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We tried to expand the Posner paradigm, a framework which links attentive effects to early events of cognitive processing. In a spatial cueing task, the influence of attentive effects on the P1 component onset was confirmed. The discussion includes speculation about the underlying neural mechanisms.
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Attention modulates P1 component onset
Research Report Cognitieve Neurowetenschappen
K. Bangel, B. Hilhorst, K. Jagersberg, D.Portain, A. Siebold,
4/8/2009
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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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.
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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 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.
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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 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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METHODS
PARTICIPANTS
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.
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Figure 1. Example of the stimuli and their temporal order as employed in the
experiment.
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.
DATA 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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TASK 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.
DATA 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
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participant, our data covered by the time window of interest between -100 and
250ms from stimulus onset were distinguished into 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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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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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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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)
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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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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.
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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 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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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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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, 174-176.
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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