The Eye: Vision as a Sensory-Motor System

Why is vision considered a sensory-motor system rather than purely a sensory system?

Vision involves continuous interaction between sensory input and motor output. Eye, head, and body movements determine what visual information is sampled from the environment. Perception results from both incoming sensory information and active movement.

What are the sensory components of vision?

Light enters the eye. Photoreceptors detect light. Visual information is transmitted to the brain.

What are the motor components of vision?

Eye movements, head movements, body movements. These movements determine which visual information is sampled.

What is the key principle underlying vision as a sensory-motor system?

What you see depends partly on how you move.

What are saccades?

Rapid eye movements that shift gaze between objects.

What are smooth pursuit movements?

Eye movements that track moving objects.

Why is the visual system not simply a camera?

The brain does not passively record images; it actively constructs perception through neural processing.

What evidence shows that perception is actively constructed?

Visual illusions involving brightness, contrast, colour, motion, line length, and angles demonstrate that perception is created by neural processing.

How does context influence visual perception?

Perception depends on background brightness, surrounding visual scene, prior expectations and knowledge.

What does relative coding mean in vision?

The visual system responds primarily to differences rather than absolute values.

How is brightness coded?

Brightness is judged relative to neighbouring regions.

How is colour perception context-dependent?

Colour constancy allows colours to be perceived differently depending on surrounding conditions.

Why is contrast important in visual processing?

The retina and brain are particularly sensitive to boundaries and changes in luminance.

What does the 'What' pathway process?

Shape and colour for object identification.

What does the 'Where' pathway process?

Distance and depth for spatial localisation.

What does the 'When' pathway process?

Motion and temporal changes.

What does the 'Who' pathway process?

Faces for social recognition and identification.

What is the function of the cornea?

Transparent outer surface that performs most of the eye's light refraction (bending).

What is the function of the lens?

Fine-tunes focus and changes shape during accommodation to focus images on the retina.

What is the retina?

Light-sensitive neural tissue that contains photoreceptors and converts light into neural signals.

What is the function of the iris?

Regulates pupil diameter.

What is the pupil?

The opening through which light enters the eye.

What is the optic disc?

Site where the optic nerve exits the eye; forms the blind spot.

What is the fovea?

Region of highest visual acuity responsible for detailed vision.

What is the optic nerve?

Cranial nerve II composed of ganglion cell axons that carry visual information to the brain.

Why is the retinal image inverted?

The optics of the eye invert incoming light before it reaches the retina.

How are visual fields represented on the retina?

Upper visual field → lower retina; Lower visual field → upper retina; Left visual field → right retina; Right visual field → left retina.

What is the blind spot?

The optic disc where the optic nerve exits the eye.

Why is the blind spot blind?

It contains no rods, cones, or photoreceptors.

Why do we not notice the blind spot?

The brain fills in missing information using surrounding visual cues.

What is the visual field?

The area visible while looking straight ahead.

Approximately how large is the visual field of one eye?

About 150°.

What is the binocular visual field?

The region of the visual world seen simultaneously by both eyes.

What is stereopsis?

Depth perception generated from binocular disparity between the two eyes.

Why is binocular vision important?

It enables depth perception, distance estimation, accurate reaching, and navigation.

What happens at the optic chiasm?

Visual information is reorganised so that each hemisphere processes the opposite visual field.

Where does information from the left visual field go?

Right cerebral hemisphere.

Where does information from the right visual field go?

Left cerebral hemisphere.

Why is knowledge of visual pathway organisation clinically important?

Damage at different points produces predictable visual field defects useful for neurological diagnosis.

Why is the retina considered a neural circuit?

It performs substantial information processing before signals reach the brain.

What are the major retinal cell types?

Photoreceptors, bipolar cells, ganglion cells, horizontal cells, amacrine cells.

What are photoreceptors?

Rods and cones that detect light.

What is the function of bipolar cells?

Relay information from photoreceptors to ganglion cells.

What is the function of ganglion cells?

Output neurons of the retina whose axons form the optic nerve.

What is the function of horizontal cells?

Lateral communication, integration, and contrast enhancement.

What is the function of amacrine cells?

Signal modulation and temporal processing.

What is the vertical pathway through the retina?

Photoreceptor → Bipolar Cell → Ganglion Cell → Brain.

Why is the retina layered?

To facilitate signal processing, contrast enhancement, and information compression.

What is retinitis pigmentosa?

A degenerative retinal disease involving progressive photoreceptor degeneration and vision loss.

Which photoreceptors are often affected first in retinitis pigmentosa?

Rods.

What are the two types of photoreceptors?

Rods and cones.

What are the characteristics of rods?

Extremely light sensitive; function in dim light; night vision; no colour detection; scotopic vision.

Why are rods highly sensitive?

Many rods converge onto relatively few ganglion cells.

What is the advantage of rod convergence?

High sensitivity to light.

What is the disadvantage of rod convergence?

Reduced spatial resolution and acuity.

What are the characteristics of cones?

Less light sensitive; function in bright light; colour vision; high acuity; fine detail detection; photopic vision.

What is light?

A form of electromagnetic radiation.

What range of wavelengths is visible to humans?

Approximately 400–700 nm.

Which wavelength corresponds to violet light?

Approximately 400 nm.

Which wavelength corresponds to red light?

Approximately 700 nm.

What is white light?

The perception produced when all visible wavelengths are mixed together.

What are photopigments made of?

Opsin (protein) and retinal (light-sensitive molecule).

What are opsins?

Proteins that bind retinal and determine wavelength sensitivity.

What is phototransduction?

The conversion of light energy into electrical signals by photoreceptors.

What makes photoreceptors unique compared with most sensory receptors?

Light causes hyperpolarisation rather than depolarisation.

What happens to most sensory receptors when stimulated?

They depolarise.

What happens to photoreceptors when stimulated by light?

They hyperpolarise.

What are cGMP levels like in darkness?

High.

What happens to cGMP-gated sodium channels in darkness?

They remain open.

What happens to sodium influx in darkness?

Continuous sodium influx occurs.

What is the membrane potential of photoreceptors in darkness?

Relatively depolarised.

What neurotransmitter is continuously released in darkness?

Glutamate.

What are the steps of phototransduction?

Light activates rhodopsin.

11-cis retinal → all-trans retinal.

Rhodopsin activates transducin.

Transducin activates PDE.

PDE lowers cGMP.

cGMP-gated Na⁺ channels close.

Na⁺ influx decreases.

Photoreceptor hyperpolarises.

Glutamate release decreases.

Bipolar cells receive the signal.

What is the high-yield phototransduction sequence?

Light → Rhodopsin → Transducin → PDE → ↓ cGMP → Channel Closure → Hyperpolarisation → ↓ Glutamate Release → Bipolar Cell Signalling.

What happens to glutamate release when light strikes a photoreceptor?

Glutamate release decreases.

What happens to ON-centre bipolar cells when a photoreceptor is hyperpolarised by light?

ON-centre bipolar cells depolarise.

What postsynaptic potential is generated in ON-centre bipolar cells when light is detected?

An excitatory postsynaptic potential (EPSP).

How does a photoreceptor hyperpolarised by light affect ON-centre bipolar cells?

It excites them.

What is the blind spot and why is it blind?

The optic disc where the optic nerve exits the eye; it lacks photoreceptors.

What is the optic nerve made of?

Ganglion cell axons.

Which photoreceptors mediate night vision?

Rods.

Which photoreceptors mediate colour vision?

Cones.

Which hemisphere processes the left visual field?

Right hemisphere.

Which hemisphere processes the right visual field?

Left hemisphere.

What wavelength range is visible to humans?

400–700 nm.

What is the most important feature of phototransduction?

Light hyperpolarises photoreceptors, reducing glutamate release.

What is one of the most important functions of the retina?

It actively preprocesses visual information by detecting light, extracting colour, enhancing contrast, compressing information, and preparing signals for higher-order visual processing.