Week 5: Sensory System 1 - Visual System

Sensory system

A body system that receives information from the environment, identifies the features of a stimulus, and carries that information to the nervous system for processing.

What is sensory transduction?

The conversion of environmental energy, such as light or pressure, into an electrical signal that the nervous system can understand.

What does a sensory receptor do?

It detects a particular type of stimulus and transduces it into a graded generator potential or an action potential.

What organizational features do sensory systems share?

They contain receptors, chains of neurons, thalamic relays, sensory regions of the cortex, and a topographical organization.

Why is the thalamus important to sensory processing?

It acts as a major relay station that organizes and sends sensory information to the appropriate areas of the cerebral cortex.

What is topographical organization in a sensory system?

Nearby locations in the body or sensory environment are represented in nearby locations within the nervous system.

What are the four major functions of the visual system?

Sight, control of eye movements, visual perception, and providing information for postural and limb-movement control.

Which cranial nerves control eye movements?

Cranial nerves Oculomotor (III), Trochlear (IV), and Abducens (VI) control the extraocular muscles that move the eyes.

What is visual perception?

The brain's ability to recognize what objects are and determine where they are located.

How does vision contribute to movement?

Visual information helps the nervous system plan and adjust posture, reaching, walking, and other limb movements.

How do the visual and vestibular systems work together?

They coordinate eye and head movements so that vision remains stable and the eyes can stay directed toward a target.

What are the two main objectives of eye movements?

To keep the eyes stable during head movement and to direct the gaze toward a visual target.

What are conjugate eye movements?

Movements in which both eyes move together in the same direction.

What are vergence eye movements?

Movements in which the eyes move in opposite directions toward or away from the midline.

What is a saccade?

A fast eye movement that rapidly switches the gaze from one object or location to another.

When would you use a saccadic eye movement?

You use a saccade when shifting your gaze between stationary targets, such as moving from one word to the next while reading.

What is smooth pursuit?

A controlled eye movement that allows the eyes to continuously follow a moving target.

When would you use smooth pursuit?

You use smooth pursuit when tracking something that is moving, such as watching a ball travel through the air.

What is convergence?

A vergence movement in which both eyes turn inward toward the midline to view a nearby object.

What is divergence?

A vergence movement in which both eyes move outward away from the midline to view a more distant object.

Which eye movements primarily direct gaze toward a target?

Saccades, smooth pursuits, and vergence movements.

Which mechanisms primarily stabilize gaze?

The vestibulo-ocular reflex and optokinetic nystagmus.

What is the vestibulo-ocular reflex or VOR?

A reflex that moves the eyes in the direction opposite the head during fast head movements, allowing the visual field to remain stable.

If the head quickly turns right, how does the VOR move the eyes?

The eyes move to the left to compensate for the rightward head movement and keep the visual target stable.

Why is the VOR important during daily activities?

It prevents the visual world from appearing blurry or unstable while the head is moving.

What is the Doll's eye reflex?

The name used for the vestibulo-ocular reflex when it is observed in newborn infants.

What is VOR suppression?

The voluntary inhibition of the vestibulo-ocular reflex when a person needs the eyes to move with the head rather than opposite it.

When is VOR suppression useful?

It is useful when the head and eyes must follow the same moving target, such as watching your thumb while turning your head and body together.

What is optokinetic nystagmus?

A gaze-stabilizing response that adjusts eye position during slow head movement or when the visual environment moves relative to the head.

What is the basic difference between VOR and optokinetic nystagmus?

The VOR primarily stabilizes vision during fast head movements, whereas optokinetic nystagmus stabilizes vision during slow movement or movement of the visual scene.

What is the pupil?

The opening in the center of the iris through which light enters the eye.

What is the cornea?

The transparent outer layer at the front of the eye that begins bending and focusing incoming light.

What is the iris?

The colored structure that changes pupil size to regulate how much light enters the eye.

What is the lens?

A transparent structure that changes shape to fine-tune the focus of light on the retina.

What do the ciliary muscles do?

They adjust the shape of the lens so the eye can focus on objects at different distances.

How are the cornea and lens different?

The cornea provides the initial bending of incoming light, while the lens changes shape to fine-tune the focus.

What is the retina?

A thin neural sheet lining the back wall of the eye that contains photoreceptors and begins visual processing.

What is the main job of the retina?

It detects light and converts it into electrical signals that can be carried to the brain.

What is the macula?

A specialized concave area of the retina responsible for detailed central vision.

What is the fovea?

The center of the macula and the retinal area with the highest concentration of cones.

Why does the fovea provide the sharpest vision?

Its very high concentration of cones allows precise detection of color and fine visual detail.

What is the nasal hemiretina?

The half of each retina located medial to the fovea, toward the nose.

What is the temporal hemiretina?

The half of each retina located lateral to the fovea, toward the temple.

What are photoreceptors?

Specialized retinal receptors that transduce light into electrical signals.

What are rods specialized to detect?

Dim light and low-light visual information.

What are cones specialized to detect?

Color and fine visual detail or visual acuity.

Rods versus cones

Rods are most useful in dim light, while cones provide color vision and high visual acuity.

What are bipolar cells in the retina?

Intermediate neurons that connect photoreceptors to retinal ganglion cells.

What are retinal ganglion cells?

The output neurons of the retina whose axons form the optic nerve.

What sequence does visual information follow within the retina?

Photoreceptors detect light, bipolar cells relay the signal, and ganglion cells carry the output toward the brain.

What is the optic nerve?

A bundle formed by retinal ganglion-cell axons that exits the eye and carries visual information toward the brain.

What is the optic disc?

The point where the optic nerve exits the retina.

Why is the optic disc called the blind spot?

It contains no photoreceptors, so light landing there cannot be detected.

What information does one optic nerve carry?

It carries information received by one eye from portions of both the left and right visual fields.

What is the optic chiasm?

The location where the two optic nerves meet and fibers from the nasal hemiretinas cross to the opposite side.

Which retinal fibers cross at the optic chiasm?

Fibers from the nasal hemiretina of each eye.

Which retinal fibers remain on the same side at the optic chiasm?

Fibers from the temporal hemiretina of each eye.

Why is crossing at the optic chiasm important?

It allows information from the same visual field of both eyes to be processed together in the opposite cerebral hemisphere.

What is the optic tract?

The visual pathway that carries information from the optic chiasm toward the lateral geniculate nucleus and other subcortical structures.

What is the lateral geniculate nucleus or LGN?

A visual relay nucleus in the thalamus that receives information from the optic tract and sends it toward the visual cortex.

What are the superior colliculi involved in?

They use visual information to help orient the eyes, head, and body toward visual stimuli.

What are the optic radiations?

Axons that emerge from the LGN and carry visual information to the primary visual cortex.

Where is the primary visual cortex located?

It is located in the occipital lobe and is also called V1.

What does the primary visual cortex or V1 analyze?

It discriminates basic visual characteristics such as the shape, size, and texture of objects.

What do secondary visual areas such as V2 and V3 analyze?

They perform additional processing of visual characteristics, including color and motion.

What do visual association areas do?

They combine and interpret visual information so a person can identify objects and use vision to guide movement.

What is the dorsal visual stream?

The action stream that processes where an object is and how vision should guide movement toward or around it.

How can you remember the dorsal stream?

Think dorsal equals doing because it helps direct actions and movements.

What is the ventral visual stream?

The perception stream that processes what an object is so it can be visually recognized.

How can you remember the ventral stream?

Think ventral equals visual identification because it helps recognize what you are seeing.

Dorsal stream versus ventral stream

The dorsal stream guides visually directed action, while the ventral stream supports object recognition.

Which visual stream helps you accurately reach for a cup?

The dorsal or action stream because it processes the cup's location and guides the movement.

Which visual stream helps you recognize that an object is a cup?

The ventral or perception stream because it identifies what the object is.

What is the visual field?

The portion of the world that can be seen at one time without moving the eyes or head.

How is the visual field represented on the retina?

The image is inverted, so light from one side of the visual field falls on the opposite side of the retina.

Where is the left visual field processed?

It is processed primarily by the right side of the brain.

Where is the right visual field processed?

It is processed primarily by the left side of the brain.

What happens after a complete lesion of one optic nerve?

Vision is completely lost in the eye on the same side as the lesion.

What does ipsilateral mean?

On the same side of the body as the structure or lesion being discussed.

What visual deficit follows an optic-chiasm lesion?

Bitemporal hemianopia, meaning loss of the outer or temporal half of the visual field in both eyes.

Why does an optic-chiasm lesion cause bitemporal hemianopia?

The lesion interrupts crossing fibers from both nasal hemiretinas, which carry information from the temporal visual fields.

What does bitemporal hemianopia look like functionally?

The person loses peripheral vision on both outer sides and may experience tunnel-like vision.

What visual deficit may follow a lesion of the optic tract, LGN, or optic radiations?

Contralateral homonymous hemianopia, meaning loss of the same visual-field half in both eyes opposite the lesion.

What does contralateral mean?

On the side opposite the structure or lesion being discussed.

What does homonymous hemianopia mean?

Loss of the same half of the visual field in both eyes.

Why does an optic-tract lesion cause a contralateral field loss?

After the optic chiasm, each tract carries information representing the opposite visual field from both eyes.

What deficit would result from a right optic-tract lesion?

Left homonymous hemianopia, or loss of the left visual field in both eyes.

What deficit would result from a left optic-tract lesion?

Right homonymous hemianopia, or loss of the right visual field in both eyes.

What visual deficit can result from a visual-cortex lesion?

Contralateral homonymous hemianopsia, sometimes with preservation of central vision called macular sparing.

What is macular sparing?

Preservation of central vision despite loss of the surrounding half of the visual field after some visual-cortex lesions.

How can you quickly localize a visual-pathway lesion?

One blind eye suggests the optic nerve, loss of both outer fields suggests the optic chiasm, and loss of the same field half in both eyes suggests a lesion behind the chiasm.

What is age-related macular degeneration?

A progressive condition in which photoreceptors in the macula are lost, impairing central vision.

What is the cause or etiology of age-related macular degeneration?

It is associated with aging.

What structure is primarily damaged in macular degeneration?

The macula, especially its photoreceptors.

What symptoms are associated with macular degeneration?

Impaired central vision, blurriness, and distortion of objects.

What type of vision is usually most affected by macular degeneration?

Central detailed vision is affected more than peripheral vision.

What is the typical prognosis of macular degeneration?

It is progressive, meaning visual impairment may increase over time.

Why is daily screening important for someone with macular degeneration?

It helps the person recognize changes or worsening distortion early and seek appropriate care.

What problem-solving process can help manage macular degeneration?

Identify occupational problems, generate possible strategies, and begin implementing the most appropriate strategies.

What computer adaptations may help someone with macular degeneration?

Large print, enlarged displays, and speech-output technology can reduce reliance on detailed central vision.