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

9. Searching for the NCC • We’ve seen several examples of visual system activity in which no awareness ensues, yet information is represented and processed • Blindsight • Object Substitution Masking • Neglect

10. 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

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

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

13. 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

14. 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

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

16. 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

17. 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?

18. 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

19. 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

20. 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

21. 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?

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

23. 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

24. 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

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

26. 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?