The First Steps in Visual Perception

Question 1

What type of energy is light a form of?

Answer 1

Electromagnetic energy

Question 2

What psychological properties do the wavelength and intensity of light correlate with?

Answer 2

• Wavelength - colour

- Intensity - Brightness

Question 3

Which part of the electromagnetic spectrum is our visual system sensitive to?

Answer 3

The visible spectrum

Question 4

How do we see shorter wavelengths and longer wavelengths?

Answer 4

• Shorter wavelength light we see as violet

- longer wavelength light we see as red

Question 5

How do we see light?

Answer 5

• Light is reflected from objects and into the eye – forming an image
• Objects absorb certain wavelengths and reflect others – we see the light that is reflected
• This is how we go from the distal stimulus to the proximal stimulus

Question 6

Where are out receptors located in the eye?

Answer 6

the retina

Question 7

What is the function of the eye?

Answer 7

To focus the image on the retina

Question 8

What do the Iris and Pupil do?

Answer 8

• Pupil is a hole letting light in
• Adjustable aperture
• Limit the amount of light passing through
• Allows us to deal with great range of light levels
   Pupil between 2mm and 9mm

Question 9

What does the Cornea and lens do?

Answer 9

• Cornea outer surface of eye
• Lens just behind cornea
• Role is to focus light on the retina
• Cornea – 80% focussing power
• Lens – 20% but can change shape due to action of ciliary muscle, this allow accommodation

Question 10

What is accommodation?

Answer 10

• Lens becomes thinner to focus far objects

- Lens becomes fatter to focus close objects

Question 11

What is the retina?

Answer 11

• Light (photo) sensitive layer at back of the eye
• Contains number of different types of cells
   Photoreceptors
   Horizontal cells
   Bipolar cells
   Retinal ganglia cells

Question 12

What are photoreceptors and what do they do?

Answer 12

• Photoreceptors = light sensitive cells
• Carry out transduction = transforming light into electrical impulses
• Transduction occurs by visual photopigments reacting to light and tigger electrical signals

Question 13

What are the two types of photoreceptors and how many of each of them are there?

Answer 13

 Rods – 120 million

 Cones – 6 million

Question 14

What are the differences between rods and cones?

Answer 14

• Number
• Sensitivity
• Involvement in colour perception
• Retinal distribution
• Neural convergence and acuity

Question 15

What is the difference in sensitivity between rods and cones?

Answer 15

• Rods
   Very sensitive, respond well in very dim light
   Most useful at night, useless in daylight
• Cones
   Less sensitive
   Work best in daylight, useless at night

Question 16

Why if we go from light to dark we can’t see very well?

Answer 16

• Bright light bleaches photopigments so photoreceptors stop responding
• Going from bright to dark the photoreceptors have to ‘recover’ or regain sensitivity
• Dark adaption = increase in eye’s sensitivity in the dark
• After 20-30 minutes in the dark, sensitivity is about 100,000 times greater than the sensitivity in light

Question 17

What is the difference in colour perception between cones and rods?

Answer 17

• Cones
   Responsible for colour vision
   3 types of cone, sensitive to different wavelengths of light:
   Red (long wavelengths)
   Green (medium wavelengths)
   Blue (short wavelengths)
• Rods
   Produce monochromatic vision
   1 type – most sensitive to medium wavelengths (green light)
   But because there is only one type we see everything in black and white and different colours of grey

Question 18

What is the fovea?

Answer 18

• a small central area of the retina directly behind the pupil contains only cones
• When looking directly at an object the image falls on the fovea.
• At the fovea – 160 000 cones per mm^2 and no rods

Question 19

What happens when you move slightly away from the fovea?

Answer 19

you get a very rapid decline in number of cones and rapid increase in number of rods, everywhere else in the retina (peripheral retina) we have more rods than cones

Question 20

What is the blind-spot in the retina?

Answer 20

where the optic nerve leaves the eye and there aren’t any receptors

Question 21

What is convergence and how does it work with rod and cone cells?

Answer 21

• one neuron receives signals from many other neurons
• A group of photoreceptors will all send signals to one ganglion cell
• On average
   120 rods send signals to 1 ganglion cell
   6 cones send signals to 1 ganglion cells

Question 22

What is acuity?

Answer 22

• the ability to detect fine details of a stimulus
   Higher acuity – can detect fine details
   Low acuity – can only detect course details

Question 23

What determines acuity?

Answer 23

Neural convergence

Question 24

What is the difference between acuity in rod and cone cells?

Answer 24

• Rods have greater convergence – lower acuity (ganglion detects information from a large number of rods so large area of retina)
• Cones have less convergence – this leads to higher acuity (ganglion will be able to find out information from small area of retina)
• Highest acuity at the fovea
• Lower acuity in the periphery

Question 25

What do eye movements do?

Answer 25

Ensure objects of interest are imaged on the fovea so that acuity is high