The Visual System

What is light?

• Form of electromagnetic radiation
• Energy produced by vibration of electrically charged particles
• Dual nature (sometimes wave or photon)
• Only wavelength of 400-700 nanometers are visible to human eye

Properties of Light

Wavelength & Colour:

• Determines colour
• Shorter = blue
• Longer = red

Intensity & Luminance:

• How much energy light wave carries
• Luminance = how much visible light is reaching eyes
• Measured in candelas

Contrast & Relative Luminance :

• Difference in luminance between objects or areas
• Difference of brightest & darkest parts / total luminance

Key Terms

Wavelength	The distance between two peaks of a light wave; determines the colour of light perceived by the eye.
Photon	A discrete packet of light energy; the basic unit of light that interacts with photoreceptors in the eye.
Luminance	The amount of visible light energy reaching the eye from a surface; measured in candelas.
Contrast	The relative difference in luminance between two areas, crucial for detecting edges and patterns.
Electromagnetic Spectrum	The full range of electromagnetic radiation, only a small portion of which is visible to humans as light.

Structure of the Eye

Part	Description	Function
Cornea	Clear, curved outer layer	Primary window for light, bending rays toward the lens
Pupil & Iris	Pupil = dar central opening allowing light to enter eye Iris = coloured ring of muscle surrounding	Regulates amount of light reaching retina
Lens	Flexible, transparent structure behind pupil	Fine tunes focus through accomodation
Retina	Thin, light sensitive layer lining back of eye	Translates light into neural signals
Aqueous/vitreous humour	Gel fluid filling the eyeball	Maintains shape, provides nutrients
Optic nerve	Bundle of nerve fibres	Transmits visual information from retina to brain

The Journey of a Photon

1. Entering the eye

Light from environment enters cornea and is refracted to retina

2. Passing through the Aqueous Humor

Light travels through aqueous humor between cornea and lens.

3. Through the Pupil

Light passes through pupil which regulates how much enters the eye

4. Focused by the Lens

Lens fine tunes focus of light through accomodation, ensuring light always stays on the retina.

5. Through the Vitreous Humor

Light continues through vitreous humor.

6. Reaching the Retina

Focused light hits retina and is turned into neural signals.

Visual Errors

Emmetropic – Ideal vision, no refractive error

Myopic – Nearsighted, eyeball too long. Corrected with concave lenses.

Hyperopia – Farsighted, eyeball too short. Corrected with convex lenses.

Astigmatism – Irregularly shaped cornea, leading to uneven focusing of light. Corrected with cylindrical lenses.

Presbyopia – Age related farsightedness, lens too rigid. Corrected with multifocal lenses.

Scotopic – only rods active (dim light)

Photopic – only cones active (bright light)

Mesopic –  both cones & rods active ( in between)

The Anatomy of The Retina

Layers of the Retina

What is the inverted retina?

• The layers of the retina which light must pass through before reaching the photoreceptors in the back
• Inverted because photoreceptors face away from the light
• Photoreceptors are at back to absorb nutrients from pigment epithelium & to manage waste
• Photoreceptors convert light to neural signals –> bipolar cells –> optic nerve

Rods & Cones

Rods

• Highly sensitive to low light
• Sight in dim light
• Low visual acuitty
• Mostly in peripheral regions of retina
• Do not detect colour

Cones

• High acuity, colour vision
• Need bright light
• Mostly in the central retina, especially in fovea
• Good for fine details

Fovea & Optic Disc

• Small, central pit in retina packed with cones
• No blood vessels
• Provides sharpest vision
• No photoreceptors in optic disc = blind spot

Transduction

The process in which photoreceptors in retina convert light into electrochemical signals.

Receptive Fields

What is a receptive field?

The area of the retina where light can change a neuron’s activity. Defines the part of the visual scene a neuron responds to.

Which retinal cells have receptive fields?

Photoreceptors, bipolar cells, and ganglion cells.

On-Centre & Off-Centre Cells

On-Centre Cells

• Increased firing when light hits centre of receptive fields
• Inhibited when light falls on surrounding area
• Sensitive to bright spots against dark background

Off-centre Cells

• Increased firing when light hits surrounding area
• Inhibited when light hits centre
• Shadows, outlines
• Sensitive to dark spots

Centre-Surround Antagonism

• Centre & surround of the cells has opposite activity
• Together, on and off cells enhance contrast, edges, patterns
• Allows eyes to respond to both increases & decreases in light

Convergence & Mapping

1. Light Activates Photoreceptors

Light entering the eye first activates rods and cones, the photoreceptors at the back of the retina.

2. Signals Converge on Bipolar Cells

Photoreceptors send their signals to bipolar cells, which act as intermediaries in the retinal network.

3. Bipolar Cells Relay to Ganglion Cells

Bipolar cells pass the processed signals to ganglion cells, the output neurons of the retina.

4. Multiple Photoreceptors Feed One Ganglion Cell

Especially in the peripheral retina, many photoreceptors converge onto a single ganglion cell, creating a larger receptive field for that cell.

5. The Receptive Field Emerges

The receptive field of a ganglion cell is defined by all the photoreceptors whose activity can influence its firing. This organization determines the cell’s sensitivity and the detail it can encode.

Fovea vs Periphery

Foveal Vision: High Acuity

• Receptive fields small, densely packed
• Ganglion cells receive input from one cone
• Precise vision, fine detials

Peripheral Vision: High Sensitivity

• Receptive fields larger
• many rods converge to one ganglion cell
• Increased sensitive to light, but less detail
• Helps us see movement in our peripheral

Visual Illusions

Illusion	What You See	Why It Happens	Main Concept It Demonstrates
Simultaneous Contrast Illusion	Two identical grey shapes look like different shades depending on the background.	Ganglion cells compare the centre with the surrounding brightness, so the same grey is perceived differently on light vs dark backgrounds.	Brightness is judged relative to the surround, not absolutely.
Hermann Grid Illusion	Grey spots appear at the white intersections of a black-and-white grid (except where you’re directly looking).	At intersections, more of the receptive field surround is illuminated, causing greater inhibition.	Centre-surround organisation and lateral inhibition enhance contrast.
Scintillating Grid Illusion	Black dots seem to flash at the intersections as your eyes move.	Similar to the Hermann Grid. The illusion disappears when the lines are made wavy, suggesting higher visual processing contributes as well as retinal mechanisms.	Visual perception depends on both retinal processing and higher brain processing.
Troxler Fading	Blurry objects in your peripheral vision gradually disappear when you stare at one point.	The visual system adapts to an unchanging image. Tiny eye movements normally refresh the image and stop objects from fading.	The visual system is specialised for detecting change over time.
Craik–O’Brien–Cornsweet Illusion	Two identical surfaces appear to have different brightness because of a small brightness change at the border.	The retina strongly signals the edge, and the brain assumes the brightness difference continues across the whole surface.	The brain uses edge information to estimate the brightness of whole surfaces.