Test on Friday!

1. Test on Friday!

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

3. Retinocollicular Pathway independently mediates orienting • Blindsight patients have 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?

4. 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 – probably from tectopulvinar pathway, but also directly from LGN

5. 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 – probably from tectopulvinar pathway, but also directly from LGN • This input guides behaviour but doesn’t support awareness

6. Searching for the NCC • Dorsal/Ventral dichotomy suggests that not all neural processes “cause” consciousness • Can we find similar dissociations in healthy brains? • Can we attribute awareness to certain classes of neural events?

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

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

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

10. 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

11. 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

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

13. 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

14. 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?

15. 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

16. 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

17. 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

18. 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?

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

20. 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

21. 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

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

23. 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 • We do know that ventral structures seem to play a critical role in visual awareness • Thus the question remains: what is special about the activity of networks of neurons that gives rise to consciousness?