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The Journal of Neuroscience, January 15, 2002, 22(2):546-553
Cerebellar Involvement in Response Reassignment Rather Than
Attention
Amanda
Bischoff-Grethe1,
Richard B.
Ivry2, and
Scott
T.
Grafton1
1 Center for Cognitive Neuroscience and the Department
of Psychological and Brain Sciences, Dartmouth College, Hanover, New
Hampshire 03755, and 2 Department of Psychology, University
of California, Berkeley, Berkeley, California 94720-1650
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ABSTRACT |
A number of functional hypotheses have recently been advanced to
account for how the cerebellum may contribute to cognition. Neuropsychological studies suggest the cerebellum is involved in
switching attentional set. We present evidence that fails to support
this hypothesis. Rather, we propose that in such tasks, the cerebellum
is involved with the remapping of response alternatives to different
types of stimuli. In our experiment, participants fixated on the center
of a screen onto which a random presentation of four visual stimuli was
presented. The stimuli were grouped along two dimensions (color: red
square or blue square; shape: white circle or white triangle).
Participants were instructed to respond with a button press only to
presented stimuli for a particular dimension (e.g., red squares), to
switch between two dimensions (where the target on the attended
dimension served both as a signal for a response and as an indicator to
shift attention to the other dimension), or to switch attention between
two dimensions but make an overt response only to targets on one of the
dimensions. Using functional imaging, we identify areas of lateral
cerebellar cortex that are recruited when subjects must reassign motor
responses to different stimuli. Furthermore, we demonstrate that
switching of attention between dimensions without a motor response does not produce stronger activation within the cerebellum compared with
conditions involving response and attention to a single dimension. These results suggest the cerebellum is involved in response reassignment.
Key words:
functional imaging; cerebellum; attention; sensorimotor; response reassignment; cognition
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INTRODUCTION |
Cerebellar function is associated
with sensorimotor coordination, adaptation, and associative learning
(Albus, 1971 ; Brooks and Thach, 1981; Ito, 1984). Human lesion and
functional imaging studies (Ivry and Keele, 1989; Petersen et al.,
1989; Fiez et al., 1996; Gao et al., 1996; Desmond and Fiez, 1998)
suggest that the functional domain of the cerebellum is not limited to
motor control. Given the anatomical connections between the
neocerebellum and association cortical areas, including prefrontal
cortex (Schmahmann and Pandya, 1997a; Middleton and Strick, 2000,
2001), various hypotheses have been proposed concerning how the
cerebellum may be part of a neural circuit involved in executive control.
One influential model proposes that the cerebellum is involved in
coordinating rapid shifts of attention, similar to the way this
structure contributes to the coordination of rapid movements. Working
with children and young adults whose cerebellar pathology was caused by
infantile autism or neurological damage (Akshoomoff and Courchesne,
1992), Courchesne et al. (1994) observed a deficit on tasks
requiring the rapid alternation of attention between different stimulus
dimensions. Imaging data in normal subjects revealed enhanced activity
in lateral cerebellar cortex when subjects paid attention to specific
sensory information (Allen et al., 1997) or switched attention between
different visual features (Le et al., 1998). Localization within the
cerebellum was lateral to the parasagittal areas and to vermal areas
associated with movements.
Ravizza and Ivry (2001) have recently challenged the attention
switching account of cerebellar function. Patients with either Parkinson's disease or cerebellar pathology exhibited similar deficits
on the attention shifting task introduced by Courchesne et al. (1994).
The lack of specificity suggests that this impairment may be a general
feature of neurological damage. More importantly, when the task was
modified so that overt responses were only required to targets on one
dimension, the deficit in the cerebellar group was largely attenuated,
even though the attention switching requirements remained the same.
Impairments in coordinating rapid shifts of attention after cerebellar
damage may thus be related to the requirement in the attention shifting
condition to alter the task-relevant stimulus-response mapping, what
we call response reassignment. Interestingly, the Parkinson patients
continued to exhibit an attention shifting deficit under the reduced
movement condition, consistent with a role for the basal ganglia in
attention shifting (Brown and Marsden, 1988; Hayes et al., 1998; Rogers
et al., 1998).
In this report we test this hypothesis using functional magnetic
resonance imaging (fMRI). Central to our experiment is the idea that
attention shifting tasks involve two component processes. One process
entails the focus of attention, establishing which stimulus dimension
is relevant for responding. The second process entails the link between
target stimuli on the selected dimension and response processes. In
most attention shifting tasks, these two processes are confounded.
However, they can be dissociated. Response reassignment requires
attentional switching, whereas attentional switching can occur without
motor intention. We hypothesize that separate neural representations
exist to handle different categories of switching. Of critical
interest, we show that switching of attention between dimensions
without a motor response does not produce stronger activation within
the cerebellum compared with a focused attention condition. These
results argue against a role for the cerebellum in attentional
switching; rather the cerebellum appears to be engaged during response reassignment.
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MATERIALS AND METHODS |
Experimental design. Ten right-handed participants
(seven male and three female) aged 21-47 years (average age, 29.9 years) gave written informed consent. Four visual stimuli (red square, blue square, white circle, and white triangle) were individually presented in random order for 84 trials. The red square and the white
circle were designated as targets; each target was presented 17% of
the time. The blue square and white triangle were designated as
distractors, and each occurred 33% of the time. Stimulus duration was
100 msec, and the stimulus onset asynchrony (SOA) varied randomly from
450 to 1450 msec. Participants were given a button box and were
instructed to press a button using the index finger of the right hand
when specific targets appeared in the center of the screen. Response
latency was recorded on-line with an optically encoding system
compatible with fMRI.
Five separate attention conditions were presented to each subject (Fig.
1). Two were focused attention
conditions, where participants were required to respond to all
occurrences of the red square (focused attention color) while ignoring
all other stimuli or to respond to all occurrences of the white circle
(focused attention shape). In the double response condition,
participants were instructed to alternate between targets by responding
to a target defined by one dimension (e.g., color), then to a target on
the other dimension (e.g., shape) and so on. In this condition, targets not only served as stimuli requiring a response but also served as a
cue to shift attention between the two dimensions. The final two
conditions were single response conditions. Here participants shifted
attention between targets as in the double response condition, but were
to respond to targets on only one dimension (e.g., single response
color or single response shape) throughout the condition. All
conditions had the same number of target stimuli (~14 per block), but
the single response conditions had half as many overt responses as the
double response and focused attention conditions.
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Figure 1.
Examples of stimulus sequences and responses for
each condition. Red squares and white
circles were the targets, and blue squares and
white triangles were the distractors.
Arrows indicate where an attention shift and/or key
press were required. Asterisks indicate that only an
attention shift was required. The five conditions were:
A, focused attention shape; B, focused
attention color; C, double-response; D,
single-response shape; and E, single-response color.
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Participants were given one or more practice sessions on each condition
to ensure they were comfortable with each task before scanning. The
order of condition presentation was counterbalanced across
participants. Each functional imaging session consisted of one
condition plus rest, and each subject performed all five conditions,
leading to a total of five imaging sessions per subject.
MRI. Five fMRI sessions of 60 scans each were obtained, with
one session per condition. Functional MRI was performed with gradient-recalled echoplanar imaging (reaction time, 2000 msec; echo
time, 35 msec; flip angle, 90°; 64 × 64 matrix; 27 5.5 mm contiguous axial slices) on a GE 1.5 T scanner (Kwong et al., 1992;
Ogawa et al., 1992). Coplanar structural T1-weighted, and high-resolution MRIs were obtained for subsequent spatial normalization.
Statistical analysis. The data were analyzed using
Statistical Parametric Mapping (SPM99; Wellcome Department of Cognitive Neurology, London, UK) (Friston et al., 1995). Motion correction to the
first functional scan was performed within subject using a
six-parameter rigid-body transformation. The mean of the
motion-corrected images was first coregistered to the individual's
high-resolution MRI using mutual information, followed by
coregistration of the structural MRI to the functional images using a
12-parameter affine transformation. The images were then spatially
normalized to the Montreal Neurologic Institute (MNI) template
(Talairach and Tournoux, 1988) by applying a 12-parameter affine
transformation followed by a nonlinear warping using basis functions
(Ashburner and Friston, 1999). The spatially normalized scans were then
smoothed with a 6 mm isotropic Gaussian kernel to accommodate
anatomical differences across participants. The data were analyzed
using a random-effects model to make statistical inferences (Friston et
al., 1999). The two focused attention tasks were combined into the
FOCUS condition for activation analyses; similarly, the single response
tasks were combined to form the SINGLE condition. The DOUBLE condition referenced the double response task.
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RESULTS |
Functional MRI data were collected during subjects' performance
of each condition, and the analyses were designed to identify areas of
activation related to movement, shifts of attention, and shifts of
attention and stimulus-response mappings. All task conditions averaged
14 targets, but the number of motor responses differed; the FOCUS and
DOUBLE conditions required motor responses to all targets, whereas the
SINGLE conditions required responses to targets on only one dimension,
or seven targets per session. The task conditions were characterized by
one or more of the following attributes: attentional switching
(switching attentional set between the two dimensions); response
reassignment (switching motor set between the target stimuli requiring
an overt response); and total movements performed (Table
1). The FOCUS conditions did not involve attention switching or response reassignment, whereas the DOUBLE condition involved both. The SINGLE conditions only involved switching attention because the motor response was always to the same target.
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Table 1.
The relationships of tasks and task comparisons to
attentional switching, response reassignment, and movement
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Subjects performed all conditions with >90% accuracy (Table
2). Paired-sample t tests of
response times (RTs) showed that the RTs were faster for the FOCUS
condition compared with both the DOUBLE (t(9) = 3.25;
p = 0.005) and SINGLE (t(9) =  1.707; p = 0.061) conditions. Moreover, the DOUBLE and SINGLE
conditions were not statistically different (t(9) = 0.188; p = 0.855). Mean RTs for these conditions
differed by only 3 msec. These findings are consistent with the idea
that the two divided attention conditions were more difficult because
of the demands to shift attention between the two dimensions. The RTs
indicate that the DOUBLE and SINGLE conditions were of comparable
difficulty.
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Table 2.
Average mean reaction time and short and long intertarget
interval (ITI) hits averaged across task conditions
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Linear contrasts between different tasks were used to examine
differences of the functional attributes. Using a threshold for
significance of p < 0.01 (uncorrected), the six
possible contrasts were defined pairwise. Of the contrasts defined,
four permitted separation of activations related to movement
(FOCUS > SINGLE, DOUBLE > SINGLE), attentional switching
(SINGLE > FOCUS, DOUBLE > FOCUS), and response reassignment
(DOUBLE > SINGLE). Table 3 lists
cortical and cerebellar regions activated using these different contrasts of interest.
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Table 3.
Regional activations associated with comparisons of task
performance in a random effects design (p > 0.01 uncorrected)
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We first localized the relative activations within cerebellum related
to the three attributes of the task. According to our hypothesis, the
cerebellum should not show activation related to the demands to shift
attention, but rather would be related to the motor demands of the task
or response reassignment. Contrasts that would reveal movement related
activity compared the DOUBLE or FOCUS tasks to the SINGLE tasks. These
contrasts revealed activation within right lobule IV and right lobule
VI (Fig. 2). The reverse comparison of
the SINGLE condition to the DOUBLE and FOCUS tasks revealed no
cerebellar activation in these regions; an expected result as the
SINGLE condition has half as many motor responses. These results are
consistent with other studies indicating motor activation within
parasagittal cerebellum (Allen et al., 1997; Grodd et al., 2001).
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Figure 2.
Statistical parametric maps showing cerebellar
regions correlating with the different contrasts applied. Coronal
slices (Talairach coordinates: Y = 66,
Y = 60, Y = 54, and
Y = 48) indicate areas associated with the
interactions (DOUBLE > FOCUS, DOUBLE > SINGLE, FOCUS > DOUBLE, FOCUS > SINGLE, SINGLE > FOCUS, and SINGLE > DOUBLE).
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We next examined contrasts that would test the hypothesis that
cerebellar activation is related to task switching. The first contrast
compares the DOUBLE condition, where shifting is present, to the FOCUS
condition, where no shifting occurs. Consistent with previous reports
(Allen et al., 1997; Le et al., 1998), activation was observed within
left lobule IV, left lobule VI, and right lobule VI. However, this
contrast involves both attention shifting and response reassignment. To
isolate attention shifting, we used the SINGLE > FOCUS contrast
because only attention switching is required in the SINGLE condition.
Here we failed to detect any difference in cerebellar activation. This
argues strongly against cerebellar involvement in attentional shifting,
but lends credence to a role for this structure in response
reassignment. It is likely that the cerebellar activation seen when
comparing the DOUBLE condition to the FOCUS condition is also
attributable to response reassignment.
To confirm this result, we further examined the
adjusted blood-oxygen level-dependent (BOLD) response for
three of the activated cerebellar foci, averaged across subjects (Fig.
3). We found increased activation related
to the DOUBLE condition in all three foci, decreased activation related
to the SINGLE condition, and a decrease within two of the foci for the
FOCUS condition. The evidence suggests these foci are involved with
response reassignment, because the SINGLE condition did not increase
activation. Furthermore, the increased activation for both the DOUBLE
and FOCUS conditions within R lobule VI suggests this area may play a
role in both response reassignment and motor response.
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Figure 3.
Graph illustrating adjusted BOLD response for
relevant cerebellar foci across condition types. L lobule VI (22,
68, 30) shows an increased activation for the DOUBLE condition,
whereas the SINGLE and FOCUS conditions show decreased activation. R
lobule VI (22, 66, 22) shows increased activation for the DOUBLE
condition and decreased activation for SINGLE and FOCUS. R lobule VI
(22, 44, 24) shows an increase in activation for DOUBLE and FOCUS,
but a decrease in activation for the SINGLE condition. Overall, this
suggests cerebellar involvement in these foci is attributable to
response reassignment rather than facilitating attention
switching.
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Attentional shifting was associated with a distributed pattern of
cortical activation (Fig. 4). The
DOUBLE > FOCUS and SINGLE > FOCUS contrasts activated
several cortical attention areas, including left middle frontal gyrus
(supplementary motor area), left anterior cingulate, and left
precuneus. In both of these comparisons, the ability to shift attention
was necessary, although only the DOUBLE > FOCUS also requires
response reassignment. We did not observe any foci within the basal
ganglia that were active in the two conditions requiring attentional
shifts.
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Figure 4.
Statistical parametric maps showing cortical
regions that correlated with the different contrasts applied. Axial
slices (Talairach coordinates: Z = 0, Z = +10, Z = +22,
Z = +38, Z = +50,
Z = +58, and Z = +62)
illustrate activations associated with the interactions (DOUBLE > FOCUS, DOUBLE > SINGLE, FOCUS > DOUBLE, FOCUS > SINGLE, SINGLE > FOCUS, and SINGLE > DOUBLE).
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As previously described in Table 1, movement-related areas are expected
to be most commonly seen with the FOCUS > SINGLE contrast,
because the FOCUS condition has more movements than the SINGLE
condition. This contrast activated left precentral gyrus (motor cortex)
and left middle frontal gyrus.
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DISCUSSION |
Various lines of evidence have linked the cerebellum to
higher-level, executive functions. However, characterizing the
functional contribution of the cerebellum has remained elusive.
Visuospatial attention studies using positron emission tomography
(Corbetta et al., 1993) or event-related evoked potentials (Yamaguchi
et al., 1998) have reported cerebellar activity in relation to motor aspects of the tasks. Our results are in concurrence with these studies, although we have proposed that the cerebellum contributes to a
specific process involved in moving in cognitive tasks. We found right
lateral cerebellum activation related to response reassignment rather
than attentional switching. Activation was highest in the DOUBLE
condition when both attention and response reassignment were required.
In contrast, no activation was found in the cerebellum when we compared
the SINGLE condition with the focused attention conditions. In the
SINGLE condition, the participants were still required to shift
attention between the two dimensions; however, response
reassignment was no longer required because overt responses were only
required to one of the two target stimuli. We also found cerebellar
activation related to motor performance. Thus, cerebellar activation
was related to both the movement demands and higher-level aspects of
visuomotor control such as the establishment of stimulus-response
associations. Such activation was not present when attentional shifting
was isolated from response reassignment.
Courchesne et al. (1994) reported that cerebellar patients had
difficulties in shifting attention between stimuli when the intertarget
intervals (ITIs) were short (<2.5 sec). This impairment is observed on
tasks in which attention must shift between different sensory channels
or between different dimensions of a single sensory channel. Cerebellar
patient performance was equivalent to control subjects for long ITIs
(2.5-30 sec). This suggested that the cerebellum was needed to ensure
that the shifts occurred rapidly once a new focus of attention was
specified. Previous imaging studies (Allen et al., 1997; Le et al.,
1998) have reported greater activation in the right lateral cerebellum
during conditions involving attention shifting compared with focused
attention conditions. We observed similar results slightly medial to
this region during our DOUBLE condition in which responses were
required to targets on both dimensions. Given that no increase was
found in the SINGLE condition, however, it is likely that the
activation reflects response reassignment. Furthermore, our response
reassignment site is lateral to the foci activated within the contrasts
comparing the FOCUS and DOUBLE conditions with the SINGLE condition.
The area activated by those contrasts is likely responsible for
movement performance. The fact that cerebellar patients struggled with
rapid attention shifts (Courchesne et al., 1994) suggests the
difficulty lay in shifting between different sensorimotor sets rather
than attention itself.
The Wisconsin Card Sort Test (WCST) is a well known technique for
studying the prefrontal cortex, and imaging studies using this task
have identified activation within right lateral cerebellum (Berman et
al., 1995; Nagahama et al., 1996). However, Daum et al. (1993) noted
that the impairment seen in cerebellar patients performing the WCST
occurs only when the cerebellar degeneration extends to brainstem
structures. Mangels et al. (1998) found that whereas cerebellar and
prefrontal patients performed similarly on neuropsychological tests
assessing initiation, fluency, and perseveration, cerebellar patients
demonstrated reduced processing speed in the presence of preserved
working memory function. Attention shifting tasks involve multiple
executive processes. It is therefore not surprising that diverse
patient populations are impaired on these tasks and that activation
observed in imaging studies is widespread and includes the cerebellum.
The involvement of the cerebellum, though, should be considered
within a hierarchy of processes needed to accomplish the task. Our
evidence argues against involvement in assisting attentional shift per
se, but rather focuses on the motor component involved with an
attention shifting task.
Recent computational theories of motor control have articulated a
cerebellar architecture in which an internal model is constructed that
consists of multiple "modules" that link forward and inverse models
(Wolpert and Kawato, 1998; Kawato, 1999; Wolpert and Ghahramani, 2000).
These modules allow for the simultaneous prediction of multiple inverse
models and rely on contextual information to provide a framework for
the appropriate module to produce an action. Our results support this
modular view of cerebellar function; the DOUBLE condition involves two
different contextual situations (respond to a red square or respond to
a white circle) that might be represented by two different
stimulus-response modules within the cerebellum. The SINGLE and FOCUS
conditions involve responding to only one target type, thus lacking the
necessity of switching between contextual representations (and
therefore modules). Furthermore, the "activation" of these modules
likely relies upon cortical information, such as that seen in
supplementary motor area and the anterior cingulate (Doya,
1999). Both areas were strongly active in the contrasts comparing the
DOUBLE condition with the FOCUS and SINGLE conditions.
The notion that the cerebellum is associated with context-response
linkage has been discussed by Thach (1997). Anatomical studies
suggest cerebellar output is divided into multiple channels (Middleton
and Strick, 1997, 2000, 2001). Lobule VI, where our response-reassignment activation occurred, receives afferents from the
motor cortex and premotor cortex, as well as a more limited input from
the parietal lobe (Schmahmann and Pandya, 1997b). This region of the
neocerebellar cortex projects to the dentate. Progressing from dorsal
to ventral within the dentate, the cortical targets shift from motor
(motor and premotor cortex) to cognitive (prefrontal cortex). Our
activation locus may conceptually lie between the cognitive and motor
channels, providing the mechanisms of linking the cognitive context
with the appropriate motor set represented in premotor cortex (Kurata
and Wise, 1988).
Our results revealed cerebral cortical activation related to attention.
Comparisons of the DOUBLE and SINGLE conditions with the FOCUS
conditions showed activation within left middle frontal gyrus, left
anterior cingulate, left precuneus, and left inferior parietal lobule.
These areas indicate that higher order processing of the attentional
shifting did take place. We believe the involvement of the cerebellum
was on a lower level, because its activation was related to different
stimulus-response pairs in the DOUBLE condition. This motor processing
would rely on cues from further up the hierarchy to dictate the
appropriate response for the presented stimulus. Previous imaging
studies of attention set-shifting have indicated activation within the
prefrontal cortex, typically in the dorsolateral region, in relation to
increasing attentional or working memory demands (Nagahama et al.,
1996; Cohen et al., 1997; Courtney et al., 1997). Our results did not
illustrate dorsolateral activation; however, comparisons of the DOUBLE
condition to the FOCUS and SINGLE conditions showed strong activation
within the right inferior frontal gyrus (areas 44 and 47). This area
may be involved in the maintenance of information for working memory (Cohen et al., 1997) and has been shown to be transient in nature (Konishi et al., 1998), possibly reflecting the transition between cognitive sets. Konishi et al. (1998) also reported a transient activation in bilateral supramarginal gyrus related to cognitive set
shifting. Our results were more anterior; the left inferior parietal
(area 40) was active in all contrasts except those comparing the FOCUS
condition with either the SINGLE or DOUBLE conditions, where set
shifting did not occur.
The parietal cortex has been implicated in attentional tasks as well,
but primarily in those involving spatial attentional shift (Corbetta et
al., 1993). Our task involved no spatial information because the
targets all appeared at the center of the screen. We did find parietal
activation within precuneus in contrasts where the DOUBLE or SINGLE
conditions were compared with the focused attention conditions. It has
been suggested that the precuneus may be active when switching
attention between object features (Nagahama et al., 1999), as well as
during memory retrieval (Fletcher et al., 1998). The preceding
contrasts all involve comparisons of attentional shift.
In sum, our results fail to support the hypothesis that the cerebellum
is involved with attentional set shifting. Rather, our results indicate
that cerebellar activation is correlated with response reassignment.
This function is in accordance with previous theories of cerebellar
involvement in motor coordination and timing. The ability to change
sensorimotor set in response to contextual stimuli is a necessary
element in motor coordination.
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FOOTNOTES |
Received Aug. 15, 2001; revised Oct. 17, 2001; accepted Nov. 1, 2001.
This work was supported by Public Health Service Grant NS33504 and the
James S. McDonnell Foundation. The data reported in this experiment
have been deposited at the fMRI Data Center with accession number
2-2001-1127J (http://www.fmridc.org/database/?accession=2-2001-1127J).
Correspondence should be addressed to Scott T. Grafton, Center for
Cognitive Neuroscience, Dartmouth College, 6162 Moore Hall, Hanover, NH
03755. E-mail: Scott.T.Grafton{at}dartmouth.edu.
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ABSTRACT
INTRODUCTION
