Neuroscience: Central Visual Pathway: Unit 2: Lecture 3

In the retina

Bipolar cells:

• ON

• OFF

• Receptive field center (circular)

• Receptive field surround (ring)

• Ganglion cells:

• ON

• OFF

• Receptive field center (circular)

• Receptive field surround (ring)

The surround is always contrarian to the center

If the center responds to light with activity

Then the surround responds to the light by being a downer

Retinal Processing and Output

Due to antagonistic receptive field structure, ganglion cell responds mostly to differences in

illumination across the receptive field

Example: An Off-center ganglion cell

Ganglion cell output does not reflect the “amount of light or dark” but their spatial differences

Ganglion cells enhance contrast at borders (luminance boundaries)

 Edge detection

Uniform light:

center and surround receive similar amounts of light → their

effects partially cancel → relatively weak response

Uniform = “nothing interesting here”

Edge:

center and surround receive different amounts of light → stronger

response

 The neuron responds strongly when the boundary falls across its receptive field

Change = “there's an edge!”

Parvo- vs. Magno- cellular retinal ganglion cells

Parvocellular (P) pathway

“Parvo” = small

 P-type

Small receptive fields → good for fine

spatial detail

~80% of retinal ganglion cells

Important for color and form

Slower responses

Sustained responses → continues

responding while a stimulus is present

 P = Precision 楰

Fine detail + color + sustained information

Magnocellular (M) Pathway

“Magno-” = large

 M-type

Large receptive fields → good for detecting

broad patterns and movement

~5–10% of retinal ganglion cells

Little/no color selectivity → primarily responds to

luminance/brightness contrast

Fast responses

Rapid adaptation → response decreases when a stimulus

remains unchanged

 M = Motion 㰰

M = Movement + fast changes + coarse information

RETINA

Optic nerve [axons]

↓

Optic chiasm [axons]

↓

Optic tract [axons]

↓

LGN (thalamus) /Superior Colliculus (midbrain) [cells]

↓

Optic radiations [axons]

↓

V1 (primary visual cortex) [cells]

↓

Higher visual areas [cells]

Visual field =

everything you can see

at a given moment

Left visual field →

• Left eye: nasal

retina

• Right eye:

temporal retina

Right visual field →

• Left eye: temporal

retina

• Right eye: nasal

retina

Upper visual field

LOWER retina

Lower visual field

UPPER retina

Lens physiology

retinal location and visual-field location are opposite

because the lens flips the image

Think of the eye like a camera

• The lens flips the image upside down and left-to-right as it projects

it onto the retina

OPTIC CHIASM

Name Origin: The word "chiasm" comes directly from

the Greek letter "Chi" (X), which perfectly describes its

literal X-shaped anatomical structure

an X-shaped brain structure where the optic nerves

from each eye meet and partial nerve fiber crossingpartial nerve fiber crossing

occurs

It sits at the base of the brain, right under the

hypothalamus and just above the pituitary glandpituitary gland

If a pituitary tumor grows, it pushes directly upward into

the optic chiasm, leading to a highly specific and unique

type of vision loss

OPTIC CHIASM continued

• NASAL retinal fibers decussate

•• TEMPORAL retinal fibers remainTEMPORAL retinal fibers remain

ipsilateralipsilateral

• The Half-Crossing Rule:

• Humans only cross about 50% of their

optic nerve fibers at the chiasm

• This partial crossing is the biological

trick that gives us stereoscopic (3D)

depth perception, allowing us to

judge distances accurately

he "Tunnel Vision" Culprit:

Because of the exact way fibers

cross, a growing tumor on the

pituitary gland presses perfectly on

the center of the "X"

• This destroys only the crossingcrossing

fibersfibers, wiping out the outer halves of

both visual fields

•  "tunnel vision" aka

bitemporal hemianopia

Lateral Geniculate Nucleus (LGN)

• Major relay station between the

retina and visual cortex

• Highly organized and participates in

regulating and shaping regulating and shaping the

information sent toward cortex

• Maintains retinotopic organization retinotopic organization

and preserves separation of inputs

from the two eye

LGN has six principal layers and maintains an

orderly representation of the visual field

• LGN is a visual relay station in the

thalamus

• It has 6 layers, roughly:

• Layers 1–2 = Magnocellular (M)

• M = motion / movement

• Good for detecting motion, contrast, and broad

patterns

• Larger neurons

• Layers 3–6 = Parvocellular (P)

• P = precision / detail

• Good for fine detail, shape, and color

• Smaller neurons

The really important part: which eye?

• The six layers keep information

from the two eyes separate

• Layers 1, 4, 6 → contralateral eye

• Layers 2, 3, 5 → ipsilateral eye

 LGN preserves information

about which eye the signal came

from, while also organizing

information according to the visual

field

• LGN receives feedback from the visual

cortex

• influenced by other brain systems, including

attention and arousal systems

• The LGN is influenced by your brain's state of

arousal

• visual processing differs between wide awake and

paying attention state vs asleep

Under stress:Changes in sensory gain:

the brain can become more responsive to sensory signals

Under stress:Changes in attention:

stress can bias processing toward information that might be relevant to threat

Under stress:Changes in visual processing:

animal and human research suggests acute and chronic stress can affect how visualhow visual

information is processedinformation is processed, although the exact effects depend on the type and duration of stress

Under stress:Sleep/arousal effects:

because the LGN is strongly connected with systems controlling arousal and sleep, stress-related

changes in sleep can alter visual processing changes in sleep can alter visual processing

Example: you're hungry and walk past a pizza

Imagine you're walking down the street after not eating all day

1. Retina: Light from the pizza reaches your retina and produces the normal visual signals

2. LGN: receives and relays those signals to visual cortex. But the LGN is not isolated—it receives

feedback from cortex and input from neuromodulatory/arousal systems

3. Hunger state: Your hypothalamushypothalamus and other motivational systems are signaling that food is

biologically relevant

4. Attention + cortical processing:

Those motivational signals can bias attention and visual processing toward food-related stimuli

 you notice the pizza faster or more readily than something visually equivalent but irrelevant,

like a gray trash can

Superior Colliculus (Midbrain):Orienting behaviors

especially directing your eyes and head toward something important

• Helps coordinate eye movements, head movements, and attention toward a visual stimulus

• It has a retinotopic map, so it maintains an organized representation of visual space

• Unlike the LGN pathway, it can support some rapid, relatively unconscious visual responses

Superior Colliculus (Midbrain):Classic example

• You're walking and suddenly see something move in your peripheral vision:

• Retina → Superior colliculus → orienting response

• Your eyes/head may automatically turn toward it before you've consciously figured out what

you're looking at

Hence:

LGN → “What am I seeing?”

Superior colliculus → “Where is it, and should I orient toward it?”

WHY DOES THE RETINA PROJECT TO THE MIDBRAIN?

An animal with no sophisticated cerebral cortex

• A fish sees something suddenly move nearby

What does it need to do?

• It doesn't necessarily need to consciously identify the object

• It needs to: DETECT → ORIENT → RESPOND

 Retina → Midbrain (Superior Colliculus) → Orienting

behavior

Optic Radiations:

axons of LGN neurons traveling to V1 in the occipital lobe

• Travel through the deep cerebral white matter

Optic Radiations: Retinotopically organized:

different portions of the visual field travel through

different parts of the radiation

Optic Radiations:Temporal lobe:

the inferior optic radiations (Meyer’s loop) carry information

from the contralateral upper visual field

Optic Radiations: Parietal lobe:

the superior optic radiations carry information

from the contralateral lower visual field

Both parietal and temporal ultimately terminate in primary visual cortex around the calcarine sulcus

Temporal lobe lesion → contralateral superior

quadrantanopia

• Often called: “pie in the sky”

• because the resulting vision loss looks exactly like

a missing slice of pie from the top corner of your

visual field

Parietal optic radiation lesion → contralateral

inferior quadrantanopia

• → “pie on the floor”

• Resulting visual loss: exactly one-fourth of the

circle is blacked out, the missing area looks like a

missing slice of pie & patient cannot see

downward

V1 (primary visual cortex)

• V1 = primary visual cortex, also called

striate cortex or Brodmann area 17

• Located in the occipital lobe,

surrounding the calcarine sulcus

• Receives the majority of its visual input

from the LGN

• Retinotopically organized → V1

contains an organized map of the visual

field

V1 Primary Visual Cortex

Cortex consists of 6 layers (numbered I-VI)

Two principal cell types: stellate cells and pyramidal cells

Only pyramidal cells can send axons to other parts of the brain

V1: every neuron has a task

Neurons are selective for:

• Orientation — vertical vs. horizontal edges

• Spatial frequency — coarse vs. fine patterns

• Location in the visual field

• Contrast

• Direction of movement in some neurons

• Binocular disparity → differences between the two eyes that contribute to depth

perception

• Separates the information into blobs (color) and interblobs (form and movement)

Occular Dominance Columns

a region of striate cortex receiving information

predominantly from one eye

V1 keeps track of which eye visual

information came from

V1 neurons receive input from both eyes, but

many neurons respond more strongly to one

eye than the other

Neurons with similar eye preferences are

arranged in vertical bands/columns through the

layers of V1

These alternating bands are called ocular

dominance columns

It allows V1 to keep information from the two eyes

spatially organized while also bringing the

information close enough together that neurons

can eventually compare the two eyes' images

Early visual experience can influence the

development of ocular dominance

• If one eye provides consistently poor or

abnormal input during a critical developmentalcritical developmental

periodperiod, cortical circuitry can become

disproportionately driven by the other eye

• This is one mechanism underlying amblyopia

(aka lazy eye)

Blobs:

spots” of elevated cytochrome oxidase

(a metabolic enzyme)

There are difference in the inputs to blobs and

so-called “interblob” regions

Location: Layers 2/3 of the primary visual cortex (V1)

• Appearance: Cylindrical/columnar patches rich in cytochrome

oxidase

• Function: Strongly involved in color processing and aspects of

luminance/brightnessluminance/brightness

• Neurons: weakly orientation-selective or unoriented; many

have relatively circular receptive fields

• Inputs: Receive prominent input from koniocellular (K) and

parvocellular (P) pathways

• Outputs: Project preferentially to the thin stripes of V2

 Blobs → Color → Thin stripes

Interblobs?

• Location: Regions between the cytochrome-oxidase blobs in

layers 2/3 of V1

• Function: Important for processing form, contours, and

orientation

• Neurons: Strongly orientation-selective; many respond to fine

spatial detail/high spatial frequencies

• Inputs: Receive substantial input from parvocellular (P)

pathways and other cortical/geniculate sources

• Outputs: Project preferentially to the pale stripes (interstripes)

of V2

 Interblobs → Form & Orientation → Pale stripes

Magnocellular (M) Pathway

Pathway:

M retinal ganglion cells → LGN M layers → V1 layer 4 B/C → MT/V5

Main functions:

•• MotionMotion

• Rapid changes

• Luminance/contrast

• Coarse spatial information

• High temporal resolution

The M pathway does not primarily map onto the blob/interblob system

Motion

Cells in V1 are selective for componentcomponent motion (oriented edges)

Cells in MT (V5) are selective for pattern motion (whole objects)

Zeki, 1993

Damage to area MT selectively disrupts the perception of motion “akinetopsia”

You can see:

楰Car here → 楰Car here → 楰Car here

•But you don't experience the car moving between those positions

•Instead, the world appears to jump from one position to another

Imagine pouring a cup of coffee:

You see the cup / You see the coffee / You see your hand

•But instead of seeing a smooth stream of coffee, the visual scene may appear

as a series of static snapshots

MOTION OUTSIDE MT  DORSAL STREAM (+ spatial processing +

visually guided action)

DORSAL STREAM — “WHERE/HOW?”

1 → V2 → MT/V5 → POSTERIOR PARIETAL CORTEX

• Motion

• Spatial location

• Depth

• Visuospatial relationships

• Guiding actions based on visual information

• Projects toward the parietal lobe

• Think: “Where is it, and how do I interact with it?”

• Example:

楰楰→ motion/location → “A mug is to my right; reach toward it”

WHERE

WHAT

V4 — Color & Object Processing

Ventral Visual Stream : V1 → V2 → V4 → Inferotemporal cortex

• Major role: Processing color, shape, contours, and surface

properties

• Color constancy: Helps maintain a stable perception of an object's

color despite changes in illumination

• Inputs: Receives processed visual information from V2 and other

early visual areas

• Outputs: Sends information toward inferotemporal cortex for

increasingly complex object representations object representations

• Clinical note: Disruption of ventral-stream color processing can

impair color perception (achromatopsia) while leaving basic visual

functions relatively intact

VENTRAL STREAM — “WHAT?”

V1 → V2 → V4 → INFEROTEMPORAL CORTEX

• Object identity

• Shape

• Color

•• Fine visual features

• Recognition of objects and faces

• Projects toward the temporal lobe

Think: “What am I looking at?”

• Example:

楰楰→ visual features → “That's a coffee mug”

Ventral-stream damage:

difficulty recognizing objects (visual agnosia) despite

relatively preserved basic vision

Dorsal-stream damage:

difficulty using visual information to guide actions guide actions or

perceive spatial relationships spatial relationships

OFA — Occipital Face Area

Located in the inferior occipital cortex

• Early cortical processing of facial features/configurationfacial features/configuration

• Feeds information into more anterior face-processing regions

FFA — Fusiform Face Area

• Located in the fusiform gyrus

• Strongly involved in face perception and identity

• Particularly important for processing who the person is who the person is

AFP2 — Anterior Face Patch 2

• An anterior temporal face-selective region

• Involved in more high-level/complex aspects of face identity and social

information

• Are they looking directly at you or looking away?

Deficit in face recognition: PROSOPAGNOSIA

People with prosopagnosia cannot recognizecannot recognize

familiar facesfamiliar faces, even their own face in a mirror

They also do not recognize this painting by

Arcimboldo as a face composed of flowers –

they only see the flowers

Test yourself

• Surround vs center—what is the relationship?

• Parvo vs Magno cellular cells, key features

• Optic chiasm, what happens there?

• LGN—what does it do and how is it regulated

• Superior Colliculus –why do we need it

• What are the pies all about?

• V1—that do the blobs do? And the interbloods?

• Magnocellular (M) Pathway does what?

• Motion/V5

• What, V4 and color  what is the connection

• Where?

• Faces ---why do some people only see flowers?