Timing of the brain events underlying access to consciousness during the attentional blink

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Timing of the brain events underlying access to
consciousness during the attentional blink

• Claire Sergent, Sylvain Baillet, Stanislas
  Dehaene

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• The attentional blink

Perception of the first target (T1) impairs
perceptionof the second (T2) if the latter
follows the former within an interval of 500 ms
(Raymond, Shapiro Arnell (1992).
Principal question what is the fate of blinked
stimuli? - Are blinked stimuli unconsciously
perceived? --- Yes. Vogel, Luck Shapiro
(1998). P3 suppression vs N4 - Can the
difference between identical blinked and
perceived stimuli be observed in the
event-related potentials? --- Sergent et al.
compare the following conditions T2
present, seen T2 absent T2 present, unseen
T2 absent T2 seen T2 unseen
Normally, Task2 requires some for of
identification, but the present study uses a
visibility rating. However, this method has been
validated in Sergent Dehaene (2004). - The 50
visibility cut-off is used for the seen/unseen
contrast. - Task1 is a 2-AFC, which is fairly
easy. Stimulus presentation of 43 ms makes up for
it.
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• Results

FCz
C1
Un-subtracted waveforms from Suppl. Figure 1 (
T2 seen T2 unseen -- T2 absent). - These
show the evoked potentials in each of the three
conditions superimposed on each other, on a
given sensor location. - Problem of overlapping
ERPs can be seen clearly from the labels on the
FCz plot.
Compare C1 plot with the one to the left -
Initial drift between -400 and 0 evens out. - No
trace of N1 P1 components. - Differences
emerge at 170 ms and 350 ms post-T2.
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Global differences between seen and unseen trials
emerge after 170 ms post-T2. - The supplementary
video gives a global overview of events specific
to T2 perception (seen absent unseen
absent) and of the differences between these two
(seen unseen right map).
The authors proceed to give a detailed overview
of the sequence of events. Source localization
is used to obtain an idea about the neural
generators underlying the observed voltage
maps. --- Treat those with benign neglect.
The difference at 170 ms is fairly specific to
one region of the voltage map. At this location,
a positive modulation is present for the seen
T2s, but not for the unseen ones. P170
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At around 270 ms a left-posterior negativity
shows up. The authors refer to this as the N2
component, associated with perceptual similarity
but also control processes (inhibition). The
authors do not emphasize a left temporal
positivity that also looks to be unique to seen
T2s.
By 300 ms post-T2 a posterior negativity,
labeled the N3, follows the N2. Note how the
difference is quite sharp the effect of
visibility on the N3 can be described as
all-or-nothing.
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Interestingly, a modulation of the N4 component
occurs around 350 ms. Given the results of Vogel
et al. this is remarkable semantic pro-cessing
of T2 should be fairly intact.The change between
seen and unseen is gradual.
Around 430 ms the P3 waves show a large
divergence, as expected. The P3 is associated
with target-related processing, and memory
consolidation in particular. The P3a (the
anterior first part of the late P3 complex)
basically appears only when T2 is well
perceived, which was also true for the N3.
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At 580 ms, the P3b (posterior) is strongly
present for seen T2s, and completely absent for
unseen ones.
Are there problems with the comparisons made
here? - One might quibble that the number of
observations in seen and unseen conditions was
not equal. In ERP studies, this can be a
major confound as variability of the average
waveforms is strongly affected by it and may
lead to the detection of spurious components. ---
The riposte is that early attentional components
match perfectly (baseline level is further
proof), which makes it highly unlikely
that an inequality in variance could account for
the results. The authors continue to look at
T1-related activity and posit that T1-evoked
potentials interfere with T2 components. This
seemed somewhat post-hoc and without a strong
contribution to the understanding of the blink
and the neural processes leading up to it. - I
do not believe the blink arises because the T1
P3b hampers the T2 N2. - In functional terms,
the consolidation of T1 is thought to pose a
bottleneck for the same process for T2. This
is already well known and somewhat beside the
main point of this present paper.
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• Conclusions

What do these ERPs tell us? Conscious report
involves large-scale late brain activity. In
tasks like this, conscious report cannot be due
to activation in early stimulus-specific
areas. The authors replicated (and strengthened)
earlier indications that unseen events can still
evoke high-level processing (N4), although N4
amplitude was suppressed somewhat. Support for
classic two-stage models of the AB was obtained.
Components associated with stage-2 processing
diverge after less than 300 ms, which strongly
suggests a relation to the observed behavioral
effect (i.e., a failure to perform
Task2). Detailed sequence of events shows how P1
N1 are preserved, N2 and N4 are linearly
modulated (reduced), and N3, P3a, P3b
eliminated for unseen T2s providing a starting
point for further functional research tapping
into these components.