VISUAL PATHWAYSVISUAL PATHWAYS

Valentin Dragoi

Department of Neurobiology and Anatomy

Parallel streams of visual processing

Outline

• Visual pathways: From retina to the cortex

• Information processing in visual cortexp g

Outline

• Visual pathways: From retina to the cortex

• Information processing in visual cortexp g

VISUAL PATHWAY

Retinal ganglion cell axonsg gleave the eye at the optic disc

The retinal ganglion axons exit the eye to form the optic nerve.

OPTIC NERVEVISUAL PATHWAY

Consists of 3° afferent axonsAxons of retinal ganglion cells

OPTIC CHIASMi fib i i i fOptic nerve fibers originating from

the nasal retina that decussate in the optic chiasmp

Optic nerve fibers originating fromthe temporal retina do not

decussate in the optic chiasmdecussate in the optic chiasm

OPTIC TRACTConsists of 3° afferent axons from the nasal half of the

contralateral eye & the temporal half of the ipsilateral eye

RETINAL GANGLION CELL AXONS: OPTIC TRACT

Optic nerve

Optic tract

Optic chiasm

Hypothalamus: regulation of

i di h th

Lateral geniculate nucleus

circadian rhythms

Pretectum: reflex t l f il

Optic radiation

control of pupil and lens

radiation

Superior colliculus: orienting the movements of head and eyes

Striate cortex

of head and eyes

LATERAL GENICULATE NUCLEUS OF THE THALAMUS

LGN

The visual system thalamic nucleus

LGNMGN

LGN organized into 6 cell layersLGN organized into 6 cell layers2 Magnocellular layers4 Parvocellular layers

with thin layers of kinocellularwith thin layers of kinocellular(dust-like) neurons interposed

All LGN neurons • are monocular - respond to stimulation of one eye only

Type mLGN neurons in LGN magnocellular layers

• have concentric (ON/OFF or OFF/ON) receptive fields

• synapse with Type M retinal ganglion axons• have large concentric receptive fields• are insensitive to colorare insensitive to color• sensitive to small changes in brightness levels (scotopic vision)• are rapidly-adapting (motion sensitive)

Type pLGN neurons in LGN parvocellular layers • synapse with Type P retinal ganglion axons• ha e small concentric recepti e fields (high acuity)• have small concentric receptive fields (high acuity)• are sensitive to color (color sensitivity)• are not sensitive to small changes in brightness levels• are slowly-adapting (indicate the duration stimulus is “on”)

Outline

• Visual pathways: From retina to the cortex

• Information processing in visual cortexp g

PRIMARY VISUAL CORTEX

Primary visual cortexcortex

Calcarine fissure

Cerebral cortex is differentiated into 6 horizontal layers

COLOR SELECTIVITY

SPECIALIZED PROCESSING

l i i f k

SPECIALIZED PROCESSING: CHARACTERISTICS OF V1 BLOB (COLOR ) CELLS

• color sensitive - target of kLGN axons• small concentric receptive fields• monocular - only one eye stimulates the blob cellsy y• indifferent to stimulus orientation• indifferent to stimulus movement (slowly-adapting)

RETINA

….

….

BLOBS

Striate cortex: V1

V1 BLOB CELL RESPONSES

Orientation selectivity

Li ht b ti l

Stimulusorientation

Stimuluspresented

Light bar stimulus projected on screen Recording from

visual cortexRecordRecord

0 1 2 3Time (sec)

ORIENTATION SELECTIVITY

SPECIALIZED PROCESSINGSPECIALIZED PROCESSING: COMMON CHARACTERISTICS OF INTERBLOB CELLS

l d i fi ld• elongated receptive fields• insensitive to color• binocular w/ ocular dominance

> one eye elicits a stronger response• most are sensitive to stimulus orientation

> strongest response to particular orientation> strongest response to particular orientation

….

….

Interblob cells

ORIENTATION SELECTIVITY

SPECIALIZED PROCESSINGSPECIALIZED PROCESSING: SHAPE/FORM & LOCATION V1 INTERBLOB CELLS

l d i fi ld• elongated receptive fields• insensitive to color• binocular w/ ocular dominance

> one eye elicits a stronger response• sensitive to stimulus orientation

> strongest response to particular orientation

•insensitive to movement - target of pLGN axons

> strongest response to particular orientation

> slowly-adapting response• location specific

> strongest response in field centerg p

DIRECTION OF MOTION

SPECIALIZED PROCESSINGSPECIALIZED PROCESSING: MOTION SENSITIVE V1 INTERBLOB CELLS

l d i fi ld• elongated receptive fields• insensitive to color• binocular w/ ocular dominance

> one eye elicits a stronger response• sensitive to stimulus orientation

> strongest response to particular orientation

• sensitive to MOVEMENT=> target of mLGN axons

> strongest response to particular orientation

=> target of mLGN axons

DIRECTION OF MOTION

SPECIALIZED PROCESSINGSPECIALIZED PROCESSING: MOTION+LOCATION SENSITIVE V1 INTERBLOB CELLS

l d i fi ld• elongated receptive fields• insensitive to color• binocular w/ ocular dominance

> one eye elicits a stronger response• sensitive to stimulus orientation

> strongest response to particular orientation

• most sensitive to motion => target of mLGN axons

> strongest response to particular orientation

=> target of mLGN axons• some location specific

DIRECTION OF MOTION

SPECIALIZED PROCESSINGSPECIALIZED PROCESSING: MOTION DIRECTION SENSITIVE V1 INTERBLOB CELLS

l d i fi ld• elongated receptive fields• insensitive to color• binocular w/ ocular dominance

> one eye elicits a stronger response• sensitive to stimulus orientation

> strongest response to particular orientation> strongest response to particular orientation

• most sensitive to motion -=> target of mLGN axons=> target of mLGN axons

• others direction specific

‘Ice-cube’ model of primary visual cortex

R

LRLR

LRLR

L

L

Optical imaging of functional domains im

ulus

ompu

ter

functional domains

Cortical vasculature

St co

Cam

era

ideo

dat

acq

uisi

tion

Amplifier

Vi

acLight guide

Single condition maps Orientation map

EXTRA-STRIATE AND ASSOCIATION AREAS

NON-HUMAN PRIMATES

VISUAL CORTICAL AREASPosterior parietal lobe

Middle & Medial Superior Temporal gyrus

Striate (Visual) cortex

Extrastriate cortex

Inferior temporal gyrus

HIGHER ORDER PROCESSINGV SENDS MOST OF ITS AXONS TO EXTRASTRIATE CORTEXV1 SENDS MOST OF ITS AXONS TO EXTRASTRIATE CORTEX

EXTRASTRIATE CORTEX (V2, V3 & V4) SEND AXONS TOASSOCIATION CORTEX (TEMPORAL & PARIETAL AREAS)ASSOCIATION CORTEX (TEMPORAL & PARIETAL AREAS)

DORSAL STREAM: > SUPERIOR TEMPORAL & PARIETAL

NECESSARY FOR• spatial orientationp• binocular fusion• depth perception • spatial location

“THE WHERE”

• spatial location• movement detection

“THE WHERE”

HIGHER ORDER PROCESSINGV SENDS MOST OF ITS AXONS TO EXTRASTRIATE CORTEXV1 SENDS MOST OF ITS AXONS TO EXTRASTRIATE CORTEX

EXTRASTRIATE CORTEX (V2, V3 & V4) SEND AXONS TOASSOCIATION CORTEX (TEMPORAL & PARIETAL AREAS)

VENTRAL STREAM: > INFERIOR “TEMPORAL” GYRUS

ASSOCIATION CORTEX (TEMPORAL & PARIETAL AREAS)

IMPORTANT FORSHAPE & COLOR

i• perception• discrimination• recognition g• memory• learning

“THE WHAT”

Motion processing

Shape processing in infero-temporal (IT) cortex

Parallel streams of visual processing

Spatial vision

Lesion to MT abolishes the

Object vision

XLesion to MT abolishes the perception of motion without affecting object vision

XLesion to V4 abolishesLesion to V4 abolishes color vision without affecting spatial vision

Suggested readingsSuggested readings

1. Neuroscience book (Purves et al.), Chapter 12