1. Searching for the NCC
We can measure all sorts of neural correlates of these processes…so we can see the neural correlates of consciousness right?
So what’s the problem?
Not all of that neural activity “causes” consciousness
We will explore some situations in which neural activity is dissociated from awareness

2. Dorsal and Ventral Pathways
V4 and V5 are key parts of two larger functional pathways:
Dorsal or “Where” pathway
Ventral or “What” pathway
Ungerleider and Mishkin (1982)
Magno and Parvo dichotomy arose at the retina and gives rise to two distinct cortical pathways

3. Agnosia
Lesions (especially in the left hemisphere) of the inferior temporal cortex lead to disorders of memory for people and things
recognition and identification are impaired
prosopagnosia is a specific kind of agnosia: inability to recognize faces
explicit (conscious) decisions about object features are disrupted

4. Agnosia Goodale and Milner – Patient DF
Patient could not indicate the orientation of a slot using her awareness
Patient could move her hand appropriately to interact with the slot
whether visually guided or guided by an internal representation in memory

5. Agnosia Single dissociation of action from conscious perception
Dorsal pathway remained intact while ventral pathway was impaired
Dorsal Pathway seems to guide motor actions, at least for ones that need spatial information
Activity within the Dorsal Pathway seems not to be sufficient for consciousness

6. Blindsight

7. Lesions of Retinostriate Pathway
Lesions (usually due to stroke) cause a region of blindness called a scotoma
Identified using perimetry
note macular sparing X

8. Retinocollicular Pathway independently mediates orienting
Rafal et al. (1990)
subjects move eyes to fixate a peripheral target in two different conditions:
target alone

9. Retinocollicular Pathway independently mediates orienting
Rafal et al. (1990)
subjects move eyes to fixate a peripheral target in two different conditions:
target alone
accompanied by distractor

10. Retinocollicular Pathway independently mediates orienting
Rafal et al. (1990) result
Subjects were slower when presented with a distracting stimulus in the scotoma (359 ms vs. 500 ms)

11. Retinocollicular Pathway independently mediates orienting
Blindsight patients have since been shown to posses a surprising range of “residual” visual abilities
better than chance at detection and discrimination of some visual features such as direction of motion
These go beyond simple orienting - how can this be?

12. Retinocollicular Pathway independently mediates orienting
Recall that the feed-forward sweep is not a single wave of information and that it doesn’t only go through V1
In particular, MT seems to get very early and direct input

13. Retinocollicular Pathway independently mediates orienting
Recall that the feed-forward sweep in not a single wave of information and that it doesn’t only go through V1
In particular, MT seems to get very early and direct input
Information represented in dorsal pathway guides behaviour but doesn’t support awareness

14. Searching for the NCC
What is needed is a situation in which a perceiver’s state can alternate between aware and unaware in ways that we can correlate with neural events
One such situation is called Binocular Rivalry

15. Rivalrous Images
A rivalrous image is one that switches between two mutually exclusive percepts

16. Binocular Rivalry
What would happen if each eye receives incompatible input?
Left Eye
Right Eye

17. Binocular Rivalry
What would happen if each eye receives incompatible input?
The percept is not usually the amalgamation of the two images. Instead the images are often rivalrous.
Percept switches between the two possible images

18. Binocular Rivalry Stimuli: Left Eye Right Eye Percept: Or
Rivalry does not entail suppression of one eye and dominance of another – it is based on parts of objects:
Stimuli:
Left Eye
Right Eye
Percept: Or

19. Binocular Rivalry
Percept alternates randomly (not regularly) between dominance and suppression - on the order of seconds
What factors affect dominance and suppression?
Time ->

20. Binocular Rivalry
Percept alternates randomly (not regularly) between dominance and suppression - on the order of seconds
What factors affect dominance and suppression?
Several features tend to increase the time one image is dominant (visible)
Higher contrast
Brighter
Motion

21. Binocular Rivalry
Percept alternates randomly (not regularly) between dominance and suppression - on the order of seconds
What factors affect dominance and suppression?
Several features tend to increase the time one image is dominant (visible)
Higher contrast
Brighter
Motion
What are the neural correlates of Rivalry?

22. Neural Correlates of Rivalry
What Brain areas “experience” rivalry?
Clever fMRI experiment by Tong et al. (1998)
Exploit preferential responses by different regions
Present faces and buildings in alternation

23. Neural Correlates of Rivalry
What Brain areas “experience” rivalry?
Clever fMRI experiment by Tong et al. (1998)
Exploit preferential responses by different regions
Present faces to one eye and buildings to the other

24. Neural Correlates of Rivalry
What Brain areas “experience” rivalry?
Apparently activity in areas in ventral pathway correlates with awareness
But at what stage is rivalry first manifested?
For the answer we need to look to single-cell recording

25. Neural Correlates of Rivalry
Neurophysiology of Rivalry
Monkey is trained to indicate which of two images it is perceiving (by pressing a lever)
One stimulus contains features to which a given recorded neuron is “tuned”, the other does not
What happens to neurons when their preferred stimulus is present but suppressed?

26. Neural Correlates of Rivalry
The theory is that Neurons in the LGN mediate Rivalry

27. Neural Correlates of Rivalry
The theory is that Neurons in the LGN mediate Rivalry
NO – cells in LGN respond similarly regardless of whether their input is suppressed or dominant

28. Neural Correlates of Rivalry
V1? V4? V5?
YES – cells in primary and early extra-striate cortex respond with more action potentials when their preferred stimulus is dominant relative to when it is suppressed
However,
Changes are small
Cells never stop firing altogether

29. Neural Correlates of Rivalry
Inferior Temporal Cortex (Ventral Pathway)?
YES – cells in IT are strongly correlated with percept

30. Neural Correlates of Rivalry
Inferior Temporal Cortex (Ventral Pathway)?
YES – cells in IT are strongly correlated with percept
Why does area IT sound familiar to you?

31. Neural Mechanisms of Consciousness?
So how far does that get us?
Not all that far – we still don’t know what is the mechanism that causes consciousness
But we do know that it is probably distributed rather than at one locus
Thus the question is: what is special about the activity of networks of neurons that gives rise to consciousness? – that’s still a very hard problem