CBNS 106 Final Lecture 2 Vision and Retina

Visual Space and Receptive fields

1. Visual Space Mapping

• The retina creates a map of the visual world.

• Different retinal locations correspond to different locations in visual space.

• Neighboring retinal cells represent neighboring visual locations.

2. Receptive Field Definition

A receptive field is the area of visual space where light changes a neuron's activity.

3. Cells with Receptive Fields

• Photoreceptors

• Bipolar cells

• Horizontal cells

• Amacrine cells

• Retinal ganglion cells (RGCs)

4. Key Properties

• All neurons in the visual system have receptive fields.

• Receptive fields are determined by retinal location.

• Photoreceptor receptive fields are circular.

• Photoreceptors respond to changes in light intensity within their receptive field.

Fovea/Central vs Peripheral Retina (property of retinal cells)

1. Fovea/Central

Characteristics:

• Center of retina

• Highest visual acuity

• Small receptive fields

• Minimal convergence

• Sharp detailed vision

2. Peripheral Retina

Characteristics:

• Large receptive fields

• Extensive convergence

• Better light sensitivity

• Lower visual acuity

3. Exam Tip

Fovea = detail

Peripheral retina = sensitivity

Receptive Field size in RGC types (property of any retinal cell)

1. Magnocellular (M) Retinal Ganglion Cells

a) Characteristics:

• Large receptive fields

• Mainly peripheral retina

• High convergence

• Lower spatial resolution

• Better motion detection

b) Function: Detect movement and broad visual patterns

---

2. Parvocellular (P) Retinal Ganglion Cells

a) Characteristics:

• Small receptive fields

• Mainly central retina

• Little convergence

• High spatial resolution

• Better detail detection

b) Function: Detect fine visual details

---

Comparison

Magnocellular (M)	Parvocellular (P)
Large RF	Small RF
Peripheral retina	Central retina
Motion	Fine detail
High sensitivity	High acuity
More convergence	Less convergence

Convergence, Divergence & Overlapping Receptive Fields (property of retinal cells)

1. Convergence

a) Definition: Many photoreceptors synapse onto one retinal ganglion cell.

b) Effects

• Larger receptive fields

• Greater sensitivity

• Lower visual detail

---

2. Divergence

a) Definition: One photoreceptor influences multiple retinal ganglion cells.

b) Effects

• Information is distributed through multiple pathways

• Increased processing options

---

3. Overlapping Receptive Fields

a) Characteristics:

• Neighboring RGCs share portions of visual space.

• Multiple ganglion cells receive information from similar retinal regions.

b) Benefits

• Better localization

• Smoother visual representation

• Increased reliability

Center Surround receptive fields

1. Structure

a) Every retinal ganglion cell receptive field contains:

b) Center

• Middle region

c) Surround

• Outer ring

---

2. Key Principle

a) Center and surround respond oppositely to light.

Example: Light may excite the center but inhibit the surround.

---

3. Purpose

a) Enhances:

• Contrast detection

• Edge detection

• Object boundaries

b) Major Function

Detect changes in brightness rather than absolute brightness.

ON-Center & OFF-Center Retinal Ganglion Cells

1. ON-Center RGC Response to Light in Center

When light hits the center

• Increased action potential firing

• Cell becomes more active

When darkness is in the center

• Decreased action potential firing

• Cell becomes less active

Function

• Detects increases in illumination

• Responds best to bright spots

---

2. OFF- Center RGC Response to Light in Center

When light hits the center

• Decreased action potential firing

• Cell becomes less active

When darkness is in the center

• Increased action potential firing

• Cell becomes more active

Function

• Detects decreases in illumination

• Responds best to dark spots

---

3. Why Both Exist

Allows the visual system to detect:

• Bright objects

• Dark objects

• Contrast changes

• Edges

ON and OFF bipolar cells

1. ON Bipolar Cells

a) Receptor: mGluR6

b) Type: Metabotropic glutamate receptor

c) Effect of Glutamate: Inhibitory

d) Creates: ON-center responses

---

2. OFF Bipolar Cells

a) Receptor: AMPA receptor

b) Type: Ionotropic glutamate receptor

c) Effect of Glutamate: Excitatory

d) Creates: OFF- center responses

---

3. Key Concept

The same neurotransmitter (glutamate) produces opposite effects because ON and OFF bipolar cells use different receptors.

How ON and OFF pathways work

Darkness

1. Photoreceptor

• Depolarized

• Releases glutamate continuously

2. ON Bipolar Cell

• Glutamate inhibits ON bipolar cell

• ON pathway is suppressed

a) Result: Low ON-center RGC firing

3. OFF Bipolar Cell

• Glutamate excites OFF bipolar cell

• OFF pathway remains active

a) Result: Higher OFF- center RGC firing

---

Light

1. Photoreceptor

• Hyperpolarized

• Releases less glutamate

2. ON Bipolar Cell

• Less inhibition

• Depolarizes

a) Result: ON-center RGC increases firing

3. OFF Bipolar Cell

• Less excitation

• Hyperpolarizes

a) Result: OFF- center RGC decreases firing

Overall

1. Receptive Field

Area where light changes a neuron's activity.

2. Fovea

Small receptive fields + low convergence = highest acuity.

3. Peripheral Retina

Large receptive fields + high convergence = greatest sensitivity.

4. Magnocellular (M) RGC

Large receptive field, motion detection, peripheral retina.

5. Parvocellular (P) RGC

Small receptive field, fine detail, central retina.

6. Convergence

Many photoreceptors → one RGC.

7. Divergence

One photoreceptor → multiple RGCs.

8. Center-Surround Organization

Improves contrast and edge detection.

9. ON-Center RGC

Light in center → increased firing.

10. OFF-Center RGC

Light in center → decreased firing.

11. ON Bipolar Cell

mGluR6 receptor (inhibitory glutamate effect).

12. OFF Bipolar Cell

AMPA receptor (excitatory glutamate effect).