INHIBITION AND SET-SHIFTING TASKS IN CENTRAL EXECUTIVE FUNCTION OF WORKING MEMORY: AN EVENT-RELATED POTENTIAL (ERP) STUDY

Signal & Image Processing : An International Journal (SIPIJ) Vol.7, No.1, February 2016
DOI : 10.5121/sipij.2016.7105 43
INHIBITION AND SET-SHIFTING TASKS IN
CENTRAL EXECUTIVE FUNCTION OF
WORKING MEMORY: AN EVENT-RELATED
POTENTIAL (ERP) STUDY
Pankaj,1
Jamuna Rajeswaran,1*
Divya Sadana1
1
Deptartment of Clinical Psychology, National Institute of Mental Health and
Neurosciences (NIMHANS), Bangalore, India 560 029
ABSTRACT
Understanding of neuro-dynamics of a complex higher cognitive process, Working Memory (WM) is
challenging. In WM, information processing occurs through four subsystems: phonological loop, visual
sketch pad, memory buffer and central executive function (CEF). CEF plays a principal role in WM. In this
study, our objective was to understand the neurospatial correlates of CEF during inhibition and set-shifting
processes. Thirty healthy educated subjects were selected. Event-Related Potential (ERP) related to visual
inhibition and set-shifting task was collected using 32 channel EEG system. Activation of those ERPs
components was analyzed using amplitudes of positive and negative peaks. Experiment was controlled
using certain parametric constraints to judge behavior, based on average responses in order to establish
relationship between ERP and local area of brain activation and represented using standardized low
resolution brain electromagnetic tomography. The average score of correct responses was higher for
inhibition task (87.5%) as compared to set-shifting task (59.5%). The peak amplitude of neuronal activity
for inhibition task was lower compared to set-shifting task in fronto-parieto-central regions. Hence this
proposed paradigm and technique can be used to measure inhibition and set-shifting neuronal processes in
understanding pathological central executive functioning in patients with neuro-psychiatric disorders.
KEYWORDS
Working Memory (WM), Central Executive Function (CEF), Inhibition Task, Set-Shifting Task, Event
Related Potential (ERP), sLORETA
1. INTRODUCTION
Working memory (WM) is a part of the higher cognitive mental functions (HMFs) [1]. Baddeley
and Hitch generated the first model of WM that represents the role of CEF [2]. WM has a
temporary storage system that involves small packets of spatially distributed information
processing systems [3]. It involves a four subsystem i.e., phonological loop, visual sketch pad,
memory buffer and central executive function (CEF) [4]. They integrate to perform the following
cognitive functions i.e., temporary storage, learning, reasoning, comprehension, manipulation and
updating. This involves various parts of the sensory-motor cortices in the frontoparietal lobes,
where the experience is recorded and integrated and subsequently brought to the conscious
domain [5]. The information of CEF drifts through these circuits and it is coordinated by neurons
in the prefrontal cortex [6]. The CEF is studied as a component of Baddeley’s WM model. It’s
further divided in four sub components, inhibition, set-shifting, dual-tasking and updating [7].

44
Electroencephalography (EEG) measures the electrical activity of the brain from the electrodes on
the scalp [8]. Event-related potentials (ERPs) are averaged EEG responses to a specific time-
locked stimuli involving complex mental processing. ERPs are used to assess brain function and
observe electro-physiological manifestations of cognitive activities in both the time domain and
spatial extents. In general, ERPs are typically described in terms of components based on polarity
and latency. ERPs provide good temporal resolution to certify studies of any cognitive processes
in real time. When the subjects were made to perform tasks involving revision of various
memories in short-term which were presented as a specific train of stimulus sequence, the
responses could be measured and spatiotemporally correlated real-time using ERP [9]. EEG
records the correct and incorrect responses of both the inhibition and set-shifting tasks and the
ERPs associated with cognitive process and control potentials in brain are derived from this
signal.
Overall, the ERPs associated with the task are correlated with CEF activity. CEF of inhibition and
set-shifting tasks is manifest in the P200 (or P2), P300 (or P3) and N200 (OR N2) components of
the human ERP and EEG maps show peak activity in the fronto-parietal cortices [10]. This study
was aimed at understanding the brain dynamics of CEF and to find the spatial neural correlates in
CEF during inhibition and set-shifting tasks. For this we designed a paradigm to study the CEF of
WM and modelled the EEG signal to find out neural spatial domains during the aforementioned
tasks.
2. MATERIAL AND METHODS
2.1. Subjects:
We recruited 30 educated healthy subjects (male: female=15:15, right handed, age: 28±5.9years)
in NIMHANS (National Institute of Mental Health and Neuroscience, Bangalore, India) for the
Event Related Potential (ERP) study. All 30 participants were selected for study after assessing
them with clinical interview and normal or corrected vision (Visual Acuity < +1.5/-1.5). A written
informed consent was obtained after explaining the tasks and recording procedure. Subjects with
history of any neurological and mental disorders and use of substances such as alcohol and
tobacco prior to session were excluded. Individuals with frequent headaches or migraine problem
were excluded from the study. Completion of tasks was mandatory for inclusion in the analysis.
Four participants were excluded due to bad electroencephalography (EEG) signal.
2.2. EEG Data Acquisitions
Electroencephalography was recorded using 32 channels Ag-AgCl electrodes arranged to the
revised 10-20 system (to improve spatial sampling) and mounted in uniform fit, highly elastic cap
(ECI, Electro Cap Int®, USA). EEG signal were acquired by SynAmps amplifier™ and recorded
by NeuroScan™ (version 4.4) software. We used mastoid electrodes as reference. Electro-
oculographic (EOG) activity was recorded by the electrodes of on the outer canthus of each eye
for horizontal and below the left eye for vertical movements. Impedance of input signal of
electrode was kept below less than 5kΩ (sampling rate: 1kHz). This study contains one hour EEG
session. Before the start of the tasks, 3 minutes rest state EEG recording was done. Both the task
paradigms were designed using “Stim2” software (Compumedics®
, Neuroscan™). A button box
with millisecond accuracy measured the reaction time (RT) for these computerized tasks.
2.3. Paradigm for the Tasks
The following two types of tasks were performed followed to eye open and eye close at relax
resting condition and measure the CEF process.

45
2.3.1 Inhibition task:
The ‘inhibition task’, subject was shown four geometric different shapes such as a triangle, circle,
plus and square in four quadrants of the monitor (Figure 1). The geometric shapes were presented
randomly in four quadrants checker board that were positioned in the center of the screen with a
white background. The shapes are presented for 800ms at intervals of 200ms. In the four
geometric shapes, if triangle ( ) geometric shape comes with black color, then the subject was
instructed to press the button 1 or else to continue to press the button 2 of the response box. The
participant was instructed to respond as quickly and accurately as possible. The whole inhibition
WM task had 160 rounds and 3 minutes of completion.
Figure 1. Experimental design for inhibition task. “S” - stimulus, which presented for 800ms, “R” - rest
period kept for 200ms, “Target” – Targeted stimuli.
2.3.2 Set-shifting task:
In the Set-shifting task, subjects were shown a set of patterns on a monitor (Figure 2). The set-
shifting task consisted of three pictures in a row and fourth picture column was left empty. There
were four options (1, 2, 3, 4) given to the subject below the pattern. Subject chooses the one of
the right option. All pictures were positioned in the center of the screen in a white back ground.
The subject were instructed to press the key either “1” or “2” or “3” or “4” with their response. In
the task, three patterns (similar, pair, process) were present and among them each pattern had a
set of 10 stimuli of similar type which were presented successively. The stimuli were presented
for 4000ms at intervals of 1000ms. The participant was instructed to respond as quickly and
accurately as possible. The whole set-shifting WM task had 30 rounds and 3minutes of
completion.
Figure 2. Experimental design for set-shifting task. “S” - stimulus, which presented for 4000ms, “R” - rest
period kept for 1000ms.
3. DATA ANALYSIS:
3.1 Pre-processing and Artifact Removal from EEG signal
Raw EEG data analysis was performed on Neuroscan™ and MATLAB®
software. The data was
down-sampled to 250Hz. A cut-off frequency of 0.01 to 60 Hz done by 4th
order band pass
Butterworth FIR filter [9] and after 50Hz notch filter was applied for power line interference [11]
[12]. Preprocessing was done by eye movement artifact correction, muscle artifact correction and

46
singular value decomposition (SVD) for artifact free data and created linear derivation (LDR)
EEG file. The artifact free EEG was epoched to a particular time window for both CEF tasks. In
‘inhibition’ task, EEG file were epoched to a window of 100ms pre-stimulus to 700ms post-
stimulus by selecting trigger and in set-shifting task. Low pass filter of 1Hz was applied on these
epoched data and were corrected to baseline. High amplitude artifacts were rejected using the
artifact rejection criteria of +/-75µV. After artifact rejection, these epochs were averaged in both
time and frequency domain. After this, EEG data was taken for further analysis.
3.2 Post Processing for EEG and ERP Signal:
ERPs were calculated from the epoched EEG data obtained by time domain averaging.
Smoothening was applied on this averaged data. In the electrodes where ERP components were
present as per the visual inspection, the ERP components (N200, P200, P300) were identified for
inhibition task and set-shifting task. The designated time window for identification of the ERPs,
which we considered in the whole experimental tasks were as follows: a) N200 - most negatively
deflecting potential occurring 150ms to 250ms after the stimulus, , b) P200 - most positively
deflecting potential occurring 150ms to 250ms after the stimulus and c) P300 - most positively
deflecting potential occurring 250ms to 350ms after the stimulus. The group averaged EEG files
contain all the ERP components to which 2D maps were generated and collected to get the
activation areas during both the CEF - Working Memory tasks. We computed the activation value
of amplitude in frontal cluster (F3, F7, Fz, F4, F8) and parietal cluster (P3, P7, Pz, P4, P8).
The neural activity for inhibition and set-shifting process of central executive function were
mapped using “spectopo” functions of EEGLAB on 2D head view. Then the signal was computed
for three dimensional cortical distributions on realistic head model with MNI152 template using
standardized low resolution brain electromagnetic tomography (sLORETA). This was followed
by an appropriate standardization of the current density, producing images of electric neuronal
activity without localization bias [13]. sLORETA images represent the electric activity at each
voxel in neuroanatomic Talairach space as the squared standardized magnitude of the estimated
current density. It was found that in all noise free simulations, although the image was blurred,
sLORETA had exact, zero error localization when reconstructing single sources, that is, the
maximum of the current density power estimate coincided with the exact dipole location. In all
noisy simulations, it had the lowest localization errors when compared with the minimum norm
solution and the Dale method [14].
3.3 Statistics data Analysis:
Electrophysiological variables included N200, P200 and P300 peak amplitudes and latencies for
Fz and Pz electrodes for both tasks. N200 to P300 latency ratio for inhibition process (Table 1)
and P200 to P300 latency ratio for set-shifting process (Table 2) were used for paired T-test
statistics. All ERP value found highly significant (>0.05).
Table 1. Inhibition task Paired T-test
Paired T-test inhibition task
frontal electrode stimulus distracter p value t value DF SED
P300 P300 0.0001 17.43 58 0.34
N200 N200 0.0001 4.83 58 0.23
parietal electrode stimulus distracter p value t value DF SED
P300 P300 0.0001 9.7 58 0.91
N200 N200 0.0482 0.76 58 0.23

47
Table 2. Set-shifting data Paired T-test
Paired T-test set-shifting task
frontal electrode value 1 value 2 p value t value DF SED
similar P300 pair P300 0.2086 1.27 58 0.61
similar P300 process P300 0.0001 5.18 58 0.56
pair P300 process P300 0.009 2.7 58 0.79
similar P200 pair P200 0.0004 3.73 58 0.18
pair P200 process P200 0.0019 3.25 58 0.212
parietal electrode value 1 value 2 p value t value DF SED
similar P300 pair P300 0.0001 4.32 58 0.421
pair P300 process P300 0.0441 2.05 58 0.648
similar P200 pair P200 0.0001 21.46 58 0.208
pair P200 process P200 0.0001 21.46 58 0.307
4. RESULTS:
4.1 Response accuracy:
The participants were performed well in CEF tasks. Accuracy and response time were analyzed in
inhibition and set-shifting working memory paradigm. Comparative curves were plotted in
between the inhibition and set-shifting task. We found that average score of correct response was
higher in ‘inhibition task’ (87.5%) in comparison to ‘set-shifting task’ (59.5%).
4.2 ERP Peaks in Inhibition and Set-Shifting Task:
(a) (b)
Figure 3 Event related Potentials in (a) Inhibition task (b) Set-shifting task.

48
Figure 3 represents the average ERPs for the N200, P200, P300 components. We measured the
amplitudes for fronto-central (Fz) and parieto-central (Pz) electrode. These electrodes were
significantly attenuated compared. We found that Fz electrode have less amplitude value than Pz
electrode. In comparison of both tasks, inhibition process had less amplitude than set-shifting
process.
4.3 EEG ERP results:
We found that lower peak amplitude activity of brain area in inhibition process than set-shifting
process, which reveals that inhibition process utilize less activation of brain areas to perform
tasks. In order to show this, we examined the frontal and parietal activity of brain in same tasks.
We found that brain uses the fronto-parietal connectivity when it performs CEF of WM related
tasks. We also found that the midline frontal (FZ, F4, F6) and parietal (PZ, P4, P6) electrodes
were activated at the same time with prominent pattern of negative potentials and other had
positive potentials in both tasks.
Figure 4 (a). Neuronal activation in EEGLab®
head-view (3D & 2D) for Inhibition task.
Figure 4 (b). Neuronal activation in EEGLab®
head-view (3D & 2D) for Set-shifting task.
Figure 5(a) . Neuronal activation in MNI template for Inhibition task computed from EEG signal.
Figure 5 (b). Neuronal activation in MNI template for Set-shifting task computed from EEG signal.
Most significant results were found in both tasks. After the post processing we got 3D and 2D
head-views in EEGLAB [Figure 4 (a,b)] and sLORETA [Figure 5 (a,b)]. In EEGLAB we

49
measured the activation on 100, 200 and 300 ms after the stumuli come. In both head-views we
found the Inhibition process have less activation on fronto-parietal lobs than set-shifting process.
5. DISCUSSION
Till today, a lot of research has been done on both central executive function of working memory
but little efforts have been made to find the behavioural and neural connections between the two
processes. This study examined the brain activation pattern. We find out ERPs and behavioural
interpretations, during the inhibition and set-shifting tasks. In this study our objective was to
understand the brain dynamics for CEF of WM functions and find out neural spatial domain in
central executive function during inhibition and set-shifting process. The behavioural results
showed that among the population of normal individuals, inhibition process score was high in
comparison to set-shifting task.
These behavioural changes were accompanied by a number of changes in transient ERPs and task
related EEG activations. This study is mainly focused on finding the EEG correlation between
components of CEF. In EEG findings, by using time domain analysis [15] and bivalent 2-D and
3-D cartoon maps, positive and negative potentials were observed across the pre-frontal, frontal,
central and parietal electrodes, during both the tasks.
Electric potential distribution on the basis of the scalp recording was measured by sLORETA that
compute the cortical three- dimensional smoothest of all possible neural current density
distribution, where neighbouring voxels have maximally similar activity. Low Resolution
Electromagnetic Tomography (LORETA) is used for source localization, based on R. D. Pascual-
Marqui and its virtual MR anatomical images are based on Montreal Neurological Institute (MNI)
of McGill University. LORETA is a linear distribution method and it calculates the current
density voxel by voxel in the brain as a linear, weighted sum of scalp electrical potential.
Computations were made in a realistic head model using the MNI 152 template with the three
dimensional solution space restricted to cortical grey matter.
The EEG method is the dipole source with fixed location and orientation method and it is
distributed in the whole brain volume or cortical surface. There are six parameters that specify the
dipole, three special coordinates (x,y,z) and three dipole components (orientation, angle, and
strength). In the LORETA, the electrode potentials and matrix X are considered to be related as
X=LS
Where S is the actual current density and L is Ne×3m matrix representation the forward
transmission coefficients form each source to the array of sensors. There are number of sensors
Ne, extra cranial measurements in the surface of brain and Nv, voxels in the brain. In the real
world applications where more concentrated focal source methods fail and choose smoothness of
the inverse solution as
S=LT
(LLT
)X
The sloreta uses the minimum norm solution is to create a space solution with zero contribution
from the source. However, the variance of the current density estimate is based only on the
measurement noise, in contrast to sLORETA , which takes into account the actual source variance
as well.
Time domain analysis of EEG and ERPs: After averaging in time domain 2-D head maps were
taken to represent the surface potential distribution patterns. Central executive WM tasks were

50
analyzed at both the retrieval and encoding phases in epochs of 800 and 1000ms. Result was seen
throughout the whole brain electrodes. In the encoding and retrieval phase of the CEF of WM
activation patterns were seen [16].
ERPs comparison studies for inhibition and set-shifting process: Visually evoked potentials
(P100) were present throughout the CEF WM task [17]. Delay in early N200 latencies in Fz of
encoding showed that visual attention took time. In late P300 peaks in Pz of retrieval phase
showed that matching system of visually attend objects. We also found the increase in the
amplitude of the frontal visual N200 has been observed in a number of studies, this response
shows the Inhibition and attention performance. These delays in ERPs are also observed in 2D
and 3D maps in EEGLAB. So we can conclude that functioning of inhibition process is found in
fronto-parietal area of brain.
In both encoding and retrieval phase of CEF WM all desired ERPs were found. Visually evoked
potentials (P100) were seen in parietal area suggesting the stimulus representation extrastriate
cortex (BA 6 and 18) [18]. N100 component was seen in frontal and pre-frontal electrodes
showing the unpredictability of stimulus. N200 component was present in parietal electrodes.
Encoding phase showing delayed N200 latencies but in retrieval phase N200 latencies were
shorter due to attending the visual stimuli. Early-latency visual ERPs are enhanced when stimuli
are presented in an attended visual field. Reaction times are also shorter to stimuli presented in an
attended visual field than to those presented in an unattended visual field. In retrieval phase,
delayed P200 latency of Fz electrodes suggesting for a matching system that compares sensory
inputs with the stored memory. P200 latencies were present in both phases but most prominently
it was elicited in retrieval phase.
ERPs analysis is suggested for activation network during CEF of WM. In Encoding phase initial
activations occurred in the visual cortex where stimuli were attended for the visual search. The
information from the stimuli was coded to the Fz and Pz electrodes as the late P300 peaks were
present in the parietal electrodes suggesting the demand of focusing. Likewise, all the ERP
patterns were seen in the retrieval phase. The only difference was the delayed P200 latencies
suggesting for a matching system related to make comparison of present stimulus with the
previous ones. This whole ERPs analysis is consistent with EEGLAB and sLORETA maps
achieved in the time domain analysis. So, a network of activation patterns from frontal to parietal
area in the both inhibition and set-shifting process of Central Executive Function of Working
Memory.
6. CONCLUSION
Findings from this study will give a better understanding of CEF and brain activity in inhibition
and set-shifting process. Our findings demonstrate a paradigm combined with a technique to be
used to measure of inhibition and set-shifting neuronal processes. This would help in under-
standing the alteration of central executive function in patient with neurological and psychiatric
disorder.
ACKNOWLEDGMENT
Preparation of this paper was supported by Department of clinical Neuro-psychology, National
Institute of Mental Health and Neuroscience (NIMHANS), Bangalore, India. We are grateful to
Ganne Chaitanya, Deepak Ullal and Rajnikant Panda for providing thoughtful remarks and
suggestions regarding EEG and ERP analysis.

51
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AUTHORS
*The Corresponding author for this paper is Dr. Jamuna Rajeswaran
Mr. Pankaj, received M.Tech degree in Cognitive Neuroscience from Centre for
Converging Technologies, University of Rajasthan, India, in 2015. He has more than one
year of research experience on clinical psychology, Neuro-imaging and EEG signal
processing on cognition at National Institute of Mental Health and Neurosciences,
Bangalore, India. He has clinical research experience with many National institutes. He
also got Mani Bhomik research-fellowship (2015) in National Institute of Advanced
Studies (NIAS), Indian Institute of Science campus, Bangalore, India.
Dr. Jamuna Rajeswaran, Additional Professor, Consultant & Head Neuropsychology
Unit, National Institute of Mental Health and Neurosciences, Bangalore, India. She
completed her M.Phil and PhD from Dept. of Clinical Psychology, National Institute of
Mental Health and Neurosciences, Bangalore, India.
Dr. Divya Sadana, Senior Research Fellow, Department of Clinical psychology,
National Institute of Mental Health and Neurosciences, Bangalore, India. She completed
her M.Phil and PhD from Dept. of Clinical Psychology, National Institute of Mental
Health and Neurosciences, Bangalore, India.

INHIBITION AND SET-SHIFTING TASKS IN CENTRAL EXECUTIVE FUNCTION OF WORKING MEMORY: AN EVENT-RELATED POTENTIAL (ERP) STUDY

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