1Department of Physiology,
Nakamura, Kae,
Katsuyuki Sakai, and
Okihide Hikosaka.
Effects of Local Inactivation of Monkey Medial Frontal Cortex in
Learning of Sequential Procedures.
J. Neurophysiol. 82: 1063-1068, 1999.
To examine the role of the
medial frontal cortex, supplementary motor area (SMA), and pre-SMA in
the acquisition and control of sequential movements, we locally
injected muscimol into 43 sites in the medial frontal cortex while
monkeys (n = 2) performed a sequential button-press task. In
this task, the monkey had to press two of 16 (4 × 4 matrix)
buttons illuminated simultaneously in a predetermined order. A total of
five pairs were presented in a fixed order for completion of a trial.
To clarify the differential contribution of the medial frontal cortex
for new acquisition and control of sequential movements, we used novel
and learned sequences (that had been learned after extensive practice).
We found that the number of errors increased for novel sequences, but
not for learned sequences, after pre-SMA inactivations. A similar, but
insignificant, trend was observed after SMA injections. The reaction
time of button presses for both novel and learned sequences was
prolonged by inactivations of both SMA and pre-SMA, with a trend for
the effect to be larger for SMA inactivations. These findings suggest
that the medial frontal cortex, especially pre-SMA, is related to the
acquisition, rather than the storage or execution, of the correct order
of button presses.
In a previous study (Nakamura et al.
1998 However, it is still possible that the pre-SMA activation could reflect
activity in other structures that contain the true learning mechanism.
In fact, it has been shown that multiple brain areas are activated
during new learning, such as the dorsolateral prefrontal cortex, the
parietal cortex (Sakai et al. 1998 General procedures
We used the same two Japanese monkeys (Macaca
fuscata) as in our recording experiment (Nakamura et al.
1998 In the 2 × 5 task (Fig.
1A), the monkey was asked to
press five pairs of buttons in the correct order. The animal began a trial by pressing the home key. Then, 2 of 16 target LEDs (4 × 4 grid) turned on simultaneously ("set"). The monkey had to press the
illuminated buttons in the correct order. A total of five sets
("hyperset") were presented in a fixed order for completion of a
trial. If the wrong button was pressed, the trial was aborted, and the
monkey had to start the trial again by pressing the home key. Each
hyperset was presented repeatedly in a block until 10-20 successful
trials had been performed. The monkeys performed "new hypersets,"
which were experienced for the first time, and "learned hypersets,"
which had been practiced extensively (almost every day) and could be
performed with few errors. The number of learned hypersets and the
duration of practice before the inactivation experiments was:
monkey BO, n = 16, >2 yr; monkey
GA, n = 10, > 8 mo.
ABSTRACT
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
INTRODUCTION
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
), we recorded neuronal activity in the medial frontal
cortex while the monkey performed a sequential button press task, known
as "2 × 5 task" (Hikosaka et al. 1995
).
Presupplementary motor area (pre-SMA) neurons were more active when the
animal performed new sequences than when it performed well-learned
sequences. The results suggested that the pre-SMA is related to the
acquisition of new sequences rather than the storage of procedural
memory. The results of our functional magnetic resonance imaging (MRI)
study (Hikosaka et al. 1996
) were also consistent with
this hypothesis.
), the lateral
premotor area, anterior cingulate area (Jenkins et al. 1994
; Jueptner et al. 1997
), cerebellum
(Friston et al. 1992
), and basal ganglia (Seitz
and Roland 1992
). To investigate this issue, we inactivated SMA
or pre-SMA locally by injecting muscimol (a GABA agonist) while the
monkey performed the sequential button press task, "2 × 5 task" (Hikosaka et al. 1995
). If the medial frontal
cortex is necessary for acquisition, not for performance of the learned
sequences, the inactivation should impair the monkey's ability to
learn new sequences but should not affect performance of well-learned sequences.
METHODS
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
): monkeys BO and GA. The surgical
procedures and the task have been described in a previous paper in
detail (Hikosaka et al. 1995
). Briefly, a head-holding
device and a chamber for unit recording and drug injection were
implanted under general anesthesia. A scleral search coil was implanted
in one eye for monitoring eye position (Judge et al.
1980
). All surgical and experimental protocols were approved by
the Juntendo University Animal Care and Use Committee and are in
accordance with the National Institutes of Health Guide for the Care
and Use of Animals.
View larger version (33K):
[in a new window]
Fig. 1.
A: 2 × 5 task. B: experimental
procedure. Performance was ensured to be normal before and on the next
day of the injection. C: task sequence and behavioral
measures. Only the events for the 1st 3 sets are illustrated.
Injection procedures
The injection sites were determined to be in the pre-SMA and SMA
by the results of microstimulation, unit recording using the electrode
attached to the injection tube, and histology (BO) or MRI
(GA) (Fig. 2). For the
pre-SMA, larger currents (40-80 µA) and more pulses (40-60 pulses)
were needed to evoke movements compared with the SMA (20-40 µA at 20 pulses) (Nakamura et al. 1998).
|
The injection device consisted of a stainless steel tube connected to a piece of a polyethylene tubing that was in turn connected to the tip of Hamilton syringe (10 µl). A tungsten microelectrode was attached to the tube's side. For each experiment, muscimol solution (5 µg/µl × 4 µl) was pressure-injected at a single site (Fig. 2).
Before the injection, we asked the monkey to perform several learned
and new hypersets to make sure that the monkey was well motivated and
the performance was within the normal range. After the injection, the
monkey performed 20-40 hypersets, which included both new and learned
hypersets. The hand used was changed alternately every four to five
hypersets (Fig. 1B). The postinjection data were analyzed
for the period after each injection for 180 min, although we obtained
the data for the period 200 min. We also obtained the data on the
next day of the experiment to see if any effects by injection remained.
Control data were obtained when no injection was made or after saline injections. Saline injections were made at several sites (4 sites for monkey BO, 5 sites for monkey GA, Fig. 2) where the muscimol injection showed a strong effect.
Data analysis
To assess the accuracy of performance, we counted the number of
errors to criterion (10 successful trials) for each block of the
experiment. To assess the speed of performance, we measured, for each
set, the button-press reaction time (BP-RT): the time from the stimulus
onset to the pressing of the first button and the movement time (MT):
the time between the releasing of the first button to the pressing of
the second button (Fig. 1C). We also calculated the
percentage of anticipatory saccades that started before the target
onset and ended within the area of the first target of the next set.
More frequent anticipatory saccades indicate the extent of long-term
learning (Miyashita et al. 1996).
![]() |
RESULTS |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
We injected muscimol at a single site in either the SMA or pre-SMA. All injection sites were located within the medial wall not including the upper bank of the cingulate sulcus. We infer that the spread of infused muscimol was restricted (probably within 2-3 mm in diameter) from the following results. First, we did not observe the effect when muscimol was injected outside of, but just beside the pre-SMA or SMA (sites indicated in Fig. 2, *). Second, after the experiment, we advanced the electrode attached to the injection tube deeper into the brain and observed neuronal activity at ~1.5 mm from the injection site.
Change of the number of errors
In the control condition, the number of errors for learned hypersets was much smaller than that for new hypersets (Fig. 3A). After muscimol injections, the number of errors for learned hypersets showed no significant changes. In contrast, the number of errors for new hypersets was significantly greater by muscimol injections into the pre-SMA compared with the control condition. A similar change was observed by the SMA injections, which, however, was not statistically significant (Bonferroni/Dunn test, P = 0.020 for monkey BO, P = 0.038 for monkey GA). The results obtained from two monkeys were very similar. Although the injection was unilateral, the effect was present regardless of which hand the monkey used. The errors took the form of pressing the other lighted button; we did not see errors made by pressing a button that was not illuminated.
|
Figure 3B shows the time course of the effect on the number of errors for new hypersets. After muscimol injections in the pre-SMA, not saline injections or no-injection experiments, the number of errors for new hypersets increased within 30 min and the effect lasted for >120 min. The number of errors returned to the normal range on the next day.
Change of the kinematic parameters
As shown in Table 1, the BP-RT increased after SMA and pre-SMA injections for both new and learned hypersets. For new hypersets, the increase in the BP-RT was significantly greater for the SMA than pre-SMA injections. For learned hypersets, we did not observe a consistent difference between SMA and pre-SMA injections. The MT became longer consistently for learned hypersets for both monkeys. The difference of MT between pre-SMA and SMA inactivations was not consistent between monkeys. The percentage of anticipatory saccades showed no change.
|
![]() |
DISCUSSION |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Effects on the performance of new sequences
The increase of errors for new hypersets, not for learned
hypersets, suggests that the pre-SMA contributes to the acquisition of
new sequential procedures. The SMA might play a similar role but less
potently, given the similar but insignificant effects of inactivation.
This result is consistent with our previous data, which show that the
pre-SMA became more active during learning of new sequences than
execution of well-learned sequences (Hikosaka et al.
1996; Nakamura et al. 1998
; Sakai et al.
1998
). However, deficient learning of new sequences could
reflect interruption of several factors other than learning per se,
such as novelty detection, selective attention, decision making, error
correction, switching motor plan, and memory coding and retrieval
(Nakamura et al. 1998
).
The effect of muscimol injections was relatively modest so that the
animals could reach the criterion after some errors. This modest effect
may be partly due to the fact that the injection was relatively small
and unilateral. Alternatively, it may indicate that the medial frontal
cortex plays only a partial role in acquisition of new sequences.
Studies from our laboratory have shown that other brain regions also
are related to learning of new sequences: the anterior part of the
striatum (Miyachi et al. 1997), dorsolateral prefrontal
cortex, precuneus, and intraparietal cortex (Sakai et al.
1998
). What is unique about the pre-SMA and SMA remains unresolved.
We also observed the prolongation of the reaction time (BP-RT)
for new hypersets. This effect was stronger for SMA injections in
contrast to the increase in the number of errors. The prolongation of
BP-RT might be due to the animal's uncertainty about the order of
button presses, but this view is not consistent with the fact that the
increase in the number of errors was greater by pre-SMA than SMA
injections. A second possibility is the slowness of movements measured
as the prolongation of movement time (MT), but this was true only for
monkey BO. A third possibility is that the SMA, rather
than the pre-SMA, is related to the acquisition of anticipatory execution of hand movements; we previously showed that the BP-RT becomes shorter with practice because the eye and hand move in an
anticipatory manner (Miyashita et al. 1996).
Execution of learned sequential procedure
We found no significant increase in the number of errors for
learned hypersets for either SMA or pre-SMA inactivation. This was true
even though there are some cells especially in the SMA that are
preferentially active for learned hypersets (Nakamura et al.
1998). However, this cannot simply be taken to indicate that
the medial frontal cortex is unrelated to long-term storage of
sequential procedures. That the SMA is related to sequential movements
has been shown repeatedly by single-unit recording studies (Tanji and Shima. 1994
) and human imaging studies
(Grafton et al. 1992
, 1994
, 1995
; Jenkins et al.
1994
; Seitz and Roland 1992
; van Mier et
al. 1998
). We consider the following possibilities to account
for the discrepancy. 1) Our localized and unilateral inactivation may have been ineffective as the intact hemisphere may
have compensated for the possible deficit. In fact, previous studies
have shown that bilateral lesions or inactivations disrupt the learned
performance of sequence tasks (Halsband 1987
;
Shima and Tanji 1998
). 2) The effect of
inactivation may vary with the kinds of sequential movements. In our
2 × 5 task, the targets of sequential movements are presented as
visuospatial patterns, unlike in the previous studies (Halsband.
1987
; Shima and Tanji. 1998
). 3)
The performance of new hypersets requires explicit selection of the
correct order similarly to those in previous studies (Barone and
Joseph 1989
; Kermadi and Joseph 1995
;
Mushiake and Strick 1993
; Mushiake et al.
1991
), whereas learned hypersets may be performed nearly
automatically as a continuous motor trajectory. This suggests that
different brain areas may mainly contribute to the performance of
learned hypersets, such as the cerebellar dentate nucleus (Lu et
al. 1998
), posterior striatum (Miyachi et al.
1997
), or M1 (Aizawa et al. 1991
).
On the other hand, BP-RT for learned hypersets increased consistently for both monkeys. This might be taken to suggest that the medial frontal cortex is necessary for the learned performance in terms of quick, anticipatory execution rather than correct execution. However, this hypothesis remains to be confirmed, because the MT also was increased, indicating the slowness of movement itself.
We showed previously that the skillful performance for learned
hypersets was associated with the co-occurrence of anticipatory saccades and anticipatory hand movements (Miyashita et al.
1996). The inactivation of the SMA or pre-SMA was not followed
by a decrease in the occurrence of anticipatory saccades, suggesting
that anticipatory saccades may be controlled by other brain areas.
![]() |
ACKNOWLEDGMENTS |
---|
We thank Dr. Makoto Kato for the computer programs, Drs. Carl Olson, Jeremy Goodridge, Longtang Chen, Rebecca Berman, and Carol Colby for extensive comments, and Drs. Jun Tanji, Kiyoshi Kurata, and their collaborators for advice on the experiment.
This work was supported by Uehara Memorial Foundation, Grant-in-Aid for Scientific Research on Priority Areas from The Ministry of Education, Science and Culture of Japan, and The Japan Society for the Promotion of Science (JSPS) Research for the Future Program.
Present address of K. Nakamura: Center for the Neural Basis of Cognition, Dept. of Neuroscience, University of Pittsburgh, 115 Mellon Institute, 4400 Fifth Ave., Pittsburgh, PA 15213-2683.
![]() |
FOOTNOTES |
---|
Address reprint requests to: O. Hikosaka, Dept. of Physiology, School of Medicine, Juntendo University, 2-1-1 Hongo, Bunkyo-ku, Tokyo 113, Japan.
The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
Received 25 February 1999; accepted in final form 13 April 1999.
![]() |
REFERENCES |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|