The document summarizes a study that investigated how attention modulates the P1 ERP component. 6 participants completed a spatial cueing task while EEG was recorded. The study found that: 1) P1 amplitude was stronger over electrode sites contralateral to the attended visual field, suggesting attention increases neural sensitivity. 2) P1 onset latency was earlier for attended stimuli, suggesting attention optimizes neural pathways to speed response. 3) Small P1 onset differences between fast and slow reaction times suggest P1 indicates reaction time before response.

1. Attention modulates P1
component onset
Research Report Cognitieve Neurowetenschappen
K. Bangel, B. Hilhorst, K. Jagersberg, D.Portain, A. Siebold,
4/8/2009

2. ABSTRACT
Posner et al proposed a three-factor model of attention addressing different cognitive and
neural correlates. Following a study from Luck et al (1998), we confirmed the link between
attention and P1 amplitude for P1 onset latency as well. In the current study, ERP recordings of
six participants executing a variant of the Posner paradigm are investigated. P1 latency was
analyzed for segments containing trials with attention paid to the left versus attention paid to
right visual field. For each of both sides, slow versus the fast responses were compared, as
obtained from individual reaction times. The results yield that P1 component is strongest on
the side contralateral to the attended visual field compared to the ipsilateral side, suggesting
that attention has a mediating effect on the P1 component. Attention might act as a
mechanism that increases neural sensitivity, leading to an increased activity during stimulus
detection. Additionally, attention might optimize neural pathways to fasten the response upon
stimulus detection. When comparing fast RT opposed to slow RT, small differences in P1 onset
latency suggest that P1 serves as an indicator of RT which can be observed before the actual
response.
Key Terms: Attentional modulation, Spatial Cueing Task, ERP, P1.
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3. INTRODUCTION
The shifting and focus of attention by humans is a topic that has been examined in depth
within the cognitive science. Different mechanisms have been identified, such as the Simon-
effect (Simon, 1969) or change blindness (Henderson and Hollingworth., 1999). Posner and
Peterson's 3-factor model of attention suggests three general attentional functions
incorporated by distinct regions of the brain: Orienting to sensory events (primarily processed
in posterior parietal cortical areas and subcortical areas), detection of target stimuli (located in
an anterior attention system) and alerting (frontal attentional systems on the right
hemisphere) (Posner & Peterson, 1990).
Attentional processes can be examined by means of event-related potentials (ERPs).
Research on attention commonly utilizes ERPs based on recorded EEG data. This technique
offers the possibility to observe ERP waveforms that are associated with attentional processes.
One of the earliest components of the evoked response to a visual stimulus is the P1
component and is visible in the ERP waveform. The P1 component typically occurs 80 to 140
ms after stimulus onset and is strongest in the Parieto-Occipital areas PO7 and PO8. Luck,
Heinze, Mangun and Hillyard (1989) lined out that the P1 component can be understood as
reflecting a facilitation of sensory processing of items at an attended location and can therefore
serve as a measure for the presence and absence of attention. It their experiment, differences
in amplitude were observed in the P1 component for attended stimuli and unattended stimuli
when subjects performed a stimulus detection task.
It is possible that not only the amplitude of the P1 component can be facilitated by attention
but its latency as well. In that case, visual attention directed at one side of the visual field may
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4. result in an earlier onset of the P1 component for attended stimuli compared to unattended
stimuli.
The task used in the study by Luck et al. (1989) required the subject to perform a visual
search for a target stimulus from a group of stimuli. As we are interested in latency differences
of the P1 component we use a task that omits this search and only requires stimulus detection,
the Posner task.
In this experiment subjects perform an altered version of the Posner spatial cueing task
paradigm. Subjects are asked to attend to either the left of the right side of a computer screen
as indicated by a visual cue. After a short interval two stimuli appear, one on each side of the
screen. The subjects have to respond to the orientation of the stimulus (which is either
horizontal or vertical) on the attended side with a corresponding key press. Since we are
interested in attention influencing the P1 component, the task does not involve invalid cueing,
unlike the original Posner task. Stimuli will be presented bilaterally in each trial. As the stimuli
will be presented in different visual fields, each stimulus will be processed in the
corresponding hemisphere, hence each trail two different P1 components will be measures
(unattended and attended).
Both reaction times and brain activity are measured by a computer and EEG measurement
requipment. ERPs will be constructed using the EEG data and will be examined on their
amplitude and latency. We expect to observe amplitude differences in the P1 component of
attended and unattended stimuli in conformity with the study by Luck et al. (1989). We also
expect to see a latency difference of the P1 component onset as well for attended or unattended
stimuli. Finally we also observe behavioral data (response times) in order to examine if the
Posner task may be subject to a Simon effect as the mapping of the response keys in relation to
the target stimulus can be either congruent or incongruent. If this effect is present, it may
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5. influence the P1 component since it is attention-related. We expect the Simon effect to be
present, possibly to a lesser degree as attention is directed by the cue.
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6. METHODS
P ARTICIPANTS
EEG recordings were made from two female and four male students aged between 21 and 28
(mean 23.8), following a practical EEG recording course at the University of Twente. All
participants were right handed, not under actual medical treatment and had no history of
psychiatric or neurological disorders. All participants had normal or corrected to normal
vision, and accurate hearing abilities. One student did not have intact color vision and two
participants were excluded from analysis due to procedural errors. All participants gave
informed consent and the experiment was approved by the local ethics committee.
Figure 1. Example of the stimuli and their temporal order as employed in the experiment.
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7. STIMULI , APPARATUS AND RECORDING PROCEDURE
All stimuli were presented on a black 17’’ computer screen. During each trial a small white
fixation circle was continuously presented in the centre of the screen, attended by two white
circles located at 12.2° to the left and right of the fixation dot. After 700ms showing the default
display the fixation dot changed into a bigger dot for 400ms, again the default display
appeared for 600ms after which the cue (two opposing red and green triangles, each pointing
to one of the circles) replaced the fixation point for 400ms. Next, the default display was
presented again for another 600ms followed by the target, presented in one of the circles (see
Figure 1). Targets consisted either of a horizontal or a vertical black line appearing for 200ms
within one of the white circles. The default display was shown again for 1800 ms after target
onset. Participants were seated in front of a screen at a distance of about 70 cm in a silenced
and partly darkened chamber. The software Presentation (version 11.0) was used for stimulus
presentation.
D ATA ACQUISITION
EEG data were recorded using 12 Ag/AgCl ring electrodes placed on a standard 10/10 cap.
The channels were recorded at the specific locations: (F3 Fz F4 C3 Cz C4 P3 Pz P4 PO7 Oz
PO8). Electrode impedance was held below 5 kΩ. Button triggers, EEG en EOG data were
amplified by a Quick-Amp (BrainProducts GmbH) and recorded at a sample rate of 1000 Hz
with BrainVisionRecorder (Version 1.4). EOG were measured above and below the left eye, and
horizontally on the outer canthi of both eyes to determine the vEOG and the hEOG.
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8. T ASK AND PROCEDURE
Participants were instructed to keep their eyes on the fixation dot while performing 320
trials, divided into four blocks. At the beginning of the first two blocks, participants were
instructed to pay attention to the direction of the green arrow, pressing the left CTRL key
whenever the indicated circle was filled with a vertical line and pressing the right CTRL button
when the indicated circle was filled with a horizontal line. Following 20 practice trails, they
completed two blocks of each 80 randomized experimental trials varying in color (green vs.
red) and direction (left vs. right) of arrow cue and type (horizontal vs. vertical) and location of
target cues. Responses with the corresponding hand were regarded as correct response,
whereas responses with the non-corresponding hand were regarded as false response.
Prior to the start of the third and forth block instructions were chanced. Participants were
then asked to pay attention to the direction indicated by the red arrow.
D ATA ANALYSIS
Behavioral data was analyzed by one-way-ANOVA. RT were splitted into two conditions:
“congruent” and “incongruent”. To account for the Simon effect, the congruent condition was
specified as directing attention to the same spatial side as the desirable button response.
Accordingly, trials including button responses after attending to contrary side were specified
as incongruent conditions.
EEG data were digitally filtered (TC = 0 s, high-cutoff filter of 200 Hz, notch filter of 50 Hz) by
Brainvision Recorder. Using the median of all RT for each participant, our data covered by the
time window of interest between -100 and 250ms from stimulus onset were distinguished into
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9. a slow and a fast condition. We determined the baseline from -100 to 0 ms before the stimulus
was presented.
Reactions which occurred 100ms after target presentation were regarded premature and
rejected. Also rejected were segments showing false responses. Segments were then removed
from EOG artifacts by rejecting horizontal EOG amplitudes greater than 60µV and vertical
EOG amplitudes greater than 120µV. Subsequently, EEG artifacts were removed by rejecting
segments with amplitudes greater than 100µV. To determine which side of the screen was
attended, we distinguished the remaining EEG data into two halves representing attentive
processes on either the left or the right target.
After creating a grand average over all subjects, ERP data was filtered through a lowpass of
16 Hz to further enhance the signal-to-noise-ratio. After selecting the most promising
electrodes (PO7 and PO8) for further analysis, we compared the segments containing trials
with attention paid to the left versus attention paid to the right screen side. For each of both
lateral sides, we compared the slow versus the fast condition.
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10. RESULTS
If participants showed more than 5% incorrect trials on basis of their behavioral data, they
were excluded from further analysis. This criterion applied for one participant, leaving 4
participants with usable data for further analysis. EEG rejection criteria ultimately lead to the
exclusion of 14.4% from all trials.
BEHAVIORAL DATA
The analysis of reaction times did not yield any significant results. In particular there were
no significant differences between trials in congruent and incongruent conditions (F<1;
p>0.05).
EEG DATA
LATERAL COMPARISON
Regarding both sides of the scalp, we observed a difference in P1 latency between conditions
in which attention was paid to the left side versus attention was paid to the right side. In both
cases, activation of the contralateral electrode showed a significant shorter latency (by about 8
ms) than the activation of ipsilateral electrode, as can be seen in Figure 2. The difference in
amplitudes regarding P1 components was insignificant.
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11. Figure 2. Comparison of P1 components in the “attend left” and “attend right” condition for the
electrodes P7 (top) and P8 (bottom)
RESPONSE SPEED COMPARISON
Only results regarding the right visual field yielded a significant shorter onset delay for the
P1 component in the fast condition. The difference in amplitudes indicated a weaker response
for the slow condition, compared to the fast condition (Figure 3 and Figure 4). These effects
were significant for both hemispheres.
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12. Figure 3. Comparison of P1 component for fast (straight line) vs. slow responses (dotted
line) in the “attend left” condition for the electrodes P7 (top) and P8 (bottom)
Figure 4. Comparison of P1 component for fast (straight line) vs. slow responses (dotted
line) in the “attend right” condition for the electrodes P7 (top) and P8 (bottom)
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13. DISCUSSION
The results obtained in this experiment suggest that attention has a mediating effect on the
P1 component when subjects perform the Posner task. This mediating effect manifests in two
ways: when comparing the P1 component on locations PO7 and PO8, we can see that the
amplitude of the P1 component is strongest on the side contralateral to the visual field that the
subject is attending to. It may be possible that attention acts as a mechanism that increases
sensitivity for a stimulus for a specific region of the brain, in this case either PO7 or PO8. This
increased sensitivity may lead to an increased neural response when a stimulus is detected,
with more neurons firing after stimulus detection resulting in increased amplitude of the P1
component. Second, the faster onset of the P1 component for attended stimuli versus
unattended stimuli may suggest that attention even serves as a mechanism that optimizes
neural pathways in order to fasten the response upon stimulus detection.
When comparing the P1 components of fast reaction times opposed to slow reaction times
for either visual field, there are some small differences in the onset of the P1 component. This
suggests that before the actual response the P1 component serves as an indicator of the
reaction time of the response. Only did this effect occur for the condition that subjects were
attending to the right visual field. This can be explained from research on hemispatial neglect
patients, indicating an attentional bias for the left visual field by default (eg. Kinsbourne, 1987),
resulting in less noticeable differences between the P1 components of fast or slow response
times.
Analysis of the response times did not provide evidence that responses were faster when a
subject was presented a congruent stimulus compared to incongruent stimuli. As such there is
no Simon-effect (Simon & Rudell, 1967) present in the Posner task, most likely due to the
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14. presence of a cue that directs attention to the relevant visual field before the stimulus is
shown.
Some considerations have to be made when drawing conclusions from the results of this
experiment. First, the experiment only featured the data of four test subjects out of a total of
six. Certain effects may be more apparent when there are more subjects participating in the
experiment. One of those effects is the LRP, which was not clearly visible for both the
stimulus-locked and response-locked waveforms. The same is true for the analysis of the P1
component for the fast and slow responses when subjects are attending the left visual field.
Second, for analyzing the P1 components for fast and slow responses, all responses were
divided into either “fast” responses (all responses faster then the median response time) or
“slow” responses (all responses slower then the median response time). A different technique
features the division of the response times into three groups rather than two and using the
fastest group as “fast” responses and the slowest group as “slow” responses. Such a division may
have yielded more pronounced effects.
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15. FUTURE RESEARCH
Following from this study there are some recommendations regarding possible future
research possibilities. As has been stated before, there are indicators that the P1 component
can be influenced by the attentional state of the subject. First, a replication study featuring can
be conducted featuring more subjects. With more subjects it is possible to test the effects for
significance and draw solid conclusions regarding the indicators of this study.
Another topic for future research is to explore to which extent the preparation interval can
influence the P1 component. It seems likely that some preparation is required on the neural
level when a subject directs his or her attention to a particular visual field and that this
preparation takes some time. In this study the time delay between the cue and the stimulus
was constant so the preparation time was the same for every trial. It is, however, possible that
the length of the preparation interval determines the degree of neural preparation and that
adjusting the length of this interval may influence the onset and amplitude of the P1
component.
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16. REFERENCES
Henderson, John M.; Hollingworth, Andrew (1999), The role of fixation position in detecting scene
changes across saccades, Psychological Science 10, 438-443.
Kinsbourne, M. (1987). Mechanisms of unilateral neglect. In M. Jeannerod (Ed.), Neurophysiological and
neuropsychological aspects of spatial neglect. Amsterdam: Elsevier.
Luck S. J., Heinze H. J., Mangun G. R., Hillyard S. A. (1989). Visual event related potentials index focused
attention within bilateral stimulus arrays. 2. Functional dissociation of P1 and N1 components.
Electroencephalography and Clinical Neurophysiology, 75, 528–542.
Posner M. I., Peterson S. E. (1990). The attention system of the human brain.
Annual Review of Neuroscience, 13, 25-42.
Simon, J. R. (1969). Reaction toward the source of stimulation. Journal of Experimental Psychology, 81, 174-
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Simon, J. R. & Rudell, A. P. (1967). Auditory S-R compatibility: the effect of an irrelevant cue on
information processing. Journal of applied psychology, 51, 300-304.
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