Vision II

THE RETINA – A VIEW THROUGH THE PUPIL

1. Anatomy of the Retina

  • Macula: A region of the retina responsible for high acuity vision.

  • Fovea: The central pit of the macula containing a high density of cones, critical for sharp central vision.

  • Optic Nerve: The nerve that transmits visual information from the retina to the brain.

VISUAL ACUITY

1. Photoreceptor Density

  • There is variability in photoreceptor density across the retina.

  • Key Types of Photoreceptors:

    • Rods: Specialized for low-light (scotopic) vision, absent from fovea.

    • Cones: Responsible for color vision and high acuity vision, concentrated in the fovea.

STRUCTURE OF THE RETINA

1. Layer Structure

  • Outer Plexiform Layer: Composed of synapses between photoreceptors (rods and cones) and bipolar cells.

  • Inner Plexiform Layer: Contains synapses between bipolar cells and ganglion cells.

  • Cell Types:

    • Horizontal Cells: Integrate and modulate the input from multiple photoreceptors.

    • Bipolar Cells: Relay signals from photoreceptors to ganglion cells.

    • Amacrine Cells: Modulate and integrate signals at the level of ganglion cells.

    • Ganglion Cells: Output neurons that transmit visual information to the brain via the optic nerve.

PERIFOVEAL RODS AND CONES

  • The density of rods and cones varies, with a higher concentration of cones in the fovea and a prevalence of rods in the peripheral regions, affecting visual sensitivity and acuity.

IS PHOTORECEPTOR DENSITY CONSTANT ACROSS THE RETINA?

  • Central fovea has a low density of cones, which increase towards the peripheral regions.

  • Rods are absent from the fovea, creating a marked difference in visual capabilities between the foveal and peripheral retina.

COMPARISON OF RODS AND CONES

Characteristic

Rods

Cones

Vision Type

Night vision

Daylight vision

Light Sensitivity

Sensitive to scattered light

Sensitive to angle of light

Blindness

Loss leads to ‘night blindness’

Loss leads to ‘legal blindness’

Foveal Density

Absent from fovea

Highest density at fovea

Visual Acuity

Low acuity

High acuity

Response Time

Slow response to light

Fast response to light

Pigment Content

Denser pigment than cones

Less dense pigment than rods

Membrane Disks

Stacks unattached to outer membrane

Membrane disks attached to outer membrane

Photopigments

One photopigment

Three photopigments

Vision Type

Achromatic vision

Chromatic vision

Quantity

90 million in human retina

4.5 million in human retina

CONTRAST AND RESOLUTION

1. Grating Patterns

  • Square Wave Gratings: Can present contrast between light and dark edges.

  • Sine Wave Gratings: Represent sensitivity to luminance and contrasts.

UNDERLYING RETINAL CIRCUITS

1. Simplest Circuit

  • Photoreceptor -> Bipolar Cell -> Retinal Ganglion Cell

  • Ganglion cells are responsible for detecting changes in luminance.

RETINAL GANGLION CELLS

1. Types

  • ON-center Ganglion Cells: Activate in response to light in the center of their receptive fields.

  • OFF-center Ganglion Cells: Activate in response to the absence of light in their receptive fields.

2. Receptive Field

  • A receptive field is defined as the region in which stimulation results in an action potential (AP).

    • Figure 11.17 Summary:

    • Shows ON- and OFF-center responses to stimulation of various regions of their receptive fields.

NEURAL RESPONSE TO LIGHT

1. Responses to Light Spots

  • Presentation of a light spot in different parts of the receptive field affects the firing rate of ON-center and OFF-center ganglion cells differently.

ADAPTIVE CHANGES IN GANGLION CELL OPERATING RANGE

1. Discharge Rate Curve

  • Shows the adaptive response of an ON-center ganglion cell to varying intensities of light against a stable background level.

  • Intensity and operating range are crucial for understanding visual adaptation.

EDGE DETECTION

1. Response Mechanisms

  • ON-center ganglion cells are particularly responsive to contrast at the edges of light and dark regions.

LIGHT ADAPTATION

1. Role of Horizontal Cells

  • Horizontal cells create connections via gap junctions across a large area of the retina.

  • They allow the detection and processing of background illumination.

2. Glutamate's Role

  • Changes in horizontal cell activity are mediated through glutamate, which causes either a depolarizing or hyperpolarizing response in photoreceptors, thereby influencing how signals are transmitted.

LATERAL INHIBITION FROM HORIZONTAL CELLS: CENTER-SURROUND RECEPTive FIELDS

1. Mechanism

  • Bipolar cells achieve their center-surround receptive fields through lateral inhibition by horizontal cells.

    • Condition A: Small light stimulates the center; horizontal cell hyperpolarizes and reduces output.

    • Condition B: Larger light covers both center and surround; reduced horizontal cell output allows greater glutamate release from the center cone.

    • Condition C: Surround light activates but the center cone does not, maximizing glutamate release due to minimal horizontal cell inhibition.

COLOR OPPONENT GANGLION CELLS

1. Responses

  • Various configurations of color opponent ganglion cells, including:

    • Red ON/Green OFF

    • Green ON/Red OFF

    • Blue ON/Yellow OFF

This setup can affect color perception through adjacent context and light conditions, demonstrating the foundation of color constancy and contrast effects.