Cortical Columns

1. Ling 411 – 11 Cortical Columns

2. Perspective – What we know so far: ISources of information about the brain • Aphasiology • Research findings during a century-and-a-half • Brain imaging • Neuroanatomy • Other research in neuroscience • E.g., Mountcastle, Perceptual Neuroscience (1998)

3. Perspective – What we know so far: IILarge-Scale Representation of Information • Subsystems and their locations • Known for some important ones: • Primary areas • Phonological recognition • Phonological production • Etc. • Interconnections among subsystems • E.g., arcuate fasciculus • The Wernicke Principle

4. What we know so far III: Connectionism • “Wernicke’s Principle” • Each local area does a small job • Large jobs are done by multiple small areas working together, by means of interconnecting fiber bundles • The basic principle of connectionism • Connectivity Rules • Consequence: Distributed processing

5. Anatomical support for connectionism • The brain is a network • Composed, ultimately, of neurons • Neurons are interconnected • Axons (with branches) • Dendrites (with branches) • Activity travels along neural pathways

6. Consequences of basic principle of connectionism • Everything represented in the brain has the form of a network • (the “human information system”) • Therefore a person’s linguistic and conceptual system is a network • (part of the information system) • It would appear to follow that every lexeme and every concept is a sub-network (next slide)

7. Example: The concept DOG • We know what a dog looks like • Visual information, in occipital lobe • We know what its bark sounds like • Auditory information, in temporal lobe • We know what its fur feels like • Somatosensory information, in parietal lobe • All of the above.. • constitute perceptual information • are subwebs with many nodes each • have to be interconnected into a larger web • along with further web structure for conceptual information

8. Connectionism and lexicon • The information pertaining to a single lexical item must be widely distributed • That is, every lexical item is represented by a large distributed network • With subnetworks for different kinds of information • Phonological (three subwebs) • Multiple subwebs for different facets of the meaning

9. What we know so far IV: Determinants of location of subsystems • Genetically determined primary areas • Motor – frontal lobe • Perceptual – posterior cortex • Somatic – parietal • Visual – occipital • Auditory – temporal • Hierarchy • Plasticity • Consequence: Higher-level areas not in genetically determined areas

10. Locations of Subsystems I • Phonology is separate from grammar and meaning • Phonology has three components • Recognition (Wernicke’s area) • Production (Broca’s area) • Monitoring (Somatosensory mouth area) • Writing likewise has three components • Phonological-graphic correspondences • Alternative pathways (cf. ‘phonics’ vs. ‘whole words’) • Angular gyrus • Meaning is all over the cortex • Different areas for different kinds of words • Different areas for the network of a single concept • Grammar depends heavily on frontal lobe • In or near Broca’s area

11. Locations of Subsystems II • Nouns and verbs are different • In some ways (what ways?) • How to explain? • Written forms are connected to conceptual information independently of phonological forms • Writing can be accessed from meaning even if speech is impaired • Conceptual information for nouns of different categories may be in different locations

12. Locations of Subsystems III: • Locations of various kinds of “information” • Primary • Visual • Auditory • Tactile • Motor • Phonological • Recognition • Production • Monitoring • Etc.

13. What we know so far V: Processing • Processing in the cortex is • Distributed • Parallel and serial • Bidirectional

14. Next on the agenda: I. Small-scale representation Cortical ColumnsII. Processing at the small scale Operation of cortical columns

15. Vernon W. Mountcastle From Wikipedia: He discovered and characterized the columnar organization of the cerebral cortex in the 1950s. This discovery was a turning point in investigations of the cerebral cortex, as nearly all cortical studies of sensory function after Mountcastle's 1957 paper on the somatosensory cortex used columnar organization as their basis. Indeed, David Hubel in his Nobel Prize acceptance speech said Mountcastle's "discovery of columns in the somatosensory cortex was surely the single most important contribution to the understanding of cerebral cortex since Cajal". 

16. Vernon Mountcastle More from Wikipedia: Mountcastle's devotion to studies of single unit neural coding evolved through his leadership in the Bard Labs of Neurophysiology at the Johns Hopkins UniversitySchool of Medicine, which was for many years the only institute in the world devoted to this sub-field, and its work is continued today in the Krieger Mind/Brain Institute. He is University Professor Emeritus of Neuroscience Johns Hopkins University. Professor Mountcastle was elected to the National Academy of Sciences in 1966.In 1978, he was awarded the Louisa Gross Horwitz Prize from Columbia University together with David Hubel and Torsten Wiesel, who both received the Nobel Prize in Physiology or Medicine in 1981. In 1983, he was awarded the Albert Lasker Award for Basic Medical Research. He also received the United States National Medal of Science in 1986.

17. Quote from Mountcastle “[T]he effective unit of operation…is not the single neuron and its axon, but bundles or groups of cells and their axons with similar functional properties and anatomical connections.” Vernon Mountcastle, Perceptual Neuroscience (1998), p. 192

18. Evidence for columns • Microelectrode penetrations • If perpendicular to cortical surface • Neurons all of same response properties • If not perpendicular • Neurons of different response properties

19. Microelectrode penetrations in the paw area of a cat’s cortex

20. Columns for orientation of lines (visual cortex) Microelectrode penetrations K. Obermayer & G.G. Blasdell, 1993

21. The (Mini)Column • Extends thru the six cortical layers • Hence three to six mm in length • The entire thickness of the cortex is accounted for by the columns • Roughly cylindrical in shape • About 30–50 m in diameter • If expanded by a factor of 100, the dimensions would correspond to a tube with diameter of 1/8 inch and length of one foot

22. Three views of the gray matter Different stains show different features

23. Layers of the cortex • I – dendritic tufts of pyramidal neurons • No cell bodies in this layer • II, III – pyramidal neurons of these layers project to other cortical areas • IV – spiny stellate cells, receive activation from thalamus and transmit it to other neurons of same column • V, VI – pyramidal neurons of these layers project to subcortical areas • Various kinds of inhibitory neurons are distributed among the layers

24. Layers of the Cortex From top to bottom, about 3 mm

25. Cortical column structure • Minicolumn 30-50 microns diameter • Recurrent axon collaterals of pyramidal neurons activate other neurons in same column • Inhibitory neurons inhibit neurons of neighboring columns

26. Columns and neurons • At the small scale.. • Each column is a little network • At a larger scale.. • Each column is a node of the cortical network • The cerebral cortex: • Grey matter — columns of neurons • White matter — inter-column connections

27. Minicolumns and Maxicolumns • Minicolumn 30-50 microns diameter • Maxicolumn – a contiguous bundle of minicolumns (typically around 100) • 300-500 microns diameter • Dimensions vary from one part of cortex to another • In some areas at least, they are roughly hexagonal

28. Cortical minicolumns: Quantities • Diameter of minicolumn: 30 microns • Neurons per minicolumn: 70-110 (avg. 75-80) • Minicolumns/mm2 of cortical surface: 1460 • Minicolumns/cm2 of cortical surface: 146,000 • Neurons under 1 sq mm of cortical surface: 110,000 • Approximate number of minicolumns in Wernicke’s area: 2,920,000 (at 20 sq cm for Wernicke’s area) Cf. Mountcastle 1998: 96

29. Simplified model of minicolumn I:Activation of neurons in a column Other cortical locations II III IV V VI Cell Types Pyramidal Spiny Stellate Inhibitory Thalamus Connections to neighboring columns not shown Subcortical locations

30. Simplified model of minicolumn II:Inhibition of competitors Other cortical locations II III IV V VI Cell Types Pyramidal Spiny Stellate Inhibitory Thalamus Cells in neighboring columns

31. Another Quotation “Every cellular study of the auditory cortex in cat and monkey has provided direct evidence for its columnar organization.” Vernon Mountcastle (1998:181)

32. Deductions from findings about cortical columns • Property I: Cortical topography • Property II: Intra-column uniformity of function • Property III: Nodal specificity • Property IV: Adjacency • Property V: Extension of II-IV to larger columns

33. Deductions from findings about cortical columns • Property I: Cortical topography • Property II: Intra-column uniformity of function • Property III: Nodal specificity • Property IV: Adjacency • Property V: Extension of II-IV to larger columns

34. Large-scale cortical anatomy • The cortex in each hemisphere • Appears to be a three-dimensional structure • But it is actually very thin and very broad • The grooves – sulci – are there because the cortex is “crumpled” so it will fit inside the skull

35. Topologically, the cortex of each hemisphere (not including white matter) is.. • Like a thick napkin, with • Area of about 1300 square centimeters • 200 sq. in. • 2600 sq cm for whole cortex • Thickness varying from 3 to 5 mm • Subdivided into six layers • Just looks 3-dimensional because it is “crumpled” so that it will fit inside the skull

36. The cortex as a network of columns • Each column represents a node • The network is thus a large two-dimensional array of nodes • Nodes are connected to other nodes both nearby and distant • Connections to nearby nodes are either excitatory or inhibitory • Connections to distant nodes are excitatory • Via long (myelinated) axons of pyramidal neurons

37. Deductions from findings about cortical columns • Property I: Cortical topography • Property II: Intra-column uniformity of function • Property III: Nodal specificity • Property IV: Adjacency • Property V: Extension of II-IV to larger columns

38. Uniformity of function within the cortical column • All neurons of a column have the same response properties • It follows that: The nodes of the cortical information network are cortical columns • The properties of the cortical column are approximately those described by Vernon Mountcastle • Mountcastle, Perceptual Neuroscience, 1998

39. Topological essence of cortical structure • The cortex is an array of nodes • A two-dimensional structure of interconnected nodes (columns) • Third dimension for • Internal structure of the nodes (columns) • Cortico-cortical connections (white matter)

40. Deductions from findings about cortical columns • Property I: Cortical topography • Property II: Intra-column uniformity of function • Property III: Nodal specificity • Property IV: Adjacency • Property V: Extension of II-IV to larger columns

41. Nodal specificity • Property III: Every column (hence every node) has a specific function • Known from the experiments for all areas that have been tested • Hypothesis: nodal specificity applies throughout the cortex • Including higher-level areas • The cortex is relatively uniform in structure • Therefore, specificity should apply generally to cortical columns • This claim needs to be investigated (soon)

42. Microelectrode penetrations in the paw area of a cat’s cortex

43. Map of auditory areas in a cat’s cortex A1 AAF – Anterior auditory field A1 – Primary auditory field PAF – Posterior auditory field VPAF – Ventral posterior auditory field

44. More quantities • Neurons per minicolumn: average 75-80 • Number of neurons in cortex: 27.4 billion • Number of minicolumns: ca. 350 million (27,400,000,000 / (75-80)) • Neurons beneath 1 mm2 of surface: 113,000 • Columns beneath 1 mm2 of surface: 14,000 – 15,000 (113,000 / (75-80)) Mountcastle 96

45. Features of the cortical (mini)column • 75 to 110 neurons • 70% of the neurons are pyramidal • The rest include • Other excitatory neurons • Several different kinds of inhibitory neurons • Findings summarized by Vernon Mountcastle, Perceptual Neuroscience (1998)

46. Findings relating to columns(Mountcastle, Perceptual Neuroscience, 1998) • The column is the fundamental module of perceptual systems • probably also of motor systems • Perceptual functions are very highly localized • Each column has a very specific local function • This columnar structure is found in all mammals that have been investigated • The theory is confirmed by detailed studies of visual, auditory, and somatosensory perception in living cat and monkey brains

47. Deductions from findings about cortical columns • Property I: Cortical topography • Property II: Intra-column uniformity of function • Property III: Nodal specificity • Property IV: Adjacency • Property V: Extension of II-IV to larger columns

48. Nodal Adjacency • Property IV: Nodes that are anatomically adjacent have closely related functions • This property extends beyond immediate neighbors • Adjacent nodes are functionally very similar • Nodes that are nearby but not adjacent have similar function • Degrees of topographic closeness correspond to degrees of functional similarity • Consequence: A cortical area forms a functional map

49. Deductions from findings about cortical columns • Property I: Cortical topography • Property II: Intra-column uniformity of function • Property III: Nodal specificity • Property IV: Adjacency • Property V: Extension of properties II-IV to larger columns

50. Columns of different sizes • Minicolumn • Basic anatomically described unit • 70-110 neurons (avg 75-80) • Diameter barely more than that of pyramidal cell body (30-50 μ) • Maxicolumn (term used by Mountcastle) • Diameter 300-500 μ • Bundle of about 100 continuous minicolumns • Hypercolumn – up to 1 mm diameter • Can be long and narrow rather than cylindrical • Functional column • Intermediate between minicolumn and maxicolumn • A contiguous group of minicolumns