PSYCH 202: Visual Pathways

Trichromatic and Opponent-Process Theories of Colour Vision Principle of Univariance

• A given receptor can be excited by multiple stimulus attributes (e.g., wavelength and intensity), but its output varies in only one dimension (firing rate).

• Therefore, a single receptor cannot distinguish which stimulus attribute caused the change in activity

Trichromatic and Opponent-Process Theories of Colour Vision How does the Principle of Univariance explain why cods cannot code colour

• Rod ganglion cell firing rate is influenced by both wavelength and light intensity, but only signals firing frequency

• The brain interprets this as changes in brightness rather than colour, producing greyscale scotopic vision

Trichromatic and Opponent-Process Theories of Colour Vision What are the 3 cone types in humans

• S-cones

• M-cones

• L-cones

Trichromatic and Opponent-Process Theories of Colour Vision How does Trichromatic Theory explain colour perception

• Any visible wavelength produces a pattern of activity across S, M, and L cones

• The brain determines colour by comparing the relative activity of the 3 cone types

Trichromatic and Opponent-Process Theories of Colour Vision Evidence for Trichromatic Theory

Every wavelength of visible light produces a unique ratio of responses across the 3 cone types, allowing humans to distinguish all visible wavelengths

Trichromatic and Opponent-Process Theories of Colour Vision Principle of Course Coding

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

• Graded response: The response decreases gradually as the stimulus differs from the preferred value

Trichromatic and Opponent-Process Theories of Colour Vision How do cones demonstrate course coding

Each cone type responds to many wavelengths, not just one

• M-cones respond most strongly near their peak wavelength but still respond to neighbouring wavelengths with lower activity

Trichromatic and Opponent-Process Theories of Colour Vision What is Population Coding

Precise stimulus information is obtained by combining the responses of multiple broadly tuned neurons

Trichromatic and Opponent-Process Theories of Colour Vision How does population coding contribute to colour vision

The brain compared activity across S, M, and L cones to precisely determine wavelength and colour

Trichromatic and Opponent-Process Theories of Colour Vision How do trichromatic theory, course coding, and population coding explain colour vision anomalies

Most colour vision deficiencies result from the absence, weakness, or abnormal functioning of one cone type, altering the normal population code used to identify wavelengths

Trichromatic and Opponent-Process Theories of Colour Vision What are Herring’s opponent colour pairs

• Red vs Green

• Blue vs Yellow

The colours are perceived as opposites and cannot be experiences simultaneously (e.g., reddish-green)

Trichromatic and Opponent-Process Theories of Colour Vision What is Opponent-Process Theory

Colour is encoded by neurons beyond photoreceptors that are excited by one colour and is inhibited by its opponent colour

Trichromatic and Opponent-Process Theories of Colour Vision What evidence supports Opponent-Process Theory

RGCs receive excitatory input from some cone types and inhibitory input from others, creating colour-opponnent responses

Trichromatic and Opponent-Process Theories of Colour Vision What is photopic vision

Daylight vision mediated by cones under high-light condition

Trichromatic and Opponent-Process Theories of Colour Vision Where are cones concentrated

Fovea: retinal region responsible for highest visual acuity

Trichromatic and Opponent-Process Theories of Colour Vision Why can trichromatic animals distinguish all visible wavelengths

Because every wavelnegth produces a unique ratio of activity across the 3 cone types

Trichromatic and Opponent-Process Theories of Colour Vision How does Trichromatic Theory explain colour blindness

Colour vision anomalies result from missing, weakened, or altered cone photopigments

Trichromatic and Opponent-Process Theories of Colour Vision What happens when one cone type is absent

The brain loses part of the normal population code, reducing the ability to distinguish certain wavelengths

Trichromatic and Opponent-Process Theories of Colour Vision Why do colour anomalies support Trichromatic Theory

Different deficiencies correspond to dysfunction in specific cone types predicted by the theory

RGCs and Colour Opponency RGCs

Specialised retinal neurons that receive visual information and transmit APs to the brain through the optic nerve

RGCs and Colour Opponency What are the 3 colour-coding ganglion cell types

• Nonopponent RGCs

• L+M- Opponent RGCs

• S+(L&M)- Opponent RGCs

RGCs and Colour Opponency What is a Nonopponent RGC

A ganglion cell receiving excitatory input from both L and M cones that primarily codes brightness (luminance)

RGCs and Colour Opponency Why are Nonopponent RGCs considered univariate

Their firing rate changes with light intensity but does not provide information about hue

RGCs and Colour Opponency Which RGCs are associated with parasol cells

Nonopponent RGCs

RGCs and Colour Opponency What is an L+M- Opponnent RGC

A ganglion cell

• excited by L-cones (red)

  • frequency of action potentials go up in this part of spectrum)

• inhibited by M-cones (green)

RGCs and Colour Opponency What colour information is carried by L+M- Opponent cells

Red-green colour information

RGCs and Colour Opponency What happens when L and M cones are equally active

Excitation and inhibition balance, producing no hue signal

RGCs and Colour Opponency What is an S+(L&M)- Opponent RGC

A RGC excited by S-cones and inhibited by L and M cones

RGCs and Colour Opponency What colour information does the S+(L&M)- cell encode

Blue-yellow colour information

RGCs and Colour Opponency What does firing rate above baseline mean in an S+(L&M)- cell

The stimulus appears blue

RGCs and Colour Opponency What does firing rate below baseline mean in an S+(L&M)- cell

The stimulus appears yellow

RGCs and Colour Opponency What does baseline firing indicate in an S+(L&M)- cell

No blue-yellow hue signal

RGCs and Colour Opponency What is the Principle of Opponnent Processing

A coding principle in which excitation and inhibition have opposite perceptual meanings

RGCs and Colour Opponency What illustrates Principle of Opponent Processing

Opponent RGCs

• e.g. Opponent L+M- RGC is inhibited by red light and excited by green light

RGCs and Colour Opponency How would a dim violet light be coded

• Weak activation of nonopponent RGCs → dim

• Strong activation of S+(L&M) RGCs → blue

• Some activation of L+M- RGCs → red

Together producing violet

RGCs and Colour Opponency How would a bright greenish-yellow light be activated

• Strong activation of nonopponnent RGCs → bright

• Strong inhibition of L+M- RGCs → green

• Strong inhibition of S+(L&M)- RGCs → yellow

RGCs and Colour Opponency What colour vision phenomena are explained by Trichromatic + Opponent Process theories together

• brightness differences

• hue perception

• colour mixing

• colour blindness

• opponent colours

• negative afterimages

RGCs: Anatomy & Physiology What are the two major classes of RGCs involved in visual perception

• Parasol (M-projecting)

• Midget (P-projecting)

RGCs: Anatomy & Physiology How do paarsol and midget cells differ anatomically

• Parasol cells → large dendritic fields

• Midget cells → small dendritic fields

RGCs: Anatomy & Physiology How does dendritic field size affact RF (receptive field) size

• Larger dendritic fields produce larger Receptive Fields

• Smaller dendritic fields produce smaller Receptive Fields

RGCs: Anatomy & Physiology What is a RF (receptive field)

A region of visual space where stimulation changes the firing rate of a neuron

RGCs: Anatomy & Physiology How are RF measured

By visual angle, eccentricity, and location within visual space

RGCs: Anatomy & Physiology How do RFs differ across the retina

• small near fovea

• large in peripheral retina

RGCs: Anatomy & Physiology Why are overlapping RFs important

• They improve spatial precision

• Improve limitations of univariance through coarse and population coding

RGCs: Anatomy & Physiology Why do parasol cells conduct APs faster

They possess larger axons with lower electrical resistance

RGCs: Anatomy & Physiology Why do midget cells conduct APs more slowly

They possess smaller axons with higher electrical resistance

RGCs: Anatomy & Physiology What consequence does this difference in AP conduction speed between parasol and midget cells have

Visual information arrives in the cortex through two temporally distinct streams

RGCs: Anatomy & Physiology What is Centre-Surround Opponency

A RF organisation where centre and surround regions have opposite effects on firing

RGCs: Anatomy & Physiology What is an On-Centre RF

• Light in the centre excites the cell

• Light in the surround inhibits it

RGCs: Anatomy & Physiology What is an Off-Centre RF

• Light in the centre inhibits the cell

• Light in the surround excites it

RGCs: Anatomy & Physiology Why are centre-surround RFs useful

They emphasise edges and changes in stimulation rather than unifrom illumnation

RGCs: Anatomy & Physiology What visual illusion demonstrates centre-surround processing

Mach Bands

RGCs: Anatomy & Physiology Why do Mach Bands occur

Edge regions receive unequal excitation and inhibition, making edges appear brighter or darker than they physically are

Lateral Geniculate Nucleus (LGN)

A visual relay nucleus in the thalamus that:

• receives retinal input

• projects to primary visual cortex

Lateral Geniculate Nucleus (LGN) How many layers does the LGN contain

6

Lateral Geniculate Nucleus (LGN) Which LGN layers are magnocellular?

1 & 2

Lateral Geniculate Nucleus (LGN) Which LGN layers are parvocellular?

3-6

Lateral Geniculate Nucleus (LGN) Which RGCs project to magnocellular layers

Parasol cells

Lateral Geniculate Nucleus (LGN) Which RGCs project to parvocellular layers

Midget cells

Lateral Geniculate Nucleus (LGN) Why do signals arrive earlier in magnocellular layers

Larger axons conduct APs faster

Lateral Geniculate Nucleus (LGN) How much earlier can magnocellular signals reach cortex compared to parvocellular signals

~10-15ms earlier in monkeys

Lateral Geniculate Nucleus (LGN) What is a tonic response

Continuous firing for the duration of a stimulus

Lateral Geniculate Nucleus (LGN) Which LGN pathway shows tonic responses

Parvocellular pathway

Lateral Geniculate Nucleus (LGN) What is a phasic responses

A brief burst of firing at stimulus onset

Lateral Geniculate Nucleus (LGN) Which LGN pathway shows phasic responses

Magnocellular pathway

Lateral Geniculate Nucleus (LGN) Why are magnocellular cells (parasol) important for motion detection

They respond strongly to changes over time but rapidly

Lateral Geniculate Nucleus (LGN) Why are parvocellular cells (midget) important for fine detail

Their smaller RFs and continuous responses allow detailed spatial analysis

Subcortical Visual Pathways Describe the main visual pathway from retina to cortex

Rerina → Optic Nerve → Optic Chiasm → Optic Tract → LGN → Optic Radiations → Primary Visual Cortex

Subcortical Visual Pathways What happens at the optic chiasm

Axons from the nasal hemiretina cross to the opposite hemisphere

Subcortical Visual Pathways What are optic radiations

Axons projecting from the LGN to primary visual cortex

Subcortical Visual Pathways What visual information travels to the superior colliculus

Info used for rapid orienting movements and reflexive attention

Subcortical Visual Pathways Why is the superior colliculus important

It rapidly directs eye and head movements toward peripheral stimuli

Subcortical Visual Pathways Which animals rely heavily on superior colliculus processing

Reptiles, fish, birds

What is Frequency Coding?

Stimulus intensity is represented by the frequency of action potentials.

What is the advantage of population coding?

Combining responses from many neurons allows highly precise stimulus representation

What is Parallel Processing?

Different neural populations simultaneously process different aspects of vision (colour, motion, form, detail).

What demonstrates Parallel Processing

• Magnocellular pathways specialise in motion and luminance

• Parvocellular pathways specialise in colour and fine detail

What is isoluminance and why is it important

• It occurs when colours differ in hue but not brightness

• Without luminance contrance, form and motion perception become difficult because centre-surround mechanisms are less effective

What is retinotopic organisation

The spatial layout of the retina is preserved throughout visual pathways, so neighbouring neurons represent neighbouring locations in visual space

Why is retinotopic organisation important?

It preserves spatial relationships and allows neighbouring neurons to process information across visual space efficiently.

How are magnocellular and parvocellular inputs kept separate in primary visual cortex?

They terminate in different subdivisions of layer 4:

• Magnocellular → Layer 4Cα

• Parvocellular → Layer 4Cβ

What is the Striate Cortex?

The primary visual cortex (V1), named for the Line of Gennari formed by heavily myelinated LGN inputs.