PSYCH 202: Light and Colour

Neuron Doctrine and Systems Neuroscience What is the Neuron Doctrine (Waldeyer, 1891)

States that:

• The neuron is the fundamental structural and functional unit of the nervous system

• Neurons are individual cells that communicate with one another

• All brain functions can ultimately be understood through neuronal activity and interactions

Neuron Doctrine and Systems Neuroscience What is the modern Neuron Doctrine (Gazzaniga, 1955)

• Brain processes such as perception, cognition, and consciousness arise from neural activity and interactions.

• Psychology can be informed by understanding neural mechanisms

Neuron Doctrine and Systems Neuroscience What is Systems Neuroscience

The study of how groups of neurons interact to perform specific functions such as vision, hearing, or movement

Neuron Doctrine and Systems Neuroscience How do sensory and motor systems differ

• Sensory system: convert environmental energy into APs

• Motor system: convert APs into movement and behaviour

Neuron Doctrine and Systems Neuroscience What is Muller’s Doctrine of Specific Nerve Energies

• We perceive the activity of our sensory nerves rather than objective reality itself.

• Different sensory receptors determine how we experience the world

Neuron Doctrine and Systems Neuroscience Why is human perception limited

• Humans can only detect forms of energy for which we possess sensory receptors

• Many forms of energy exist that we cannot perceive

Proximal and Distal Stimuli Distal stimulus

The actual object or event in the external world

Proximal and Distal Stimuli Proximal stimulus

The pattern of energy that reaches a sensory receptor (e.g., the image formed on the retina)

Proximal and Distal Stimuli Why is the distinction between distal and proximal stimuli import

The brain never directly accesses the external world; it only interprets information arriving at sensory receptors

Physical Properties of Light Light

Electromagnetic radiation that travels in packets called photons (quanta)

Physical Properties of Light Visible Light

Electromagnetic radiation with wavelengths approximately between 390-700 nm

Physical Properties of Light What is a photon (quantum)

A discrete packet of electromagnetic energy

Physical Properties of Light Wavelength

The distance between peaks of a light wave, measured in nm

Physical Properties of Light What determines the wavelength of light

The distance between successive peaks of an electromagnetic wave

Physical Properties of Light Why do humans see only a small portion of the electromagnetic spectrum

• water in atmosphere absorbs much of the EM spectrum, leaving visible wavelengths available for vision

• photoreceptor pigments are tuned to these wavelengths

Physical Properties of Light What can happen when light strikes an object

• reflection

• absorption

• transmission

Appearance of Light What is brightness

The perceived intensity of light

Appearance of Light What is hue

The subjective experience of colour associated with wavelength

Appearance of Light Is colour a physical property of objects

No. Colour is a psychological and physiological construct produced by the visual system

Appearance of Light How does wavelength affect hue

Equal-intensity wavelengths can appear to differ in brightness because photoreceptors are more sensitive to some wavelengths than others

Appearance of Light Why do brightness perception differ between scotopic and photopic vision

Rods and cones have different wavelength sensitivities, causing peak brightness to shift between low-light and daylight conditions

Scotopic vs Photopic Vision Scotopic Vision

Vision under low-light conditions, mediated by rods

Scotopic vs Photopic Vision Photopic Vision

Vision under bright-light conditions, mediated by cones

Scotopic vs Photopic Vision Why do some wavelengths appear brighter in scotopic vision

Rods absorb some wavelengths more efficiently than others, producing stronger neural responses

Colour Mixing & Colour Vision Additive colour mixing

Combining different wavelengths of light to produce new colour perceptions

Colour Mixing & Colour Vision What colour is produced by mixing red and green light

Yellow

Colour Mixing & Colour Vision What is a metamer

Two physically different light stimuli that appear identical

• Red + Green light can appear identical to pure 580 nm yellow light

Colour Mixing & Colour Vision Why can computer monitors produce any colour using only red, green, and blue pixels

Because human colour vision is based on three cone types

Colour Vision Anomalies What are the main types of colour vision deficiency

• Red-green deficiencies

• Blue-yellow deficiencies

• Monochromy

• Achromatopsia

Colour Vision Anomalies causes of colour blindness

missing or altered cone photopigments

Opponent Colours According to Hering, what are the opponent colour pairs

• Red/Green

• Blue/Yellow

Opponent Colours What evidece supports opponent colours

Negative afterimages and the inability to perceive reddish-green or yellow-ish blue colours

Opponent Colours Why can’t we perceive reddish-green or yellow-ish blue

Opponent colours are encoded by opposing neural mechanisms that cannot be active simultaneously

Opponent Colours What is a negative afterimage?

A perceptual illusion where staring at one colour produces the perception of its opponent colour when looking away.

Examples:

• Red → Green afterimage

• Yellow → Blue afterimage

Opponent Colours What causes a negatve afterimage

Adaptation of colour-sensitive neurons, producing a perception of the opponent colour when looking away

Frequency Coding

A neural coding principle where stimulus intensity is represented by the frequency of APs in cells that respond to that stimulus

Frequency Coding What evidence supports frequency coding in VISION

RGCs increase firing frequency when stimulated by wavelengths strongly absorbed by rhodospin

Frequency Coding How do RGCs code stronger visual stimulation

By firing APs more frequently, not by just increasing AP size

Frequency Coding Why is it necessary

APs are all-or-none events, so stimulus intensity must be encoded through firing rate

Univariance

A receptor’s output varies along only one dimension (firing rate), even though multiple stimulus attributes can affect it

Univariance Why can't a single photoreceptor determine wavelength independently?

The same firing rate can result from:

• a bright light at one wavelength or

• a dimer light from another wavelength

Univariance evidence

RGCs respond only by changing firing frequency, even though both wavelength and intensity affect that response

Univariance How does the Principle of Univariance create a problem for colour vision?

A single receptor cannot distinguish colour from brightness.

Course Coding

Neurons respond to a broad range of stimuli but respond most strongly to a preferred stimulus

Course Coding evidence

Each cone type responds to a wide range of wavelengths rather than a single wavelength

Course Coding How do M-cones demonstrate course-coding

They respond most strongly near their preferred wavelength but still respond to nearby wavelengths with graded decreases in activity

Course Coding why is it useful

A small number of receptor types can represent a large range of wavelengths

Population Coding

Precise information is represented by combining activity across multiple neurons

Population Coding evidence

Colour perception depends on comparing activity across S, M, and L cones

Population Coding how does it solve the problem of univariance

Comparing responses across multiple cone types allows the brain to distinguish wavelength from intensity

Population Coding why are 3 cone types sufficient for colour vision

The pattern of activity across S, M, and L cones uniquely specifies most wavelengths

Application of Neural Coding to Visual Coding Rods

Photoreceptors specialised for low-light (scotopic) vision

Application of Neural Coding to Visual Coding How many rods are in each retina

~120 million

Application of Neural Coding to Visual Coding Rods location

throughout the retina except the fovea

Application of Neural Coding to Visual Coding Rods photopigment

rhodospin

Application of Neural Coding to Visual Coding what is special about rhodospin

it can be activated by very few photons, making rods highly sensitive to dim light

Application of Neural Coding to Visual Coding what is dark current in rods

In darkness, Na⁺ channels remain open, keeping rods partially depolarised and continuously releasing glutamate.

Application of Neural Coding to Visual Coding what happens when light striles rhodospin

Na+ channels close —> rod hyperpolarises —> GLu release decreases

Application of Neural Coding to Visual Coding how does light ultimately generate APs

Reduced Glu release disinhibits bipolar cells, which activate RGCs that generate APs

Application of Neural Coding to Visual Coding what wavelengtj is rhodospin most sensitive to

~500 nm

Application of Neural Coding to Visual Coding why does rhodospin sensitivity explain brightness differences in scotopic vision

Wavelengths absorbed more strongly by rhodospin produce higher RGC firing rates and appear brigher

Application of Neural Coding to Visual Coding Cones

Photoreceptors specialised for daylight (photopic) vision and colour perception

Application of Neural Coding to Visual CodingCone types

• S-cones

• M-cones

• L-cones

Application of Neural Coding to Visual Coding where are cones concentrated

fovea

Application of Neural Coding to Visual Coding what is Trichromatic Theory

Colour perception arises from comparing activity across 3 cone types with different wavelength sensitivities

Application of Neural Coding to Visual Coding what supports Trichromatic Theory

Additive colour mixing and the existence of 3 cone types

What are midget ganglion cells specialised for?

High-acuity detailed vision

What are parasol ganglion cells specialised for

Detecting broader visual patterns and movement, with lower spatial detail