Timing of the brain events underlying access to consciousness
during the attentional blink Claire Sergent, Sylvain Baillet, &
Stanislas Dehaene Slide 2 2 The attentional blink 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. 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 Perception of the first target (T1) impairs perception of
the second (T2) if the latter follows the former within an interval
of 500 ms (Raymond, Shapiro & Arnell (1992). Slide 3 3 Results
FCzC1 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. Slide 4 4 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 Slide 5 5 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. Slide 6 6
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. Slide 7
7 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. Slide 8 8 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.
