July, 2005
In Press, Neuropsychologia
L-Dopa Impairs Learning, But Spares Generalization, in
Parkinson’s Disease
Daphna Shohamy1, Catherine E. Myers2, Kindiya D. Geghman3, Jacob Sage4,
and Mark A. Gluck3
1
Department of Psychology, Stanford University
2
Department of Psychology, Rutgers University
3
Center for Molecular and Behavioral Neuroscience, Rutgers University
4
Department of Neurology, UMDNJ/Robert Wood Johnson Medical School, New
Brunswick, New Jersey
Corresponding Author: Daphna Shohamy
Department of Psychology
Stanford University
Jordan Hall, Bldg. 420
Stanford, CA
USA
Tel: (650) 724-9515
Fax: (650) 725-5699
Word Count: 5617
Dopamine and Learning in Parkinson’s Disease
ABSTRACT
In this study we examined the effect of dopaminergic modulation on
learning and memory. Parkinson’s patients were tested on vs. off dopaminergic
medication, using a two-phase learning and transfer task. We found that
dopaminergic medication was associated with impaired learning of an
incrementally acquired concurrent discrimination task, while patients withdrawn
from dopaminergic medication performed as well as controls. In addition, we
found a dissociation of the effect of dopamine within a single two-phase task:
patients tested on medication were not impaired at the ability to generalize based
on learned information. The deficit among medicated patients appeared to be
related specifically to the concurrent, incremental, feedback-based nature of the
task: such a deficit was not found in a version of the task in which demands for
concurrent error-processing learning were reduced. Taken together with a
growing body of evidence emphasizing a role for midbrain dopamine in errorcorrecting, feedback-based learning processes, the present results suggest a
framework for understanding previously conflicting results regarding the effect of
medication on learning and memory in Parkinson’s disease.
Keywords: Dopamine, Learning, Memory, Basal Ganglia, Cognition
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Dopamine and Learning in Parkinson’s Disease
INTRODUCTION
Converging evidence suggests that the midbrain dopamine system plays an
important role in learning and memory. Electrophysiological studies have shown
that midbrain dopamine neurons may contribute to reward-related or noveltyrelated learning (Schultz et al., 1997; Schultz, 2000, 2002; Horvitz, 2000).
Functional imaging studies in humans have also indicated a role for midbrain
dopamine regions in several aspects of incremental learning, such as in the
processing of reward, of expectancy of reward, and of error-correcting feedback
(Poldrack et al., 2001; Delgado et al., 2000,2004; Knutson et al., 2001, Aron et
al, 2004).
Neuropsychological studies of patients with dopamine dysfunction have also
shown that midbrain dopamine may play an important role in particular types of
learning and memory. In Parkinson’s disease, there is a profound loss of
dopamine-containing neurons in the substantia nigra compacta (SNc), leading to
dopamine depletion in the striatum. Studies have shown that the loss of
dopamine that occurs in Parkinson’s disease leads to a variety of learning and
memory deficits, particularly on tasks that involve incremental, feedback-based
learning of cue-outcome associations (Gotham et al., 1988; Canavan et al., 1989;
Vriezen and Moskovitch, 1990; Knowlton et al., 1996; Swainson et al., 2000;
Cools et al, 2001a, 2001b; Myers et al., 2003; Shohamy et al., 2004a, 2004b). By
contrast, Parkinson’s patients are generally not impaired on performance of tasks
which involve declarative, non-feedback based learning, or tasks that require
3
Dopamine and Learning in Parkinson’s Disease
flexible use of knowledge (Knowlton et al., 1996; Myers et al., 2003, Shohamy et
al., 2004a) - functions which are thought to rely on the medial temporal lobe
(Squire and Zola, 1996; Eichenbaum, 2002; Gluck and Myers, 1993; Gabrieli,
1998; Robbins, 1996). Taken together, these findings imply that modulation of
dopamine levels in Parkinson’s disease should have selective effects on learning
and memory function depending on the specific task demands.
Studies examining the effect of dopaminergic medication on cognitive function
in Parkinson’s disease are generally consistent with this idea. Parkinson’s
disease is most commonly treated with L-dopa, a dopamine precursor
synthesized into dopamine in the brain leading to increased dopamine levels.
Studies which specifically examined the effect of L-dopa treatment on cognition
suggest that the effect of L-dopa depends on the specific task demands – with Ldopa sometimes remediating, sometimes having no effect, and sometimes
impairing cognition (Gotham et al., 1988; Swainson et al., 2000; Cools et al.,
2001a, Fournet et al., 2000; Mattay et al., 2002; Frank et al., 2004). However,
most of these prior studies focused on ‘frontal’-like executive function tasks (such
as working memory, planning, and set-shifting) and did not directly examine
learning and memory per se. For example, Parkinson’s patients are impaired on
the Tower of London task and associated spatial working memory tests, and Ldopa ameliorates this deficit (Lange et al., 1992; Owen et al., 1992; Owen et al.,
1993). Overall, there is considerable evidence suggesting that L-dopa often
improves cognitive performance on tasks that depend on ‘frontal’ executive or
working memory processes, especially in mild to moderate Parkinson’s patients.
4
Dopamine and Learning in Parkinson’s Disease
By contrast, less is known of the impact of L-dopa on learning and memory, and
most studies reporting learning and memory impairments in Parkinson’s disease
have tested only medicated patients (e.g., Canavan et al., 1989; Knowlton et al.,
1996; Myers et al., 2003, Shohamy et al., 2004a, 2004b). Recent studies have
begun to examine the effect of L-dopa on learning and memory. These have
shown that L-dopa sometimes improves and sometimes worsens performance,
depending on the specific task demands (Frank et al., 2004; Swainson et al.,
2000; Cools et al., 2001; Czernecki et al., 2001). For example, Cools et al. (2001)
demonstrated that L-dopa impaired performance on a probabilistic reversal task,
but facilitated task-switching performance in the same patients. Frank et al.
(2004) examined the effect of L-dopa on a reinforcement based learning task,
and found that L-dopa impaired learning that was based on negative outcomes,
but facilitated learning that was based on positive outcomes. These findings
emphasize the fact that the effects of L-dopa can differ even within a single task,
depending on highly specific modifications to the task demands.
Understanding the circumstances under which L-dopa facilitates or impairs
learning and memory is important not only from a clinical perspective, but could
also potentially provide important insights into the neural mechanisms underlying
the role of dopamine in learning and memory. In particular, electrophysiological
studies demonstrate that midbrain dopamine neurons respond to behaviorally
important stimuli in a temporally-specific, stimulus-specific manner: the signal
occurs only in response to certain stimuli, and it is rapid and brief (Schultz et al.,
1997; Schultz, 2002; Horvitz, 2000). These studies suggest that phasic dopamine
5
Dopamine and Learning in Parkinson’s Disease
signals (as opposed to tonic, ongoing dopamine release) may be critical for
learning that involves incremental acquisition of stimulus-outcome associations
via error-correcting feedback.
L-dopa, however, is thought to cause global increases in tonic dopamine
levels in target areas, such as the neostriatum, consistent with recent
pharmacological studies in rodents suggesting that L-dopa acts via nondopaminergic neurons (Miller & Abercrombie, 1999; Yamato et al., 2001; Tanaka
et al., 1999). If midbrain dopamine signals are indeed critical for providing
stimulus-specific, feedback-based information, enhanced levels of dopamine in
the striatum coming from the ‘wrong’ neurons at the ‘wrong’ time may disrupt or
mask critical stimulus-specific and temporally-specific signals essential for
feedback-based error-correction learning.
The purpose of the present study was to examine the effect of L-dopa on
learning and memory in patients with mild to moderate Parkinson’s disease,
using an incremental learning task. In this task, subjects are presented with a
series of pairs of objects, and are required to learn to respond to the rewarded
object in each pair. This task is similar to other incremental learning tasks
previously shown to be impaired in Parkinson’s patients (e.g. Canavan et al.,
1989; Myers et al., 2003). In addition, we sought to assess whether the effects of
medication are specific to incremental learning. To that end, following acquisition,
subjects were tested on a transfer/generalization phase, in which they were
required to use what they have learned in the first phase to predict rewarded
objects among a new set of stimuli. This kind of transfer has been shown to rely
6
Dopamine and Learning in Parkinson’s Disease
on the medial temporal lobe (Myers et al., 2003; Eichenbaum et al., 1989;
Preston et al., 2004), and is expected to be intact in patients with Parkinson’s
disease. In addition, given that transfer is not based on trial-by-trial feedback,
rather presumably on representational changes that occur over time,
performance on the transfer phase would not be expected to be affected by Ldopa.
Finally, we sought to assess which specific aspects of incremental learning
might be most critical in contributing to learning deficits in Parkinson’s disease.
Drawing on electrophysiological, modeling and neuroimaging evidence for the
role of midbrain dopamine regions in error-correcting feedback-based learning,
we hypothesized that L-dopa would impair learning processes that rely on such
error-correcting feedback, but might spare learning that does not involve such
processes. To that end, in Experiment 2 we manipulated the degree to which
learning involved error-processing and compared learning under concurrent
learning conditions, with learning of the same task in a shaping (reduced error)
condition. We predicted that while the concurrent incremental learning task might
be impaired with L-dopa, the reduced-error shaping version would be spared.
Overall, we expected this study to shed light on the effect of L-dopa on
incremental learning, on the degree to which this effect is specific to incremental
feedback-based learning, as opposed to transfer, and the degree to which it is
affected by error-correction processes.
7
Dopamine and Learning in Parkinson’s Disease
Experiment 1
METHODS
Participants
Participants included 24 individuals with a diagnosis of idiopathic
Parkinson’s disease, randomly assigned to be tested ‘on’ medication (n=12; 7
men, 5 women), or ‘off’ medication (n=12; 8 men; 4 women). Patients for this
study were recruited from the Parkinson’s disease clinic at ColumbiaPresbyterian Medical Center (New York) and from the motor disorders clinic,
Robert Wood Johnson University Hospital (New Jersey), having met diagnostic
criteria for Parkinson’s disease as assessed by a neurologist and having given
informed consent to participate.
Parkinson’s patients were in the mild to moderate stages of the disease,
with scores on the Hoehn-Yahr scale of motor function (Hoehn and Yahr, 1967)
that ranged from 1 to 3 (in the ‘on’ state). All Parkinson’s patients were nondemented. Parkinson’s patients were also screened for clinical depression, as
indicated by scores below 15 on the Beck Depression Inventory (Beck, Steer and
Brown, 1996). All patients included in the study were treated with L-dopa, were
stable on their medication doses for at least 3 months, and were responding well
to the medication. Four subjects were also receiving treatment with dopamine
agonists (two each in the off/on medication subgroups, either pramipexole or
ropinirole). None of the patients were being treated with anti-cholinergic
medication, nor with anti-depressants). Patients in the ‘on’ medication group
8
Dopamine and Learning in Parkinson’s Disease
were tested within 2 hours since their last dose of medication. Patients tested ‘off’
medication had refrained from taking medication for a minimum of 16 hours.
An equivalent number of age-matched healthy controls (n=12; 5 male; 7
female) were recruited and were screened for the presence of any neurological
disorder or history of psychiatric illness including depression. Patient and control
information is presented in Table 1. Controls did not differ significantly from the
Parkinson’s ‘on’ or ‘off’ groups on age, education, or mini-mental state exam
(MMSE; Folstein, Folstein and McHugh, 1975) [ANOVA, group ('on','off','control')
by age, education, or MMSE, p>0.5].
All studies conformed to research guidelines established by Rutgers
University and the Federal Government.
Disease
Duration
PD on
PD off
Control
Hoehn
and Yahr
(at test)
2.3
UPDRS
(on)
6.1
Hoehn
and Yahr
(‘on’)
2.3
Age
Education
MMSE
BDI
26.4
UPDR
S
(at test)
26.4*
64.5
16.0
29.6
6.8
(1.2)
(0.1)
(0.1)
(4.4)
(4.4)
(1.5)
(1.0)
(0.2)
(1.0)
6.7
2.2
2. 6
27.5
46.1*
62.1
16.8
29.4
6.7
(1.2)
(0.2)
(0.2)
(3.8)
(4.2)
(2.3)
(0.5)
(0.4)
(1.2)
-
-
-
-
-
65.0
15.7
28.9
-
(1.9)
(0.9)
(0.4)
Table 1. Demographic and disease information for patients (PD) and
controls (MMSE = Mini Mental State Exam; UPDRS = Unified Parkinson’s
Disease Rating Scale; BDI = Beck Depression Inventory; Duration, age,
and education in years. SE in parentheses). *Significantly different at
p=0.05 level.
9
Dopamine and Learning in Parkinson’s Disease
Behavioral Task
General Description
The task consisted of two phases. In Phase 1 (acquisition) subjects learned a
concurrent discrimination. Subjects were presented with a series of pairs of
objects, and on each trial were required to predict which of two objects was
associated with reward. Each pair of objects differed in either color or shape, but
not both, so that there was one relevant and one irrelevant dimension to the
discrimination. In phase 2 (transfer), the pairs of objects continued to differ along
the previously relevant dimension, but the irrelevant dimension changed. Sample
trial events are shown in Figure 1.
Apparatus
Behavioral experiments were automated on an iBook computer
programmed in the SuperCard language (Allegiant Technologies, San Diego,
CA). Testing took place in a quiet room, with the participant seated in front of the
computer at a comfortable viewing distance. The keyboard was masked except
for two keys, labeled “LEFT” and “RIGHT” which the participant could press to
record a response.
10
Dopamine and Learning in Parkinson’s Disease
Stimuli
The stimuli and procedures of Experiment 1 replicated those used in an
earlier study (Myers et al., 2002). Phase 1 of the experiment was a concurrent
discrimination. Stimuli consisted of sixteen colored shapes, organized into eight
discrimination pairs. Four of the pairs differed in color (relevant feature) but not in
shape (irrelevant feature); four pairs differed in shape (relevant feature) but not
color (irrelevant feature). Within each discrimination pair, one stimulus was
designated as rewarded. Assignments of particular color, shape and reward to
discrimination pairs were made according to a pseudorandom procedure, but
were held constant across the experiment. The full stimulus set is shown in
Figure 2.
Phase 2 of the experiment was a transfer test. Stimuli consisted of
sixteen colored shapes which were partial recombinations of the shape and color
features in phase 1: Each of the eight discrimination pairs was organized around
the same relevant features as in phase 1; only the irrelevant features were novel.
The features that were rewarded in phase 1 were also rewarded in phase 2.
Thus, a set of response rules that emphasized the relevant features in phase 1
would perfectly predict the rewarded stimuli in phase 2. Alternatively, a set of
response rules that emphasized the entire stimulus (including relevant and
irrelevant features) in phase 1 would not transfer well to the new feature
combinations in phase 2.
Insert Figure 1 about here.
11
Dopamine and Learning in Parkinson’s Disease
Procedure
At the start of the experiment, the following instructions appeared on the
screen: “Welcome to the experiment. You will see pairs of objects. Each time,
there is a smiley face hidden under one of the two objects. It looks like this:
.
Find as many as you can.” The experimenter read these instructions aloud and
then clicked the computer mouse button to begin phase one of the experiment.
On each trial of phase 1, participants saw one of the eight discrimination
pairs. Trials were organized into blocks, each containing 16 trials: one
presentation of each discrimination pair in each possible left-right ordering. Trials
in a block occurred in a pseudorandom but fixed order. Figure 1A shows screen
events in a typical trial. Below the stimuli, a prompt appeared: “Which object is
the smiley face under? Use the “LEFT” or “RIGHT” key to choose.” Participants
then responded by pressing one of the two labeled keys. If it was the rewarded
stimulus, a smiley face icon was revealed underneath and displayed for one
second. The object then returned to its original position, obscuring the smiley
face icon below. The objects were then removed and a new trial initiated. There
was no limit on response time. Phase 1 continued until the participant completed
16 consecutive trials correctly, or for a maximum of 96 trials (6 blocks).
Insert Figure 2 about here.
As soon as phase 1 terminated, phase 2 began without any warning that
task demands had shifted. The screen events were identical to phase 1 (Figure
12
Dopamine and Learning in Parkinson’s Disease
1B) except that the discrimination pairs were altered as described above. Again,
trials were organized into blocks of 16 trials, one with each discrimination pair in
each possible left-right ordering, in a pseudorandom but fixed order. Phase 2
continued until the participant completed 16 consecutive trials correctly, or to a
maximum of 48 trials (3 blocks).
The entire procedure, including phase 1 and phase 2, took approximately
15-20 minutes to complete.
Data Collection
On each trial, the computer recorded the discrimination pair, its left-right
ordering, the desired response, and the participant’s response. For both phases,
the total errors in each phase was recorded.
RESULTS
Acquisition (Phase 1)
All healthy control participants, and all but one participant in the 'off'
medication group, reached performance criterion of phase 1 within the 96 trial
maximum. By contrast, seven participants in the ‘on’ medication group failed to
13
Dopamine and Learning in Parkinson’s Disease
reach the performance criterion in phase 1. Overall, this was a significant
difference [Chi-square, χ2 (2)= 13.8, p<0.001)].
Figure 3A shows the mean total errors for each group in phase 1. An
analysis of variance (ANOVA) with phase 1 errors as the dependent variable and
group (Parkinson’s ‘on’ med, Parkinson’s ‘off’ med, controls) as the independent
variable revealed a significant difference in phase 1 performance [F(2,33)=9.3,
p<0.001]. Post-hoc Tukey pairwise comparisons revealed that this effect was
due to significantly more errors in the Parkinson’s ‘on’ medication group
compared with either the Parkinson’s ‘off’ group (p<0.01) or the control group
(p<0.001), while the Parkinson’s ‘off’ group did not differ significantly from the
control group (p=0.7). There was no effect of gender, age, or motor score on
performance (all p>0.05).
Although too small a number of subjects in the PD ‘on’ group reached
criterion on phase 1 (n=5) to allow separate statistical analyses of phase 1
performance in this subgroup, a comparison of the mean number of errors
among this group suggested that, similar to those subjects that failed to reach
criterion performance, they made more errors during acquisition than either the
control or the PD ‘off’ group (mean number of errors 24.1, SE = 4.9). This
subgroup of non-learners also did not differ substantially on any demographic or
medication measures (no differences in age, education, MMSE, stage of disease,
or years since onset; the distribution of gender and of subjects treated with
agonists was the same in both subgroups).
Insert Figure 3 about here.
14
Dopamine and Learning in Parkinson’s Disease
Transfer (Phase 2)
Following prior studies (Myers et al., 2002), phase 2 data from those
subjects who failed to reach criterion performance in phase 1 were excluded from
phase 2 analysis. This exclusion was necessary since any analysis of transfer
phase performance is illogical for a participant who failed to learn the
associations in phase 1. Indeed, those subjects who failed phase 1 also failed
phase 2 (mean number of errors among non-learners was 37.0, SE=6.04).
Among the remaining subjects (5 subjects tested ‘on’, 11 subjects tested
‘off’’, 12 control subjects), all subjects reached criterion performance in phase 2.
Figure 3B shows that the mean phase 2 errors was similar among all groups; an
analysis of variance (ANOVA) with phase 2 errors as the dependent variable
found no significant effect of group (F (2,25)=0.56, p=0.6), no effect of subject
gender, age, or motor score (all p>0.05).
Experiment 1: Discussion
Experiment 1 found that Parkinson’s patients tested ‘on’ L-dopa
medication were significantly impaired on an incremental learning task. This
impairment was not found in a group of matched patients who were tested while
withdrawn from dopaminergic medication for approximately 16 hours; these ‘off’
medication patients learned the task as well as healthy controls.
15
Dopamine and Learning in Parkinson’s Disease
This effect does not appear to be due to any general effects of L-dopa or
L-dopa withdrawal on motor or cognitive functioning. Withdrawing patients from
their medication in this manner does result in a temporary worsening of motor
symptoms (as evidenced by the difference in motor scores, shown in Table 1).
However, the effects of L-dopa do not appear to be due to general cognitive
changes, since L-dopa has been previously shown to either enhance or impair
cognitive function, depending on the task demands (e.g. Cools et al., 2001; Frank
et al., 2004).
Preliminary evidence suggests that the L-dopa related deficit was
selective to the incremental acquisition phase of the task, and was not found for
the transfer phase, where subjects were required to generalize what they had
learned to a set of new stimuli. This result replicates previous findings on a
similar task (Myers et al., 2003), which reported that Parkinson’s patients
(medicated) were slow to learn, but those that did learn were able to transfer as
well as control subjects (while individuals with hippocampal atrophy showed the
opposite pattern). The present study extends these findings and suggests that
the patients’ deficit on acquisition is associated specifically with the effects of
medication, while medication does not impair transfer. However, examining
transfer performance is dependent on the fact that subjects were able to reach
criterion learning in phase 1. Because many medicated patients in the present
study failed to reach criterion performance in phase 1, any conclusions regarding
performance on the transfer phase are limited, given that it is not clear to what
16
Dopamine and Learning in Parkinson’s Disease
degree the intact transfer performance might be biased by the fact that only
phase 1 learners were included in the analysis.
Experiment 2 aimed to address this issue, as well to gain a better
understanding of the specific cognitive processes affected by L-dopa in
Experiment 1. In particular, we sought to evaluate the extent to which the effect
of L-dopa on learning is modulated by error-correcting feedback. To that end, we
reduced the error load by developing a version of the task where subjects are
shown the correct outcome to each pair, and then each discrimination pair is first
trained to criterion prior to the introduction of the next pair (Experiment 2,
Shaping condition), and we compared performance on this reduced-error version
to a concurrent discrimination version (as in Experiment 1; Experiment 2,
Concurrent condition). In addition, with the aim of gaining better insight into
performance on the generalization/transfer phase, we reduced the memory load
of the task in both conditions, to allow more subjects to reach criterion
performance on phase 1.
Experiment 2
As suggested by recent elecrophysiological, neuroimaging and
neuropsychological studies, one possible interpretation of the L-dopa related
impairment found in Experiment 1 is that L-dopa selectively impairs errorcorrecting, feedback-based learning processes. Prior neuroimaging studies have
shown that while incremental trial-and-error learning depends on midbrain
dopaminergic regions, learning the same information without error-correcting
17
Dopamine and Learning in Parkinson’s Disease
feedback (i.e. by simply observing stimuli and outcomes) relies on distinct brain
regions, particularly the medial temporal lobes (Poldrack et al., 2001; Aron et al.,
2004). Consistent with these findings, we have shown recently that training
Parkinson’s patients on an ‘observational’ version of a probabilistic learning task
remediates learning impairments, while having no impact on performance among
control participants (Shohamy et al., 2004a). Therefore, we hypothesized that
modifying the present task by reducing demands for error-correcting feedback
might alleviate the L-dopa related deficit.
To evaluate the extent to which the effect of L-dopa on learning is modulated
by the role of error-correcting feedback, we revised the concurrent discrimination
task of Experiment 1 as follows. On the first trial with a new discrimination pair,
the subject was shown the correct answer. Additionally, initial training was done
by shaping; instead of interleaving all the discrimination pairs, subjects were
trained on one pair to criterion (several consecutive correct responses), then a
new pair was added and training continued until the subject reached criterion on
both, and so on until all the pairs were learned. These changes were intended to
reduce the need for trial-and-error learning and also to reduce the chances that
subjects would "guess" incorrectly on their first trial with a new stimulus.
18
Dopamine and Learning in Parkinson’s Disease
METHODS
Participants
Participants included 24 individuals with a diagnosis of idiopathic
Parkinson’s disease tested ‘on’ medication and an equivalent number of agematched healthy controls, randomly assigned to participate in the Concurrent
condition or the Shaping condition (n=12 for each group, each condition). Patient
recruitment and screening procedures were identical to those described in
Experiment 1, and subjects were taken from the same patient pool as
Experiment 1. Because the intention of Experiment 2 was to explore the basis of
the impairment found in Experiment 1 among Parkinson’s patients tested on
medication, all patients in Experiment 2 were tested on medication. As in
Experiment 1, all patients were being treated with L-dopa; a small number of
patients were additionally treated with dopaminergic agonists (n=3 in the
Concurrent condition; n=2 in the Shaping condition; either pramipexole or
ropinirole).
Patient and control information is presented in Table 2. There were no
significant differences in age or education between the groups or the conditions
[ANOVA with age or education as dependent variables and condition
(Concurrent, Shaping) and group (Patients, Controls) as independent variables,
all p>0.5].
19
Dopamine and Learning in Parkinson’s Disease
PD
Concurrent
PD
Shaping
Controls
Concurrent
Controls
Shaping
Parkinson’s
Disease
Duration
6.3
Hoehn
andYahr
UPDRS
Age
Educatio
n
MMSE
BDI
2.0
25.5
65.0
16.3
28.8
7.2
(1.1)
(0.2)
(3.9)
(3.0)
(0.9)
(0.4)
(1.5)
5.9
2.1
24.2
63.4
16.3
28.5
7.1
(0.3)
(0.1)
(2.4)
(1.7)
(0.2)
(0.1)
(3.6)
-
-
-
61.0
15.6
29.7
-
(3.0)
(0.5)
(0.1)
64.5
16.4
28.9
(3.3)
(0.7)
(0.4)
-
-
-
-
Table 2. Demographic and disease information for patients and
controls (MMSE = Mini Mental State Exam; UPDRS = Unified Parkinson’s
Disease Rating Score; BDI = Beck Depression Inventory; Duration, age,
and education in years. SE in parentheses).
Behavioral Task
Apparatus & Procedure
Concurrent condition
In this condition, subjects were required to learn a concurrent
discrimination task identical to that described in Experiment 1, except that this
version required subjects to learn a reduced number of stimulus-outcome
associations - 6 object pairs in the present experiment, compared to 8 in
20
Dopamine and Learning in Parkinson’s Disease
Experiment 1. All other procedures were identical to those described in
Experiment 1.
Shaping condition
In this condition, subjects were required to learn the same 6 stimulusoutcome associations as in the concurrent condition, but here the associations
were learned using a shaping paradigm: For each stimulus, subjects first
observed a single trial where they saw a pair of objects, and saw the correct
answer revealed by the computer (without making a response; “observational”
trial). Subsequently, the subject was presented with “standard” response-based
trials, for that particular pair (for each trial, the subject responded “left” or “right”
based on what they thought the correct object was, followed by responsecontingent feedback). After reaching a criterion of 4 correct consecutive
responses (or a maximum of 12 trials) for each object pair, subjects were
presented with a new pair introduced by a single observational trial, subsequently
followed by response-feedback trials. For each sub-phase of this task, subjects
were tested on the new pair, as well as on all previously learned pairs, gradually
building up towards the full set of 6 pairs. Thus, importantly, the last phase of
acquisition on the Shaping task was identical to all phases of the Concurrent
task.
All other procedures were identical across conditions.
21
Dopamine and Learning in Parkinson’s Disease
RESULTS
Acquisition (Phase 1)
In the Concurrent condition, one control and six of the Parkinson’s patients
failed to reach the performance criterion in phase 1. This was a near-significant
difference [Chi-square, χ2 (1)= 3.23, p<0.07)]. By contrast, in the Shaping
condition, all subjects in both groups reached criterion performance in phase 1.
Figure 4A shows the mean errors for Parkinson’s patients and controls on
acquisition of the Concurrent condition, compared with the Shaping condition.
Consistent with our prediction, Parkinson’s patients were impaired at learning the
Concurrent condition, but were not impaired at learning the Shaping condition.
An ANOVA on number of errors (dependent variable) by group and condition
(independent variables) revealed a significant main effect of condition
[F(1,44)=25.52, p<0.001], a main effect of group [F(1,44)=6.26, p<0.05] and a
significant group X condition interaction [F(1,44)=13.12, p<0.001]. Post hoc
Tukey analyses confirmed that this was due to a significant difference between
Parkinson’s and controls on the Concurrent condition (p<0.001), but not on the
Shaping condition (p=0.8). Post hoc analyses of performance across conditions
showed that Parkinson’s patients were significantly worse on the Concurrent
condition compared to the Shaping condition (p < 0.001), whereas there was no
difference between the conditions for control subjects (p = 0.7).
Insert Figure 4 about here.
22
Dopamine and Learning in Parkinson’s Disease
Transfer (Phase 2)
Phase 2 data from those subjects who failed to reach criterion
performance in phase 1 were excluded from phase 2 analysis. Among the
remaining subjects (6 Parkinson’s patients and 11 controls on the Concurrent
condition, 12 Parkinson’s patients and 12 controls on the Shaping condition), all
subjects reached criterion performance in phase 2 for both conditions. Figure 4B
shows that the mean phase 2 errors was similar among patients and controls and
across conditions. An ANOVA on number of errors (dependent variable) by group
and condition (independent variables) revealed no significant main effects or
interactions [Main effect of group, F(1,37)=2.0, p=0.2; main effect of condition,
F(1,37)=0.7, p=0.4; group by condition interaction, F(1,37)=0.04, p=0.8].
Experiment 2: Discussion
In Experiment 2, we sought to evaluate the extent to which L-dopa related
impairments on a concurrent learning task are modulated by error-correction
processes. The findings from this experiment replicate those from Experiment 1,
showing that medicated Parkinson’s patients are impaired at a concurrently
trained incremental learning task even when the task involves reduced memory
load. More importantly, we found that Parkinson’s patients were not impaired on
23
Dopamine and Learning in Parkinson’s Disease
this task when they were trained on a ‘shaping’ version designed to involve
reduced error-correcting processes.
It is important to note that in both versions subjects ultimately learn to
make the correct responses to an identical number of concurrent discriminations,
with the final phases of the shaping version identical to the concurrent version.
Thus, the critical difference between the conditions lies in the learning process,
and the degree to which this process relies on trial-by-trial error processing. It
seems unlikely that the differences between Parkinson’s patients’ performance
on the two tasks are due to reduced loads in a general learning mechanism,
given that performance among controls did not differ significantly between the
two versions (and given that the errors among controls do not indicate a ceiling
effect).
In contrast to the L-dopa related learning impairment on acquisition, we
found that both medicated and non-medicated patients performed normally on
the transfer phase of the task, when they were required to generalize what they
had learned to a novel context.
These findings are consistent with recent electrophysiological and
neuroimaging studies implicating midbrain dopamine in error-correcting feedback
processes (e.g. Schultz, 2000; Aron et al., 2004). [add more discussion and refs
to imaging papers]. These results are also consistent with recent studies with
Parkinson’s patients demonstrating that while Parkinson’s patients are impaired
on a feedback-based incremental learning task, they are not impaired on a nonfeedback ‘observational’ version of the same task (Shohamy et al., 2004a). It is
24
Dopamine and Learning in Parkinson’s Disease
worth noting that in the prior study, the observational version eliminated stimulusdependent responding, in addition to feedback; in the present study, by contrast,
subjects were still required to produce stimulus-related responses to learn the
correct outcome.
GENERAL DISCUSSION
The present study found that Parkinson’s patients tested in a dopamine
replete state, shortly after receiving dopaminergic medication, were impaired on
an incremental learning task, while patients tested off medication, in a dopamine
deplete state, performed as well as controls on the same task. These detrimental
effects of dopaminergic medication were not found when subjects were required
to learn same task in a ‘shaping’ version which involved decreased errorprocessing demands (while control subjects performed similarly under both
conditions). Furthermore, the effects of medication were specific to learning:
when the same subjects were challenged to generalize what they had learned to
a novel context, Parkinson’s patients performed as well as controls, regardless of
whether they were tested ‘on’ or ‘off’ medication. These findings suggest that Ldopa is associated with learning impairments which are selective to concurrent,
feedback-based learning of incrementally acquired associations.
The results of the current study provide behavioral evidence from humans
that dopaminergic systems are critically involved in incremental learning. The
25
Dopamine and Learning in Parkinson’s Disease
present findings converge with recent electrophysiological, computational and
neuroimaging evidence for the role of midbrain dopamine systems in errorcorrecting, feedback-based learning processes (Schultz et al., 2002; Poldrack et
al, 2001; Aron et al., 2004; Knutson et al., 2001). Specifically, recent imaging
studies have suggested that the dorsal striatum, in particular (which is particularly
affected by early stages of Parkinson’s disease) is important in reward and
feedback-based learning (Haruno et al., 2004; Delgado et al., 2005; King-Casas
et al., 2005). As such, our findings propose a framework for understanding
previously conflicting results regarding the effect of L-dopa on learning and
memory function in Parkinson’s disease.
Specifically, based on the wealth of recent evidence indicating that the
midbrain dopamine system plays an important role in stimulus-specific, feedbackbased learning, we have hypothesized that global increases in dopamine
following L-dopa treatment may obstruct the learning-related temporally-specific,
stimulus-specific dopamine signal in mild to moderately affected Parkinson’s
patients, by providing the ‘wrong’ signal at the ‘wrong’ time. The results of the
present study are consistent with this hypothesis, demonstrating an L-dopa
related impairment on a concurrent, incremental learning task, which is alleviated
when the error-processing, feedback-based demands of the learning task are
reduced (despite the fact that both tasks involve learning identical sets of
associations, and despite the fact that control subjects perform similarly on both
tasks). Further, the detrimental effects of L-dopa are selective to the learning
26
Dopamine and Learning in Parkinson’s Disease
phase of the task, and do not appear when the same patients are required to
transfer what they have learned to a novel context.
This pattern of impaired learning and spared transfer is exactly opposite to
the pattern of impairments observed in individuals with damage to the
hippocampal system on this task (Myers et al., 2002) and other tasks (e.g. Myers
et al., 2003; Schacter, 1985). Thus, these findings fit in with recent evidence
suggesting that cortico-striatal and hippocampal brain systems play distinct roles
in learning and memory, with the cortico-striatal system contributing to
incremental, stimulus-response learning, and the hippocampal system
contributing to the formation of flexible, episodic, stimulus-stimulus
representations (e.g. Gabrieli, 1998, Squire and Zola, 1996; Poldrack et al.,
2001, Myers et al., 2003, Shohamy et al., 2004a).
The present results further emphasize the role of dopamine in modulating
incremental learning, and suggest that the incremental learning deficits found in
Parkinson’s patients in prior studies (e.g. Knowlton et al., 1996; Myers, 2003;
Shohamy et al., 2004a, 2004b) may be due, at least in part, to disruption of
dopaminergic transmission with L-dopa, rather than the disruption of striatal
function caused by the disease itself.
The finding that L-dopa differentially impacts cognitive function depending
on task demands is consistent with recent findings. Cools et al. have proposed
that L-dopa mediated dopamine “overdose” may account for the differential
effects of L-dopa on different attentional and executive function tasks, with Ldopa alleviating deficits in dopamine-depleted circuits, but causing impairments
27
Dopamine and Learning in Parkinson’s Disease
in non-depleted circuits (Cools et al., 2001a). In support of this hypothesis, they
found that L-dopa impaired probabilistic reversal learning but enhanced taskswitching performance. It is interesting to note that in the Cools et al. study, the
two tasks differ not only in the neural circuitry they are presumed to rely on, but
also in the kinds of learning processes they involve. In particular, while the
probabilistic reversal (impaired with L-dopa) involves feedback-based learning
that relies on temporally specific, stimulus-specific information, the task-switching
ability (remediated with L-dopa) does not. It is worth emphasizing that these two
hypotheses regarding the impact of L-dopa on cognitive function are not mutually
exclusive and could both be factors in understanding how and where L-dopa
improves or impairs cognitive function in Parkinson’s disease. In fact, given that
L-dopa is provided systemically, one would expect effects on cognition to be
mediated at the synaptic level within the midbrain, as well as more globally in
widespread neural circuitry linking the striatum with frontal cortex.
Examining the effects of L-dopa at both these levels will be critical for fully
understanding why L-dopa sometimes impairs and sometimes facilitates
performance. Several studies have reported positive effects of L-dopa on
cognitive performance on varying tasks (e.g. Cools et al., 2001; Frank et al.,
2004; Shohamy et al., 2005). We have previously reported positive effects of Ldopa on a sequence learning task, where subjects were required to learn to
predict chains of events leading to reward (Shohamy et al., 2005). The
dissociation of the effect of L-dopa on concurrent learning vs. sequence learning
indicates that performance of these two tasks relies on dissociable cognitive and
28
Dopamine and Learning in Parkinson’s Disease
neural processes. The present hypothesis taken together with the Cools et al.
(2001) hypotheses regarding effects of L-dopa on frontal function suggest that
one reason for this dissociation may be that the sequence learning task relies
more heavily on frontal-based working memory and attention processes, as
compared to the present task. Future studies will examine potential interactions
between these two proposed consequences of L-dopa medication, as well as
how such interactions may explain differences in the effect of L-dopa on different
learning tasks.
The present results are also generally consistent with a more recent report
demonstrating differential effects of dopaminergic medication on positive vs.
negative reinforcement-based learning (Frank et al., 2004; Frank et al., in press).
This study used a different paradigm in which a series of probabilistic
competitions were held between two alternative stimuli, one of which was always
the winner, allowing separate analyses of positive vs. negative outcome based
learning. This study found that Parkinson’s patients tested off medication were
particularly impaired at learning from positive outcomes, compared to negative
outcomes, while dopaminergic medication reversed this effect: patients tested on
medication were particularly impaired at learning based on negative outcomes
compared to positive outcomes. These findings are conceptually similar to those
in the present study, demonstrating that Parkinson’s disease and dopaminergic
medication interfere with patients’ ability to process feedback. Future studies are
necessary to address the degree to which differential positive vs. negative based
29
Dopamine and Learning in Parkinson’s Disease
learning may contribute to the effects of medication on the kind of incremental
learning paradigms described here.
Conclusions
The present results suggest that dopaminergic treatment in Parkinson’s
disease is associated with impairments to learning and memory function. The
detrimental effect of dopaminergic medication can be understood in the context
of the role of midbrain dopamine systems in reward-related, error-correcting
incremental learning processes. As such, the present findings suggest a means
by which to understand the varied pattern of facilitated vs. impaired learning
processes in Parkinson’s patients following L-dopa treatment. In addition, the
present results shed light on the differential contributions of different brain
systems to learning and memory function, with a dopaminergic modulated
cortico-striatal system contributing to incremental, error-correcting feedbackbased learning, and a medial temporal lobe system supporting formation of
episodic, flexible, stimulus-stimulus representations.
30
Dopamine and Learning in Parkinson’s Disease
Acknowledgments
For his guidance and assistance with patient recruitment, the authors wish to
thank Dr. Lucien Cote of the Columbia Presbyterian Medical Center.
31
Dopamine and Learning in Parkinson’s Disease
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Figure Legends
Figure 1. Stimulus set used for concurrent discrimination and transfer. Each pair
of objects differed either by color or by shape. For transfer, the relevant
dimension stayed the same, while the irrelevant dimension was changed.
Figure 2. (A) Screen events on a sample trial of phase 1. Top: On each trial, the
discrimination pair is presented in either left-right order and a prompt appears.
Bottom: If the participant responds correctly (in this case the mushroom-like
shape), the chosen object is raised to reveal a smiley face icon underneath. (B)
Screen events on a sample trial of phase 2: events are similar to phase 1, but the
objects are changed so that the relevant dimension (here the shape) is the same,
whereas the irrelevant dimension (here the color approximated by grayscale) is
novel.
Figure 3. Performance on the incremental learning task described in Experiment
1; (A) Mean total errors (+/- SEM) on acquisition of the concurrent discriminations
(phase 1); patients tested on L-dopa were impaired, but those tested off L-dopa
were not (B) All groups performed equally well and made few errors on the
transfer phase (phase 2).
38
Dopamine and Learning in Parkinson’s Disease
Figure 4. Total errors (+/- SEM) on Concurrent versus Shaping conditions of the
incremental learning task (A) Parkinson’s patients tested on medication were
impaired on the Concurrent condition, but performed as well as controls on the
Shaping version (B) Both groups performed equally well on the transfer phase of
the task, for both the Concurrent and the Shaping conditions.
39
Dopamine and Learning in Parkinson’s Disease
A.
B.
Phase 2
Phase 1
Figure 1
40
Dopamine and Learning in Parkinson’s Disease
Discrimination pairs
Probe
Train
vs.
vs.
vs.
vs.
vs.
vs.
vs.
vs.
vs.
vs.
vs.
vs.
vs.
vs.
vs.
vs.
Phase 3 retraining
vs.
vs.
vs.
Figure
2
vs.
41
Dopamine and Learning in Parkinson’s Disease
B.
Phase 1
40
40
35
35
30
30
25
25
Total # Errors
Total # Errors
A.
20
15
20
15
10
10
5
5
0
0
CON PD ‘on’ PD ‘off’
Phase 2
CON PD ‘on’ PD ‘off’
Figure 3
42
Dopamine and Learning in Parkinson’s Disease
Phase 1
Phase 2
A.
B.
Controls
Controls
40
PD
35
35
30
30
25
25
Total # Errors
Total # Errors
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Concurrent
PD
Shaping
Concurrent
Figure 4
43
Shaping