NRSC 1110 Ch 10

Decussation

Optic nerves from both eyes form optic chiasm:

Ganglion cell axons from nasal retina cross in \__________ then form \______________

optic chiasm, optic tract

Ganglion cell axons terminate in: \_____________

• Lateral geniculate nucleus (LGN) of thalamus

• Superior colliculus

• Pretectum

• Hypothalamus (SCN)

Lateral geniculate nucleus (LGN) of thalamus

• Axons from LGN give rise to optic radiation \>>>projects to primary visual cortex

Suprachiasmatic nucleus (SCN) of the hypothalamus

biological (circadian) rhythms\>>>sleep and wakefulness

Pretectum

located between thalamus and midbrain

• control size of the pupil (pupillary light reflex)

Superior colliculus (retinotectalprojection):

• saccades

• orients the eyes in response to new stimuli—move fovea to objects of interest

• 10% of ganglion cells (100,000 neurons) project here

Visual field

the entire region of space that can be seen with both eyes looking straight ahead

Hemifield

an imaginary line in the middle of the visual field divides into right and left

Binocular visual field

viewed by both retinas

Monocular visual field

viewed by only one retina

Optic nerve contains info from a \________ eye

single

Optic tract is formed after \___________

optic chiasm

Optic tract contains info from \________ eyes

both

Left hemifield

imaged on left nasal and right temporal retina

•projects to right side of brain

Right hemifield

imaged on right nasal and left temporal retina

•projects to left side of brain

Objects in the temporal part of the visual field are seen by the \____________________

nasal part of the retina

Ganglion cell axons from nasal retina cross in \_____________

optic chiasm

Lesion to left optic nerve

Abolishes input from left temporal retina and left nasal retina

• Hint: close left eye

Lesion to left optic tract

• Abolishes input from left temporal retina and right nasal retina

• vision will be lost in the right VF of each eye

• Right hemifield is lost

Bisect the optic chiasm

• Abolishes input from left and right nasal retinas

• all peripheral vision will be lost

Right and left LGN

major targets of the optic tract

LGN

gateway to the visual cortex

Number of layers that LGN is layered with

6

At the LGN, input from the two eyes is kept \_______

separate

Right LGN receives info about \________

the left visual field

Layers 1 and 2

Magnocellular LGN layers (input from M-type GCs)

Layers 3-6

Parvocellular LGNlayers (input from P-type GCs)

Koniocellular LGN layers

input from nonM-nonP cells

Each koniocellular layer receives input from the \_________ as the \______________

same eye, overlying M or P layer

Info from three types of RGCs from both eyes remains \_____________

segregated (Streams of info are processed in parallel)

Within each layer of LGN:• neurons are activated by only \______________________________ • ON-center and OFF-center cells are \_________

one eye (monocular), intermixed

Magnocellular LGN neurons

(like M-type ganglion cells):• large, center-surround RFs with transient bursts• Insensitive to differences in wavelengths

Parvocellular LGN cells

(like P-type ganglion cells):• Small, center-surround RFs with sustained responses• Color opponency

Koniocellular LGN cells

• Center-surround RFs• Light/dark or color opponency

LGN's major synaptic target

primary visual cortex (aka: BA 17, V1, or striate cortex)

Much of V1 lies on the \________________ of each hemisphere (surrounds calcarine fissure)

medial surface

Retinotopy

neighboring cells in the retina send info to neighboring places in their targets (in LGN and V1)• a mapping of retinal images on V1• Visual space is not sampled uniformly by retina

Cortical magnification

• more GCs with RFs in fovea\>>> more neurons in V1 that receive input from central retina• Central retina is over-represented in V1

V1

typical six layer (really, 9) arrangement of neo-cortex

Layer IV has \____________ subdivisions

three (A,B,C)

IVC further sub-divided into \____ others \______

two (A, B)

Spiny stellate cells

layer IVC, spine-covered dendrites radiating from cell body

Pyramidal cells

outside of IVC, spines and thick apical dendrite—layers III, IVB, V, VI• only cells that send axons out ofV1• glutamatergic

Layer 4C

primary recipient of LGN afferents

Inputs

LGN axons terminate in layer IVC(mostly)• Magnocellular LGN neurons: layerIVCa• Parvocellular LGN neurons: layerIVCb• Information from each eye remains segregated

Outputs

Radial connections• Intra-cortical connections that extend perpendicularly through layers• layer IVCa to IVB; IVCb to layer III

Hubel and Wiesel (1981 Nobel Prize)

injected radiolabelled amino acids into one eye of a monkey

Ocular dominance columns

axon terminals are split into equally spaced patches showing alternating inputs from 2 eyes, left and right eye inputs to layer IV are laid out as a series of alternating bands

Stereopsis

the sensation of depth that arises from viewing nearby objects with one eye

all layer IVC neurons receive input from...

only one eye

Layer II, III, and IVB cells project to the

other cortical areas

Layer V cells project to the

superior colliculus and pons

Layer VI cells project back to the

LGN

Layer IVC

similar to LGN cells that project to them

Monocular RFs (layer IVC beta and alpha)

Layer IVC: insensitive to wavelength (M cells)

Layer IVC beta: center-surrounding color opponency (p cells)

Binocular Receptive Fields

Most neurons in layers above IVC are binocular (respond to light in either eye), layer II and III neurons are more strongly driven by the eye represented by IVC but still binocular

Orientation selectivity

highest response is given to a bar with a partcular oreitnation

Orientation columns

radical columns from layer II to Layer VI, selectivity shifts acrss

analysis of motion

hallmark of neurons receiving input from the magnocellularlayers of LGN

Magnocellular Pathway

M-type GCs\>>Magnocellular layer of LGN\>>layer IVCa\>>IVB• Critical for tasks that require high temporal resolution(evaluating location, speed, and direction of objects)

Parvocellular Pathway

P-type GCs\>>Parvocellular layer of LGN\>>layerIVCB\>>layer II and III• Critical for tasks that require high spatial resolution(analysis of size, shape, and color of objects)

Koniocellular Pathway

• nonM-nonP GCs\>>koniocellular layer of LG

Beyond V1 \>>

~30 areas of extrastriatecortex that have unique RFproperties

Two large-scale cortical streams

1. Dorsal stream• Analysis of motion
2. Ventral stream• Object recognition

Area MT (V5-temporal lobe)

most cells are direction-selective (respond more to the motion of objects than their shape), arranged into direciton-of-motion columns analogous to orientation columns in V1, have large receptive fields

3 roles proposed for cells in area MST (parietal lobe)

1. Navigation 2. Directing eye movement 3. Motion perception

Area V4

shape and color perception

Achromatopsia

caused by damageto area V4—partial or complete lossof color vision

Area IT

• Major output of V4• Farthest extent of processing in ventral stream• Receptive fields respond to a wide variety of colors and abstract shapes• May be important for both visual perception and visual memory

Fusiform Face Area (FFA)

area in the human brain that is more responsive to faces than other stimuli

Prosopagnosia

difficulty recognizing faces but with otherwise normal vision

Visual perception

Identifying andassigning meaning toobjects

In RGCs

center-surround structure,sensitive to contrast and wavelength of light

in V1

orientation selectivity, direction selectivity, and binocularity

Extrastriate cortical areas

selective responsive to complex shapes (faces) and motion