Alpha and beta changes in cortical oscillatory activity in a go/no go randomly-delayed-response...

Alpha and beta changes in cortical oscillatory activity in a go/no

go randomly-delayed-response choice reaction time paradigm

M. Alegrea,b, L. Imirizaldub, M. Valenciaa, J. Iriartea,b, J. Arcochab, J. Artiedaa,b,*

aNeurophysiology Laboratory, Neuroscience Area, Centre for Applied Medical Research, Universidad de Navarra, Pamplona, SpainbClinical Neurophysiology Section, Department of Neurology, Clı́nica Universitaria de Navarra, Avda. Pio XII, 36, Pamplona, Navarra 31008, Spain

Accepted 12 August 2005

Available online 28 November 2005

Abstract

Objective: Predictable movements induce oscillatory changes over the contralateral motor cortex that begin before the movement, but their

significance has not been fully established. We studied non-phase-locked changes in cortical oscillatory activity in a S1-centred double-

stimulus go/no go paradigm with random interstimulus interval.

Methods: About 58 reference-free EEG channels were analyzed by means of Gabor transforms in a group of 10 healthy subjects. A 2000 Hz

tone (S1go, 84% probability) indicated the subject to make a brisk wrist extension after a second 2000 Hz tone (S2go). The S1–S2 interval

was either 1.5, 3 or 4.5 s. A 1000 Hz tone (S1 no go, 16% probability) indicated the subject not to move (and wait for another S1 tone).

Results: A frontal 15 Hz synchronization was observed after S1 in all conditions. No further significant changes were observed in the no go

condition. A small pre-S2 alpha and beta desynchronization could be observed only in the 3 and 4.5 s-interval go conditions, being larger in

the latter.

Conclusions: These results suggest that the predictability of the timing of a movement influences the appearance of the pre-movement

oscillatory changes; not only motor planning (the ‘go’ decision) is necessary, but also an estimation of when to move.

Significance: Our findings provide new insight on the relationship between the decision-making process, movement, and cortical oscillatory

activity.

q 2005 International Federation of Clinical Neurophysiology. Published by Elsevier Ireland Ltd. All rights reserved.

Keywords: Event-related desynchronization; Event-related synchronization; Go/no go; Predictability; Timing; Frontal

1. Introduction

The possible role of cortical oscillatory activity as a

substrate of the binding mechanisms involved in sensory,

motor and cognitive processing has attracted much attention

during the last decade (Singer, 1993). Voluntary movements

are accompanied by changes in oscillatory activity, not only

limited to cortical structures (Crone et al., 1998), but also

occurring in the thalamus (Paradiso et al., 2004) and the

basal ganglia (Cassidy et al., 2002). Some of these changes

1388-2457/$30.00 q 2005 International Federation of Clinical Neurophysiology.

doi:10.1016/j.clinph.2005.08.030

* Corresponding author. Address: Clinical Neurophysiology Section,

Department of Neurology, Clı́nica Universitaria de Navarra, Avda. Pı́o XII,

36, 31008 Pamplona, Navarra, Spain. Tel.: C34 948 255400; fax: C34 948

296500.

E-mail address: [email protected] (J. Artieda).

can be observed in normal subjects without the use of any

further analysis, like the disappearance of the central mu

rhythm (in the alpha range) during movement (Gastaut

et al., 1954). With the help of different mathematical tools, a

well-defined pattern of alpha (8–12 Hz) and beta

(15–30 Hz) oscillatory changes during self-initiated move-

ments has been characterized in the EEG. In the alpha band,

a decrease in energy begins up to 2 s before the movement,

and lasts till 2–3 s after its end (Derambure et al., 1993;

Pfurtscheller and Aranibar, 1979). In the beta range, a fall in

activity which begins more than 1 s before movement and

lasts till the end of muscle contraction (beta event-related

desynchronization, ERD), and a post-movement rebound

over baseline level (beta event-related synchronization,

ERS), have been thoroughly described (Alegre et al.,

2003b; Derambure et al., 1999; Pfurtscheller, 1981;

Clinical Neurophysiology 117 (2006) 16–25

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Published by Elsevier Ireland Ltd. All rights reserved.

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M. Alegre et al. / Clinical Neurophysiology 117 (2006) 16–25 17

Pfurtscheller et al., 1996). The number of studies, however,

is lower for stimulus-induced movements. Oscillatory

changes in these paradigms begin after the stimulus, unless

it is rhythmic and therefore predictable (Alegre et al.,

2003a). In this latter case, changes begin before the

stimulus, and are similar to those observed in self-initiated

movements. Go/no go paradigms are series of stimulus-

induced movements, in which the subject decides to move

or not depending on the characteristics of the stimulus.

These paradigms have been intensively used to study the

processes linked to decision-making and response inhibition

(Lai et al., 1997). An extended variation of this paradigm

may include pairs of stimuli (S1 and S2); the decision (go or

no go) is taken after S1, and motor preparation/inhibition

occur afterwards, while S2 carries the information on the

timing of the movement (but only in the go condition).

ERD/ERS changes in different go/no go paradigms have

been addressed in several studies (Filipovic et al., 2001;

Leocani et al., 2001). In a recent paper, our group described

the different pattern of alpha and beta changes depending on

whether the decision to move was taken after the first or the

second stimulus (Alegre et al., 2004). Alpha and beta ERD

only began after the decision to move had been taken, and

lasted through the fixed interstimulus interval (1.5 s).

However, as commented above, the predictability of the

timing of the movement also affects pre-movement beta

ERD (Alegre et al., 2003a). This activity could be related to

motor planning or be due to the use of motor predictive

strategies.

We studied alpha and beta changes in a S1-centred

double stimuli go/no go reaction time paradigm with

random interstimulus interval (1.5, 3 or 4.5 s), in order to

separate the decision to move or not from the decision of

when to move. Our analysis was therefore mainly focused

on the differences in the pre-movement alpha and beta-ERD

between the 3 go conditions.

Fig. 1. Schematic representation of the 4 conditions of the paradigm

studied. Vertical bars represent the 1000 or 2000 Hz tones. Horizontal

arrows indicate the interstimulus interval and the two fragments obtained

from each sweep for the offline statistical analysis.

2. Materials and methods

2.1. Signal recording

A total of 10 right-handed healthy volunteers (1 man, 9

women) with an age range between 27 and 31 years

participated in the study. All of them gave their written

consent after a detailed explanation of the procedure. The

protocol was approved by the institutional ethics committee.

The subjects were comfortably sat in a dimly-lit sound-

attenuated room with eyes opened looking at a fixed point.

A standard electrode cap with 58 surface electrodes placed

according to the 10–10 system was used for the recordings

(ElectroCap Int.). All impedances were systematically kept

below 8 kU. Both linked earlobes were used as initial

reference. Current source density (reference-free) values

were obtained for each electrode afterwards using BrainVi-

sion Analyser software. One electrooculogram (EOG) and

one bipolar EMG channel (right extensor carpi) were

simultaneously recorded. EEG channels were amplified

!20,000 and digitised at 500 Hz using BrainVision

Recorder software, with low-pass and high pass filters set

at 100 and 0.3 Hz, respectively. After downsampling data to

200 Hz to reduce computing time, all channels were stored

for offline analysis.

2.2. Description of the stimulation paradigm

Sequences of 70 ms, 90 dB tone bursts were generated by

means of the Neuroscan Stim module synchronized with the

recording equipment, and delivered by means of bilateral

intracanalicular earphones.

Fig. 1 shows the experimental paradigm. A minimum of

300 S1 stimuli was delivered to each subject. S1 was either a

1000 Hz (16% probability, S1no go) or a 2000 Hz (84%

probability, S1go) tone. S1no go indicated the subject not to

move, and wait for another S1 tone. S1go indicated the

subject to make a wrist extension after S2go, another

2000 Hz tone that always followed S1go with a random

interstimulus interval of 1.5, 3 or 4.5 s (33% probability

each). The interval between S1no go or S2go, and the next S1

stimulus was randomized between 6 and 10 s.

2.3. Signal processing

An offline segmentation was performed on the continu-

ously recorded data. S1 stimuli (S1go or S1no go) were used

as triggers, in 4 different conditions: no go, go with 1.5 s

S1–S2 interval (go1.5), go with 3 s S1–S2 interval (go3),

and go with 4.5 s S1–S2 interval (go4.5). For the no go and

go1.5 conditions, 10-s sweeps were obtained, from 5 s

before to 5 s after S1. For the go3 condition, 11.5 s sweeps

were obtained, from 5 s before to 6.5 s after S1. Finally, for

the go4.5 condition, 13 s sweeps were obtained, from 5 s

M. Alegre et al. / Clinical Neurophysiology 117 (2006) 16–2518

before S1 to 8 s after S1. All sweeps were visually reviewed,

and those with visible artefacts or incorrect responses were

manually rejected. Reaction times were averaged in each go

condition. The minimal number of artefact-free trials per

subject and condition was 40 for the no go condition and 70

for any go condition. For the statistical analysis, an off-line

pre-processing was performed, obtaining two asymmetrical

sections from each sweep in each condition. The first

sections (named ‘rest-nogo’, ‘rest-go1.5’, ‘rest-go3’ and

‘rest-go4.5’), 3 s of duration, were taken from the beginning

of the sweeps after discarding the initial 500 ms. The second

group of sections (named ‘no go’, ‘go1.5’, ‘go3’, and

‘go4.5’) included the period between 0.5 s before S1 and 3 s

after S2 (or 4.5 s after S1 in the no go condition).

2.4. Time–frequency and statistical analysis

Oscillatory activity associated with perceptual or motor

phenomena may appear with a certain jitter in latency from

trial to trial. When all trials are averaged (as in the classic

evoked potentials), all activity not coincident in phase tends

to disappear. Therefore, the time–frequency representation

of the average of all trials (TF of Av) only represents

oscillatory activity clearly phase-locked to the trigger of the

sweep. However, if the time–frequency transform is

computed for every single trial and averaged afterwards

(Av of TF), all oscillatory activity is added despite its phase,

as the values of energy are always positive. Processing each

response separately, without averaging, implies that the

interesting elicited components are several orders of

magnitude smaller than other rhythmical activities in the

EEG, so efficient mathematical tools are required for a good

visualisation.

Our method used the Gabor transform (Cohen, 1989) to

represent the time–frequency distribution of the energy in

the signal in the 7–37 Hz range. The signal is expressed as a

weighted sum of time–frequency-shifted versions of an

elementary function g(t)

sðtÞ ZXX

Gt;f gn;kðtÞ; with gn;k Z gðtKnTÞej2pðkf Þt

where the Gabor coefficients Gt,f are defined as

Gt;f Z

ð ðsðtÞeKt2=s ej2pft dtdf

Each coefficient Gt,f can be interpreted as the energy

present in the signal around time t for the components

around frequency f and when displayed as a 3-dimensional

colored graph gives a complete picture of the temporal

evolution at different frequencies. A more detailed

description of the procedure can be found elsewhere

(Gurtubay et al., 2001).

After obtaining the current source density from each

channel, the average of the single-trial transforms for each

of the 4 conditions was computed. The values obtained at

each frequency were normalized to baseline (and expressed

as relative percentual increase or decrease), using the mean

energy level during the first 3 s as reference. The energy

changes in the most significant frequency bands were

linearly displayed for an easier visualisation. Also, the

average of the single-trial transforms for each section group

(go1.5, go3, go4.5, no go, rest-go1.5, rest-go3, rest-go4.5,

rest-no go) was computed separately. The energy in the

individual sweeps from different pairs of section groups was

compared using a Wilcoxon test (energy values do not have

a gaussian distribution (Tallon-Baudry et al., 1996)), and the

statistical significance of the comparison for small windows

of time–frequency domain represented in a logarithmic

scale using a dual colour scheme (blue for decreases, red for

increases). Both percentual change values and significance

values of the same time–frequency range in different

electrodes were also represented over a 2D head model. A

Bonferroni correction was optionally applied to the

statistical significance of the windows, given the high

number of comparisons.

Finally, a Gabor transform was also performed on the

average of all sweeps per condition, in order to assess those

changes that were strictly phase-locked to the stimuli

(related to the evoked potentials).

The mathematical procedure (TF transform averaging

and normalisation, and statistical comparison of energy

values using the Wilcoxon test) was applied to each

individual subject and to each group as a whole. Latencies

and energy values from the most significant changes

observed were measured in each individual subject and

compared between conditions using repeated-measures

ANOVA tests under SPSS 11.0.

3. Results

All subjects performed the task correctly, with less than

5% of errors. Mean RTs were significantly higher in the

go1.5 condition (1.5 s interval) than in the go3 and go4.5

conditions (240 versus 207 and 206 ms, respectively; one-

way ANOVA, FZ17.09, P!0.001; Tukey post hoc test:

go1.5–go3 and go1.5–go4.5, P!0.001, go3–go4.5, PZns).

Figs. 2 and 3 show the normalised phase-independent

energy changes in the C3 and F1 electrodes. Fig. 4 shows

the statistical significance of the changes observed in C3.

3.1. S1-related changes

The changes immediately after S1 were very similar in

all conditions. A significant desynchronisation of alpha and

beta activity could be observed in the left central region

(minimum at 270 ms). In the lower beta band (17–20 Hz),

this central desynchronisation was followed by a central

bilateral ERS, maximally significant in C3 at 750 ms. In the

higher beta band (20–35 Hz), it just returned to baseline

after 300 ms (Fig. 5). The duration of the alpha-ERD had a

high intersubject variability. In 3 subjects, it was maintained

Fig. 2. Normalized average of the single-sweep time–frequency transforms,

representing the energy changes in the 7–47 Hz band, in the 4 conditions

(electrode C3). The colour scale common to the 4 plots represents

percentual energy changes with respect to the baseline period (decreases are

blue and increases are red). The 4 graphs are aligned with S1 (time 0).

Fig. 3. Normalized average of the single-sweep time–frequency transforms,

representing the energy changes in the 7–47 Hz band, in the 4 conditions

(electrode F1). The colour scale common to the 4 plots represents

percentual energy changes with respect to the baseline period (decreases are

blue and increases are red). The 4 graphs are aligned with S1 (time 0).


through all the S1–S2 interval in the 3 go conditions. In the

no go condition, and in the go conditions from the remaining

7 subjects, the alpha ERD returned to baseline around 0.8 s

after S1.

In the left-midline frontal region (F1, Fz), a marked

ERS was observed after S1, partially overlapping with the

central ERD/ERS, around 15 Hz (Figs. 3 and 6, left and

right columns). This frontal ERS peaked around 600 ms

(individual range between 500 and 870 ms), and lasted

until 1.1 s after S1 (Fig. 7). The ERS was significant in 8

out of the 10 subjects individually considered, as well as

in the grand average, in the Wilcoxon test (Fig. 4). There

were no differences in the amplitude of the ERS nor in

the latency values between the 4 conditions (repeated-

measures ANOVA). The interindividual variability in the

frequency of this ERS was very narrow, between 13 and

16 Hz.

The only difference between the 4 conditions in this

period was the longer duration of the ERS after S1 in C3 in

the no go condition, up to 2 s after S1 (Fig. 2, top), although

not significant due to the lower number of sweeps in this

condition. In the no go condition, no further significant

changes could be observed (Figs. 2, 3, 5, 7 and 8).

In the lower limit of the graphs (7–8 Hz), a small energy

increase could be observed immediately after S1 in all

conditions. This increase was coincident with the mid-

latency auditory evoked potential, and was also present with

the same frequency and timing when the time–frequency

plot of the averaged signal (measuring phase-locked energy

changes) was computed.

3.2. S2-related changes

Fig. 5 displays linearly the changes in the 22–27 Hz

band in all conditions in C3, aligned to S1. In the go3

and go4.5 conditions, but not in the go1.5 condition, a

small ERD could be observed before S2, longer and more

significant in the go4.5 condition. This pre-movement

beta ERD was only present over the left central area

(Fig. 9, first and second columns). In fact, in the go3 and

go4.5 conditions, the period between 2.5 and 3 s after S1

was very similar, showing a beta ERD (Fig. 5, dashed

and dotted lines). In the go3 condition, this small ERD

was followed by a large peri-movement ERD after S2. In

the go4.5 condition, the ERD continued until S2

increasing slightly in amplitude; after S2 the typical

Fig. 4. Statistical significance of the time–frequency energy changes during the 3 go conditions, in C3, aligned at S2. Vertical arrows indicate the timing of the

sounds (S1 green, S2 blue). The colour scale represents the uncorrected statistical significance in a logarithmic scale (G log P; a value of C2/K2 means PZ0.01, uncorrected). The small horizontal arrows in the colour scale indicate a corrected P value of 0.01 (Bonferroni correction for multiple comparisons).

Positive values (red) indicate significant increases, and negative values (blue) indicate significant decreases.


large peri-movement ERD could be observed. When the

individual latencies of the beta-ERD (measured as the

interval between the beginning of the beta-ERD and S2)

were compared between the 3 conditions, significant

differences were found (FZ11.24, PZ0.001). The direct

comparison between pairs showed significant differences

between the go3/go4.5 conditions and the go1.5 condition

(P!0.001), while there was only a trend (PZ0.059) for

Fig. 5. Linear representation of the relative (percentual) energy changes in the 22–

(vertical arrow). The horizontal line indicates baseline level (100%). Please note th

the text for specific frequencies within this same band.

higher latencies in the go4.5 condition compared to the

go3 condition.

The peri/post movement ERD/ERS pattern, as shown in

Figs. 5 and 9, was very similar in the 3 go conditions. The

beta-ERD was shorter and had clearer left predominance

than the alpha-ERD (Figs. 2 and 9, 3rd and 4th columns). It

was followed by an ERS in the 15–22 Hz range, maximal in

the left central region (Fig. 9, last column). This ERS was

26 Hz band, during the 4 conditions, in C3. The traces are aligned with S1

at the values represented in the figure may be different from those quoted in

Fig. 6. Topographical representation of the S1-related energy changes (ERD/ERS) in different time–frequency windows (columns) and conditions (rows).

Please note that colour scales (relative increase/decrease with respect to 100% baseline) are different for each column. Left: Simultaneous frontal ERD and

central ERD at 15 Hz, 200 ms after S1. Centre: Left central alpha ERD at 10–12 Hz, 400–600 ms after S1. Right: Midline–left frontal ERS at 14–16 Hz, 600–

800 ms after S1.


more lateralized and had a wider frequency range than the

ERS observed in the same area in the lower beta band after

S1 (Fig. 2).

As described above, only 3 subjects showed a

significant sustained alpha-ERD through all the S1–S2

interval. Five subjects showed, as in the beta band, a

small pre-S2 alpha ERD only in the go3 and go4.5

conditions. In the grand average (Fig. 8), a S1–S2

sustained ERD could be observed in all go conditions,

although it was only statistically significant in the go3

and go4.5 s conditions shortly before S2 (as a result of

the summation of the 3 subjects with sustained ERD and

Fig. 7. Linear representation of the relative (percentual) energy changes in the 14

(vertical arrow). The horizontal line indicates baseline level (100%). Please note th


the 5 subjects with a pre-S2 ERD, Fig. 4). After S2, a

highly significant peri-movement alpha-ERD could be

observed in the 3 go conditions (Fig. 9, 4th column).

Compared with the peri-movement beta-ERD, the alpha-

ERD lasted longer (1.5 s), was more bilateral, and was

not followed by a clear ERS within the limits of the

sweep (Fig. 2). As in the beta band, there were no

differences in the amplitude of the alpha-ERD between

conditions.

In the 7–8 Hz range, a small ERS could be observed after

S2, corresponding to the auditory evoked potential, similar

to the activity described after S1.

–16 Hz band, during the 4 conditions, in F1. The traces are aligned with S1

at the values represented in the figure may be different from those quoted in

Fig. 8. Linear representation of the relative (percentual) energy changes in the 10–13 Hz band, during the 4 conditions, in C3. The traces are aligned with S1

(vertical arrow). The horizontal line indicates baseline level (100%). Please note that the values represented in the figure may be different from those quoted in



4. Discussion

The main findings in our study can be summarized in 3

aspects: the frontal ERS at 15 Hz in F1 and Fz only present

after S1 in all conditions, the low beta left central ERD/ERS

also present after S1, and the sustained alpha and beta S1–

S2 ERD, clearly observed in the two go conditions with

Fig. 9. Topographical representation of the S2-related energy changes (ERD/E

(rows). Please note that colour scales (relative increase/decrease with respect to

left central beta ERD in the go4.5 condition (1.2–0.9 s before S2, 24–26 Hz

conditions (0.5–0.2 s before S2, 24–26 Hz). Third column: Peri-movement be

column: Peri-movement alpha ERD in the 3 go conditions (0.4–0.7 s after S2,

conditions (1.5–1.8 s after S2, 16–20 Hz).

longer interstimulus interval. These 3 aspects will be

discussed separately.

4.1. Frontal changes after S1

After S1, a frontal ERS with maximum at 15 Hz could be

observed in the 4 conditions (Fig. 7). This ERS was

RS) in different time-frequency windows (columns) and go conditions

100% baseline) are different for each column. First column: Early pre-S2

). Second column: Pre-S2 left central beta ERD in the go3 and go4.5

ta ERD in the 3 go conditions (0.3–0.6 s after S2, 22–26 Hz). Fourth

10–13 Hz). Fifth column: Post-movement central beta ERS in the 3 go


significantly present in 8 subjects in Fz and F1, and was not

preceded by any ERD. No similar activity was seen after S2.

This activity is similar to the ERS described in a previous

study using S1-centred and S2-centred go/no go paradigms,

although the higher number of channels used in the present

study allows a better topographic delimitation of these

changes (Alegre et al., 2004).

Although the maximum of this ERS was around 600 ms

after S1, it began immediately after the stimulus. The

latency of the ERS component of the central ERD/ERS was

about 60 ms later; the earlier part of the frontal ERS

overlapped partially with the central ERD, so a simul-

taneous central ERD and frontal ERS could be observed

until 0.5 after S1.

Alpha and beta ERS have classically been considered as

an indication of an ‘idling’ or inactive area (Chen et al.,

1999; Leocani et al., 2001; Pfurtscheller, 1992; Pfurtscheller

et al., 1996). A recent study (Sauseng et al., 2005) suggests

that at least in some instances a synchronization of cortical

activity in the alpha range may represent an activation of an

area, as it may enable a tight functional coupling with other

areas that show a simultaneous desynchronization. Such a

coupling has been shown to occur between the sensorimotor

cortex and frontal mesial areas (Spiegler et al., 2004). The

simultaneous occurrence of the frontal ERS and the central

ERD after S1 might represent therefore some kind of

coupling between midfrontal and central (motor) areas,

related to the decision to move (or not). The appearance of

the frontal ERS only after S1, together with the findings in

the aforementioned go/no go study (Alegre et al., 2004),

suggests that it may be related to the decision-making

process.

In a recent fMRI study in a go/no go paradigm, an

increase of activity in the anterior cingular cortex was found

in both conditions (Liddle et al., 2001). Some neurophy-

siological data also support the idea of a role of frontal

mesial areas in decision-making. Activity in this area in

both conditions of go/no go paradigms has also been

detected in electrocorticographic recordings (Ikeda et al.,

1996). The beta changes observed in our study might be

originated in this area as well.

4.2. Central ERD/ERS after S1

In the left central region (maximal at C3), an alpha ERD

and a beta ERD/ERS could be observed after S1. The

topography of these changes, strictly limited to the central

region contralateral to the hand to move, suggests that they

may have their origin in the motor cortex. All sweeps with

muscular response to S1 were manually discarded; however

it cannot be discarded that the subject inhibited an already-

prepared response, as S1 and S2 were identical. The possible

relationship of the central ERD with the frontal ERS has

already been discussed above. Alternatively, these changes

could indicate the loading of the motor program in

preparation of the response that is surely going to happen.

The ERD/ERS pattern in the low beta range suggests a

transitory process (like the two mentioned). The sustained

changes in the higher beta band may have a different

meaning, as will be discussed below.

4.3. Sustained ERD in the S1–S2 interval

In our previous study on go/no go paradigms (Alegre

et al., 2004), we found a sustained alpha and beta ERD in the

1.5 s S1–S2 interval when the decision to move was taken

after the first stimulus. The results in the present study point

in the same direction, as a sustained alpha and beta ERD

could be observed. However, this sustained ERD was only

statistically significant in the go3 and go4.5 conditions,

especially in the last one. The topography of these alpha and

beta ERDs (central contralateral to the hand moved) was

similar to the early pre-movement ERDs described in self-

initiated or rhythmic stimulus-induced movements. Mag-

nani et al. (1998) described an alpha-ERD that began

1500 ms before the imperative stimulus in a CNV paradigm

with a 2 s interstimulus interval. The S1–S2 alpha ERD in

the go4.5 condition became significant also 1.5 s before S2

(although only 0.5 s before S2 in the go3 condition).

The predictability of the stimulus may influence the

appearance of the pre-movement beta ERD in stimulus-

induced movements, as shown in a previous study (Alegre

et al., 2003a). In this study, no significant pre-stimulus alpha

ERD could be found, even when the stimulus was

predictable. Several authors have studied the relationship

between alpha-ERD and attention and expectancy (Bas-

tiaansen et al., 2001; Klimesch et al., 2005; Vazquez et al.,

2001). A recent study in a go/no go paradigm showed a

sequence of parallel alpha desynchronizations over different

central areas related to the expectancy to the stimuli

(Babiloni et al., 2004). In the present study, a S1–S2 alpha

ERD was observed in most subjects, but the statistical

significance was lower than in the beta band because of the

interindividual variation in the pattern of this ERD. The

variability was maintained when the alpha frequency was

individually selected in each patient (Klimesch, 1999). This

high interindividual variation suggests that additional

factors may influence on the appearance of the alpha-ERD

(besides attention and expectancy), and the strategies used

by the subjects may have been slightly different.

Our finding of an increasing ERD through the S1–S2

period, more clearly observed with the longer interstimulus

interval, supports the influence of the predictability on the

alpha and especially the beta desynchronization. Although

they were not specifically informed about the possible

interstimulus intervals, the subjects learned through the test

that the maximum interstimulus interval was about 6 s, so if

S2 did not appear after 4.5 s they were sure that it would

appear 1.5 s later (and thus it became predictable). Also, if

S2 did not appear 1.5 s after S1, the probability of a 3 or

4.5 s interval raised from 33 to 50%. Reaction times were

significantly higher in the go1.5 condition than in the other


two go conditions. This may suggest that the expectance or

the predictability for S2 was higher in these two conditions,

thus correlating with the presence of the alpha and beta S1–

S2 ERD.

In delayed motor tasks, a sustained neuronal activity

during the delay has been observed in different cortical

regions, mainly prefrontal and parietal (Andersen, 1995;

Fuster, 1973). This sustained activity might represent a

preparatory set for the forthcoming movement (Curtis et al.,

2004). A recent study in monkeys has shown that the build-

up pattern of discharge in the pre-SMA neurons observed in

motor delayed tasks, is modulated by time predictability, as

it only appears when the delay interval is fixed (Akkal et al.,

2004). Quintana and Fuster (1999) found neurons (set

neurons) both in the prefrontal cortex and the parietal cortex

of monkeys that tended to accelerate their firing in

proportion to the predictability of the movement. The S1–

S2 sustained beta ERD observed over the primary motor

cortex in the two longer conditions might be related to these

activities.

4.4. Other findings

The alpha and beta ERD/ERS found after S2 (peri-

movement) in the 3 go conditions had the same character-

istics and topography described in previous works with

either stimulus-induced or self-initiated movements (Alegre

et al., 2003a,b; Derambure et al., 1999; Leocani et al., 1997,

2001; Pfurtscheller et al., 1999).

The changes observed after S1 (the only stimulus) in the

no go condition were similar to those observed in the go

conditions, with the exception of a longer ERS in C3. In our

previous study in go/no go paradigms, a longer ERS after

the no go signal was also found (Alegre et al., 2004). The

topography of this ERS was similar in the go and no go

conditions, with a frontal and a central components, but

different from the post-movement beta ERS observed after

S2, more limited to the contralateral central region. In our

previous work, an origin in the frontal dorsolateral region

was suggested for the no go ERS; our present data do not

support this hypothesis, although the number of sweeps

(lower) and the low probability of the no go stimulus may be

limiting the results.

5. Final remarks

In summary, our results have confirmed previous

findings on the presence of a midline frontal beta ERS

(15 Hz) after the stimulus that makes the subject decide

whether to move or not, and have given further insight on

the influence of the predictability of a movement on the

appearance of cortical oscillatory activity changes related to

a simple ballistic movement.

Acknowledgements

This study was partially funded by the ‘UTE project

CIMA’.

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