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The Neural Correlates of Consciousness

The Neural Correlates of Consciousness. NRS 495 – Neuroscience Seminar Christopher DiMattina , PhD. Outline. Introducing consciousness Neural substrates of consciousness Neural correlates of consciousness. Introducing Consciousness. Definitions and Science.

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The Neural Correlates of Consciousness

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  1. The Neural Correlates of Consciousness NRS 495 – Neuroscience Seminar Christopher DiMattina, PhD

  2. Outline • Introducing consciousness • Neural substrates of consciousness • Neural correlates of consciousness NRS 495 - Grinnell College - Fall 2012

  3. Introducing Consciousness NRS 495 - Grinnell College - Fall 2012

  4. Definitions and Science • In everyday usage, consciousness means a variety of things • Not being in a coma, deep sleep, anesthesia • Being aware of feelings, thoughts, perceptions • Dreaming • One might be tempted to find a precise definition before embarking on scientific study • The history of science shows this would be a huge mistake NRS 495 - Grinnell College - Fall 2012

  5. Fire • Before science is mature, one classifies similar things together • Once science matures and we have new theoretical frameworks, we re-classify things in light of the framework NRS 495 - Grinnell College - Fall 2012

  6. Greek Astronomy • Greeks thought there were two realms following different laws • Newton showed that one can explain astronomy and motion on earth with one set of laws NRS 495 - Grinnell College - Fall 2012

  7. Modern definitions • What is a “gene” precisely? • Many things make it hard to define • Removal of introns • Post-translational editing of proteins • Some genes have regulatory roles • Lack of precise definition does not impede molecular biologists NRS 495 - Grinnell College - Fall 2012

  8. Definitions are quite often inexact “I shall not attempt today to further define pornography. But I know it when I see it.” - Justice Potter Stewart NRS 495 - Grinnell College - Fall 2012

  9. Eliminative Materialsm • Our folk psychology will eventually be replaced by a mature cognitive neurobiology • Psychological terms like ‘attention’, ‘memory’, etc… will eventually be supplanted as we learn more about how the brain actually works NRS 495 - Grinnell College - Fall 2012

  10. Francis Crick • There should be some difference in neural activity when a stimulus is presented and the subject is aware of it then when the stimulus is presented and the subject is unaware NRS 495 - Grinnell College - Fall 2012

  11. Philosophers • Group into two camps on the issue of consciousness • Reductionists • anti-Reductionists NRS 495 - Grinnell College - Fall 2012

  12. The Hard Problem • Many anti-reductionist philosophers argue that explaining consciousness in terms of brain states ultimately leaves something out • Qualia, experience, “what it is like”, etc… • Logical possibility of Zombies NRS 495 - Grinnell College - Fall 2012

  13. Neuroscience • Consciousness is ultimately a problem for neuroscience • Experiments (behavioral and biological) and neural models • Bringing neuroscience concepts and discoveries to bear on philosophical debates NRS 495 - Grinnell College - Fall 2012

  14. Neural basis of P • P is any psychological property, B is brain property • B must be necessary and sufficient for P • Necessary: Without B, P is impossible • Sufficient: With B, and nothing else, P occurs NRS 495 - Grinnell College - Fall 2012

  15. MT and motion perception • Lesion MT and monkey cannot discriminate motion • Stimulate MT and monkey sees illusory motion A naïve person might think that this data shows MT activity is necessary and sufficient for motion perception NRS 495 - Grinnell College - Fall 2012

  16. Why does this fail necessity? • MT activity might simply be input to higher brain area where motion perception actually occurs • Unknown to us, some other brain area may be the real basis of our motion perception NRS 495 - Grinnell College - Fall 2012

  17. Why does it fail sufficiency? • Stimulating MT may simply be stimulating higher areas which are sufficient for motion perception • MT might be part of a neural coalition including other areas, and by itself does not give rise to perception NRS 495 - Grinnell College - Fall 2012

  18. Neural correlates of visual awareness • Crick and Koch make the problem tractable by focusing their efforts on finding the minimum set of neural events which is needed to give rise to a visual percept • Neural correlates of consciousness (NCC) NRS 495 - Grinnell College - Fall 2012

  19. Neurocomputational approach to consciousness NRS 495 - Grinnell College - Fall 2012

  20. How might we proceed • List some properties of consciousness • Find what brain states correlate with those properties NRS 495 - Grinnell College - Fall 2012

  21. Consciousness • Involves short-term memory • Independent of sensory inputs • Steerable attention • Disappears in sleep • Reappears in dreaming • Single, unified experience NRS 495 - Grinnell College - Fall 2012

  22. Thalamo-cortical loops NRS 495 - Grinnell College - Fall 2012

  23. Properties of Thalamo-cortical loop • Forms a recurrent neural network • This can account for several known properties • Short term memory • Sensory independence • Widespread awareness NRS 495 - Grinnell College - Fall 2012

  24. Pieces of the puzzle: 40 Hz • ILN neurons tend to oscillate intrinsically at 40 Hz • Cortical regions also oscillate at 40 Hz during wakefulness • These cortical oscillations disappear in deep sleep when ILN neurons are inactive • Oscillations reappear in REM sleep NRS 495 - Grinnell College - Fall 2012

  25. ILN lesions abolish consciousness • Lesion left ILN, one neglects right visual field • Lesion ILN entirely, one looses consciousness NRS 495 - Grinnell College - Fall 2012

  26. A Neural hypothesis • I have just outlined a simple, testable hypothesis of consciousness which accounts for our list of known properties • Of course it is not the whole story since one can get spatial neglect with cortical damage (Posterior parietal) • Also, the relevant recurrent interactions may be within the cortex, with ILN just functioning as a timekeeper NRS 495 - Grinnell College - Fall 2012

  27. Neural correlates of consciousness NRS 495 - Grinnell College - Fall 2012

  28. Crick and Koch • Neural correlates of consciousness (NCC) • Focus on visual awareness NRS 495 - Grinnell College - Fall 2012

  29. V1 and visual awareness • Crick and Koch hypothesize that V1 activity does not correspond directly to visual awareness NRS 495 - Grinnell College - Fall 2012

  30. V1 activity independent of perception • Patients with parietal cortex damage (extinction) • Activity same in V1 whether stimulus is perceived or extinguished Vuilleumier et al. (2001) NRS 495 - Grinnell College - Fall 2012

  31. Blink • We blink constantly yet do not perceive interruption of vision • V1 neuron responses do get interrupted NRS 495 - Grinnell College - Fall 2012

  32. Adaptation • Subjects adapt to grating they cannot consciously perceive • Similar results for gratings that they perceive as uniform fields NRS 495 - Grinnell College - Fall 2012

  33. Imagery • Vivid mental imagery possible without V1 [Bridge et al, J. Neurol. 2011] NRS 495 - Grinnell College - Fall 2012

  34. Binocular Rivalry • The vast majority (~80%) of V1 neurons follow the physical stimulus, not the percept NRS 495 - Grinnell College - Fall 2012

  35. Other lines of evidence • Color: Color-tuned V1 neurons track a flickering iso-luminant two color stimulus subjects perceive as uniform color • Anatomical: V1 does not project to prefrontal cortex as do ventral and parietal areas NRS 495 - Grinnell College - Fall 2012

  36. Alternative viewpoints on V1 • Other experiments have suggested V1 may play a role in visual awareness (Tong 2003) NRS 495 - Grinnell College - Fall 2012

  37. Binocular Rivalry NRS 495 - Grinnell College - Fall 2012

  38. Feedback from MT to V1 needed for motion perception • TMS stimulation of MT gives motion percept • Stimulating V1 shortly after MT abolishes motion percept NRS 495 - Grinnell College - Fall 2012

  39. Ventral stream + awareness • Several lines of evidence suggest neural correlates of visual awareness in the ventral stream NRS 495 - Grinnell College - Fall 2012

  40. Binocular rivalry in monkeys • Sheinberg and Logothetis (1997) NRS 495 - Grinnell College - Fall 2012

  41. Rivalry in people NRS 495 - Grinnell College - Fall 2012

  42. Attending different stimuli activates different brain regions NRS 495 - Grinnell College - Fall 2012

  43. FFA and PPA NRS 495 - Grinnell College - Fall 2012

  44. Imagery neurons • Krieman et al. (2000) showed that medial temporal neurons selective for visual images exhibit the same selectivity during vision and imagery NRS 495 - Grinnell College - Fall 2012

  45. Activity in higher cortical areas may not by itself be enough • Neurons in IT can respond to objects even when the animal is under anesthetic • One can obtain responses in human extra-striate cortex to stimuli which are not perceived due to masking • May get activation of higher ventral visual areas to unseen or extinguished stimuli in subject with parietal lesion NRS 495 - Grinnell College - Fall 2012

  46. Frontal and Parietal areas • Damage is associated with visual neglect • Awareness in visual extinction is associated with co-variation of activity between visual cortical areas and undamaged parietal and prefrontal cortex NRS 495 - Grinnell College - Fall 2012

  47. Percepts of bi-stable figures • Reversals: Enhanced activation of frontal and parietal areas NRS 495 - Grinnell College - Fall 2012

  48. Percepts of bi-stable figures • Lumer, Friston and Rees (1998, Science) • Frontal and parietal areas differentially activated during rivalry and non-rivalry conditions NRS 495 - Grinnell College - Fall 2012

  49. Co-variation of activity • Frontal and parietal areas showed more co-variation of activity with visual areas during binocular rivalry than stable viewing NRS 495 - Grinnell College - Fall 2012

  50. Summary • One hypothesis is that the NCC involve a variety of areas • Areas must interact in both top-down and bottom-up manner • Crick & Koch propose NCC may be activity in ventral stream which interacts with activity in frontal and parietal areas NRS 495 - Grinnell College - Fall 2012

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