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Searching for the NCC

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

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Searching for the NCC

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  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 • 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

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