Perception WK 3 - Structure of Visual System

Properties of light

Wavelength

Intensity (number of photons)

Spatial distributions of light is characterised by…

Sources of light

Reflectors of light

Optic Array

Structued observation of changing patterns of light in an environment, allowing us to infer depth, distance, layout etc

Structure of the visual system

Eye → Retina → Optic Chiasm → LGN → Striate Cortex → Extrastriate Cortex

Function of the visual system

Converts a structured pattern of light taken in from the retina into a perception of a 3D world

Encoding principles

Principle of least committment - Don’t throw away things you might need later

Principle of least redundancy - Do things as efficiently as possible

Principle of graceful degradation - If the system breaks, it should still be usable

Gross anatomy of retina (outer layer to inner layer; optic nerve)

(RCHBAG)

Rods and Cones (photoreceptors) - transmit signals

Horizontal Cells - modulate signal processing

Bipolar Cells - transmit signals

Amacrine Cells - modulate signal processing

Ganglion Cells - transmit signals

Rods and cones → Bipolar Cells → Ganglion cells → Optic nerve

Photoreceptor: Rods

100+ million

Provide black and white night vision

Photoreceptor: Cones

6+ million

Provide high-acuity colour vision in bright light

(Visual) Receptive field

Area of the retina that, when stimulated, influences the firing rate of neurons

Phototransduction

When photoreceptors convert light into electricity

Light is taken in by visual pigment molecules

The subsequent chemical changes results in electrical signals

Spectral sensitivities of photoreceptors

L, M and S (Long, Short and Medium) cones are responsible for colour vision

S cones are responsible for blue light

M cones are responsible for green light

L cones are responsible for red light

Blue/Green/Red (upwards)

Determining different colours

Different levels of each cone photoreceptor determines the colour

Can be fooled by METAMERS (making things look like a particular colour when it isn’t)

Visual electrophysiology

Experiments studying the activity of an individual neuron

Tiny electrode placed close to or inside a visual neuron

(Paralysed and anaesthetised) animals were shown visual stimuli

Electrical activity of the neuron is recorded and investigated

Action potentials on Oscilloscopes

We can see action potentials on Oscilloscope screens

This can look like a ‘spike’ if compressed

We measure the amount of action potentials/spikes given in a particular time

Centre/surround structure of ganglion cells (Kuffler, 1953)

Experiment on horse shoe crabs - areas outside the centre had lower responses

Retinal ganglion cells have an excitatory centre and an inhibitory surround (on-center cell)

Off-center cells have inhibitory centres and excitatory surrounds

When light shines in on the centre, this excitates the cell, whereas light on the surround inhibits the cell

This allows us to enhance edges and contrast by highlighting changes in light

Retinal function

Photoreceptors → Bipolar cells → Retinal Ganglion cells

Photoreceptors process light and turn it into an action potential

This is then synapsed into bipolar cells which segregate this into on (light-activated) or off (dark-activated) pathways which is sent to the ganglion cells

The ganglion cells axons form the optic nerve and sends signals to the brain

There are 1 million axons in the optic nerve → the 100+ million rods must compress this somehow

Horizontal and amacrine cells help to shape retinal ganglion cells’ receptive fields

Retinal ganglion cells

Respond with a series of nerve impulses (spike train)

More stimulation → Faster firing rate (spikes per second)

These cells react to contrast on the centre/surround rather than light

This means it responds more to high contrast colours or images (images that are “high information”)

Respond to blue/yellow, red/green and black/white

Structure of Laterate Geniculate Nucleus (LGN)

Size of a peanut, on both sides of the brain (bilateral nucleus)

Has 6 layers:

• Layers 1, 4 and 6 process contrateral (opposite side as eye) visual information

• Layers 2, 3 and 4 process ipsilateral (same side as eye) visual information

Retinotopic organisation - precise mapping of visual input from the retina onto the visual field (V1), where neurons located near eachother activate neighbouring neurons, creating a map, like a flipped picture

“The Club Sandwich” refers to the laminated structure of the LGN (with a red line as a toothpick)

Contains parvocellular, koniocellular and magnocellular layers

Function of Laterate Geniculate Nucleus (LGN)

Similar centre/surround structure to ganglion cells

Little modification of response, however many regulate information from retina to cortex:

• Modificatory inputs from thalamus and brainstem

• Feedback pathways from visual cortex

Parvocellular, magnocellular and koniocellular layers

These layers are specialised retinal ganglion cells forming parallel pathways for visual processing

• Parvocellular (retinal P cells) - Dorsal layers (3, 4, 5, and 6), small receptive field, fine detail, red/green vision, → “what” an object is

• Magnocellular layers (retinal M cells) - Ventral layers (1 and 2), large receptive field, motion detection, colourblind → “where” an object is

• Koniocellular (b/y cells) - Interlaminar regions, smallest, blue/yellow vision

Encoding along the optic nerve

The optic nerve contains 3 channels via Retinal Ganglion Cells. These channels contain a centre-surround structure that computes colour contrast rather than absolute colour

• Achromatic channel (Black and white → L + M)

• “Red-Green” channel (L vs M)

• “Blue-Yellow” channel ((L + M) vs S)

Structure of Striate Cortex (V1)

Striate = stripey

Connected to 1.5 million axons from LGN

Contains over 250 million neurons

6 layers (but not like LGN)

Function of Striate Cortex (V1)

Function is still hotly debated as highly complex

No conscious access

More refined than the LGN → deals with orientation, edge detecton, motion, contrast and eye-of-origin

No longer seen as the “seat” of visual perception → perception is more spaced out in brain

Essentially blind without it

Contains receptive fields → Simple cells, Complex cells and End-stopped cells (hypercomplex)

Simple cells

Neurons responsible for detecting orientation → orientation tuning

Responds optimally to bars or specific edges within narrow, spatially mapped receptive fields