Failure to Control Prepotent Pathways in Early Stage Dementia of the Alzheimer's Type: Evidence From Dichotic Listening

Neuropsychology
Copyright 2005 by the American Psychological Association
2005, Vol. 19, No. 5, 687– 695
0894-4105/05/$12.00
DOI: 10.1037/0894-4105.19.5.687
Failure to Control Prepotent Pathways in Early Stage Dementia of the
Alzheimer’s Type: Evidence From Dichotic Listening
Janet M. Duchek and David A. Balota
Washington University
The authors examined the right ear advantage in a dichotic listening task in healthy aging and very mild
and mild stages of Alzheimer’s disease. Subjects were simultaneously presented 3 pairs of digits to the
left and right ears (e.g., left ear: 4, 3, 1; right ear: 9, 2, 5) for immediate ordered recall. Four lists of triads
were presented, varying in presentation rate between digit pairs within a triad (0.5, 1.0, 1.5, 2.0 s). Results
indicated that the very mild and mild Alzheimer’s groups showed a larger right ear advantage in free
recall compared with the healthy controls, indicating a tendency to respond to the prepotent left
hemisphere pathway for language processing. Also, the right ear advantage and proportion of switches
made during recall were correlated with psychometric measures of frontal lobe function in the mild
Alzheimer’s group but not in the very mild or healthy control groups.
Keywords: Alzheimer’s disease, attention, dichotic listening
Dementia of the Alzheimer’s type (DAT) is characterized by a
reported in dual-task paradigms, with greater costs in dual-task
generalized breakdown in cognitive performance. In particular, in
performance relative to single-task performance in DAT compared
addition to the memory breakdowns, there are de?cits in perfor-
with healthy aging (e.g., Baddeley et al., 2001; Baddeley, Logie,
mance in tasks that demand attentional processing, and these
Bressi, Della Sala, & Spinnler, 1986).
de?cits occur even in the early stages of the disease (Balota &
Balota and Faust (2001) have proposed a generalized break-
Faust, 2001; Parasuraman & Haxby, 1993; Perry & Hodges, 1999).
down in attentional control in early stage DAT. Speci?cally, DAT
Perry and Hodges (1999) have suggested that attention is the ?rst
individuals have dif?culty controlling attention to select the ap-
nonmemory aspect of cognition that declines in DAT prior to any
propriate pathway when confronted with competing information.
de?cits in language or visuospatial abilities, and this declining
For example, in the Stroop study by Spieler, Balota, and Faust
attentional capacity may underlie the dif?culty with daily activities
(1996), individuals with DAT showed a breakdown in the ability
often seen in the early stages of the disease.
to inhibit the word code when naming colors in a Stroop task
In this light, it is critical to fractionate the components of
compared with healthy older adults (also see Balota & Duchek,
attention to better understand the precise nature of this attentional
1991; Faust, Balota, Duchek, Gernsbacher, & Smith, 1997). Thus,
de?cit. The available literature indicates that various subcompo-
there appears to be a de?cit in early stage DAT in the ability to
nents of attention may be differentially affected in DAT. For
control attention in order to select an appropriate processing path-
example, there is some evidence that sustained and focused atten-
way and/or control or inhibit the prepotent pathway (e.g., naming
tion appear to be relatively preserved in early stage DAT (e.g.,
the word rather than the color in the Stroop task). It has been
Baddeley, Baddeley, Bucks, & Wilcock, 2001; Nebes & Brady,
argued that such attentional breakdowns may be related to the
1993). However, de?cits in selective attention in DAT have been
memory de?cits observed in early stage DAT (Balota & Faust,
reported across several tasks, such as Stroop interference (Spieler,
2001; Becker, 1988; Perry & Hodges, 1999) and that attentional
Balota, & Faust, 1996), visual–spatial attention (Greenwood, Para-
selection may be subserved by frontal control systems (Balota &
suraman, & Alexander, 1997; Greenwood, Parasuraman, & Haxby,
Faust, 2001; Dempster, 1992; Shallice, 1982; Stuss & Benson,
1993), visual search (Nebes & Brady, 1989), and negative priming
1986). Finally, it has been suggested that the neuropathology seen
and ?anker paradigms (Faust, Balota, & Duchek, 1995; Sullivan,
in the frontal lobes in early DAT (e.g., J. C. Morris et al., 1996)
Faust, & Balota, 1995). De?cits in divided attention have also been
may be related to such de?cits in attentional control (Balota &
Faust, 2001).
The classic dichotic listening task affords a unique experimental
Janet M. Duchek and David A. Balota, Department of Psychology,
paradigm to address the ability to select a nondominant processing
Washington University.
pathway and control a prepotent processing pathway (e.g., Broad-
This work was supported by National Institute on Aging Grants
bent, 1952; Cherry, 1953; Treisman, 1960). Speci?cally, a right ear
AG10145, AG03991, and AG05681.
advantage in free recall is commonly reported in dichotic listening
We thank the Clinical and Psychometric Cores of the Washington
tasks (e.g., Carter & Wilson, 2001; Strouse, Wilson, & Brush,
University Alzheimer’s Disease Research Center for their diagnostic and
2000a) and has been explained by the superiority of the left
testing assistance, Martha Storandt for providing the psychometric perfor-
hemisphere in processing language. If DAT individuals have dif-
mance, Mark Law for helping develop the stimulus materials, and Keith
?culty controlling the prepotent processing pathway, then they
Hutchison for comments on a previous version of this article.
should have more dif?culty controlling the left hemisphere dom-
Correspondence concerning this article should be addressed to Janet M.
Duchek, Department of Psychology, Washington University, One Brook-
inance for language when presented with competing information to
ings Drive, St. Louis, MO 63130. E-mail: jduchek@artsci.wustl.edu
both auditory channels. Thus, DAT individuals might actually
687

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688
DUCHEK AND BALOTA
show a pattern of an exaggerated right ear advantage in recall
In the present study, healthy older adults, individuals with very
performance and a decreased ability to switch attention and report
mild DAT, and individuals with mild DAT participated in a
from the left ear.
dichotic listening task, similar to the classic Broadbent (1954)
Mohr, Cox, Williams, Chase, and Fedio (1990) directly exam-
study, wherein the presentation rate of the dichotic message was
ined the ability to selectively allocate attention to one channel and
varied. We expected a larger right ear advantage in early stage
switch attention in a cued dichotic listening paradigm in DAT. In
DAT due to the inability to switch attention between channels, and
an ordered recall condition, subjects were given a precue indicat-
we expected performance to be primarily driven by the prepotent
ing which channel (left vs. right) to report ?rst. The results indi-
linguistic channel. The inclusion of two groups that vary in terms
cated that overall accuracy was lower in DAT relative to older
of dementia severity (i.e., very mild vs. mild DAT) allowed us to
controls. More interesting, in the ordered recall condition, DAT
examine the progression of this attentional breakdown across
individuals were unable to selectively direct attention to the pre-
groups at varying levels of the disease process. If individuals in the
cued ear and instead showed consistent right ear preferences in
earliest stages of the disease exhibit an overreliance on the prepo-
recall regardless of the precue. Although de?cits in dichotic lis-
tent linguistic pathway, this could potentially serve as an early
tening performance in early stage DAT also have been reported in
marker for cognitive impairment. Finally, the relationship between
other studies (e.g., Gates et al., 1995; Grady et al., 1989; Grimes,
attentional selection and frontal control systems in the early stages
Grady, Foster, Sunderland, & Patronas, 1985), these studies did
of DAT was explored by correlating various psychometric and
not directly address the right ear advantage and attentional control
dichotic listening measures.
changes.
On the basis of the Mohr et al. (1990) study, it appears that the
Method
ability to selectively allocate attention in a dichotic listening task
may be impaired in DAT. DAT subjects were unable to use a
Subjects
precue to direct attention to the appropriate channel and instead
A total of 94 subjects (55 men, 39 women) were recruited from the
performance was driven by the stronger tendency to report from
Washington University Alzheimer’s Disease Research Center for this
the right ear. Of course, one alternative possibility is that the DAT
study. All subjects were originally screened for depression, hypertension,
subjects simply did not process the precue in the Mohr et al. study
reversible dementias, and other disorders that could potentially produce
to the same extent as the control individuals, and thus the right ear
cognitive impairment. All subjects were reevaluated every 6 months for
advantage was not necessarily indicative of an inability to selec-
changes in cognitive status and medical comorbidities that may affect
tively allocate attention. The present study will expand on the
cognition. Thus, subjects were considered to be free of medical comor-
Mohr et al. ?ndings by directly testing whether DAT individuals
bidities that could affect cognition at the time of testing. The inclusionary
can control the tendency to report from the prepotent pathway (i.e.,
and exclusionary criteria for DAT are consistent with the National Institute
of Neurological and Communicative Diseases and Stroke–Alzheimer’s
right ear) when instructed to recall the order of presentation of the
Disease and Related Disorders Association criteria (McKhann et al., 1984).
items during recall and hence switch attention between the ears.
The severity of dementia was staged according to the Washington Univer-
This study also will examine the time course for switching atten-
sity Clinical Dementia Rating (CDR) Scale (Berg, 1988; Hughes, Berg,
tion by varying the presentation rate of the dichotic message. It is
Danziger, Coben, & Martin, 1982; J. C. Morris, 1993). According to this
possible that the right ear advantage in DAT may be attenuated
scale, scores of 0, 0.5, 1, 2, and 3 represent no dementia, very mild
when the dichotic message is presented at a slower rate and
dementia, mild dementia, moderate dementia, and severe dementia, respec-
subjects have more time to switch attention between the ears.
tively. The CDR is based on a 90-min interview with both the subject and
In support of this latter contention, Broadbent (1954) found that
a collateral source. This interview assesses the subjects’ cognitive abilities
the ability to switch attention between the ears improved as pre-
in the areas of memory, orientation, judgment and problem solving, com-
sentation rate slowed down. In the Broadbent (1954) study, young
munity affairs, home and hobbies, and personal care. Both the reliability of
the CDR and the validation of the diagnosis (based on autopsy) by the
adults were presented three pairs of dichotic digits and were asked
research team have been excellent (93% diagnostic accuracy) and well
to report the digits in the order in which they were presented, thus
documented (e.g., Berg et al., 1998).
forcing a strategy of switching attention between channels. The
Of the 94 subjects, 44 were healthy older controls (CDR
0; 26 men, 18
presentation rate between pairs of digits within a triad varied from
women; mean age
74.50 years, SD
7.94, range
61–91; mean
among 0.5, 1.0, 1.5, and 2.0 s across trials. The results indicated
education
15.00 years, SD
3.06), 28 were diagnosed with very mild
that subjects’ ability to report across the ears (i.e., percentage
DAT (CDR
0.5; 16 men, 12 women; mean age
74.00 years,
correct order recall) increased at the longer 1.5- and 2.0-s presen-
SD
7.67, range
61– 88; mean education
14.10 years, SD
3.01),
tation rates. Broadbent (1954) concluded that a time interval of 1–2
and 22 were diagnosed with mild DAT (CDR
1; 13 men, 9 women;
s was necessary to shift attention back and forth between channels.
mean age
71.20 years, SD
8.84, range
58 –95; mean educa-
The primary purpose of the present study was to investigate the
tion
14.00 years, SD
3.42). There were no signi?cant differences
among the groups in age or education (all ps
.14). Overall, 96.4% of the
ability to control a prepotent pathway, as re?ected by left hemi-
total sample was right-hand dominant (97.7% for CDR 0, 95.8% for
sphere linguistic processing, with a secondary purpose to examine
CDR 0.5, 94.1% for CDR 1).
the ability to switch attention across ears as a function of healthy
aging and dementia severity. Although previous studies have
shown de?cits in DAT in the dichotic listening task, the in?uence
Psychometric Testing
of the prepotent right ear advantage has not been directly investi-
Each subject from the Washington University Alzheimer’s Disease
gated. Moreover, we are particularly interested in whether we can
Research Center was administered a 2-hr comprehensive psychometric
isolate an impairment in the ability to control the prepotent path-
battery that assessed various aspects of memory, intelligence, and lan-
way from overall levels of performance in this task.
guage. Memory was assessed with the Wechsler Memory Scale (Wechsler

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DICHOTIC LISTENING AND ALZHEIMER’S DISEASE
689
& Stone, 1973) and scored accordingly: Logical Memory (immediate, with
no delayed recall; recall of Scoring Units 0 –23), Forward and Backward
Digit Span (number of correct digits, 0 – 8 or 0 –7, respectively), Paired
Associate Learning Recall (sum of correctly recalled pairs over three trials,
0 –18 easy pairs, 0 –12 hard pairs), Mental Control (scored 0 –9). The Word
Fluency Test (Thurstone & Thurstone, 1949) was administered in which
subjects had to name as many words as possible that started with the letter
S or P in a 60-s period. General intelligence was assessed with the
Information (scoring range
0 –29), Block Design (scoring range
0 – 48), and Digit Symbol (scoring range
0 –90) subtests of the Wechsler
Adult Intelligence Scale and scored according to the manual (Wechsler,
Figure 1.
Free recall as a function of group, presentation rate, and ear. Error
1955). Visual perceptual-motor performance was assessed with the Benton
bars represent standard errors. DAT
dementia of the Alzheimer’s type.
Visual Retention Test and the Benton Copy Test (number correct, number
errors; Benton, 1963) and Part A of the Trail Making Test (number of
seconds to complete; Armitage, 1946). Finally, the Boston Naming Test
(ANOVA) on the percentage correct free recall yielded a signi?-
(Goodglass & Kaplan, 1983) was administered as a test of semantic–lexical
cant main effect for group, F(2, 91)
38.34, MSE
860.10, 2
retrieval (number correct out of 60). Psychometric tests are scored such that
.46, p
.0001; presentation rate, F(3, 273)
19.43, MSE
greater scores indicate better performance with the exception of Trail
Making (Part A) and Benton Copy errors, for which higher scores indicate
160.50,
2
.18, p
.0001; and ear, F(1, 91)
34.32, MSE
slower and hence poorer performance. Psychometric testing always oc-
811.50, 2
.27, p
.0001. There was also a signi?cant Group
curred within a 2-month window of the dichotic listening task.
Presentation Rate interaction, F(6, 273)
3.13, MSE
160.50,
2
.06, p
.005, indicating that the mild DAT group did not
show an increase in free recall across presentation rates, F(3,
Materials and Procedures
63)
2.27, MSE
73.53,
2
.10, p
.09, as did the healthy
All stimulus materials were constructed using SoundEdit on a Macintosh
control, F(3, 129)
14.94, MSE
250.00,
2
.26, p
.001,
computer and then recorded and presented on an Optimus SCT-7500 stereo
and very mild DAT groups, F(3, 81)
16.87, MSE
85.55, 2
cassette deck and Realistic Nova 40 stereo headphones.
.39, p
.001. A signi?cant Presentation Rate
Ear interaction
One practice list and four test lists were constructed for this study. Each
indicated a slightly larger right ear advantage across presentation
list contained eight dichotic messages. Each dichotic message consisted of
rates, F(3, 273)
3.38, MSE
101.40,
2
.04, p
.019.
three digit pairs (e.g., left ear: 4, 3, 1; right ear: 9, 2, 5). The triads of digits
Finally, there was a highly reliable Group
Ear interaction, F(2,
were created using the numbers 1–9 (excluding the two-syllable Number 7)
91)
12.96, MSE
811.50,
2
.22, p
.0001. Post hoc
without repeating the same digit within a triad. Each list of eight triads was
analyses comparing the healthy control and very mild DAT groups
blocked according to the presentation rate among digit pairs within a triad
yielded a signi?cant Group
Ear interaction, F(1, 70)
4.99,
(0.5, 1.0, 1.5, and 2.0 s). For example, in List 1 the digit pairs within each
triad were presented at 0.5-s intervals. In List 2, the digit pairs within each
MSE
357.20,
2
.07, p
.003, indicating a right ear
triad were presented at 1.0-s intervals and so forth The ordering of
advantage in the very mild group ( 2
.25, p
.006), but the
presentation for the four test lists was counterbalanced across subjects in
right ear advantage was not reliable for the healthy control group
each group. The presentation rate for the practice list was 1.0 s between
( 2
.01, p
.45). Although both the very mild ( 2
.25, p
digit pairs.
.006) and mild ( 2
.42, p
.001) DAT groups produced a
All subjects were tested in a quiet room with the headphones and volume
highly reliable right ear advantage, the mild DAT group produced
on the stereo ampli?er adjusted according to the subject’s preferences.
a larger right ear advantage than the very mild DAT group, as
After each triad, subjects were instructed to recall the digits in the order in
re?ected by a reliable Group
Ear interaction, F(1, 48)
7.32,
which they were presented. Additional practice was given if necessary to
MSE
1,278.60, 2
.13, p
.01. Thus, there was a larger right
ensure the subjects understood the task, and the instructions were repeated
ear advantage in the mild DAT group compared with the healthy
before each of the four test lists. Following each triad, the experimenter
recorded the subject’s responses.
control and very mild DAT groups. Finally, the overall Group
Presentation Rate
Ear interaction did not approach signi?cance,
F(6, 273)
0.39, MSE
101.40,
2
.008, p
.89.
Results
Of course, one might be concerned that the Group
Ear
interaction is due to the near-ceiling, free-recall performance of the
Percentage Correct Free Recall
healthy control and to some extent the very mild DAT groups. It
To examine the right ear advantage, we scored the data for (a)
is possible that the right ear advantage seen in the mild DAT group
percentage correct free recall by ear regardless of input order and
may simply re?ect this group’s overall lower free-recall perfor-
(b) percentage correct ?rst item output by ear. Percentage correct
mance. To address this concern, we conducted a 3 (group)
4
free recall was calculated as the number of digits recalled out of
(presentation rate)
2 (ear) analysis of covariance, with overall
three for the right ear and the left ear, respectively. Percentage
recall performance as a covariate. The results of this analysis
correct ?rst item output was calculated by determining the number
yielded a signi?cant main effect of presentation rate, F(3,
of trials on which the ?rst correctly recalled item came from the
270)
13.03, MSE
143.10,
2
.13, p
.001, and ear, F(1,
right ear versus the left ear. Both of these measures address
90)
7.75, MSE
787.40, 2
.08, p
.007. Again as indicated
whether there is a right ear advantage in recall.
in the previous ANOVA, there was a signi?cant two-way interac-
Figure 1 displays the percentage correct free recall as a function
tion between group and presentation rate, F(6, 270)
7.36,
of group, presentation rate, and ear (left vs. right). The results of
MSE
143.10, 2
.14, p
.001, but not presentation rate and
a 3 (group)
4 (presentation rate)
2 (ear) analysis of variance
ear, F(3, 270)
0.81, MSE
101.30,
2
.009, p
.48. Most

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690
DUCHEK AND BALOTA
important, there was a signi?cant Group
Ear interaction, F(2,
90)
3.31, MSE
787.40, 2
.07, p
.04, indicating a larger
right ear advantage in the mild DAT group compared with the
healthy control and very mild DAT groups after using overall
recall performance as a covariate.
To further examine the reliability of the Group
Ear interac-
tion, we performed a median split on each group to equate free-
recall performance across groups. Figure 2 displays the percentage
correct recall as a function of group, high–low performers, and ear
(collapsed across presentation rates for ease of presentation). To
equate overall performance across groups, we performed a sepa-
Figure 3.
First item output. Error bars represent standard errors. DAT
rate 3 (group)
4 (presentation rate)
2 (ear) ANOVA compar-
dementia of the Alzheimer’s type.
ing the low healthy control, low very mild DAT, and high mild
DAT groups (mean recall
74.4%, 71.2%, and 68.0%, respec-
tively), F(2, 43)
2.01, MSE
632.30,
2
.09, p
.15. The
rate, the important point is that this effect was constant across
results of this analysis yielded a signi?cant main effect of presen-
groups. The Group
Presentation Rate interaction did not ap-
tation rate, F(3, 129)
6.90, MSE
269.90, 2
.14, p
.0002,
proach signi?cance (F
1.00,
2
.01).
and ear, F(1, 43)
21.70, MSE
650.10,
2
.34, p
.0001.
As indicated in the previous ANOVAs, there were signi?cant
Proportion of Switches in Output
two-way interactions between group and presentation rate, F(6,
To further examine the reliance on the prepotent processing
129)
2.23, MSE
269.90, 2
.09, p
.05, and Presentation
pathway (i.e., the right ear) during dichotic listening, we analyzed
Rate
Ear, F(3, 129)
2.99, MSE
116.20, 2
.07, p
.03.
the proportion of ear switches during recall. Subjects were in-
In addition, there was a marginally signi?cant Group
Ear
structed to report the digits in the order in which they were
interaction, F(2, 43)
2.87, MSE
650.10, 2
.12, p
.067,
presented (i.e., alternate between the ears). The proportion of
indicating a larger right ear advantage in the mild DAT group
switches was computed by totaling the number of times digits were
compared with the healthy control and very mild DAT groups even
correctly recalled across ears divided by the total number of digits
when performance has been equated, as displayed in Figure 2.
correctly recalled (i.e., number of switches / total number re-
Further right ear advantage analyses were conducted by exam-
called). Thus, this measure controls for group differences in over-
ining the percentage of time the ?rst item output came from the
all recall performance.
right ear. Because this analysis is based on only one item being
Figure 4 displays the proportion of switches across ears during
recalled, it is relatively impervious to overall group differences in
recall relative to the total correct recall as a function of group and
recall, because virtually all subjects recalled at least one item on all
presentation rate. The results of a 3 (group)
4 (presentation rate)
trials. These data are displayed in Figure 3. The results of a 3
ANOVA yielded a signi?cant main effect of group, F(2,
(group)
4 (presentation rate) ANOVA yielded two signi?cant
89)
38.36, MSE
421.27,
2
.46, p
.0001. Post hoc
main effects. There was a signi?cant main effect for group, F(2,
analyses indicated there were fewer switches made in the mild
89)
4.47, MSE
1,042.30,
2
.09, p
.014. Post hoc
DAT group (.34) compared with the healthy control (.57) and very
analyses revealed a signi?cant difference between healthy control
mild DAT (.55) groups, F(1, 62)
56.47, MSE
524.90,
2
and mild DAT groups, F(1, 62)
8.11, MSE
1,142.90,
2
.48, p
.0001, and F(1, 48)
33.67, MSE
629.30,
2
.41,
.12, p
.006, but no difference between healthy control versus
p
.0001, respectively. Because this measure takes into account
very mild DAT ( 2
.03, p
.13) or very mild DAT versus mild
overall correct recall, this effect does not simply re?ect group
DAT ( 2
.04, p
.16) groups. There also was a signi?cant main
differences in overall performance. There was no difference in the
effect of presentation rate, F(3, 267)
4.80, MSE
258.79, 2
proportion of switches between the healthy control and very mild
.05, p
.003. Post hoc analyses indicated lower ?rst item recall
DAT groups, F(1, 68)
1.38, MSE
180.00, 2
.02, p
.25.
from the right ear at the 1.0-s presentation rate, compared with all
Second, as discovered by Broadbent (1954), there was an increase
other presentation rates ( 2
.10, all ps
.002). Although it is
in the proportion of switches with increasing (i.e., slower) presen-
unclear what produced this lower recall performance at the 1.0-s
Figure 4.
Proportion of switches as a function of group and presentation
Figure 2.
Free recall as a function of group, high–low performers, and ear.
rate. Error bars represent standard errors. DAT
dementia of the Alzhei-
Error bars represent standard errors. DAT
dementia of the Alzheimer’s type.
mer’s type.

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DICHOTIC LISTENING AND ALZHEIMER’S DISEASE
691
Table 1
Psychometric Means and Standard Deviations as a Function of Group
Healthy control
Very mild DAT
Mild DAT
Group
M
SD
M
SD
M
SD
Logical Memory
10.03
3.18
7.30
3.49
2.83
2.59*ˆ
Digits Forward
6.81
1.09
6.54
1.14
5.70
1.35
Digits Backward
5.29
1.40
4.64
1.16
3.76
1.04ˆ
WMS—Paired Associate
Learning Recall easy
17.19
1.33
15.11
2.88
13.20
2.59*
WMS—Paired Associate
Learning Recall hard
6.69
3.47
3.00
3.84
0.80
1.67*
WMS Mental Control
7.90
1.88
7.11
1.85
4.95
2.18ˆ
Word Fluency Test (Letter
S or P)
34.57
12.10
28.54
11.30
15.71
11.60ˆ
WAIS Information
22.31
3.87
17.75
5.09
10.81
5.83*ˆ
WAIS Block Design
33.86
9.37
27.75
8.84
15.86
12.3*ˆ
WAIS Digit Symbol
49.62
11.30
40.82
11.90
21.74
10.3*ˆ
Benton Recalla
6.64
1.68
5.21
1.71
2.89
1.85*ˆ
Benton Copy Test
9.74
0.54
9.68
0.61
9.21
1.44
Benton Delay Errors Ca
5.60
3.67
8.43
3.77
15.58
4.89*ˆ
Benton Copy Errors Db
0.26
0.54
0.32
0.61
0.95
1.87
Trail Making Test (Part A)
42.40
14.30
43.18
13.00
87.76
42.90ˆ
Boston Naming Test
55.21
4.55
49.89
10.80
41.71
9.03*ˆ
Note.
DAT
dementia of the Alzheimer’s type; WMS
Wechsler Memory Scale; WAIS
Wechsler Adult
Intelligence Scale.
a Measure from the Benton Visual Retention Test. b Measure from the Benton Copy Test.
* p
.01 for the comparison between healthy controls versus very mild DAT.
ˆ p
.01 for the comparison
between very mild DAT versus mild DAT.
tation rates, F(3, 267)
18.26, MSE
54.93,
2
.17, p
subjects showed a larger right ear advantage when they made
.0001. There was no signi?cant interaction between group and
fewer switches between the ears. This correlation was not statis-
presentation rate, F(6, 267)
0.72, MSE
54.93, 2
.02, p
tically signi?cant for the healthy controls (r
.15, p
.34) but
.63. Thus, all subject groups made more switches with slower
was in the same direction.
presentation rates, but the overall proportion of switches was
To examine the relationship between psychometric performance
consistently lower in the mild DAT individuals, thereby re?ecting
and attentional mechanisms, we correlated the measures of the
an inability to switch between channels.
right ear advantage and the proportion of switches with composite
psychometric scores that were created as a function of differences
Psychometric Performance and Correlations Among Right
in possible underlying neural systems (i.e., medial–temporal, pa-
Ear Advantage, Switches, and Psychometric Data
rietal, and frontal). On the basis of previous literature (e.g., Chase
The means and standard deviations for the psychometric mea-
et al., 1984; Glisky, Polster, & Routhieaux, 1995; Lezak, 1995;
sures for each of the groups are presented in Table 1. A series of
Milner, 1972; Mishkin, 1978) and the factor analysis work of
one-way ANOVAs, with group as a between-subjects factor, in-
Kanne, Balota, Storandt, McKeel, and Morris (1998), we de?ned
dicated that performance on all of the psychometric measures,
medial–temporal measures as Wechsler Adult Intelligence Scale
except Benton Copy errors, was signi?cantly different among
information, Boston Naming Test, Logical Memory, and Paired
groups (all ps
.05). Post hoc comparisons between the healthy
Associate Learning Recall. Parietal measures were de?ned as
control versus very mild DAT groups and the very mild DAT
Benton Copy, Trail Making (Part A), Block Design, and Digit
versus mild DAT groups generally indicated decreasing psycho-
Symbol (e.g., Chase et al., 1984; Kanne et al., 1998; Lezak, 1995).
metric performance with increasing dementia severity (see
Frontal measures were de?ned as Digits Forward, Word Fluency,
Table 1).
and Mental Control1 (e.g., Glisky et al., 1995; Kanne et al., 1998;
To examine the relationship between the right ear advantage and
Parks et al., 1988). Kanne et al. (1998) found that not only did
general psychometric performance, we correlated a single com-
these tests load on the appropriate factor structure, but these factors
posite measure of the right ear advantage (i.e., percentage correct
were also related to neuropathology in the targeted areas at
right ear / percentage correct left ear) and the proportion of
autopsy.
switches (i.e., the number of ear switches made during recall
output / total correct recall) with all of the psychometric measures
1 Clearly, there are recent measures that may be more appropriate to
for each group. First, it should be noted that there is a highly
assess frontal functioning, however, these measures (i.e., Digits Forward,
reliable correlation between the right ear advantage and the num-
Word Fluency, Mental Control) were available on the present sample and
ber of switches for the very mild DAT (r
.57, p
.001) and
have been reported to be sensitive to frontal involvement in DAT (Kanne
mild DAT (r
.67, p
.001) groups, indicating that the DAT
et al., 1998).

---

692
DUCHEK AND BALOTA
For each subject, a z score was computed for each psychometric
vation by showing that this pattern is not limited to a cueing
test based on the healthy control subject’s group mean.2 Then three
manipulation. Furthermore, the present study afforded an analysis
overall z scores (medial–temporal, parietal, frontal) were created
of the right ear advantage as a function of dementia severity in
for each subject that were based on the average of the four
early stage DAT. In contrast, the Mohr et al. study reported that
medial–temporal measures, the four parietal measures, and the
their subjects were at the mild to severe stage of DAT. Thus, the
three frontal measures, respectively. The correlations between the
present study extends the use of the dichotic listening task to
right ear advantage and the proportion of switches and the medial–
discriminate early stages of dementia. Moreover, the present re-
temporal, parietal, and frontal scores are presented in Table 2 as a
sults clearly indicate that the right ear advantage increased with
function of group. For the healthy control group, only the corre-
dementia severity. That is, the very mild DAT group showed a
lation between the right ear advantage and the medial–temporal
larger right ear advantage than the healthy controls, even though
score approached statistical signi?cance ( p
.055). None of the
the proportion of switches did not differ between these two groups.
correlations reached statistical signi?cance for the very mild DAT
Also, switching ability was correlated with the right ear advantage
group. Most interesting, the right ear advantage was marginally
in the very mild DAT group but not in the healthy controls. Thus,
correlated ( p
.087), and the proportion of switches was reliably
although very mild DAT individuals could switch attention be-
correlated with the frontal measure ( p
.01) but not the parietal
or medial temporal measures for the mild DAT group.
tween channels like the healthy controls, they still showed a
tendency toward a larger right ear advantage. Similarly, the mild
DAT group showed a larger right ear advantage than the very mild
Discussion
DAT group. Thus, the reliance on the prepotent linguistic channel
The purpose of the present study was to examine the right ear
increased with dementia severity.
advantage in early stage DAT as a measure of attentional control.
It is also interesting to note that the healthy older adults did not
A clear right ear advantage in free-recall performance was found in
show a reliable right ear advantage in free recall in the present
early stage DAT. In fact, this right ear advantage was quite
study, even though there has been evidence of a right ear advan-
dramatic in the mild DAT group. Moreover, the large right ear
tage in dichotic listening in healthy older adults (Alden, Harrison,
advantage in mild DAT did not appear to be due to the overall
Snyder, & Everhart, 1997; Carter & Wilson, 2001; Strouse, Wil-
lower free-recall performance in this group because the advantage
son, & Brush, 2000b). The lack of a right ear advantage in the
remained when overall recall was used as a covariate and in the
present older adults is likely due to the emphasis in this study to
matched subjects analysis. Further analyses also supported this
report the digits in the order in which they were presented. Healthy
?nding. That is, the ?rst item output in free recall was more likely
older adults were able to switch attention between the ears and thus
to come from the right ear in mild DAT compared with the healthy
report from both ears. This is evidenced by the ?nding that
control and very mild DAT groups, and also there was evidence of
only 47.5% of the time the ?rst item output in free recall was from
a decrease in switching across ears in the mild DAT group.
the right ear for the healthy controls, compared with 60.3% for the
The larger right ear advantage in early stage DAT is consistent
mild DAT group.
with Claus and Mohr (1996) and the Mohr et al. (1990) study,
Along these same lines, there is evidence that the right ear
which reported an inability of DAT subjects to strategically allo-
advantage seems to be modulated by the complexity of the dichotic
cate attention to the left ear on precue, resulting in a strong right
listening task. Speci?cally, there is some literature that suggests
ear preference in recall. The present results extended this obser-
that the right ear advantage increases as task complexity increases.
That is, Mondor (1991) found that when subjects are confronted
with competing information and attention needs to be selectively
allocated, the left hemisphere processing is more likely to drive
Table 2
performance. It is possible that the present paradigm did not
Correlations Between Medial-Temporal, Parietal, and Frontal
suf?ciently push healthy older adults to produce the right ear
Psychometric Measures and Proportion of Switches and Right
advantage.
Ear Advantage
We would argue that the large right ear advantage in mild DAT
Measure
Medial-temporal
Parietal
Frontal
re?ects a greater reliance on the prepotent pathway for language
processing (i.e., the left hemisphere) when confronted with com-
Healthy control
peting information (i.e., from the left and right ear simulta-
neously). This notion is further supported by two other ?ndings.
Right ear advantage
.30
.09
.09
Proportion of switches
.09
.18
.18
First, when one compares the proportion of switches made during
recall across the groups (which controls for overall recall perfor-
Very mild DAT
mance), it is clear that the mild DAT group is making fewer
switches between the ears compared with the very mild DAT and
Right ear advantage
.10
.08
.13
Proportion of switches
.10
.01
.17
healthy control groups (.34, .55, and .57, respectively). The pro-
Mild DAT
2 The overall pattern of the correlations between the right ear advantage
Right ear advantage
.18
.01
.38ˆ
and the proportion of switches and the medial–temporal, parietal, and
Proportion of switches
.23
.13
.57*
frontal scores presented in Table 2 did not change when the z scores were
Note.
DAT
dementia of the Alzheimer’s type.
also calculated on the basis of each group’s respective means, as opposed
ˆ p
.087.
* p
.01.
to the healthy control group’s means.

---

DICHOTIC LISTENING AND ALZHEIMER’S DISEASE
693
portion of switches made by these latter two groups is remarkably
general cognitive slowing (at least as measured by digit symbol
similar. It also should be noted that there is an increase in the
performance) cannot fully account for the larger right ear advan-
proportion of switches during recall across presentation rates (i.e.,
tage in mild DAT. Of course, one might more directly address this
more switches with slower presentation rates) across all groups,
issue by examining longer presentation rates than those used in the
precisely as Broadbent (1954) found in his original study. How-
present study.
ever, the mild DAT group still exhibited a large right ear advantage
It is also possible that the DAT individuals may have had
at the slowest presentation rates. Thus, the left hemisphere is
dif?culty maintaining the instructions to switch between the ears
clearly driving output during recall in these individuals.
over the course of the experimental task and thus simply relied on
Second, there was a strong correlation between the right ear
the prepotent right channel for recall. This seems unlikely for two
advantage and the proportion of switches in both DAT groups. A
reasons: (a) subjects were reminded of the instructions to switch
decreasing ability to switch attention between the ears was related
prior to the presentation of each list, and (b) the mild DAT group
to an increased reliance on reporting from the language-dominant
was able to switch on some trials (proportion switches
.34), and
right ear in early stage DAT. This was not the case in the healthy
they did recall digits from both ears (72.5% right ear recall
control group. As previously mentioned, we have reported ?ndings
and 44.5% left ear recall across presentation rates).
from other cognitive tasks in which DAT subjects are unable to
Finally, one limitation of this study was that there was no
control attention to select a speci?c pathway when confronted with
measure of hearing sensitivity available for these subjects. One
competing prepotent information compared with healthy controls,
might argue that the results were due to differential left–right ear
such as Stroop performance (e.g., Spieler et al., 1996). We have
hearing loss. Of course, there would have to be differential left–
also extended this perspective to reading (e.g., Balota & Ferraro,
right hearing loss as a function of group to account for the larger
1993, 1996; Duchek, Balota, & Thessing, 1998) and memory (e.g.,
right ear advantage found in the mild DAT group. We think that
Balota et al., 1999) performance. Of course, control of prepotent
this is unlikely because studies indicate that early stage DAT is not
pathways is intimately involved in working memory (see Engle,
associated with greater hearing loss than healthy aging using both
Kane, & Tuholski. 1999). Along these lines, Conway, Cowan, and
behavioral and psychophysiological measures (e.g., otoacoustic
Bunting (2001) reported that low-span subjects were more likely to
emissions) of hearing sensitivity (Gates et al., 1995; Sommers,
report hearing their own name in an unattended irrelevant message
1998).
during a shadowing task, indicating that low-span subjects have
Also, the right ear advantage ( p
.087) and the proportion of
dif?culty inhibiting prepotent information. Thus, the results of the
switches during recall were reliably correlated with a composite
present study are likely to converge on the working memory
score from frontal measures in the mild DAT group but not parietal
de?cits in DAT (R. G. Morris, 1994). In this light, one might
and medial–temporal measures. These attentional measures were
question whether working memory capacity plays a role in the
not correlated with frontal psychometric measures in either the
present results. It is possible that the reduced capacity of the DAT
very mild DAT or healthy control group. This ?nding is suggestive
subjects induces subjects to attend to the prepotent channel (right
that de?cits in attentional control in early stage DAT may re?ect a
ear) to maximize their recall performance. In an attempt to address
breakdown, at least in part, in frontal lobe functioning.
this issue, we examined the correlation between the right ear
In sum, the present study is consistent with previous reports of
advantage and Digits Forward and Digits Backward performance,
impaired dichotic listening performance in DAT (e.g., Claus &
as measures of working memory capacity, for both the DAT
Mohr, 1996; Mohr et al., 1990). A large right ear advantage in free
groups. The results indicated that although these correlations were
recall is evident in mild DAT, providing further support that
in the predicted direction, none of the correlations were reliable
attentional processing, like other cognitive processes, is affected
(all ps
.144). Thus, it appears that a reduced working memory
early in the disease process, and de?cient attentional control may
capacity, at least as re?ected by these measures, cannot fully
force responding based on familiar, prepotent information path-
account for the larger right ear advantage in mild DAT.
ways. One can speculate about the importance of attentional con-
Other alternative interpretations for the larger right ear advan-
trol in everyday tasks, such as driving. Indeed, early studies by
tage in mild DAT need to be addressed. For example, it is possible
Kahneman and colleagues indicated that dichotic listening perfor-
that the presentation rate may have been too fast for the mild DAT
mance was a good predictor of accident rates in commercial bus
subjects to process the digits in each ear or manipulate items in
drivers (Kahneman, Ben-Ishai, & Lotan, 1973; Mihal & Barret,
working memory before recall and thus they were forced to rely on
1976). More recent studies of driving performance in DAT suggest
the prepotent channel due to general cognitive slowing. To address
that aspects of attentional selection and control are better predic-
this concern, we reanalyzed the free-recall data (i.e., percentage
tors of safe driving than general cognitive status and neuropsycho-
correct free recall by ear), with digit symbol performance as a
logical test performance (Duchek, Hunt, Ball, Buckles, & Morris,
covariate. Digit symbol performance was chosen as a measure of
1998).
general processing speed on which the mild DAT subjects showed
de?cient performance relative to the very mild DAT and healthy
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