CBNS 106 Final Lecture 3 Center Surround antagonism

What is center-surround antagonism? → How is it generated? → Why is it useful?

Recall: Center-Surround Receptive Fields & Antagonism

1. ON-Center RGC

• Light in center → ↑ firing

• Light in surround → ↓ firing

2. OFF-Center RGC

• Light in center → ↓ firing

• Light in surround → ↑ firing

3. Antagonistic Responses

Center and surround always oppose one another.

4. Key Idea

The strongest responses occur when center and surround receive different illumination.

ON Center RGCs

Rule 1: Photoreceptor response (ONLY CELLS RESPONSIVE TO LIGHT)

a) light: Photoreceptor is hyperpolarized LESS GLUTAMATE

b) dark: Photoreceptor is depolarized MORE GLUTAMATE

** directly affected by light

Rule 2: ON Bipolar cell response

a) Less glutamate: activated DEPOLARIZED

b) More glutamate: inhibited

**respond opp to glu

Rule 3: ON Center RGC response

a) On activated: more firing

b) On inhibited: less firing

**are what you expect

ON Center RGC: Response to Light in Center

Center Light→ Center Photoreceptor (LESS GLUTAMATE)→ ON Bipolar ACTIVATED→ RGCS FIRE MORE

ON Center RGC: Response to Light in Surround

Surround Light→ Surround Photoreceptor (LESS GLUTAMATE)→ Horizontal Cells (LESS ACTIVE)→ Center Photoreceptor (MORE GLUTAMATE)- ON Bipolar (inhibited)→ RGC FIRES LESS

***The surround pathway affects the center indirectly through horizontal cells.

OFF Center surround Pathways

Rule 1: Photoreceptor response (ONLY CELLS RESPONSIVE TO LIGHT)

a) light: Photoreceptor is hyperpolarized LESS GLUTAMATE

b) dark: Photoreceptor is depolarized MORE GLUTAMATE

** directly affected by light

Rule 2: OFF Bipolar cell response

a) Less glutamate: inhibited

b) More glutamate: activated (depolarized)

**what you expect (diff from ON CENTER)

Rule 3: OFF Center RGC response

a) On activated: more firing

b) On inhibited: less firing

**are what you expect

OFF center RGCs: Response to light in center

OFF center light→ OFF center PR (hyperpolarized LESS GLU) → OFF center Bipolar Cells (inhibited)→ OFF center RGCs fire less

OFF Center: Response to light in surround

OFF Center darkness→ OFF center PR (depolarized MORE GLU)→ OFF center Bipolar (acitvated)→ OFF center RGC fires more

Lateral Inhibition & Horizontal Cells

Lateral Inhibition

a) Definition: An excited photoreceptor inhibits neighboring photoreceptors.

b) Mediated By: Horizontal cells (HCs)

c) Function

• Enhances contrast

• Sharpens edges

• Improves visual acuity

• Helps detect borders

• Helps track moving objects

Receptive Field Structure

Receptive Field = Center + Surround

Important Concept

Horizontal cells Horizontal cells compare neighboring photoreceptors and inhibit the center photoreceptor

Exam Sentence

Horizontal cells generate center-surround receptive fields through lateral inhibition.

Why Does the Retina Use Contrast?

Major Principle: Visual perception depends more on contrast (differences in light intensity) rather than absolute illumination.

a) Bright Environment: Object still looks recognizable.

b) Dark Environment: Object still looks recognizable.

Exam Statement

The visual system is more sensitive to contrasts in illumination than total brightness.

Retinal Output & Major Visual Pathways

1. Major Targets of Retinal Output

a) Lateral Geniculate Nucleus (LGN): Main visual relay to cortex.

b) Primary Visual Cortex (V1): Conscious visual perception.

c) Superior Colliculus (SC): Eye movements and visual orientation.

d) Suprachiasmatic Nucleus (SCN): Circadian rhythms and biological clock.

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2. Overall Pathway

Retina → LGN → V1 → Higher Visual Areas

V2 → V3 → V4 → V5

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3. Anatomy (Pathway zoomed in)

Definitions

a) Optic Nerve: Axons leaving one eye.

b) Optic Chiasm: Location where nasal retinal fibers cross.

c) Optic Tract: Fibers after the chiasm.

d) Optic Radiation: LGN projections to visual cortex

Anatomical Route

Retina

↓

Optic Nerve

↓

Optic Chiasm

↓

Optic Tract

↓

LGN

↓

Optic Radiation

↓

V1

Visual Field Organization

1. Key Rule

a) Nasal Hemiretina: Crosses at optic chiasm.

b) Temporal Hemiretina: Does NOT cross.

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2. Visual Field Representation: Left Visual Field

a) Projects to:

• Left nasal retina

• Right temporal retina

b) Then travels to:

• Right optic tract

• Right LGN

• Right visual cortex

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3. Right Visual Field

a) Projects to:

• Right nasal retina

• Left temporal retina

b) Then travels to:

• Left optic tract

• Left LGN

• Left visual cortex

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4. High-Yield Rules

a) Nasal Retina: Crosses.

b) Temporal Retina: Stays ipsilateral.

c) Right Brain: Processes left visual field.

d) Left Brain: Processes right visual field.

c) Each Optic Tract Contains: Information from the contralateral visual field.

Example:

• Right optic tract = Left visual field

• Left optic tract = Right visual field