Cognitive Tasks Augment Gamma EEG Power

Cognitive tasks augment gamma EEG power

S.P. Fitzgibbona,b, K.J. Popec, L. Mackenziea, C.R. Clarkb, J.O. Willoughbya,*

aCentre for Neuroscience and Department of Medicine (Neurology), Flinders University, P.O. Box 2100 Adelaide, SA, AustraliabCognitive Neuroscience Laboratory, School of Psychology, Flinders University, P.O. Box 2100 Adelaide, SA, Australia

cSchool of Informatics & Engineering, Flinders University, P.O. Box 2100 Adelaide, SA, Australia

Accepted 3 March 2004

Abstract

Objective: Gamma EEG oscillations are low amplitude rhythms in the 30–100 Hz range that correlate with cognitive task execution. They

are usually reported using time-locked averaging of EEG during repetitive tasks. We tested the hypothesis that continuous gamma EEG

would be measurable during mental tasks.

Methods: We investigated sustained human gamma EEG oscillations induced by 8 cognitive tasks (Visual Checkerboard, Expectancy,

Reading, Subtraction, Music, Expectancy, Word learning, Word recall, and a Video Segment) in 20 subjects using standard digital EEG

recording and power spectral analysis.

Results: All of the cognitive tasks augmented gamma power relative to a control condition (eyes open watching a blank computer screen).

This enhancement was statistically significant at more than one scalp site for all tasks except checkerboard. The Expectancy, Learning,

Reading and Subtraction tasks expressed the most impressive gamma response, up to 5 fold above the control condition and there was some

task-related specificity of the distribution of increased gamma power, especially in posterior cortex with visual tasks.

Conclusions: Widespread gamma activation of cortical EEG can easily be demonstrated during mental activity.

Significance: These results establish the feasibility of measuring high frequency EEG rhythms with trans-cranial recordings, demonstrate

that sustained gamma EEG activity correlates with mentation, and provides evidence consistent with the temporal binding model.

q 2004 International Federation of Clinical Neurophysiology. Published by Elsevier Ireland Ltd. All rights reserved.

Keywords: Spectral analysis; Theta; Alpha; Beta

1. Introduction

Gamma EEG oscillations (low amplitude rhythms in the

30–100 Hz range) became a topic of intense interest in

humans (Aoki et al., 1999; Joliot et al., 1994; Keil et al.,

1999; Revonsuo et al., 1997; Sauve et al., 1998; Tallon-

Baudry et al., 1998) after it was established in animal

models that synchronous recurrent discharge bursts within

the gamma range are involved in perception and cognition

and are correlated with cognitive task execution (Engel and

Singer, 2001).

In several animal studies, gamma EEG derived from

cortex using small electrodes has been closely correlated

with both local multi-unit and single-unit discharges.

Synchronous gamma discharges have been identified in

the visual, auditory, somatosensory, olfactory, motor and

memory modalities in a wide range of animal species (Engel

and Singer, 2001). Within these areas, there were synchro-

nous bursts within groups of neurons in different cortical

columns (Gray et al., 1990), spatially distributed within the

same hemisphere (Engel et al., 1991a; Frien et al., 1994),

between inter-hemispheric sites (Engel et al., 1991b) and in

sub-cortical structures (Alonso et al., 1996). Other animal

experiments have demonstrated the functional significance

of synchronous gamma oscillations by showing that

synchroneity of gamma discharges correlates with cognitive

function (Fries et al., 1997; Murthy and Fetz, 1996;

Roelfsema et al., 1997) and that disrupted synchroneity of

gamma discharges correlates with diminished cognitive

function (Roelfsema et al., 1994; Stopfer et al., 1997).

In humans, trans-cranial EEG integrates measures

summed electrical fields which are volume conducted

from a large population of neurons within an extensive

region around a recording site. Such measurement is

unlikely to detect EEG rhythms derived from small cell

Clinical Neurophysiology 115 (2004) 1802–1809

www.elsevier.com/locate/clinph

1388-2457/$30.00 q 2004 International Federation of Clinical Neurophysiology. Published by Elsevier Ireland Ltd. All rights reserved.

doi:10.1016/j.clinph.2004.03.009

* Corresponding author.

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

http://www.elsevier.com/locate/clinph

assemblies and it would be even less likely to permit

identification of synchronous rhythms from small cell

assemblies in different cortical areas. However, the presence

of gamma oscillations in trans-cranial EEG indicates

significant synchronicity in large populations of subjacent

neurons and, therefore, that there are relatively large local

cell assemblies exhibiting rhythmic bursting, possibly

synchronous with cell groups elsewhere.

Synchronous gamma activity between widely distributed

cell groups, sometimes referred to as ‘binding’ gamma, is

thought to integrate (bind) information processed in dis-

tributed neurons and/or neural circuits and/or cortical areas

into a coherent cognitive process/percept (Engel and Singer,

2001). Thus synchronous cortical gamma EEG provides a

measure of binding activity and has been observed for short

durations, for example in auditory discrimination (Joliot et al.,

1994), somatosensory discrimination (Sauve et al., 1998),

stereoscopic fusion (Revonsuo et al., 1997), the formation of

percepts (Keil et al., 1999), working memory (Tallon-Baudry

et al., 1998), and sensory-motor processing (Aoki et al.,

1999). In many studies such as these, the involvement of

gamma has been demonstrated using repetitive tasks, time-

locked averaging, and short post-stimulus time windows.

We determined if sustained gamma EEG oscillations,

induced by a variety of complex mental tasks in human

subjects, could be measured trans-cranially and if the strength

and spatial pattern of enhancement would be task-dependant.

2. Method

2.1. Subjects

Twenty adults (8 male and 12 female) free of psychiatric

or neurological disorder participated in the study, approved

by the Flinders Clinical Research Ethics Committee, and all

subjects gave informed and written consent. They were

recruited as age, gender and education-matched controls for

patients as part of a larger clinical study (published

elsewhere (Willoughby et al., 2003)).

2.2. EEG

Sixty-four channel EEG was recorded continuously

(linked-ear reference, 512 samples per second, 16-bit analog

to digital conversion, 107 Hz low-pass filter) using a

commercial EEG acquisition system (Compumedics,

Victoria, Australia). A 64-channel electrode cap with tin

electrodes provided uniform scalp coverage. Electrode

impedances were kept below 5 kV.

2.3. Cognitive tasks

EEG was recorded whilst participants performed the

following 8 tasks, chosen to activate mental activity in a

variety of circumstances, and a Control procedure:

2.3.1. Visual checkerboard

The participant was instructed to fixate for 20 s on a red

dot located in the centre of an alternating, rectangular black-

and-white checkerboard pattern for 20 s (check size

1.5 £ 2 cm, alternation rate 8 Hz).

2.3.2. Story reading

The participants were instructed to read silently for a

period of 28 s from the beginning of page 46 of ’Politically

Correct Bed-Time Stories’ by James Finn Garner (Souvenir

Press, London, 1994). The book contained only text in size

12 point font.

2.3.3. Subtraction task

The participants were instructed to serially subtract 7

from 1000 and, in a practice session, the participants were

intermittently interrupted to check their accuracy and to

ensure compliance with the task. The recorded Subtraction

period was not interrupted. A small number of subjects who

were unable to serially subtract 7 from 1000 were given a

simpler serial Subtraction task.

2.3.4. Music

The participants listened to a segment of Pachelbel’s

Cannon in D for 28 s.

2.3.5. Expectancy

The participant was presented with a series of 11

expectancy trials, each involving stimulus pairs. Each pair

consisted of a visual direction cue presented at eye height in

the centre of the computer screen followed 4 s later by a

lateralised visual target. Each cue consisted of an arrow

pointing equally probably to the left or right. The target was

a cross, located on the side indicated by the preceding

arrow. The period between each direction cue and the

subsequent target was 4 s. Participants were required to

respond by a button press as soon as a target was presented.

The expectancy period recorded was the interval between

each directional cue and the subsequent target.

2.3.6. Learning

The participants were instructed to memorise a set of 10

words that were presented simultaneously on a computer

monitor for 20 s. The words were medium frequency

(Kucera and Francis, 1967) concrete nouns of 4–7 letters

in length. In order to induce intentional learning, partici-

pants were advised that they would be tested on them

subsequently.

2.3.7. Recall

Three lists of 10 words were presented sequentially for

10 s each, with an inter-list interval of 2 s. The first list

contained the same words as the Learning list but in

different order. The other two lists contained some of the

words in the first list. Following the presentation of all 3 lists

the participant was asked to identify which list contained

S.P. Fitzgibbon et al. / Clinical Neurophysiology 115 (2004) 1802–1809 1803

the words that they had previously learnt. EEG was recorded

during the presentation of the first list.

2.3.8. Strictly ballroom

The participant was required to watch a 28 s segment of

the film ‘Strictly Ballroom’ (with permission of Andrew

Pike, Ronin Films, Canberra, Australia). The segment

selected contained a complex mix of intense colour,

movement, speech, drama and music.

Subjects also completed a Control task requiring them to

relax with eyes open and to look at a blank computer screen.

During the experiment, participants were seated 1.5 m in

front of a computer monitor on which the various task

materials were presented. Participants were instructed on

the requirements of each task immediately beforehand.

Because the subject group were controls for a study of

patients with different forms of epilepsy and the numbers of

patients were not known, it was not feasible in advance to

undertake a Latin square design in administering the tasks.

In addition, there were two tasks that required a fixed order,

namely Learn and Recall. The tasks were therefore

administered in fixed order: Checkerboard, Reading,

Subtraction, Music, Expectancy, Learn, Recall, Strictly

Ballroom, Control and there was a half to 1 min rest

between each task.

2.4. EEG analysis

All uncontaminated EEG for each task (usually around

20 s) was epoched into consecutive 1 s blocks. Each epoch

was transformed from the temporal domain to the frequency

domain using fast-Fourier transform (1 Hz resolution, 512

point block-size, Hanning window, 1–100 Hz). While

artifact due to eye blinks was present, eye blinks frequency

and expression in the power spectra could not be detected

because of their brief duration and infrequent occurrence

(around once every 10 s) even during the Strictly Ballroom

task. Muscle activity, if present, occurred in isolated regions

and did not prevent the use of recordings from other leads.

When muscle activity affected electrodes in a widespread

distribution, all leads were edited from the analysis.

The frequency epochs were averaged within each task for

each subject to yield an averaged power spectrum and then

divided by the power spectrum for the Control task for that

subject. This provided a relative spectral response profile for

each subject for each task. The data was banded into theta

(3–7 Hz), alpha (8–12 Hz), beta (13–29 Hz) and gamma

(30–100 Hz) frequency bands. For visualisation purposes

the relative spectra were averaged across subjects within

each task to provide group mean spectra relative to Control

for each task. To ensure data were not contaminated by

50 Hz mains frequency and 60 Hz computer monitor refresh

rate, power values in the 50 ^ 1, 60 ^ 1, 99 and 100 Hz

values were omitted from analysis.

2.5. Statistical analyses

The Kolmogorov–Smirnov test was used to assess

normality of power estimates, which was achieved when

the power estimates were log-transformed. Significant

differences in spectral power between experimental tasks

and the Control task were calculated for each power band

using a single factorial analysis of variance for each

electrode. Significances were corrected form multiple

comparisons using the Modified Bonferonni procedure.

3. Results

Relative to the Control condition there were widespread

increases in EEG power during all tasks except Checker-

board. We illustrate in Fig. 1 the average relative power-

response to Learning at each electrode as a montage of

power spectra with 1 Hz resolution.

3.1. Gamma power

All of the tasks other than Checkerboard exhibited

significantly increased gamma band power ðP , 0:05Þ

relative to the Control condition (Fig. 2). These increases

were evident in all cases at more than one scalp site and their

spatial distribution was task-specific.

The most striking enhancement of gamma power was

expressed in the Expectancy, Learning, Reading, and

Subtraction tasks. All 4 of these tasks induced widespread

2–5 fold increases in gamma power relative to Control at a

large number of posterior and central scalp sites (Fig. 2).

The Recall, Music and Strictly Ballroom tasks also induced

increases in gamma power, but of lower order and at fewer

electrodes. The spatial distribution of enhanced gamma

power for Recall was similar to that for Expectancy and

Learning.

3.2. Theta, alpha and beta power

There were no significant increases in alpha band power

relative to the Control condition for any of the experimental

tasks.

The distribution of beta band power for each experimen-

tal task relative to the Control condition was similar to the

gamma power distribution, however, the relative enhance-

ment was weaker than for gamma and tended to occur at

frequencies close to the gamma frequency band. Increased

beta power therefore very likely reflects an increase in

power of an equivalent phenomenon to gamma activity. The

only significant increases in apparent beta power were

observed during the Reading and Subtraction tasks ðP ,

0:05Þ and these were localised to a small subset of the scalp

sites that were significant for the gamma band.

There were 2–5 fold increases in theta power at

bilateral and midline frontal scalp sites for all tasks except

S.P. Fitzgibbon et al. / Clinical Neurophysiology 115 (2004) 1802–18091804

Fig. 1. Montage display of individual (a) and mean (with SEM) of 20 subjects (b) EEG power increases during Reading relative to the Control state (ordinate

scale: from 1 to 5 fold) between 0 and 100 Hz (abscissa) recorded over the scalp. Decreases relative to the Control state (values below 1) are not shown.


Checkerboard. The Subtraction task produced the most

striking augmentation of theta power with a large number

of central, temporal and frontal sites reaching significance

relative to the Control condition (Fig. 3). In addition,

there were 1–3 fold increases in theta power in the Read,

Recall, Strictly Ballroom and Subtraction tasks at bilateral

and midline occipital sites.

4. Discussion

The key finding is that mental activity can easily be

demonstrated to augment gamma activity using trans-

cranial recordings. We recorded ongoing EEG during

continued mental activity without time-locked averaging

of repeated tasks, revealing increased gamma during all

Fig. 2. Topographic maps of group mean gamma power for each Experimental task relative to the Control condition. The maps were scaled from 1 (black) to 5

fold (white) increases in gamma power relative to the Control condition. Each greyscale increment represents a 0.125 fold increase. A relative power of 1

indicates no difference to the Control condition. The transparent red overlay marks scalp sites at which the increase in gamma power relative to Control was

significant (P , 0.05).

Fig. 3. Topographic maps of mean theta power for each Experimental task relative to the Control condition. The maps were scaled from 1 (black) to 5 fold

(white) increases in theta power relative to the Control condition. Each greyscale increment represents a 0.125 fold increase. A relative power of 1 indicates no

difference to the Control condition. The transparent red overlay indicates scalp sites at which the increase in theta power relative to Control was significant

ðP , 0:05Þ:


tasks other than Checkerboard. Increases were widespread

but quite variable from individual to individual, as indicated

by the maximal mean increases not always reaching

statistical significance. Increases were most striking for

tasks of Expectancy, Learning, Reading, and Subtraction in

which significant, 2–5 fold increases in gamma power were

obtained at widespread posterior and central scalp sites. In

addition, the distributions of augmented gamma EEG

demonstrated some specificity for individual tasks. The

tasks constitute very complex stimuli but we used a ‘blunt’

approach to inducing gamma activity that many would have

hypothesised we would not find. Clearly, given our findings,

the methodology is open to be used in almost unlimited

kinds of studies that will address the detail of ‘how and

where gamma correlates with thinking’, both in health and in

disease. We have interpreted the findings generally and with

an eye to avoiding over-interpretation.

The Expectancy task was similar to tasks used to elicit

the contingent negative variation (CNV). The CNV is a

steady slow negative shift in the EEG observed in the period

prior to the presentation of an expected stimulus. Therefore

the CNV is thought to reflect a state of anticipation by the

brain for the expected stimulus (Walter, 1968). It is this state

of anticipation that is likely to be inducing the enhanced

gamma power response observed in the Expectancy task.

The Expectancy task is a visual and motor paradigm so that

visual and motor areas are obvious candidates for

contributing to the anticipatory state. The distribution of

the gamma power response at posterior and central sites is

consistent with the implied role of the visual and motor

systems.

The Learning task is an intentional episodic memory-

encoding task. Cabeza and Nyberg (2000) conducted a large-

scale review of the many functional neuro-imaging studies

that have investigated intentional episodic memory encod-

ing. They concluded that the key cortical areas associated

with episodic memory encoding are the prefrontal, cerebellar

and medial-temporal brain regions. We did not record from

medial temporal cortex nor from cerebellum. While there

was some pre-frontal gamma augmentation, this was not a

consistent finding and did not reach statistical significance.

However the comparisons made in most encoding studies

contrast a condition involving encoding with a very similar

condition involving less encoding. In this study, we

contrasted a visual language-encoding task with a non-

language visual control condition, and as such the observed

responses may be language related gamma power enhance-

ment at posterior sites related to reading as opposed to

episodic encoding. This task, like others we used, is very

complex and we think it would be difficult to reach consensus

on what might be an appropriate control task. This

experiment also emphasises an obvious limitation of surface

EEG recordings: they do not provide information about brain

regions remote from the scalp.

The distribution of gamma power in the reading task

relative to the Control condition is consistent with

neuro-imaging studies that have consistently demonstrated

temporal, parietal and occipital involvement in written

word recognition and comprehension (Cabeza and Nyberg,

2000). Language studies using event-related potentials

(ERP) have demonstrated that early ERP components first

appear at occipital sites followed closely by the expression

of the subsequent components at occipital-temporal sites

(Vitacco et al., 2002). We report gamma power for Word

Reading relative to Control that is sustained for the

duration of the task and as such we cannot delineate the

timing at which various cortical regions are recruited,

however our distribution of gamma power is supportive of

their involvement.

The Subtraction task exhibited the greatest increase in

gamma power relative to the Control condition and it was

significant at more scalp sites than any of the other

conditions. This may be related to the inherent difficulty

of the task as task complexity correlates with gamma EEG

power (Simos et al., 2002). It is also the only task to exhibit

extensive frontal augmentation of theta power, a correlate of

the augmented attention requisite for this task. It would be

useful in future work to obtain measures of difficulty from

the subjects for each of the tasks. The distribution of the

significant gamma power response at occipital scalp sites is

curious given that subtraction was not a visual task. As the

primary visual cortex has been reported to be involved in

visual imagery (Cabeza and Nyberg, 2000), occipital

gamma enhancement may point to the use of visual imagery

during serial subtraction.

Mental tasks led to augmented theta activity in central

frontal leads and, in addition to frontal sites, Subtraction

was associated with a marked increase in theta power in

temporal sites. In this study, Subtraction was the most

powerful in enhancing gamma power and, intuitively, it

would be expected to be the most mentally challenging of

our tasks. Frontal theta correlates with mental tasks

requiring attention as originally demonstrated with arith-

metic and reasoning (Ishihara and Yoshii, 1972). Recently,

the probable source of this activity has been demonstrated

by magneto-encephalography to be medial prefrontal cortex

(Ishii et al., 1999). Theta activity is generally associated

with cognition and memory (Klimesch, 1999). Further,

intensified theta activity has previously been reported in

humans during recall and other tasks, with some correlation

with task effort (Gundel and Wilson, 1992; Grunwald et al.,

2001; Schober et al., 1995). Theta activation is generated in

hippocampus and related structures in response to alerting

stimuli in rabbit, rat and other species (Blessing, 1997).

Thus enhanced theta activity may be reflective of mental

arousal, analogous to the findings in the hippocampus in

animals. Its temporal prominence may also be partially

supportive of a hippocampal to temporal process, consistent

with the possibility of arousal-induced hippocampal theta

generation in humans. There was a small, but significant,

augmentation of theta power during Reading, Recall and

Strictly Ballroom, an observation that is difficult to account


for in terms of occipital cortical involvement in mental

processing given the absence of this finding in the Learn

task, which was a visual task.

The Recall, Music and Strictly Ballroom tasks expressed

significant increases in gamma power relative to Control in

only a small number of sites, whilst Checkerboard expressed

no significant gamma power enhancement relative to

Control. These results may be related to lack of complexity

in the task. With the exception of Recall, these tasks were

passive, as they did not require the subject to actively

engage in the task. This is in contrast to the Expectancy,

Learning, Reading and Subtraction tasks all of which

required active involvement by the participant and there-

fore, we propose, induced far more impressive gamma

power responses. While the Recall task was not passive, it

was still simple as the participants were presented with lists

from which they had to select the list they had previously

learnt. They were not required to spontaneously recall the

learnt words un-cued. Although gamma enhancement with

Music was left-sided, a subset of subjects undertook a

similar task, viz listening to Mozart, in which the gamma

augmentation was bi-temporal (unpublished). We are

therefore reluctant to interpret the apparent lateralization

with Pachelbel as a robust finding at this stage.

There were different distributions between the areas of

augmented gamma EEG determined by significance versus

areas determined by maximal amplitude. Maximal mean

power was sometimes markedly influenced by powerful

gamma responses in a few subjects with little or no

enhancement at the same area in others. This observation

points to striking individual variability in the brain areas

recruited into mental activity, possibly correlating with the

widely differing strategies individuals use in solving mental

problems. The areas most consistently activated were

parietal and central, an observation that fits with the

known general involvement of fronto-parietal networks

during working memory processes in humans (Cabeza and

Nyberg, 2000).

How do the findings from this study relate to the

temporal binding model of cortical processing? The

model proposes that increased gamma power, as

presented here, reflects large-scale integration of many

coactive cell assemblies synchronously discharging in

recurrent bursts at different periodicities within the

gamma band. The discharges would serve the purpose

of binding assemblies, both local and distant, so that the

information processed in both could be integrated into a

coherent whole. While we have not attempted to define

different regions with synchronous (same-phase) gamma

oscillations, enhanced synchroneity of neuronal bursting

locally is a prerequisite for enhancement of gamma EEG

activity, and we observed 2–5 fold increases in mean

gamma power, strongly supportive of gamma involve-

ment in cerebral processing and consistent with the

temporal binding model. In the Reading task, for

example, the gamma response reflects binding in

the various visual perception and language comprehen-

sion assemblies across the occipital, parietal and temporal

areas to form the story. In the temporal binding model,

task complexity demands utilisation of more cognitive

resources. This requires more binding and subsequently

results in increased gamma power. From this viewpoint,

Subtraction and Reading would be the most complex of

the tasks we administered, intuitively something that

seems likely.

Using spectral analysis to examine EEG correlates of

mental processing permits measurement of oscillatory

phenomena only. While evidence that oscillatory activity

is an important aspect of cortical processing and reflects

synchrony and binding, it is not established that oscillatory

activity is essential for all mental processing nor for

mediating all synchronous (bound) neuronal activity. For

example, Newsome et al. (1990) demonstrate that firing

rates of neurons correlate with the perception of motion

(control condition 10–45 spikes per 2 s versus activated

condition 45–90 spikes per 2 s). Konig et al. (1995) have

also observed that synchronous neural discharges may be

achieved over distances less than 2 mm with or without

oscillating firing patterns. Our methodology does not permit

measurement of any aspect of such non-rhythmic activity if

it occurs during the mental tasks we used.

In conclusion, all of the cognitive tasks we administered

clearly increased gamma power relative to a Control

condition. This enhancement was significant at more than

one scalp site for all tasks with the exception of Checker-

board. The Expectancy, Learning, Reading and Subtraction

tasks were the most complex tasks and expressed the most

impressive gamma power response. Finally, different tasks

led to different distributions of gamma EEG power increase.

These results establish the easy feasibility of examining

sustained high frequency EEG activity without time-locked

averaging and demonstrate some of the EEG correlates of

mentation. It provides evidence of the involvement of

gamma EEG rhythms in these processes and demonstrates

some specificity in the distribution of gamma EEG

activation. The findings are consistent with the temporal

binding model. This method also offers a simple means of

defining the distribution of gamma over the cerebral

convexity correlating with thought processes in individuals,

as well as in health and in disease.

Acknowledgements

Funded by National Health and Medical Research

Council.

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Cognitive Tasks Augment Gamma EEG Power

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