L2 The eye and retina

What form of energy is light

electromagnetic energy

What are the two main properties of light?

Wavelength and intensity

Psychological property of wavelength

colour

Psychological property of intensity

brightness

Name this diagram

the electromagnetic spectrum

How does light enter the eye

light is reflected from objects and into the eye

What is the function of the eye in perception?

The eye focuses an image on the retina for perceptual processing.

Where are receptors located in the eye

in the retina

label the pupil and iris

14 and 2

label the cornea and lens

1 and 3

what is label 7

the fovea

what is label 5

the retina

Role of iris in receiving light

adjustable aperture to limit or extend the amount of light passing through

Diameter range of the pupil

2mm – 9mm

What is the role of the cornea and lens?

to focus light on the retina

% of focusing power provided by the cornea

80%

% of adjusting power provided by the lens

20% (but can change)

Why can the shape of the lens change

due to the action of ciliary muscles

What is accommodation in the eye?

The lens changes shape to focus on near or distant objects.

Explain how lens accommodates for nearer vs further objects

becomes fatter vs becomes thinner

What are refractive errors

when the lens does not operate correctly, do not focus correctly on the retina

What are two common refractive errors?

Myopia and hyperopia

What is myopia

nearsightedness, when near objects are clear but far objects are blurry

What is hyperopia

farsightedness, when near objects are blurry but far objects are clear

What is the retina?

A light-sensitive layer at the back of the eye that contains photoreceptors

What part of the eye is associated with the receptor processes

retina, and rods and cones

Role of photoreceptors

carry out transduction

What are photoreceptors

light sensitive cells

What are the two types of photoreceptors?

Rods and cones

What is transduction?

The process where photoreceptors convert light into electrical impulses

What is the role of visual photopigments in transduction

to react to light and trigger electrical signals

How many rods and cones are there in the human eye?

About 120 million rods and 6 million cones.

which is rod and cone

1st is rod, 2nd is cone (note different lengths/shapes)

6 ways in which rods and cones differ

number, sensitivity, involvement in colour perception, retinal distribution, acuity and neural convergence

Are rods or cones more sensitive to light (work better in dim lights)

rods

Are rods or cones more useful in daylight

cones

3 ranges of lighting levels

scotopic, photopic and mesopic

Which lighting levels are rods active

scotopic and mesopic

Which lighting levels are cones active

photopic and mesopic

Why do photoreceptors stop responding in bright light

because bright light bleaches photopigments, need to recover / regain sensitivity

What is dark adaptation

increase in eye’s sensitivity in the dark

Where are rods mostly located?

Rods are mostly located in the peripheral retina.

Where are cones mostly located?

Cones are mostly located in the fovea (central retina).

Are rods or cones responsible for colour vision

cones

Why can’t we see colour at night?

Cones, which detect colour, do not function well in low light

What are the three types of cones (and their wavelengths)?

Red (long wavelengths), Green (medium wavelengths), and Blue (short wavelengths).

What colour perception do rods produce

monochromatic vision (black and white)

Which type of wavelengths are rods more sensitive to

medium wavelengths (green light)

Number of types of cones vs rods

3 vs 1

What is the Purkinje shift?

At night, red looks darker than green

Nature of retinal distribution of photoreceptors

not evenly distributed

What is the fovea (location + consists of?)

small central area of the retina that contains only cones

Explain blindspot

area of the retina with no photoreceptors

where does an image of an object fall when looking directly at it

on the fovea

What is neural convergence?

when one neuron receives signals from many other neurons

Which photoreceptors have greater convergence?

Rods have greater convergence than cones

What determines acuity

neural convergence

What is acuity

the ability to detect fine details of a stimulus

Which photoreceptors have higher acuity

cones

Why does less convergence lead to higher acuity

because it allows for more precise spatial representation of visual info (each individual photoreceptor sends signal to dedicated pathway)

Why is vision sharpest in the fovea?

Each cone in the fovea connects to a single ganglion cell, enhancing detail perception.

Why do we move our eyes to look directly at objects of interest?

To position the image on the fovea for maximum acuity.

What is mesopic vision?

Vision in intermediate light levels when both rods and cones are active.

Why does acuity decrease in low lighting conditions

because rods, which dominate night vision, have low acuity due to high neural convergence