PERCEPTION PSY247
Electromagnetic
radiation/wave
Rays
Particle
Define
Wave with periodic changes in the electric and magnetic field - wavelength
measured in nanometers
Light travels in straight lines at very high speech – wave as electrical field
strength varies
Light comes in separate packets – light comes in chunks/ quanta – light
particles called photons
Vision – Light wavelength
• Short wavelengths = cosmic & gamma rays long wavelengths include radio, in middle is visible light –
only difference between visible light & others is differing wavelengths
Vision – Light intensity
• Amplitude/height of the wavelength = light intensity
• Humans cant see wavelengths outside of visible light – thus don’t measure intensity of these
• Luminance = intensity of light we can see measured in candelas/m (more candelas more bright)
• Human vision operates in large range of luminance (sunlight – starlight) - dynamic range = 10 million
• Objects reflect differing amounts of light: 75% light reflected by white, 5% reflected by black objects
– 15:1 ratio between white and black, constant despite differing luminance (how dark or bright it is)
2
Vision – Light contrast
C = (Lmax – Lmin)/ (Lmax + Lmin)
• To detect objects there has to be contrast between object & its background
• Contrast (C): difference between luminance of objects and background
• Varies between 0 – 1: no diff between object & background [Lmax=Lmin] = C is 0 (invisible)
Vision – The Eye
Part
Describe structure
Cornea
Transparent window, curved,
material denser than air
Aqueous Watery liquid from ciliary body
humor
Pupil
Dark circular opening at center of
iris in the eye (black bit)
Iris
Coloured part of eye
Lens
Physical transparent part, curved
Describe function
• Where light enters - slows light down upon entry
• Acts as lens doing majority (3/4) of focusing
• Pressure of liquid important
• Where light goes through
• Adjustable part of eye – controls light intake
- Light level high: iris constricts & pupil gets smaller
reducing light entering
- Light low: iris relaxes so light can enter via the pupil
• Slows light upon entry
& denser than surrounding media
Vitreous Gelatinous substance
humor
Retina
Contains many photoreceptors, at
back of eye
Optic
Big bundles of cables absent of
nerve
photoreceptors (has blind spot)
• Focuses light onto retina via ciliary muscles
• Keeps eyeball in shape & retina pinned to back of eye
• Where light lands at back of eye – light sensitive due to
photoreceptors
• Takes information to brain – as ganglion cell axons leave the
eye here
Vision – Lens & focusing
• Light rays disperse in all directions from single point – thus focusing (unifying rays to 1 point) done
by cornea and lens
How it focuses
Cornea • Curved & does ¾ of focusing but cant change shape (isn’t adjustable)
• Puts light in right ballpark, has greater refractive power
Lens
• Fine tunes light through accommodation (stretched or contracted)
• For close objects: light needs more bending - Ciliary muscles contract, zonular
fibers slacken, lens more round
• For far objects: light parallel (less bending) – C-muscles relax, fibers tighten - lens
stretched thin
Vision – Lens & focusing: errors
Describe Focusing
Emmetropic
Normal refractive condition – near & far images land on retina
Myopic
Optics too strong – distant things bent too much/blurry, near
objects look sharp, focal length short (falls short of retina)
Hyperopia
Optics/lens too weak – not enough bending done – focal length
/Hypermetropia too long (rays converge after retina)
Presbyopia
Old age: lens looses elasticity & we relax our ciliary muscles,
thus lens unable to accommodate
Astigmatism
Due to cornea shape lens does different focusing – different
focal lengths/focusing power for different orientations
Vision
Normal
Short sightedness
Long sightedness
Closest point
(near point)
moves away
Horizontal lines
appear myopic
Vision – Transduction: Photoreceptors Rods & Cones
• As light focused on the retina – photoreceptors (rods & cones) are involved in transduction
Rods
Cones
Describe
• Rod shaped outer segment,
• Cone shaped outer pigment
• Contain photopigment (bit that absorbs
• Different photopigment for different light
light) rhodopsin/ visual purple (As purple)
wavelengths/ types
Types
• All the same – only one type
• 3 different types – red, blue, green – red =
long wavelength, blue = short, green = middle
Respond
• Respond to very dim light “night vision”
• Respond to lots of light “daytime vision”
photopic
scotopic
• Very sensitive: only need small amount • Less sensitive: need a lot of light to fire –
of light intensity to make function (low
higher threshold
threshold) – become overly sensitive in
daylight “dark adaptation”
Sensitive
• Most sensitive to green light
• Most sensitive to yellow light
Distribution • Absent from fovea/macula where what
• Heavily concentrated in fovea on retina
your looking on lands – most densely
• NO blue cones on fovea
packed 12-15 degrees into periphery
• NONE ON OPTIC DISC “blind spot”
• NONE ON OPTIC DISC “blind spot”
Simple
cell
Detect/ role
• Respond to
orientated
stimulus in
particular location
within their RF
• Bar or edge
detectors
Complex • Respond to
cell
orientation edge
anywhere in RF
• Wire together
response from
simple cells
Hypercomplex
cell
Properties
Picture
• Formed from adding inputs from LGN
cells
• Long, thin and rectangular with distinct
off & on regions
• Phase sensitive: impacted by position of
line – if bar to far right or left can
produce inhibitory effect
• Formed by adding responses of many
simple cells in different
• Do NOT show discrete on and off
regions
• Phase insensitive – give same response
across RF
• Give bigger response as bar convers
more RF but when too large and
outside RF leads to no further response
!
• End-stopped cells: • Formed from adding inhibitory &
Respond to
excitatory responses of complex cells
orientation + cells • Thus allows to detect larger bars by as
of certain length
they hit inhibitory input of complex cells
within their RF
Vision – illusions: detecting edges that aren’t there
• Kianza triangle: presence of illusionary edge could be due to interconnected
hypercomplex cells
• Some cells in V2 act this way: Allows detect of subthreshold edges (ones hard to
detect)
Vision – illusions: the tilt-after-effect
• Illusion: first see vertical lines and then adapt to picture of slightly tilted lines after straight lines look tilted in opp. direction
• Explain: we adapt to slightly tilted lines – neurons become less sensitive to
this orientation (reduce firing rate), then staring to line of similar orientation
(straight line) the response distribution is shifted to tilt on opposite side
• NB: distance paradox – effect works best with changing bars of similar/
nearby orientation
• SAME principle for simultaneous tilt illusion – vertical lines appear
tilted due to being surrounded by tilted (We adapt)
• Same principle applies with size after effect and size illusions
– neurons in V1 also have receptive fields tuned to different sizes for each
orientation – thus for e.g. cells thus adapt to thin bar, then upon showing
them a slightly bigger bar, previous one looks thinner (distance paradox also
applies.
Vision – spatial frequency
• Spatial frequency refers to how many bars can fit in certain distance