The Structure of the Eye

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Questions and Answers

What is the term for the ability to see in dim light?

Photopic vision

Central vision

Peripheral vision

Scotopic vision (correct)

What is the function of the fovea in the human eye?

To maintain the shape of the eye

To provide detailed central vision (correct)

To focus light onto the retina

To regulate the amount of light entering the eye

What is the term for the protein chain part of the rhodopsin molecule?

11-cis

Retinal

Opsin (correct)

All trans

What is the role of rhodopsin in the eye?

To absorb photons and initiate visual signal transduction Signup and view all the answers

How many types of photoreceptors are found in the human eye?

Two Signup and view all the answers

What is the wavelength of light that blue cones respond most to?

419nm Signup and view all the answers

What happens to the rhodopsin molecule when light enters the eye?

It breaks apart Signup and view all the answers

What is the term for the region in the centre of the macula?

Fovea Signup and view all the answers

What is the purpose of cGMP in rods?

It keeps sodium channels open Signup and view all the answers

What type of potentials do photoreceptors produce?

Graded potentials Signup and view all the answers

What is the function of horizontal cells?

To integrate information from nearby photoreceptors Signup and view all the answers

What is the role of amacrine cells?

To integrate visual messages across the retina Signup and view all the answers

What is the function of retinal bipolar cells?

To begin identifying contrast in light Signup and view all the answers

What is the purpose of ganglion cells?

To produce action potentials Signup and view all the answers

How many ganglion cells integrate communication from photoreceptors?

1.25 million ganglion cells from 103 million photoreceptors Signup and view all the answers

What is the function of the optic nerve?

To transmit information from ganglion cells to the brain Signup and view all the answers

What is a characteristic of anomalous trichromacy?

Having three cone pigments with peak response wavelengths at different locations than typical Signup and view all the answers

What is the term for the ability of an object's colour to remain constant despite changes in illumination?

Colour constancy Signup and view all the answers

What is the term for the condition where the lens accommodates more slowly to changes in focal distance in middle age?

Presbyopia Signup and view all the answers

What is the effect of the lens growing throughout life on colour perception?

It changes the perception of blue and green colours Signup and view all the answers

What is the effect of ageing on the pupil size?

It decreases Signup and view all the answers

What is the term for the condition where one eye cannot focus on objects?

Amblyopia Signup and view all the answers

What is the term for the phenomenon where colours can look different depending on their context?

Colour contrast Signup and view all the answers

What is the effect of ageing on the cortical processing of visual information?

It decreases the activity of the cortical processing Signup and view all the answers

Study Notes

The Macula and Photoreceptors

The macula has a pit called the fovea, specialized for detailed vision
There are two types of photoreceptors: rods and cones
Rods:
- 100 million in the human eye
- Responsible for scotopic vision (seeing in dim light)
- Contain the photopigment rhodopsin, responding to 502nm (bluish-green)
Cones:
- 3 million in the human eye
- Responsible for photopic vision (seeing in bright light)
- Three classes of cones:
  - Blue or short-wave cones (respond to 419nm, violet)
  - Green or middle-wave cones (respond to 531nm, green)
  - Red or long-wave cones (respond to 502nm, yellow)

Transduction by Photoreceptors

Rhodopsin has two parts: opsin (protein chain) and retinal (made from Vitamin A)
When light enters the eye, rhodopsin changes structure, breaking apart and hyperpolarizing the photoreceptor
In the dark, rods have a resting potential of -30mV due to open sodium channels maintained by cGMP
When rhodopsin breaks apart, cGMP is broken down, and sodium channels close, hyperpolarizing the photoreceptor

Processing by Retinal Interneurons

Horizontal cells:
- Receive input from photoreceptors, integrating information from nearby photoreceptors
Bipolar cells:
- Receive input from photoreceptors and horizontal cells, communicating to amacrine cells
- Two types: diffuse and midget cells
- Identify contrast by responding to light falling on photoreceptors in their receptive fields
Amacrine cells:
- Form connections with bipolar cells, ganglion cells, and other amacrine cells
- Typically release GABA and glycine, integrating visual messages and processing changes in light over time
Ganglion cells:
- Receive input from bipolar and amacrine cells, producing action potentials
- Axons form the optic nerve, integrating communication from 103 million photoreceptors through their receptive fields

Colour Contrast and Constancy

Colour contrast: colours appear different depending on their context
Colour constancy: an object's colour appears the same despite changes in lighting
Edwin Land suggests that the brain normalizes the responses of all cones exposed to a scene before making local comparisons, maintaining colour constancy

Life-Span Development of the Visual System

Development of contrast sensitivity: unclear
Vision as we age:
- Presbyopia (old sight): lens accommodates more slowly to changes in focal distance
- Decreased response to changes in lighting
- Perceptions of blue and green change due to lens growing and taking on a yellow hue
- Pupil size decreases, affecting vision quality
- Ageing affects cortical processing of visual information negatively