Cognitive Neuroscience of LanguageLecturer: Marielle Lange

October 8, 2004

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5From eye to brain

Cognitive Neuroscience ofLanguage

Marielle Langehttp://homepages.inf.ed.ac.uk/mlange/teaching/CNL/

Today’s goals

Look at the pathways that conduct thevisual information from the eye to thevisual cortex

Today’s reading

Leff, A.P., Crewes, G.T., Scott, S.K., Kennard, C., Wise,R.J.S. (2001). The functional anatomy of single-wordreading in patients with hemianopic and pure alexia. Brain,124, 510-521

Available at the ITO.Visual Perception of Print. Source to identify.

The Eye

Cognitive Neuroscience of LanguageLecturer: Marielle Lange

October 8, 2004

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Structures of the Human Eye Light: The physical stimulus

http://www.nipissingu.ca/stange/courses/P1106/SANTROCKPP/Chapter05.pptPsychology: Second Canadian Edition by Santrock and Mitterer.

Light is a form ofradiant energy.

This energy isradiated in wavesthat have acharacteristicwavelength.

Retina, as seen through pupil

On-line book, Anatomy and Physiology, Martini.http://media.pearsoncmg.com/ph/esm/esm_martini_fundanaphy_5/bb/obj/14/CH14/html/ch14_4_1.html

The retina is a tissuethat is an extension ofthe brain. Photoreceptors

Cognitive Neuroscience of LanguageLecturer: Marielle Lange

October 8, 2004

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Structure of the retina

http://www.arts.uwaterloo.ca/~cellard/teaching/PSYC261/vision/vision.ppt

Characteristics of Rods and Cones

http://www.nipissingu.ca/stange/courses/P1106/SANTROCKPP/Chapter05.ppt Psychology: Second Canadian Edition by Santrock and Mitterer.

120 million per eye 8 million per eye

Three types of cones(reacting to blue, red,green ranges ofwavelength).

Their stimulation invarious combinationsprovides the perception ofdifferent colours.

Distribution of rods and cones

http://www.physiology.wisc.edu/neuro524/vision.htm

Ganglion cells

on/off surround

Cognitive Neuroscience of LanguageLecturer: Marielle Lange

October 8, 2004

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Ganglion Cell, receptive fields

On-line book, Anatomy and Physiology, Martini.http://media.pearsoncmg.com/ph/esm/esm_martini_fundanaphy_5/bb/obj/14/CH14/html/ch14_4_1.html

In the visual pathway, the messagemust cross two synapses before itheads toward the brain.

In other sensory pathways at mostone synapse lies between a receptorand a sensory neuron.

The extra synapse adds to thesynaptic delay, but it provides anopportunity for the processing andintegration of visual informationbefore it leaves the retina.

http://www.arts.uwaterloo.ca/~cellard/teaching/PSYC261/vision/vision.ppt

Centre-surroundantagonism

On-center ganglion cells:excited when light fallsin the center of theirreceptive field. Inhibitedwhen light falls on thesurround.

(Only a weak response isevoked by a uniform field oflight. )

Hubel & Wiesel

10% are magnocells (large) : fast responses - fortiming visual events, visual motion, controlling eyemovements, coarse features (low ‘spatialfrequencies’), high contrast sensitivity

80% are parvocells (small) : for colour, high acuity,fine detail (high spatial frequencies), low contrastsensitivity

Visual magnocellular pathways control eye movements,and are particularly important for maintaining steadyfixation

Retinal Output(Ganglion) Cells

Cognitive Neuroscience of LanguageLecturer: Marielle Lange

October 8, 2004

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Optic disk, opticnerve, optic chiasm,

optic tract

Optic disc & start ofoptic nerve

Optic nerves, Optic chiasm,optic tract

http://www.neuromod.org/courses/np2000/disorders-

attention-awareness-kok/disorders-attention-

awareness-kok.ppt(pupillary reflex,orient eyes towardsobjects)

LateralGeniculate

Nucleus (LGN)

Lateral GeniculateNucleus (LGN)

Cognitive Neuroscience of LanguageLecturer: Marielle Lange

October 8, 2004

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Lateral geniculate nucleus(LGN)

http://www.physiology.wisc.edu/neuro524/vision.htm

The LateralGeniculate Nucleus(LGN) deals withvisual information,sending some toreflex centers in thebrain stem, other tothe the visual cortex.

Functions: Enhanceinformation aboutcontrast, organizesinformation, receivesfeedback from otherareas.The Thalamus is mostly a relay center.

Axonal pathway to the LGN

http://www.neuromod.org/courses/ecba1999/perception-and-attention/perception-and-attention.ppt

The LGN hassix layers eachof which getsindependentinput fromeither the left orthe right eyebut not both.

Left eyeRight eye

Magnocellular andparvocellular projections

The magno cells (large) are part ofthe m-pathway -- primarilyresponsible for processing informationabout motion and flicker.

The parvo cells (small) are part of thep-pathway -- primarily responsible forprocessing information about form,colour, and texture.

Large ganglion cellsColour insensitiveLarge Receptive FieldsLow resolutionFast, transient response.More sensitive at lowcontrast

Small ganglion cellsColour sensitiveSmall Receptive FieldsHigh resolution`Slow, sustained resp.More sensitive at highcontrast

Can receive info from asmany as 1000 rods -coarse coding

In fovea,monitor cones,with 1:1connections

Cognitive Neuroscience of LanguageLecturer: Marielle Lange

October 8, 2004

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Visual Cortex (orstriate cortex)

Primary andassociation areas

Striate cortex (V1, Area 17)

http://www.physiology.wisc.edu/neuro524/vision.htm

Retinotopic maps

Retina, geniculate-striatesystem

http://www.driesen.com/retino-geniculate-striate_system.htm

1,000,000 axons!

Axons carryingsignals fromneighbouring partsof the retina arenext to one anotherwithin the opticnerve.

Cognitive Neuroscience of LanguageLecturer: Marielle Lange

October 8, 2004

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V1: Topographicrepresentation

Bottom image: Slice through area V1. Thecells that have stained dark are those thatwere responding while the animal viewedthe stimulus shown above.

Preservation of spatial structuretopographic representation.

Important because of the vast numbers ofcells (~100,000,000 in each hemisphere'sV1).

Note also how the cortex expands therepresentation of the fovea relative to theperiphery (cortical magnification).

“Cortical” image

“Retinal” image

Original image

What Do Images Look Likein Cortex?

Cortical organizationin V1:

Layered and ColumnarOrganization

LGN to V1 connections

Information travels from the LGNprimarily to layer 4 of V1 but not all ofthe information goes to the same partof layer 4.

Magnocellular layers of the LGN projectto an upper subdivision of layer 4 in V1and the he parvocellular layers of theLGN to a lower subdivision of this layer.

Separation of information (e.g., motionvs. colour) so that it can be processedseparately.

Cognitive Neuroscience of LanguageLecturer: Marielle Lange

October 8, 2004

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Columnarorganization

The surface of the cortexis divided into functionallydistinct regions ormicrocolumns, eachabout 30 µm in diameter.

Neurons within a columnwill tend to increase ordecrease their firing ratestogether.

5: motor cortext,6: feedback cx

2: -> V2

1: few cells(‘molecular’ layer)

Columnar organization

http://www.physiology.wisc.edu/neuro524/vision.htm

Each column canbe seen as acomputationalunit that codes aspecificinformation(orientation,direction, colour).

Here: orientationcolumns (Pinwheel demo

on course’swebsite)

Selectivity

Cells in V1 do not stick to the same sort of circular,center/surround organization as the ganglion cells

They have more complex organizations that allow for newsorts of selectivity that does not occur before we reach thevisual cortex.

http://www.arts.uwaterloo.ca/~cellard/teaching/PSYC261/vision/vision.ppt

Hubel and Wiesel’shierarchical model

of visual corticalprocessing

Simple cells respond to lines in a particular

orientation

Complex cells respond to lines in a particular

orientation, which move in acertain direction

Cognitive Neuroscience of LanguageLecturer: Marielle Lange

October 8, 2004

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Orientation selectivity

The figure shows receptive field mapsfor several different sorts of simple cells.

The + symbols show positions wherethe cell responds well to a light spot,and the - symbols show positions wherethe cells responds well to a dark spot.

Orientation selectivity in V1 V1: Position selectivity

Position selectivity in simplecells in V1. This cell respondsbest when the stimulus of theappropriate orientation andsize is in one particularposition.

V1: Direction selectivity

This cell responds best whenthe stimulus moves up and tothe left.

This response is not based onsolely the orientation of themoving bar, because when thesame bar moves in theopposite direction, the cellresponds at a significantlyreduced level.

V1: Colour selectivity

This cell responds best when a bluebar of the appropriate orientation,size, and position is presented on ayellow background.

Note that this cell is not responding tojust the combination of blue andyellow, because when we reverse thecolours of the bar and thebackground, the cell's response isreduced significantly.

Cognitive Neuroscience of LanguageLecturer: Marielle Lange

October 8, 2004

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Beyond V1,Functional

specialization:

Different areas specializedin different types of

processing of the visualinformation

0.2°

3°

25° ant.6° post.

Beyond V1: what and wherepatwhays

http://www.physiology.wisc.edu/neuro524/vision.htm

The “Where” (dorsal)pathway: deficits inlandmark task

The “What” (ventral)pathway: deficits inobject discrimination

PET study in normals(Zeki et al., 1991)

Cognitive Neuroscience of LanguageLecturer: Marielle Lange

October 8, 2004

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Two visual streamshypothesis

Milner and Goodale (1993, 1998)

Number ofcorticalstructuresarranged ina quasi-hierarchicalstructure

Connectivity in monkey cortex(Felleman and Van Essen 1991)

Reentrant signalingInfluence of context on letter

perception

http://ibs.derby.ac.uk/~kpat/Israel_cognitive/cognitive_reading.shtml

Cognitive Neuroscience of LanguageLecturer: Marielle Lange

October 8, 2004

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Eye movementswhen reading

Occulomotor nerves

On-line book, Anatomy and Physiology, Martini.http://media.pearsoncmg.com/ph/esm/esm_martini_fundanaphy_5/bb/obj/14/CH14/html/ch14_4_1.html

Patterns of eye movements Viewer perspective

Cognitive Neuroscience of LanguageLecturer: Marielle Lange

October 8, 2004

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Foveal view

A “visual” account ofreading impairment

A “visual” account of readingimpairment

Impairment of the magnocellular pathways

Poor readers have been shown to have 30%smaller magnocellular neurons.

This can lead to unsteady eye fixation andvisual confusion of letter order. This leads topoor memory of the visual form of wordsimpeding orthographic skills.

Stein, Talcott, & Witton (2001)

Conclusions

Retinotopic maps: what is close to each other in the stimulus iscoded close to each other in the brain. This remains true allalong the way from photoreceptors to cortex.

Selectivity in information processing at various levels oforganization in the visual system (Photoreceptors, M-, P-ganglion cells, M- P- layers in the LGN, Columns in V1;Dorsal/where & ventral/what pathways beyond V1)

Parallel pathways: information from each eye initiallyprocessed separately.

Canonical microcircuit hypothesis: each cortical areaconducts computations of the same form using similar circuitrybut different inputs to each area convey different functions. Inspite of differences in cytoarchitecture, the types, arrangements,and connections of cortical neurons is similar throughout cortex.