1. Effects of Attention on Functional
Connectivity of Visual Cortex
Submitted by
Francesca Bocca
Thesis
Presented to the Department of Psychology / Neuro-Cognitive
Psychology of the Ludwig Maximilian University Munich
For the degree of
Master in Neuro-Cognitive Psychology (M.Sc.)
Ludwig Maximilian University Munich
September 2011

2. Effects of attention on functional connectivity of visual cortex
Francesca Bocca
Abstract
The present experiment aims at studying the orienting of visual attention through endogenous
cueing while employing Transcranial Magnetic Stimulation (TMS) and Electroencephalography
(EEG). While spatial visual attention has been widely studied with many neuropsychological
techniques, the combined usage of TMS and EEG provides an optimal tool for this field, with the
advantages of high temporal resolution and causal power. The participants were asked to orient
their attention towards one of two placeholders, at equal eccentricity compared to the central
fixation cross. While in some trials a visual stimulus could appear and a discrimination task would
be performed, in the majority of trials the participant would just be asked to orient his attention.
During this covert attention task, a TMS pulse was sent 600 ms after cue onset. TMS could be
delivered to occipital cortex, a parietal control site, or while holding the coil orthogonally to the
participant’s scalp. No effect of stimulation site was observed on behavioural results, while the
ERPs showed a late interaction between TMS stimulation site and cue direction (340-390 ms after
TMS pulse). This effect is later than the one reported in previous literature (Taylor et al., 2010), in
which no cueing was employed. Further analyses and follow-up studies will be run in order to
verify the hypothesis that this late difference might reflect a top-down attentional modulation.

3. INTRODUCTION
In every circumstance of our life, we need attention to properly select relevant information and
perform even the most basic of daily duties. In psychological research, the most studied type of
attention is without doubt visual attention, and more particularly the orienting of this kind of
attention to different elements of the environment (called spatial attention). The present study will
employ a classical experimental paradigm, the Posner cueing, while registering EEG and employing
transcranial magnetic stimulation (TMS) to different brain areas. We are interested in further
clarifying the effects of attentional shifts on the functional connectivity of visual cortex, and for this
research question the combining of EEG and TMS provides an ideal method, combining high
temporal resolution and making possible causal inferences on the resulting effects.
A classical method to investigate covert (i.e. not including eye movements) attention is the Posner
cueing paradigm (Posner, 1980). In this experimental situation, the observer is presented with a
central fixation cross and is asked to fixate on it for the whole trial duration. After a variable
stimulus-onset asynchrony (SOA) a cue in the form of an arrow indicates to the participant on
which side of the display to expect the appearance of a visual stimulus. The participant is asked to
perform some kind of detection or discrimination task on the target. The classic finding of Posner
with such paradigm is that reaction times (RTs) are substantially faster when the cue is valid (i.e.
the cue indicates to the same direction where the target will appear) than when it is invalid (i.e. the
cue indicates the opposite direction of where the target will appear).
An important parameter in the Posner paradigm is the cue validity ratio, which is to say how many
times out of the total number of trials the cue points to the target location. Eimer and colleagues
(Eimer et al. 1994, Eimer et al. 1997) have investigated the changes in ERPs component while
varying the cue validity ratio in a given block. Even when using totally uninformative cues (50%
validity) he found an effect in central electrodes like CZ, and with more informative cues (75%
validity) the effect in the CZ electrode became greater. The conclusion of Eimer is that cueing

4. involves both voluntary and involuntary components, whose disentangling might prove difficult. In
order to limit the involuntary components of cueing, it is thus better not to employ 100% cue
validity. The present experiment employs 80% cue validity, in order to obtain robust effects that
also include voluntary effort by the participant to direct his attention.
Another important variable in experiments employing cueing is the type of cue employed: the main
distinction is between exogenous and endogenous cueing. In exogenous cueing, a reflexive shift of
attention is obtained by a salient cue (e.g. an increase in luminance or a change in colour) at the
location of the expected target, while in endogenous cueing a central cue is presented (e.g. in the
form of an arrow or even a more arbitrary symbol) requiring the participant to purposefully
allocating his attention to the location indicated by the cue. The distinction between this two cues is
relevant for our experiment: in an fMRI study (Mayer et al. 2004) it has been found that the two
kinds of cues result in differential patterns of brain activation: endogenous cueing produces a wider
cortical activation than exogenous cues. In our experiment we are interested in stimulating one side
of the visual cortex and to interact with attentional related activity in the brain: the more widespread
activity resulting from endogenous cues thus maximises our chance of interacting with an area that
is being differentially activated by attentional shifting.
Many studies have employed the ERPs technique as a way of measuring the attention allocated at a
certain location and the resulting facilitation of processing the stimulus (that is behaviourally shown
in reduced reaction times for cued locations) caused by cueing. The ERPs technique presents many
appealing characteristics that make it suitable to study visual attention: first of all, shifts of
attentions are very fast – almost at a single millisecond scale – and thus the high temporal resolution
of ERPs provide the experimenter with the possibility of following this shifts with a sufficient
temporal grain. Furthermore, ERPs can be registered from the whole cortex, even though it is
extremely hard to know where exactly the source of an ERP is located; given that the exact location
and movements of attention throughout the cortex are not clear, the bad spatial resolution isn’t a
serious limitation for studies of attentions, while the possibility of registering from many electrodes

5. (typically 64 or 128) allows for a higher probability of locating an effect connected to attention,
even if small and widely distributed.
ERPs components related to attention are many; we will review a few of them that could be present
also in our experiment. Luck and others (1990) used a visual search task and highlighted how the
P1 component can reflect the facilitation of sensory processing at a certain location and can thus be
used as a measure of the presence or absence of attention. Hillyard and colleagues (Hillyard et al.,
1994) showed an effect of cueing on both P1 and N1 ERP components, suggesting that cueing
exerts the strongest effects at short latency (80-200 ms after stimulus). The greatest effect
magnitude was found on posterior visual-cortical electrodes, and is consistent across different kinds
of tasks like attentional orienting and sustained attention. Later (Hillyard et al., 1995), this finding
has been used to argue for an early theory of visual attention selection, stating that active selection
of relevant elements happens early in time, based on physical attributes such as location
(Broadbent, 1958). More recent studies have investigated also later components that are thought to
reflect attentional effects (for a review, see Luck et al., 2000). A limitation on the EEG
methodology, however, is that it is very difficult to localise with precision the source of an ERP
component; only the combination of ERP and other techniques (typically fMRI and PET) can help
identifying an unambiguous location.
The other method that we will employ in the present study is transcranial magnetic stimulation
(TMS). TMS is a relatively young tool in the cognitive neurosciences, being introduced in 1985
(Barker et al, 1985); however the working principle of TMS has been known for much longer. TMS
is based on the principle of electromagnetic induction introduced by Faraday; exploiting a magnetic
field changing fast in time, a secondary current can be generated. Placing the coil on the head of the
subject then causes stimulation on his brain because of the flowing current. Pascual-Leone and
colleagues (2000) have outlined some of the possible applications of TMS in a review. They
explain TMS as acting like a “virtual lesion” in the participant brain, allowing for controlled studies
of neurophysiology. The biggest difference between TMS and other neuroscientific techniques like

6. fMRI, PET, MEG and others is its causal nature and the fact that it can provide a functional
connection between brain and behaviour.
Stimulating different areas of the brain with TMS results in different patterns of behaviour and
perception. We will focus on the stimulation over the visual cortices that frequently results in the
perception of phosphenes. Phosphenes are one of the two commonly used indices of cortical
excitability in TMS studies, the other being motor threshold. These two measures do not correlate
despite being stable in time; suggesting how phosphenes and motor twitches should be used for
visual and motor cortex (Stewart et al., 2001). This observation is important because different
cortices have different excitability properties, and the correct threshold should be chosen to
investigate them, in order to have a meaningful baseline.
Phosphenes have been studied in relationship to attention. A recent study suggested that spatial
attention can influence the phosphene threshold through overall excitability modulations of the
visual cortex, resulting in lower phosphene threshold for the attended side of the visual field. These
results have been observed employing exogenous cueing on one side of the visual field, and
stimulating either the cued side of the brain or the non-cued one (Bestmann et al. 2007).
However, employing TMS alone in the study of attention allows only for behavioural observations
to be conducted. Recently, the combination of TMS and EEG has been tested and perfected. The
first problem that is faced when TMS is applied during EEG registration is the TMS-induced
artefact, typically resulting in saturation of the EEG amplifiers. Using a phantom head to record this
kind of artefacts and to exclude cortical response, it has been found that already after 5ms from a
TMS pulse the EEG signal is sufficiently clean to be recorded and analysed (Veniero et al., 2009).
The interactions between TMS and ERP have also been recently studied (Reichenbach et al. 2010);
while delivering TMS to early visual areas, different interactions between the early components of
the two evoked potentials have been observed (Visual-evoked potentials, or VEP and TMS-evoked
potentials, or TEP). The strength of the overall effect seemed to depend on the early overlap of TEP

7. and VEP. A review by Komssi and Kähkönen (2006) described the advantages that the
combination of TMS and EEG can provide for neuroscientific research. The possibility of
generating a measurable and consistent cortical reactivity at substantially sub-threshold intensities
(as low as 40% of the motor threshold) makes EEG a very sensitive method for the assessment of
cortical excitability. In our study, we are interested in stimulating sub-threshold (in our case, 90%
of the phosphene threshold) in order to investigate only attentional cortical processes while not
being disturbed by perceptual or motor components.
The neural characteristics of phosphene perception have been investigated through concurrent usage
of TMS and EEG in a recent paper by Taylor and colleagues (2010): in this study TMS (with
intensity set at the value of phosphene threshold) has been applied over the occipital or parietal
parts of the brain, and trials in which phosphenes were seen were compared with trials where
phosphenes were absent. The result of their study was that occipital stimulation resulting in
phosphene perception was characterised by a bigger positivity than when phosphenes were absent.
It is also interesting to note that stimulation of occipital TMS where no phosphenes were perceived
was more negative than stimulation of parietal areas that was, in turn, more negative than trials in
which no TMS was employed. The difference between conditions was not only in terms of
amplitude, but also of scalp topography. While a TMS-evoked potential (TEP) was present in all
TMS conditions, occipital and parietal TMS have very different topographical effects: occipital
stimulation resulted in a late positivity while parietal stimulation in an early negativity.
In the present study we employ the EEG and TMS techniques technique in the study of covert
visual attention. We use an endogenous cue with 80% validity indicating towards one side of the
visual field. While in some trials there is a visual stimulus on which the participant is asked to
perform a discrimination task, on the majority of trials (66%) no visual stimulus is present. In this
way, we are able to investigate the effect of TMS over the covert orientation of visual attention
without the possible confound of ERPs caused by the visual stimulus itself. Three TMS stimulation
locations are employed for each participant: a condition in which TMS is directed on the visual

8. cortex of the participant, on the phosphene hotspot. In a second condition, the parietal control site,
the TMS coil is placed over the parietal cortex, as lateral as the phosphene hotspot. A third
condition, the orthogonal coil orientation control, is mainly an auditory and electrical control: the
TMS coil will be placed perpendicularly on the head of the participant, touching the phosphene
hotspot. The rationale of this condition is to compensate for the auditory and part of the tactile
effects of TMS, without the pulse itself; furthermore, such coil location produces the TMS artefact
in the EEG electrodes, making the recorded data of this condition more comparable with the other
two.
The most important dependent measure is the TMS-evoked potential. In previous literature, in
which the task was to report phosphene presence or absence after each TMS pulse, hemispherical
differences both in early and late TEP were found. The interaction of this TMS-evoked potential
and covert attentional modulation is the main interest in our study.

9. METHODS
Stimuli and Procedure
12 observers took part in this study (mean age 26 + 3 years, 2 males). All participants had normal or
corrected to normal visual acuity, were right-handed and reported no history of neuropsychiatric
illness or epilepsy. All gave informed consent in accord with local ethics approval.
Before accepting the participant into the study, we tested whether we could obtain right hand
twitches by stimulating his left primary motor area and phosphenes in the left visual field by
stimulating his right visual cortex. Details of this procedure are described in the TMS section of the
methods below.
Prior to the beginning of the experiment, phosphenes and motor twitches were obtained and
thresholds were measured through an adaptive staircase algorithm.
In the case of motor threshold procedure, the subject was instructed to keep his hand lifted at the
height of his chest, and keeping his fingers muscles at a moderate tension (about 30% of the
maximal tension possible). At each trial, the participant received a TMS pulse on the area
previously defined as being the optimal for eliciting hand twitches, and the experimenter had to
report whether a twitch was observable or not. A sigmoidal function was fitted to the measured
responses, and the stimulator output intensity of 50% ‘yes’ responses was taken as the motor
threshold. At least 30 trials or 8 reversals were considered sufficient to terminate the thresholding
algorithm.
In the case of visual phosphenes thresholding the subject fixated on a central point on blank screen
of the same colour and luminance as the one used in the experiment, and reported the presence or
the absence of a phosphene after each TMS pulse (‘yes/no’). A sigmoidal function was fitted to the
measured responses, and the stimulator output intensity of 50% ‘yes’ responses was taken as the
phosphene threshold. Participants had both to report whether they saw a phosphene and where did

10. they see it. At least 100 trials or 20 reversals of the staircase algorithm were considered sufficient to
terminate the thresholding procedure. The information about location was used to position
placeholders during the rest of the experiment.
After threshold measures, the participant was trained on the behavioural task until its performance
reached below 30% of errors, and until the effect of cue validity was bigger than 50 ms. After
training, each participant underwent a total of 15 experimental blocks, each consisting of 60 trials.
The structure of the trial is reported in Figure 1; while training was constituted only by visual
stimuli present trials, in the real experiment visual stimuli could be both present and absent.
Figure 1: structure of individual trials. Top panel: time line of visual stimuli present trials. Bottom panel: time line of
visual stimulus absent trials.

11. Each trial began with a black fixation cross and two placeholders (in symmetrical positions respect
to the vertical midline of the screen) on a grey background; both the fixation and the placeholders
remained on screen for the whole trial duration.
After a random fixation ranging from 1000 ms to 1400 ms, a white cue in the form of an arrow
appeared in the centre of the screen, lasting 100 ms. The cue could point to the left of to the right.
Participants were instructed to keep their gaze stable on the fixation cross, but to move their
attentional focus on the placeholder indicated by the cue. Independently of whether visual
stimulation was present or absent in the trial, after 600 ms from cue offset, a TMS pulse was sent.
In trials in which visual stimulation was present, after a variable stimulus onset asynchrony (SOA)
from 400 to 800 ms, a visual stimulus appeared inside one of the two placeholders. Visual stimuli
consisted in a 9-squares black and white checkerboard, as in Figure 1, and its negative image. The
stimulus stayed on screen for 100 ms, and then the subject had to discriminate which of the two
checkerboards appeared, through a key-press. Keys were adjacent on the keyboard, and subjects
were instructed to press them with the index and middle finger of their right hand, respectively. The
stimulus-response key combination was counterbalanced across subjects.
In visual stimulation absent trials instead, after the cue offset, fixation cross and placeholders
remained on screen for a total of 1200 ms, before initiating the new trial. After each trial, the
participant had to respond to whether he saw a phosphene or not. After this response, an 800 ms
inter-trial interval (ITI) was presented, in which the subject could close his eyes if he needed to, or
take a small break.
In each of the 15 experimental blocks, 40 trials (66%) were of target absent type, and 20 (33%) of
target present. In target present trials, 16 times (80%) the cue was valid, and 4 times (20%) invalid.
Cue direction, visual stimuli positions and visual stimuli colour were balanced within each block.
The block type was randomized and balanced across participants using a Latin square technique.
During the experiment, each observer sat in a darkened room, 57 cm from a on a cathode ray tube

12. monitor, wore earplugs in order to diminish the distraction caused by the TMS pulse sound, and
stable viewing and head position were ensured with a chin-rest.
Transcranial Magnetic Stimulation (TMS)
TMS was applied using a figure-of-eight coil (Mag & More Gmbh). Both the coil and a plot
showing focal stimulation properties of this type of coil are shown in Figure 2. The usage of a
double coil is better for focal stimulation of small areas, or for high precision because of the sharp
field distribution due to the coils summation effect in the centre.
a) b)
Single coil Double coil
Figure 2. Panel a (left): a figure of the coil used in the present experiment. Panel b (right): A comparison between the
magnetic field generated by a single coil (left plot) and the double coil (right plot). Adapted from Mag & More research
TMS user manual.
The coil was held with the handle pointing towards the right (and the current therefore flowing
lateral-to-medial, from right to left), an efficient coil orientation for inducing phosphenes (Kammer
et al., 2001).

13. Preliminary to the beginning of the phosphene and motor threshold calculation procedures, the
areas that could elicit a best motor twitch or phosphene were individuated using the following
screening procedure. In order to screen the motor area, starting from a TMS output intensity value
of 50% and from 5 cm lateral and 2 cm frontal to the centre of the head, stimulation intensity was
increased in 5%-10% steps until a visible twitch was observed, or until the stimulation intensity was
at 90%. Other points were tested, spanning a 2cm x 2cm grid around the beginning point of the
procedure. The point that showed a greater motor twitch at the same level of stimulation was
marked on the cap of the participant and used as the motor threshold hotspot. After this first
screening the motor threshold was measured. This measure was taken because it can be helpful,
when trying to induce phosphenes in a naïve participant, to employ repeated TMS (rTMS).
However, safety limits impose that rTMS can only be delivered at 120% of the motor threshold.
A similar procedure was employed to screen for phosphene presence: the starting point was 2 cm
medial and 1 cm lateral from the inion. Participants wore a blindfold and were asked to keep their
eyes closed and to report whether they perceived any visual stimulus following the TMS pulse. If
the participant was not able to perceive any phosphene at 90% intensity in any point spanning the
grid of 2cm x 2cm around the starting point, then rTMS at 10Hz was applied. This frequency has
been shown to produce particularly vivid phosphenes (Reference needed here). Participants were
excluded if no phosphenes were perceived or if the stimulation of the right visual cortex didn’t
evoke reliable phosphenes on the left visual field. Once the participant experienced phosphenes,
the point of most vivid phosphenes was individuated, marked on the cap and employed for both
phosphene threshold and experiment stimulation location.
Thresholding was computed using a custom-made staircase threshold. After setting a maximum and
a minimum intensity output for the machine, the algorithm started a one up – one down staircase,
with step size decreasing arriving to 1%.
During the whole experiment duration, output intensity of the TMS machine was set at 90% of the
visual phosphene threshold. This intensity was chosen because we an intensity high enough to elicit

14. differential activations among the hemispheres and small enough to avoid the participants seeing
phosphenes throughout the experiment.
Event-related potentials
EEG was DC-recorded continuously at 5,000 Hz with a TMS-compatible ERP amplifier (BrainAmp
DC, Brainproducts, Germany) capable of recording a veridical EEG without TMS or recharging
artefacts within 50 ms after TMS pulse (Veniero et al., 2009). EEG was recorded with minimal
filtering (DC-1000Hz, no notch) from a montage of 29 Ag-AgCl electrodes (at positions Fpz, F3,
F4, C3, C4, P3, P4, PO7, PO8, F7, F8, T7, T8, P7, P8, Fz, Pz, Cz, Po4, Po3, Poz, Fc5, Fc6, Cp5,
Cp6, Fcz). Horizontal EOG was recorded from the left and right temples. The ground was at AFz
and the active reference on the left earlobe. Electrode impedance was kept below 5 kΩ. Data were
re-referenced to the average of the left and right earlobes, and an HEOG signal was formed from
linear derivation of the left and right EOG electrodes. Data were then epoched to form 500-ms
segments containing the TMS-evoked potential. Then the TMS artefact was removed from the data
through linear interpolation of the data between 1 ms before and 60 ms after the TMS pulse
(Fuggetta et al., 2006; Taylor et al., 2008). Filtering used a notch 50-Hz filter and then a
Butterworth zero phase filter with high cut-off of 40 Hz (12 dB/octave). Data were then epoched
into 500-ms periods, starting 100 ms before the time of TMS onset. Baseline correction used the
100 ms prior to the TMS pulse. Automated ERP artefact rejection removed trials with eye
movements by eliminating trials where the HEOG signal exceeded ± 30 µV. Blinks were removed
by deleting trials in which the amplitude signal at Fpz exceeded ± 60 µV, and other movement-
related artefacts were removed by eliminating any trials where the signal from any electrode
exceeded ± 80 µV. A minimum criterion of 30 trials per condition was set to ensure a sufficiently
high signal-to-noise ratio of the ERP averages, which were time-locked to TMS onset.

15. Neuronavigation
7 (mean age 25 + 3 years, 2 males) out of the 12 participants had their brain scanned in previous
experiments. For them, at the end of the experimental session, the location of the stimulated sites
was co-registered with the participant’s brain scan. This procedure was accomplished using
Brainsight (Rogue Research Inc.) stereotactic infrared registration to the participant’s structural
MRI scan; With Brainsight technology a 3D reconstruction of an MRI scan can be made and
landmarks on the subject’s head are co-registered with landmarks on the structural MRI. Figure 3
shows the stimulation locations on the brain of a representative participant.
Parietal TMS stimulation Occipital TMS stimulation Figure 3: Stimulation locations
co-registered with the brain of
a representative participant.
The left panel shows the
location of the parietal TMS
stimulation place, with the top
figure showing a sagittal plane
and the bottom figure a coronal
plane. The right panel shows
the location of the occipital
TMS stimulation place, with
the top figure showing a
sagittal plane and the bottom
figure an axial plane.
Statistical procedures
Behavioural data were analysed using the statistical software R. In error analyses, target present
data were employed and no outlier rejection was performed on the data. In reaction times analyses,
target present data were filtered according to two criteria: the first one is correctness of the trial, and
the second was an automatic outlier rejection procedure, in which all reaction times larger than the

16. average plus three standard deviations or smaller than the average minus three standard deviations
were rejected.
EEG data analyses were performed on Brain Vision Analyser 2.0 and R. The figures were generated
through Brain Vision Analyzer, while statistical tests of significance for activation data were
conducted in R. All statistical procedures related to EEG data excluded the first 60ms after the TMS
pulse.

17. RESULTS
Reaction times
Figure 5 shows the distribution of reaction times for all subjects depending on cue validity. The
notches in each side of the boxes are a statistical index (+/-1.58 * IQR/√͞n , where IQR represents
the inter-quartile range, and n is the numerosity of the sample): if the notches of two plots do not
overlap this is “strong evidence” that the two medians differ (Chambers et al., 1983, p.62). This
method is based on asymptotic normality of the median and roughly equal sample sizes for the two
medians being compared, and is reasonably insensitive to the underlying distributions of the
sample. The notches provide then an equivalent to a 95% confidence interval for the difference in
two medians.
Figure 5. Distribution of Reaction Times depending on cue
validity. The median RT for valid cue trial was 650 ms
(average = 690 ms), while the median RT for valid cue trials
was 710 ms (average = 754 ms)
A Wilcoxon rank sum test with continuity correction was also conducted on the reaction times,
revealing a significant effect of cue validity, with RTs in invalid cueing condition being higher (W =
41537700, p<0.05). The effect size was of 60 ms, with valid cue median RT value being 650 ms and
invalid cue RT being 710 ms.
Figure 6 shows the averages of the same data depending on both cue validity and block type.

18. Figure 6. Averages of reaction times
depending on cue validity (x axis)
and block type (line colour).
A two-ways 3*2 ANOVA showed as an only significant main effect cue validity (F(1,30)=28,
p<0.001), while block type and interaction did not reach significance.
Errors
Error rates were calculated on visual stimulus present trials and the average of overall errors was
15%. Figure 4 shows the error rate averages depending on block type and cue validity.
Figure 4. Averages of error
distribution depending on cue
validity (x axis) and block type
(line colour).

19. A two-ways 3*2 ANOVA showed a significant effect of cue validity (F(1,30)=20.8, p<0.001); while
block type and the interaction between cue validity and block type did not reach significance. Mean
error for valid cue trials was 14% and mean error for invalid cue trials was 16%.
The TMS Evoked Potential
TMS was applied at 90% of the threshold for phosphene perception either to occipital cortex, to a
parietal site as lateral as the occipital site on the scalp, or with the coil held orthogonally to the
scalp. Figure 7 shows the TMS-evoked potentials (TEP, time-locked to TMS onset) for the three
TMS conditions.
Figure 7. The mean amplitude of the ERP after TMS stimulation on occipital cortex (top panel) orthogonal coil condition (middle
panel), and parietal control site (bottom panel). Each head map represents the average activation inside a 57 ms time bin.

20. The TEP varied depending on the place of stimulation. As shown in Figure 7 (top panel), after 60
ms from the TMS pulse, it was possible to notice a right-side negativity. From 230 ms until the end
of the epoch (400 ms) a generalised negativity, starting from the right parietal electrodes and
expanding to the fronto-central regions, was shown. In the orthogonal coil orientation control
blocks, after a weak negativity on the right hemisphere following the TMs pulse (60-173 ms) a left
occipital positivity was shown, lasting the rest of the epoch (173-400 ms) but peaking in the period
from 287 to 343 ms. Finally, in the parietal TMS stimulation block, a weak central positivity was
shown in the first time bin after TMS stimulation (60-117 ms), followed by a right occipital
positivity (117-230 ms) and finally a left occipital positivity (287-400 ms) partially overlapping in
time with a right temporal negativity (343-400 ms).
From this qualitative description, we can notice some similarities and differences between the
effects of TMS stimulation on different sites. The most consistent effect is an extremely lateral right
occipital negativity, resent in all three kinds of blocks, but longer in the orthogonal coil orientation
control condition. The most striking difference is in the latest time bin (343-400 ms): while the
occipital stimulation caused an overall negativity, the other two conditions seem to cause a left
occipital positivity (strictly occipital in the case of orthogonal coil control, parieto-occipital in the
case of parietal TMS stimulation) in concurrence with a right parieto-occipital negativity.
Thus our analyses verged on the effect of stimulation site on the electrode Pz. The detailed
activation of this electrode time-locked to the TMS onset is represented in Figure 8; the top panel
illustrates parietal TMS stimulation blocks, the middle one is for occipital stimulation block and the
bottom one for orthogonal coil orientation blocks. Black lines represent right cue trials, and red
lines left cue ones.

21. Orthogonal coil Occipital TMS Parietal TMS
Figure 8: TMS-evoked potential for different conditions: top panel represents parietal TMS stimulation blocks, middle panel
represents occipital TMS stimulation blocks, while the bottom panel represents orthogonal coil control blocks. In all panels, right
TEP is drawn in black while left TEP is drawn in red. The yellow highlighted zone (340-390 ms) represent the significant interaction
between cue direction and block type revealed by a two-ways ANOVA.
A two-ways 3*2 ANOVA on the time bin 340-390 ms revealed a significant interaction between
TMS stimulation site and cue direction (F(2,20)=5.2, p<0.05). Paired t-tests were conducted to
further clarify the effect; the only significant effect was the difference between left and right cue for
parietal TMS stimulation trials (t(11)=-2.6, p<0.05. meanLEFT=8.6, meanRIGHT=9.8).

22. DISCUSSION
The present experiment aimed at investigating the relationship between endogenous attentional
modulation – obtained through a Posner cueing paradigm – and TMS stimulation. While no effect
of TMS was obtained on reaction times and error rates, a late interaction between TMS stimulation
site and cue direction was found on the ERP data.
More in detail, the present experiment replicated the classical Posner effect of cue validity, that is to
say that error rates are faster for valid cue trials than for cue absent trials. The present experiment
replicated this effect, meaning that the kind of cue effectively guides attention towards the location
to attend, and that the visual discrimination task employed is hard enough to be modulated by
attention. A small effect of cue validity was found, but despite its significance, its effect size of only
2% of error made it not worthy of interpretation.
Analyses on both reaction times and error rates didn’t show any effect of block type, nor interaction
between block type and cue direction. This finding means that any differential activation caused by
TMS was either not-related to the task, or small enough not to perturb the behavioural outcome in a
significant way. Furthermore, the fact that orthogonal coil control didn’t differ significantly from
trials in which TMS was actively employed on the scalp means that the haptic component of the
TMS pulse, caused by the electric activation of the scalp muscles, doesn’t cause distraction or
disrupt performance on the discrimination task employed.
The EEG analyses focused on the TMS-evoked potential. A late interaction (340-390 ms after TMS
pulse) between TMS stimulation site and cue direction was found. Previous studies (Taylor et al.,
2010) have shown effects of TMS stimulation region both in an early time period (160-200 ms after
pulse) and in a late one (280-400 ms). While our dataset differs concerning the first effect, it is
possible that our significant effect on the parietal electrode Pz captures the late component
described by Taylor and colleagues (2010). The fact that the component we observed is later than
the one of Taylor and colleagues (2010) could be explained in term of different attentional tasks in

23. the two studies. In our experiment, an endogenous orienting of attention was necessary in order to
accomplish the task; this kind of attentional orienting is known to be slower and to exert more
widespread effects in the cortex compared to exogenous orienting (Mayer et al., 2004). In the study
of Taylor and colleagues (2010), however, the only task of the participant was to report whether a
phosphene was perceived or not; this task clearly involves no endogenous orientation of attention,
but an exogenous one that is known to be faster. The absence of an early component and the delay
of the late component compared to the previous literature (Taylor et al., 2010) could be thus
explained in terms of exogenous vs. endogenous orienting of attention, or in other words, top-down
vs. bottom-up attentional modulation.
Further data analysis will be conducted on the present data. A first aspect that we are planning to
explore further is the grouping of electrodes in broad regions and a consequent more broad analysis
of the data. Also the cue related potential (CRP) will be investigated: the advantage of studying the
interaction between CRP and TMS stimulation is the huge amount of studies (see for a review:
Luck et al., 2000) who used the CRP as a measure of attentional allocation.
Follow-up studies will be conducted in which the proportion of visual stimulus present trials will be
higher. Participants often reported a difficulty in orienting their attention because the appearance of
the visual stimulus was too rare. It is then possible that an increase in visual stimulus presence will
increase the attentional effect of the cue, and improve the overall experiment results.
In conclusion, the combined use of TMS and EEG is a promising methodology for the study of
visual attention. The present study showed a differential activation in a late parietal component in
relationship to TMS stimulation site and cue direction; this component could correlate with
endogenous attention. Further studies will aim at improving attentional effect of the present
paradigm, and investigate broader cortical regions.

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