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

1. Sergent C. Baillet S. & Dehaene S.(2005) Timing of the brain events underlying access to consciousness during attentional blink Nature Neuroscience, Vol.8, No.10, 1391-1400 Reporter: 曾昱翔 心理四

2. Outline • Attentional Blink • Experiment • Results • Discussions • Summary

3. Attentional blink …It shows that there is a brief time after paying attention to one stimulus where attention cannot be focused on a subsequent stimulus. This duration is called an attentional blink because it is analogous to being unable to see objects during an eye blink. Reference: CogLab: Attentional Blink. [Online] Retrieved Dec 25, 2005, from http://coglab.wadsworth.com/experiments/AttentionalBlink/

4. Attentional Blink • An optimal situation to study: • The fate of stimuli not consciously perceived • The difference between conscious and nonconscious processing • The differences between consciousness and nonconsciousnesss: • Different levels of activation in early stimulus-specific areas • Conscious access is a late, optional triggering of a ‘second stage’ of processing involving a distributed frontoparietal network.

5. Experiment • Goal: To disentangle theories predict an early correlate of conscious access and theories predicting late divergence. • Design: 2(SOA, 258ms/688ms) X 2(Task, Single/Dual) X 2(T2, presence/absence) • DV: • ERPs • Sbj’s Conscious Reports • (T1 Task Report)

6. Experimental Procedure Design: SOA(2) X Task(2) X T2(2) DV: ERPs / Sbj’s Conscious Reports / (T1 Task Report)

7. Results • Conscious Reports • ERPs of seen and unseen stimuli • Neural events correlating with bimodal conscious report • ERPs evoked by the task on T1

8. Conscious Reports

9. Conscious Reports Responses obtained during attentional blink as being a mixture of seen and unseen states

10. Results • Conscious Reports • ERPs of seen and unseen stimuli • Neural events correlating with bimodal conscious report • ERPs evoked by the task on T1

11. ERPs of seen and unseen stimuli • ERPs evoked by T2(=T2 presence (seen/unseen) – T2 absence) • No sig. difference in early visual P1(96ms) and N1(180ms) wave • Divergence occurred around 270ms: • Seen T2, (N2, 276ms), (N3, 300ms) (P3a, 436ms), (late P3b, 576ms) • Unseen T2, (N4, 348ms), (early P3b, 480ms)

12. ERPs of seen and unseen stimuli

13. Results • Conscious Reports • ERPs of seen and unseen stimuli • Neural events correlating with bimodal conscious report • ERPs evoked by the task on T1

14. unchanged linear non-L non-L Reduced but not suppressed Neural events correlating with bimodal conscious report Identify which events varied linearly with the rating and which showed a nonlinear change related to the bimodal character of the response distribution

15. Results • Conscious Reports • ERPs of seen and unseen stimuli • Neural events correlating with bimodal conscious report • ERPs evoked by the task on T1

16. ERP evoked by the task on T1 • Principally the same • Partially parallel processing • Competition between P3b(T1) and N2(T2)

17. ERP evoked by the task on T1 When T2 was seen, the T1-evoked P3b seemed to reach its peak earlier in time and then decreased at a faster rate.

18. Discussion • Early visual events and consciousness • Two stages of processing • Competition of two targets • Use of a continuous scale

19. Early visual events and consciousness • Early visual P1 and N1 are preserved during the blink, suggesting these brain events are not the primary correlates of conscious perception. • Contrast finding • Identical low-contrast grating patches are presented at the sbj’s threshold. • P1 wave is larger when the patch is detected than when it is missed. • P1 is the primary correlate of consciousness.

20. Early visual events and consciousness • Alternative Interpretation • When visual stimuli are degraded and close to threshold, stochastic variation in P1 amplitude might be sufficient to occasionally prevent conscious access. • Conscious perception of simple grating patches resides in early visual processing, whereas consciousness of more complex stimuli such as words resides in higher-level areas.

21. Early visual events and consciousness • Alternative Interpretation • P1 is sufficient to prevent access … • Conscious perception of simple grating patches resides in early visual processing, whereas consciousness of more complex stimuli such as words resides in higher-level areas. • Although T2 is a word, but sbj are only requested to detect the presence. • Sbj use zero of visibility even when it was discouraged. However, P1, N1 are still preserved in the zero-visibility situation.

22. Discussion • Early visual events and consciousness • Two stages of processing • Competition of two targets • Use of a continuous scale

23. Two stages of processing • Two-stage model of the attentional blink • 1st stage: When ‘blinked’, T2 received normal perceptual processing. • 2nd stage:T2 is prevented from accessing a second capacity-limited stage which is already engaged by T1 processing. • Where is the brain location of 2nd stage • From current study: • 200~300ms: left temporal and inferior frontal regions. • 300~440: lateral prefrontal and anterior cingulate • 440~580: posterior regions. • It is consistent with previous model that attribute an important role to distributed prefrontal, parietal and cingulate activity in relation to conscious perception.

24. Two stages of Processing • Consistency with ‘Global neuronal workspace’ • Suprathreshold stimuli gain access to consciousness by mobilizing a global workspace of multiple distant associative areas such as prefrontal parietal and anterior cingulate, (as mentioned above) • which in turn send top-down signals to stimulus-encoding processors in visual regions.(P3b shows a late reactivation of occipital areas, suggesting top-down amplification.)

25. Discussion • Early visual events and consciousness • Two stages of processing • Competition of two targets • Use of a continuous scale

26. Competition of two targets • The similar components • T1 and T2 affect similar ERP components, namely N2, P3a and P3b.→index a capacity-limited stage. • P3-N2 Competition • Attentional blink might be caused by a competition between P3b evoked by T1, and N2 evoked by T2. • Depending on stochastic variations of P3b, T2 would either succeed or fail to enter second-stage processing. • Partially parallel processing • Conscious and nonconscious processing can proceed along partially distinct and parallel anatomical pathways, and their time courses may overlap.

27. Discussion • Early visual events and consciousness • Two stages of processing • Competition of two targets • Use of a continuous scale

28. Use of a Continuous Scale • The Scale conflates several aspects of perception, from detection to identification. • Further experiments will be needed to specify the content of the state that was labeled as ’seen’ in the present work.

29. Summary • Early P1, N1 waves were evoked by seen and unseen targets, suggesting these brain events are not the primary correlates of conscious perception. • Transition toward access to consciousness related to the optional triggering of a late wave of activation that spreads through a distributed network of cortical association areas. • This study supports the idea that there is a capacity-limited stage capable of processing only one task at a time. It is indexed by the similar components(N2, P3a, P3b waves) evoked by T1 and T2.

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