The Eye R.J.S. (2001). The functional anatomy of single-word - - PowerPoint PPT Presentation

Cognitive Neuroscience of Language Lecturer: Marielle Lange October 8, 2004 1

5 From eye to brain Cognitive Neuroscience of Language

Marielle Lange

http://homepages.inf.ed.ac.uk/mlange/teaching/CNL/

Today’s goals

 Look at the pathways that conduct the

visual information from the eye to the visual 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-word reading 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 Language Lecturer: Marielle Lange October 8, 2004 2

Structures of the Human Eye Light: The physical stimulus

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

 Light is a form of

radiant energy.

 This energy is

radiated in waves that have a characteristic wavelength.

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 tissue

that is an extension of the brain.

Photoreceptors

Cognitive Neuroscience of Language Lecturer: Marielle Lange October 8, 2004 3

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/Chapter0 5.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 of wavelength).

 Their stimulation in

various combinations provides the perception of different colours.

Distribution of rods and cones

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

Ganglion cells

• n/off surround

Cognitive Neuroscience of Language Lecturer: Marielle Lange October 8, 2004 4

Ganglion Cell, receptive fields

On-line book, Anatomy and Physiology, Martini. http://media.pearsoncmg.com/ph/esm/esm_marti ni_fundanaphy_5/bb/obj/14/CH14/html/ch14_4_1 .html

 In the visual pathway, the message

must cross two synapses before it heads toward the brain.

 In other sensory pathways at most

• ne synapse lies between a receptor

and a sensory neuron.

 The extra synapse adds to the

synaptic delay, but it provides an

• pportunity for the processing and

integration of visual information before it leaves the retina.

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

Centre-surround antagonism

 On-center ganglion cells:

excited when light falls in the center of their receptive field. Inhibited when light falls on the surround.

 (Only a weak response is

evoked by a uniform field of

• light. )

Hubel & Wiesel

 10% are magnocells (large) : fast responses - for

timing visual events, visual motion, controlling eye movements, coarse features (low ‘spatial frequencies’), high contrast sensitivity

 80% are parvocells (small) : for colour, high acuity,

fine detail (high spatial frequencies), low contrast sensitivity

 Visual magnocellular pathways control eye movements,

and are particularly important for maintaining steady fixation

Retinal Output (Ganglion) Cells

Cognitive Neuroscience of Language Lecturer: Marielle Lange October 8, 2004 5

Optic disk, optic nerve, optic chiasm,

• ptic tract

Optic disc & start of

• ptic nerve

Optic nerves, Optic chiasm,

• ptic tract

http://www.neuromod.org/ courses/np2000/disorders- attention-awareness- kok/disorders-attention- awareness-kok.ppt (pupillary reflex,

• rient eyes towards
• bjects)

Lateral Geniculate Nucleus (LGN)

Lateral Geniculate Nucleus (LGN)

Cognitive Neuroscience of Language Lecturer: Marielle Lange October 8, 2004 6

Lateral geniculate nucleus (LGN)

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

 The Lateral

Geniculate Nucleus (LGN) deals with visual information, sending some to reflex centers in the brain stem, other to the the visual cortex.

 Functions: Enhance

information about contrast, organizes information, receives feedback from other areas. The Thalamus is mostly a relay center.

Axonal pathway to the LGN

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

The LGN has six layers each

• f which gets

independent input from either the left or the right eye but not both.

Left eye Right eye

Magnocellular and parvocellular projections

 The magno cells (large) are part of

the m-pathway -- primarily responsible for processing information about motion and flicker.

 The parvo cells (small) are part of the

p-pathway -- primarily responsible for processing information about form, colour, and texture.

Large ganglion cells Colour insensitive Large Receptive Fields Low resolution Fast, transient response. More sensitive at low contrast Small ganglion cells Colour sensitive Small Receptive Fields High resolution` Slow, sustained resp. More sensitive at high contrast Can receive info from as many as 1000 rods - coarse coding In fovea, monitor cones, with 1:1 connections

Cognitive Neuroscience of Language Lecturer: Marielle Lange October 8, 2004 7

Visual Cortex (or striate cortex)

Primary and association areas

Striate cortex (V1, Area 17)

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

Retinotopic maps

Retina, geniculate-striate system

http://www.driesen.com/retino-geniculate- striate_system.htm  1,000,000 axons!  Axons carrying

signals from neighbouring parts

• f the retina are

next to one another within the optic nerve.

Cognitive Neuroscience of Language Lecturer: Marielle Lange October 8, 2004 8

V1: Topographic representation

 Bottom image: Slice through area V1. The

cells that have stained dark are those that were responding while the animal viewed the stimulus shown above.  Preservation of spatial structure topographic representation.

 Important because of the vast numbers of

cells (~100,000,000 in each hemisphere's V1).

 Note also how the cortex expands the

representation of the fovea relative to the periphery (cortical magnification).

“Cortical” image “Retinal” image Original image

What Do Images Look Like in Cortex?

Cortical organization in V1:

Layered and Columnar Organization

LGN to V1 connections

 Information travels from the LGN

primarily to layer 4 of V1 but not all of the information goes to the same part

• f layer 4.

 Magnocellular layers of the LGN project

to an upper subdivision of layer 4 in V1 and the he parvocellular layers of the LGN to a lower subdivision of this layer.

 Separation of information (e.g., motion

• vs. colour) so that it can be processed

separately.

Cognitive Neuroscience of Language Lecturer: Marielle Lange October 8, 2004 9

Columnar

• rganization

 The surface of the cortex

is divided into functionally distinct regions or microcolumns, each about 30 µm in diameter.

 Neurons within a column

will tend to increase or decrease their firing rates together.

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 can be seen as a computational unit that codes a specific information (orientation, direction, colour). Here: orientation columns

(Pinwheel demo

• n course’s

website)

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 new

sorts of selectivity that does not occur before we reach the visual cortex.

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

Hubel and Wiesel’s hierarchical model

• f visual cortical

processing

Simple cells

 respond to lines in a particular

• rientation

Complex cells

 respond to lines in a particular

• rientation, which move in a

certain direction

Cognitive Neuroscience of Language Lecturer: Marielle Lange October 8, 2004 10

Orientation selectivity The figure shows receptive field maps for several different sorts of simple cells. The + symbols show positions where the cell responds well to a light spot, and the - symbols show positions where the cells responds well to a dark spot.

Orientation selectivity in V1 V1: Position selectivity

 Position selectivity in simple

cells in V1. This cell responds best when the stimulus of the appropriate orientation and size is in one particular position.

V1: Direction selectivity

 This cell responds best when

the stimulus moves up and to the left.

 This response is not based on

solely the orientation of the moving bar, because when the same bar moves in the

• pposite direction, the cell

responds at a significantly reduced level.

V1: Colour selectivity

 This cell responds best when a blue

bar of the appropriate orientation, size, and position is presented on a yellow background.

 Note that this cell is not responding to

just the combination of blue and yellow, because when we reverse the colours of the bar and the background, the cell's response is reduced significantly.

Cognitive Neuroscience of Language Lecturer: Marielle Lange October 8, 2004 11

Beyond V1, Functional specialization:

Different areas specialized in different types of processing of the visual information

0.2° 3° 25° ant. 6° post.

Beyond V1: what and where patwhays

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

The “Where” (dorsal) pathway: deficits in landmark task The “What” (ventral) pathway: deficits in

• bject discrimination

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

Cognitive Neuroscience of Language Lecturer: Marielle Lange October 8, 2004 12

Two visual streams hypothesis

Milner and Goodale (1993, 1998)

Number of cortical structures arranged in a quasi- hierarchical structure

Connectivity in monkey cortex (Felleman and Van Essen 1991)

Reentrant signaling Influence of context on letter perception

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

Cognitive Neuroscience of Language Lecturer: Marielle Lange October 8, 2004 13

Eye movements when 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 Language Lecturer: Marielle Lange October 8, 2004 14

Foveal view

A “visual” account of reading impairment

A “visual” account of reading impairment

 Impairment of the magnocellular pathways  Poor readers have been shown to have 30%

smaller magnocellular neurons.

 This can lead to unsteady eye fixation and

visual confusion of letter order. This leads to poor memory of the visual form of words impeding orthographic skills.

Stein, Talcott, & Witton (2001)

Conclusions

 Retinotopic maps: what is close to each other in the stimulus is

coded close to each other in the brain. This remains true all along the way from photoreceptors to cortex.

 Selectivity in information processing at various levels of

• rganization 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 initially

processed separately.

 Canonical microcircuit hypothesis: each cortical area

conducts computations of the same form using similar circuitry but different inputs to each area convey different functions. In spite of differences in cytoarchitecture, the types, arrangements, and connections of cortical neurons is similar throughout cortex.