used as instruments to measure the degree of hemispheric dominance for language, they must satisfy the same high standards of construction, procedure, and scoring as any other psychological test. These standards may be derived from methodologieally oriented research in the laboratory, from theoretical analyses of the task situation, and from general test-theoretical
principles. Many of these requirements are not sufficiently met by dichotie tests as they are now widely
720
J. Acoust.Soc. Am., Vol. 62, No. 3, September1977
Copyright¸ 1977 by the AcousticalSocietyof America
720
Redistribution subject to ASA license or copyright; see http://acousticalsociety.org/content/terms. Download to IP: 131.111.164.128 On: Fri, 19 Dec 2014 07:03:53
721
B.H. Repp: Measuringlateralityeffects
721
which requires only a single response to each dichotic stimulus, relies on the phenomenon of dichotic (or
binaural) fusion. Section II briefly discusses the factors that make
two dichotic
stimuli
fuse
more
or less
com-
pletely into a single perceived stimulus, as well as the methodological consequences of such fusion. Section III derives an index of the ear advantage for the single-response paradigm. Here, the phenomenon of stimulus dominance (perceptual dominance of one stimulus over the other in a fused dichotic pair) exerts constraints on the ear score difference similar to that exerted by performance level in the two-response paradigm. However, these constraints can be dealt with and actually become a crucial factor in deriving an unbiased index of the ear advantage. Section IV is devoted to a survey of additional topics and problems in dichotic testing: stimulus intelligibility, selective attention, blend responses, test reliability, homogeneity, and validity. Since the. concern of this paper is exclusively methodological, any discussion of the physiological factors that may underly dichotic ear advantages is avoided. The aim is to develop methods for measuring the dichotic ear advantage with maximum precision. Before we can attempt to answer the more fundamental questions about the structures and processes underlying the ear asymmetry, we must be able to obtain
valid
and reliable
measurements
from
dichotic
tests. There is much room for improvement in existing methods with respect to this goal. I.
LATERALITY
INDICES
FOR THE TWO-RESPONSE
POC'
•'øJ(c)/• • o.51- / • •
POE' ß
1.o
Method
In the two-response paradigm, two different stimuli are presented simultaneously to the two ears, and the subject is asked to identify both, typically without any constraint on the order of report. The two responses must be different from each other, and guessing is en-
o
0.5
i.o
Performance Level(Po) FIG. 1. The numerical ranges of four indices of the ear advantage as a function of performance level.
!
P0= •(P•+ PL) ß
(1)
B. The simple difference score (d) The simplest index of the ear advantage is the difference between the proportions of correct responses for the two ears,
d=P•-PL
.
(2)
The vast majority of dichotic listening studies have reported the ear advantage as d. (There is no commonly accepted name of the index; it is called d here simply for notational convenience.) Although useful as a descriptive statistic,
PARADIGM
A.
•
d has severe limitations when the
results of different subjects, different tests, or different experimental conditions are to be compared. These limitations arise from the constraint imposed on differences between proportions by their absolute size-a fact that is often neglected and therefore constitutes one of the primary fallacies of descriptive statistics. In the context of measuring laterality effects, Halwes
couraged.
This is the standard situation that will be
(1969)was the first to point out that the overall perfor-
considered
in this
mance level P0 sets an upper limit to d,
section.
The results of a standard two-response test may be summarized in a 2)
(Received 3 March 1977; revised5 May 1977)
This paper discusses methodologicalissuesand problemsrelated to measuringlaterality effectsin dichotic listening.SectionI describesthe standarddichotic two-responseparadigm,as well as a number of indices of the degreeof ear advantageproposedin the literature. The numericalrange of most of theseindicesis constrainedby performancelevel; only one particular index avoids theseconstraints.However, this does not make this index necessarilythe optimal one. First, a correction for guessingis proposed.Then, analogiesto signal-detectiontheory are discussed,as well as theoretical and empirical criteria for
choosing the correctindexof laterality.While this indexis still unknown,the indexcalledhere egis proposedas the best solutionat the presentstate of knowledge.SectionII discusses the phenomenonof dichotic fusion and the dichotic single-response paradigm, which offers many methodologicaladvantages over the two-responseparadigm. Section III discussesthe factors of ear dominance and stimulus dominancein the perceptionof fused stimuli. An index of ear dominanceis derived by again taking advantage of analogiesto signal-detectiontheory. In Sec. IV, a number of remaining problems are discussed:stimulus intelligibility, guessingand selectiveattention, blend responses,test reliability, validity, and homogeneity. PACS numbers:43.70.Dn, 43.66.Rq, 43.66.Yw
INTRODUCTION
Since Kimura's (1961) demonstration of an average
right-ear advantage(REA) in the recognitionof dichotic verbal stimuli, many researchers have used dichotic listening tasks to measure hemispheric dominancefor
language. Kimura's hypothesisthat hemispheric dominance for-language is the major factor underlying the ear asymmetries has been almost universally accepted. While some studies have been content with diagnosing the
mere direction of the average ear advantage (left or right) and testing its significance, many recent studies have attempted to compare different individuals, different tests, or different experimental conditions with respect to the observed magnitude of the ear asymmetry. Underlying these attempts has been the belief that cerebal lateralization, like handedness, is a matter of degree and can be measured on a continuous scale (Zangwill, 1960; Shankweiler and Studdert-Kennedy, •07•).
In order to yield meaningfulandreliable measurements, dichotie testing must meet certain formal and methodological requirements which have been given rela-
gation or have not been dealt with at all in the past. Although the discussion is restricted to dichotic listening, many of the issues should also apply to other situations
in which lateral asymmetries are measured (e.g., visual hemifield differences,
binocular rivalry,
or ocu-
lar dominance), and therefore may be of interest to a wider
audience.
One focus of the present discussion is the problem of choosing a numerical index of the ear advantage. This problem is fundamental to the measurement of lateralization; unless it is solved, no meaningful comparisons between subjects, tests, or experimental conditions are possible. Section I--which heavily relies on earlier
discussionsby Halwes (1969) and Marshall, Caplan, and Holmes (1975)--discusses a number of indices that have been proposed and used in the past in conjunctionwith
the dichotic two-response paradigm (which requires the listener to identify both stimuli in a dichotic pair). All but one of these indices
fail
to take into account
the con-
straints imposed by performance level on the range of differences
between
the scores
for the two ears.
In ad-
dition, none of them corrects for guessing, despite the
used.
fact that most dichotic studies use only a few different stimuli, which results in substantial guessing probabilities. An index is described that takes both performance level and guessing into account. However, the correct index must be based on a correct theory and empirical evidence of how scores for the two ears change with performance level and how guessing operates, and this theoretical and empirical basis is not available at present. The index proposed here is merely based on plausible assumptions; the question whether it is the correct index remains open.
The present paper summarizes the issues that must be dealt with in constructing an effective dichotic test for the purpose of measuring hemispheric dominance. The dichotic listening situation is remarkably complex. In the discussion that follows, I provide some suggestions but point out many problems that need further investi-
The second focus of the present paper is finding ways of simplifying dichotic testing and circumventing some of the problems encountered in the standard two-response paradigm. Sections II and III describe an approach to dichotic listening that in many ways seems simpler than the two-response paradigm. This method,
tively little attention in the past. If dichotic listening tasks are used as instruments to measure the degree of hemispheric dominance for language, they must satisfy the same high standards of construction, procedure, and scoring as any other psychological test. These standards may be derived from methodologieally oriented research in the laboratory, from theoretical analyses of the task situation, and from general test-theoretical
principles. Many of these requirements are not sufficiently met by dichotie tests as they are now widely
720
J. Acoust.Soc. Am., Vol. 62, No. 3, September1977
Copyright¸ 1977 by the AcousticalSocietyof America
720
Redistribution subject to ASA license or copyright; see http://acousticalsociety.org/content/terms. Download to IP: 131.111.164.128 On: Fri, 19 Dec 2014 07:03:53
721
B.H. Repp: Measuringlateralityeffects
721
which requires only a single response to each dichotic stimulus, relies on the phenomenon of dichotic (or
binaural) fusion. Section II briefly discusses the factors that make
two dichotic
stimuli
fuse
more
or less
com-
pletely into a single perceived stimulus, as well as the methodological consequences of such fusion. Section III derives an index of the ear advantage for the single-response paradigm. Here, the phenomenon of stimulus dominance (perceptual dominance of one stimulus over the other in a fused dichotic pair) exerts constraints on the ear score difference similar to that exerted by performance level in the two-response paradigm. However, these constraints can be dealt with and actually become a crucial factor in deriving an unbiased index of the ear advantage. Section IV is devoted to a survey of additional topics and problems in dichotic testing: stimulus intelligibility, selective attention, blend responses, test reliability, homogeneity, and validity. Since the. concern of this paper is exclusively methodological, any discussion of the physiological factors that may underly dichotic ear advantages is avoided. The aim is to develop methods for measuring the dichotic ear advantage with maximum precision. Before we can attempt to answer the more fundamental questions about the structures and processes underlying the ear asymmetry, we must be able to obtain
valid
and reliable
measurements
from
dichotic
tests. There is much room for improvement in existing methods with respect to this goal. I.
LATERALITY
INDICES
FOR THE TWO-RESPONSE
POC'
•'øJ(c)/• • o.51- / • •
POE' ß
1.o
Method
In the two-response paradigm, two different stimuli are presented simultaneously to the two ears, and the subject is asked to identify both, typically without any constraint on the order of report. The two responses must be different from each other, and guessing is en-
o
0.5
i.o
Performance Level(Po) FIG. 1. The numerical ranges of four indices of the ear advantage as a function of performance level.
!
P0= •(P•+ PL) ß
(1)
B. The simple difference score (d) The simplest index of the ear advantage is the difference between the proportions of correct responses for the two ears,
d=P•-PL
.
(2)
The vast majority of dichotic listening studies have reported the ear advantage as d. (There is no commonly accepted name of the index; it is called d here simply for notational convenience.) Although useful as a descriptive statistic,
PARADIGM
A.
•
d has severe limitations when the
results of different subjects, different tests, or different experimental conditions are to be compared. These limitations arise from the constraint imposed on differences between proportions by their absolute size-a fact that is often neglected and therefore constitutes one of the primary fallacies of descriptive statistics. In the context of measuring laterality effects, Halwes
couraged.
This is the standard situation that will be
(1969)was the first to point out that the overall perfor-
considered
in this
mance level P0 sets an upper limit to d,
section.
The results of a standard two-response test may be summarized in a 2)
