Week 4 - The Visual system

Question 1

What is decussation?

Answer 1

Decussation refers to the process by which nerve fibers cross over from one side of the body to the other. In the visual system, decussation occurs in the optic chiasm, which is a region located at the base of the brain where the optic nerves from the eyes meet.

Question 2

What are the three parts involved in the eye, forming an image?

Answer 2

Cornea
* Transparent outer layer
* Most light bending (refraction) occurs
here

Lens
* Fine tunes image formation
* Adjustable
* Accommodation reflex
* Stiffens with age

Iris and Pupil
* Size of the opening (pupil) regulated
by contractile tissue (iris)
* Varies light, but more importantly
focal length
* Reflex

Question 3

How does the eye generate a neural signal? List two parts.

Answer 3

Transduction
Retina
* Receptors to transduce light signal to
neural signal
* Layers of neurons for early processing
of the signal
* Retinal ganglion cells (RGCs) final layer
- axons to the brain

Fovea
* Small specialised high acuity central
vision
* Solves the “backward wiring” problem

Question 4

How does the eye transmit neural signal to the brain for processing?

Answer 4

Optic disc
* Point on the retina where RGC axons
leave to become the optic nerve
* Blind spot – no receptors

Optic nerve
* Neural transmission to thalamus
* Partial decussation at the optic chiasm
* Optic tract beyond the optic chiasm

Question 5

How do we know that vision is constructed?

Answer 5

Each eye has a blind spot but there is no black hole in vision
* VISION IS CONSTRUCTED!!
* Completion
* Receptors around the blind spot provide information to fill in the
gaps.
* Edges are continued
* Surfaces are interpolated
* Best guess at what is in the blind spot based on what is around it.

Question 6

What are cones?

Answer 6

Cones are photoreceptors in the retina of the eye that are responsible for colour vision and function best in bright light.

• Lower sensitivity
• High positional acuity
  due to low convergence
• 3 types – short (S),
  medium (M), and long
  (L) wavelength
• Photopic vision (well lit)
• Colour perception
• 6 - 7 million per retina

Question 7

What are rods?

Answer 7

Rods are another type of photoreceptor in the retina of the eye. rods are responsible for black and white (grayscale) vision and function best in low light conditions.

High sensitivity
* Low positional acuity
due to high
convergence
* Scotopic vision (low
light)
* 120 million per retina

Question 8

What is Fovea?

Answer 8

The fovea is a small, central pit in the retina of the eye that is responsible for detailed, high-resolution vision. It is located at the center of the macula, which is a small, pigmented area in the retina that is responsible for central vision. The fovea is the part of the eye that is used for reading, driving, and other activities that require detailed vision.

The fovea is unique because it is the only part of the retina where the photoreceptors (rods and cones) are densely packed and arranged in a single layer. This allows the fovea to achieve the highest visual acuity (sharpness) of any part of the eye.

Question 9

What is the role of thalamus in the visual system?

Answer 9

Relay station – all sensory
except smell relays through
thalamus
* Also receives many cortical projections
* Integration of bottom-up
inputs and top-down
* Many separate nuclei
* Lateral Geniculate Nucleus first synapse after the optic nerve
leaves the eye
* Medial Geniculate Nucleus
Auditory relay
* Ventro Posterior Nucleus - Sensorimotor relay