The First Steps in Visual Perception Flashcards
What type of energy is light a form of?
Electromagnetic energy
What psychological properties do the wavelength and intensity of light correlate with?
- Wavelength - colour
- Intensity - Brightness
Which part of the electromagnetic spectrum is our visual system sensitive to?
The visible spectrum
How do we see shorter wavelengths and longer wavelengths?
- Shorter wavelength light we see as violet
- longer wavelength light we see as red
How do we see light?
- 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
Where are out receptors located in the eye?
the retina
What is the function of the eye?
To focus the image on the retina
What do the Iris and Pupil do?
- 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
What does the Cornea and lens do?
- 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
What is accommodation?
- Lens becomes thinner to focus far objects
- Lens becomes fatter to focus close objects
What is the retina?
- Light (photo) sensitive layer at back of the eye
- Contains number of different types of cells
Photoreceptors
Horizontal cells
Bipolar cells
Retinal ganglia cells
What are photoreceptors and what do they do?
- Photoreceptors = light sensitive cells
- Carry out transduction = transforming light into electrical impulses
- Transduction occurs by visual photopigments reacting to light and tigger electrical signals
What are the two types of photoreceptors and how many of each of them are there?
Rods – 120 million
Cones – 6 million
What are the differences between rods and cones?
- Number
- Sensitivity
- Involvement in colour perception
- Retinal distribution
- Neural convergence and acuity
What is the difference in sensitivity between rods and cones?
- 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
Why if we go from light to dark we can’t see very well?
- 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
What is the difference in colour perception between cones and rods?
- 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
What is the fovea?
- 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
What happens when you move slightly away from the fovea?
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
What is the blind-spot in the retina?
where the optic nerve leaves the eye and there aren’t any receptors
What is convergence and how does it work with rod and cone cells?
- 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
What is acuity?
- the ability to detect fine details of a stimulus
Higher acuity – can detect fine details
Low acuity – can only detect course details
What determines acuity?
Neural convergence
What is the difference between acuity in rod and cone cells?
- 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
What do eye movements do?
Ensure objects of interest are imaged on the fovea so that acuity is high
