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Lesions of Retinostriate Pathway

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

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Lesions of Retinostriate Pathway

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

  2. 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?

  3. 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

  4. 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

  5. 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

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

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

  8. 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

  9. 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

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

  11. 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

  12. 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?

  13. 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

  14. 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

  15. 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

  16. 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?

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

  18. 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

  19. 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

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

  21. 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?

  22. 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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