The Eye & Sight Flashcards
Wavelength change in visual light =
colour change
Amplitude change in visual light =
brightness change
Role: Iris/Pupil
Control the amount of light passing through to the retina via an adjustable aperture
Role: Cornea/Lens
To focus light on the lens
myopia
nearsighteness
hyperopia
farsightedness
Daylight vs night-time rods and cones
DAY- rods: not so good, cones: work well
NIGHT- cones: useless, rods: work well
Stages of light levels and which cells are active
- Scotopic – only rods active
- Photopic – only cones active
- Mesopic – both rods and cones active
3 Types of cones for colour perception
– Red (long wavelengths)
– Green (medium wavelengths)
– Blue (short wavelengths)
Cones vs rods function
cones: colour
rods: light
Purkinje shift
at night red looks darker than green as the rods are more sensitive to green light
fovea
small central area of the retina that contains only cones
Convergence
when one neuron receives signals from many other neurons
which has higher convergence, rods or cones?
rods
what type of cell do rods and cones converge at
ganglion
Acuity
the ability to detect fine details of a stimulus
does high convergence lead to high or low acuity
low acuity
do rods or cones have higher acuity?
cones
Do ganglion cells have a baseline action potential activity?
yes
Does low convergence increase or decrease light sensitivity?
Low convergence decreases light sensistivity
Does low convergence increase or decrease visual acuity?
Low convergence increases visual acuity
Which cells transmit lateral inhibition?
Horizontal and Amacrine Cells
Lateral Inhibition
It is the capacity of an excited neuron to reduce the activity of its neighbours.
What is lateral inhibition good for?
It creates a contrast in stimulation that allows increased sensory perception.
Function of ganglion cells
They respond to changes in patterns of light
The Herman grid illusion gives an example of what element of ganglion cells?
Lateral inhibition creating centre-surround antagonism depending on excitation or inhibition of ganglion cells.
In ganglion cells, less firing is perceived as lighter or darker?
darker
How does the simultaneous contrast illusion work in regards to central centre surround antagonism and lateral inhibition?
Light shining on the light surroundings of a square (an on-centre receptive field) will cause the central cell tracts to produce an OFF response to the centre. Less firing interpreted as less bright so we perceive a darker square
Whereas light shining on the dark lateral surroundings of a square (of an off-centre receptive field) will cause the central cell tracts to produce an ON response. More firing interpreted as more bright so we perceive a lighter square.
Optic chiasm
crossover point of the 2 optic nerves.
When does the optic nerve become the optic tract?
After the optic chiasm
What part of the brain does the optic tract lead to?
Lateral Geniculate Nucleus (LGN)
Is the LGN bilateral or unilateral?
bilateral
Does the LGN have centre-surround antagonism or orientation selectivity?
centre-surround antagonism
What does V1 signify?
The primary visual cortex
Does the primary visual cortex have centre-surround antagonism or orientation selectivity?
orientation selectivity
Retinotopic mapping in V1
Objects close together in the visual scene are analysed by neighbouring parts of V1.
Cortical magnification in V1
Amount of cortex devoted to representing each part of the retinal field is distorted (not even).
What proportion of V1 does the fovea take up?
8-10%
Orientation columns in V1
Orientation preferences of V1 cells arranged in ordered way.
Are cells in LGN monocular or binocular?
monocular
Are cells in V1 predominantly binocular or monocular?
binocular
Ocular dominance
V1 cells respond better to one eye than the other
Put these in chronological order in visual processing:
LGN cells, complex cells, ganglion cells, hypercomplex cells, simple cells, photoreceptors
photoreceptors, ganglion cells, LGN cells, simple cells, complex cells, hypercomplex cells
Simple cells in V1 response depends on…
position of the bar within the receptive field
Complex cells in the V1 response depends on…
moving oriented bars and edges, responding best to a particular direction of movement
hypercomplex cells in V1 response depends on…
particular orientation of a bar AND movement in a particular direction AND a particular length
What does V3 specialise in?
form
What does V4 specialise in?
colour
What does V5 specialise in?
motion
What part of the brain is important in determining what an object is (object discrimination)?
inferotemporal cortex
What part of the brain is important in determining where an object is (landmark discrimination)?
posterior parietal cortex
What is the predominant symptom of visual form agnosia
Inability to identify objects
Who would be better at posting a block through a letterbox, someone with visual form agnosia or someone with optic ataxia?
someone with visual form agnosia because they’re better at acting on stuff, optic ataxia mean’s people have damage to the where/how stream
What is the main symptom of optic ataxia?
Problems with landmark distinction
Marr’s approach to form perception
Concerned with the representation of edges, contours and other areas of contrast change
Gestalt approach to form perception
concerned with rules of perceptual organisation
Is Marr’s approach top-down or bottom-up
bottom-up
Does Marr’s approach start with the input into the receptor fields or the brain’s preconceptions?
The receptor field, it is a bottom up approach so it starts with input to perceptual system. After this, each stage takes as its input from the information from the previous stage and transforms it into a more complex description or representation.
What does the computational model of form perception focus on?
The goal of the model, the algorithm it needs and the mechanisms to achieve this.
Order these levels of image analysis:
grey level description, 3D representation, primal sketch, retinal image, 21/2D sketch
- Retinal Image
- Grey level description – measuring intensity of light at each point in the image.
- Primal sketch -representation of contrast change (blobs, edges, bars etc) over range of spatial frequencies)
- 21/2D sketch - representation of orientation, depth, colour relative to the observer. It’s not 3D because it is observer-orientated. (unseen parts of scene and objects)
- 3D representation - representation of objects independent of observer. 2 1/2 sketch analysed for 3D volume primitives (cylinders, cones, cubes etc).
Produces 3D representation that is independent of observer. Conscious experience of vision.
Is Gestalt’s approach to form perception top-down or bottom up?
top-down
what is multistability?
when a single physical stimulus produces alternations between different subjective perceptions, eg the necker cube
What is Gestalt’s law or perceptual organisation in reference to similarity?
similar things appear to be grouped together
What is Gestalt’s law or perceptual organisation in reference to conjugation?
Points that, when connected, result in straight or smoothly curving lines, are seen as belonging together, and the lines tend to be seen in such a way as to follow the smoothest path.
What is Gestalt’s law or perceptual organisation in reference to proximity?
Things that are near to one another appear to be grouped together
What is Gestalt’s law or perceptual organisation in reference to connectedness?
Things that are physically connected are perceived as a unit
What is Gestalt’s law or perceptual organisation in reference to closure?
Of several geometrically possible perceptual organisations, a closed figure will be preferred to an open figure.
what is reification?
the act of changing something abstract into something real -eg the kanza triangle
What is Gestalt’s law or perceptual organisation in reference to common fate?
things moving in the same direction are grouped together
What is Gestalt’s law or perceptual organisation in reference to familiarity?
Things are more likely to form groups if the groups appear familiar or meaningful.
What is Gestalt’s law or perceptual organisation in reference to invariance?
Recognising that many different images are the same object just distorted
What are the oculomotor cues we receive to tell us about or eye’s position to help with depth perception?
convergence: as your eye muscles cause you eyes to look inward (the closer the object the greater the convergence)
accommodation: as the lens bulges to focus on a near object
What is interposition/occlusion in relation to monocular depth cues?
One object obscures part of another, or overlaps with it
What is texture gradient in relation to monocular depth cues?
Texture becomes smaller/finer as distance increases
What is deletion & accretion in relation to motion-produced depth cues?
As one object moves in front of another, deletion occurs whereby the front object covers more of the back object. As one object moves away from another, accretion occurs whereby the front object covers less of the back object.
What is motion parallax in relation to motion-produced depth cues?
Motion parallax refers to the fact that objects moving at a constant speed across the frame will appear to move a greater amount if they are closer to an observer (or camera) than they would if they were at a greater distance
What is binocular disparity/ binocular stereopsis?
A cue that depends on the fact that slightly different images of a scene are formed on each eye.
Does red light have a long or short wavelength?
Long
Does violet light have a long or short wavelength?
Short
DEFINE: hue
the attribute of a colour which is dependent on its dominant wavelength and independent of intensity or lightness.
Which colour photoreceptors are on the retina and which wavelengths do they respond best to?
Blue-sensitive cones maximally responsive to short wavelengths. (S-Cones)
Green-sensitive cones maximally responsive to medium wavelengths. (M-Cones)
Red-sensitive cones maximally responsive to long wavelengths. (L-Cones)
What is the significant belief from Opponent Process Theory of colour perception?
There are 3 battles going on depending on which type of light there was more of there was more of: yellow v blue, green v red & black v white. Eg: a Red-Green Receptor will signal either Red or Green but not both.
Trichromacy vs opponent process model - which one takes place in the photoreceptors and which on takes place in the LGN?
trichromacy at the level of the cones, opponent processes at the level of LGN
What are anopias in relation to colour blindness?
An insensitivity to L, M or S wavelengths of light.
(missing a type of cone)
What are anomalies in relation to colour blindness?
Misalignment of L or M in trichromats. (distribution or deficiency)
Which cone pigment is missing in protanopia?
L-cone
Which cone pigment is missing in deutanopia?
M-cone
Which cone pigment is missing in tritanopia?
S-cone
Which cone has an anomaly is missing in protanomaly?
L-cone
Which cone has an anomaly is missing in deuteranomaly?
m-cone
What us the correspondence problem in relation to motion perception?
How the visual system knows if an object seen at Time 1 is the same object at Time 2.
DEFINE: Velocity transposition
when two images moving across the visual field at different speeds can be perceived to be moving at the same speed
DEFINE: Induced movement
an object appears to move or to move differently because of other moving objects nearby in the visual field.
DEFINE: autokinetic movement
- apparent motion in stationary images (eg visual illusions)
define: movement aftereffects
- illusion of motion in the visual image caused by prior exposure to motion in the opposite direction
