Neural Correlates of Visual Awareness

1. Neural Correlates of Visual Awareness

2. A Hard Problem • Are all organisms conscious?

3. A Hard Problem • Are all organisms conscious? • If not, what’s the difference between those that are and those that are not? • Complexity? • Language? • Some peculiar type of memory? • All of these?

4. A Hard Problem • Really what we’re asking is: What is it about our brains that makes us conscious?

5. A Hard Problem • Neuroscientists have deferred some of the difficulties of that problem by focusing on a subtly different one: • What neural processes are distinctly associated with consciousness? • That is still a pretty hard problem! What are the neural correlates of consciousness (NCC)

6. Searching for the NCC • When a visual stimulus appears: • Visual neurons tuned to aspects of that stimulus fire action potentials (single unit recording) • Ensemble depolarizations of pyramidal cells in various parts of visual cortex (and elsewhere) (ERP, MEG) • Increased metabolic demand ensues in various parts of the visual cortex (and elsewhere) (fMRI, PET) • A conscious visual even occurs

7. 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?

8. 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

9. Dorsal and Ventral Pathways • But first recall some details about visual pathways

10. Dorsal and Ventral Pathways • Different visual cortex regions contain cells with different tuning properties represent different features in the visual field • V5/MT is selectively responsive to motion • V4 is selectively responsive to color

11. Dorsal and Ventral Pathways • V4 and V5 are doubly-dissociated in lesion literature: Achromatopsia and Akinetopsia, respectively

12. 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

13. Dorsal and Ventral Pathways • do both of these pathways equally contribute their “contents” to visual awareness? V5 V4

14. 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

15. 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

16. 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

17. Blindsight

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

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

20. 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

21. 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)

22. 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?

23. 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

24. 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

25. 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

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

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

28. 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

29. Binocular Rivalry • 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

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

31. 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

32. 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?

33. 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

34. 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

35. 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

36. 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?

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

38. 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

39. 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

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

41. 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?

42. 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