Vision Flashcards
(80 cards)
1
Q
Detection threshold:
A
the ability to detect the mere presence of the stimulus.
2
Q
Absolute threshold:
A
the weakest level of stimulus that can be detected.
3
Q
Difference threshold:
A
smallest detectable change in a stimulus.
4
Q
sclera:
A
eye wall made of tough white material (except for clear cornea)
5
Q
Cornea:
A
acts as a fixed lens
has anterior chamber filled with clear fluid (‘aqueous humour’) which separates cornea from the iris
6
Q
Iris:
A
a ring of muscle controlling the size of the pupil and hence the amount of light entering the eye.
7
Q
What is ‘accommodation’?
A
flattening the lens -> bring distant objects into focus
making the lens rounder -> bring near objects into focus.
8
Q
retina
A
a thin rim of neural tissue responsible for encoding patterns of light and shade.
9
Q
‘Acuity’:
A
ability to distinguish fine detail in the image
10
Q
Why do light receptors have good acuity?
A
greater numbers and smaller receptive fields (areas of the visual field from which they receive light input)
11
Q
optic disk
A
the area of the retina where nerve fibres exit the eye projecting to the brain. There are consequently no receptors there, and we thus have a ‘blind spot’ in each eye
12
Q
How many layers of cell is the retina and why?
A
5 distinct layers of cells
But only 0.2mm
13
Q
What happens to photoreceptor information? Other area that help with transmission?
A
->transmitted to bipolar cells -> Transmitted to Ganglion cells
14
Q
How do we measure ganglion cell responses?
A
NOT graded like photoreceptors, all or nothing, action potentials
15
Q
What is the baseline rate of firing of a ganglion cell?
A
Can be 10-12 impulses per second
16
Q
What are the two types of cell that process information in the retina?
A
horizontal cells - integrate information from several photoreceptors
amacrine cells - form links to several different ganglion cells
17
Q
Range of retinal Ganglion cells?
A
0-200 spikes per second
18
Q
What did Hecht (1937) do?
A
Tested sensitivity to light with two types of light:
- Red flashes -> sensitivity increased by 2 log units in 10 minutes, then no better
- Violet flashes -> sensitivity increased by 2 log units in 10 minutes, then again by 4 log units
19
Q
What does Hecht’s study imply?
A
Two systems:
Phototopic = light adapted, high acuity
Scotopic = dark adapted
20
Q
How many cones in human vision?
A
Three, all colour blind
21
Q
Problem with colour blind cones?
A
Can confuse single frequencies with white light (a mixture of frequencies)
22
Q
What have we done to help with colour blind cones?
A
Three types of cone have peak absorption at long-, middle- and short- wavelengths respectively
23
Q
Evolution of three types of cone theories?
A
- Young (1807) first suggested it based on metamaric matching -> matching appearance of any single wavelength using mixtures of three primary colours
- Brown & Wald (1966) then used microspectophotometry -> shining a thin monochromatic beam through individual receptors in dissected retina, found that the peak absorption of cones cluster around three wavelengths.
24
Q
What is the 4 colour theory thing?
A
-Hering (1978) used the opponent process theory of colour -> suggested four primaries in two opponent relationships (Red versus Green, Blue versus Yellow).
○ Complementary colours: Mix two complimentary colours and you get neutral not a mixture of the two colours (i.e., no reddishgreen, no yellowish-blue).
-Agreed that vision was trichromatic, but suggested that our subjective experience of colour is from 4 colours, not 3 wavelengths
25
Evidence in support of Herring?
- De Valois et al. (1966) - Recording from monkey LGN, they found colour opponent cells: e.g. +R-G, +B-Y
- Support for transition a trichromatic receptoral stage and four primaries at post-receptoral stage
- L-cones provide ‘red' input
- M- cones ‘green’
- S cones the ‘blue’ input.
- L+M cones together provide ‘Yellow’ inputs to colour opponent processes.
26
What do retinal ganglion cells fire for?
Ganglion cells don't respond to the overall level of light, but to the relative stimulation of the centre vs. the periphery ('the surround')
27
What is centre-surround antagonism?
the tendency for stimulation of the centre of a cell’s receptive field to have the opposite effect to stimulation of the surround, and vice versa
28
Who first demonstrated centre-surround antagonism?
Harline and Graham (1932) in the horshoe crab
Kuffler (1953) in the cat
29
On-centre cell excitatory inputs?
Centre
30
Off-centre cell excitatory inputs?
in the surround
31
What are the three principles of neural coding?
- Neurons are tuned to particular features
- The principle of adaptive independence
- The principle of univariance
32
Neurons are tuned to particular features?
E.g. one cell will be tuned to 45 degree angle bar, another cell to 15 degree angle bar
33
The principle of adaptive independence
- We saw how this happens with light, can adapt to other things too.
e. g. Afterimages - seeing an image on a blank background after its gone
e. g. Aftereffects - adaption to a stimulus one way alters our perception of the next stimulus
34
The principle of univariance
- A neuron's firing rate is ambiguous - it only varies along one dimension.
- Can't tell from the firing of one cell, but can combine the responses of multiple cells (pattern coding)
35
What is orientation coding?
In order to detect the orientation of a bar, information from multiple cells must be coded (pattern coding).
36
Evidence for orientation coding?
The tilt after-effect: adapt to patch of lines 10-15 degrees -> shown vertical lines and they appear tilting in the opposite direction
37
What units does orientation coding work in? Evidence?
10-15 degrees orientation, because this is when the sharpest peaks are seen in the tilt after-effect
- Hubel and Wiseel (1977) found that there are 'hypercolumns' in the cortex in which cells in neighboring regions tend to code orientations that were 10-15 degrees apart.
- Therefore, there may be inhibitory interactions between neighbouring regions
38
What is a heuristic?
Applying logical and rational rules to make sense of ambigous stimuli
39
Problem with ganglion cells coding for edges?
Edges can have different scales - some sharp, some gradual
40
What is 'spatial frequency'
the number of Cycles Per Degree (CPD) of visual angle
41
What is the contrast sensitivity function (CSF)
When contrast sensitivity is mapped against spatial frequency.
Shows our window of visability
42
What spatial frequencies are we more sensitive to?
who discovered this?
further evidence?
Mid-range frequencies
Campbell & Robson (1968), by adapting observers to dif spatial frequencies
Concluded that there are a limited number of spatial frequency ‘channels’ in the visual system that are tuned to a small range of frequencies.
Evidence that the visual cortical neurons in cats and monkeys are 'tuned to' spatial frequencies
43
What is Fourier analysis, and why is it relevant to SF?
demonstrated that any repeating pattern can be constructed from a series of sinusoidal wave functions of different frequencies
It is possible that early vision may employ this to decompose each scene into constituent sinusoidal wave functions
44
How do we perceive depth?
Using depth cues:
○ Binocular disparity
○ Motion Parallax
Static, pictorial cues
45
Binocular disparity
When an object at a middle distance is focused on the fovea in each eye, the images objects that are at the same distance away all appear at corresponding points in the two retinas
46
And why are objects closer + further away from the focused object blurry?
They do not appear at corresponding points on the two retinae
Objects further away = displaced leftward in the left eye relative to the right eye. (UNCROSSED DISPARITY)
Objects nearer = displaced rightward in the left eye relative to the right eye (CROSSED DISPARITY)
47
What is the horopter?
Horopter: the imaginary ellipse around the retina, where both eyes estimate the same distance for fixation
48
What is panum's area?
Panum's area: the location around the horopter when the image of each retina is fuses into one
Anywhere closer or further away, not in this area, and the eyes see diplopic (unfused) images
49
Motion Parallax
When moving in one direction, nearer objects (e.g. train tracks) move in the opposite direction very quickly, background objects (e.g. distant hills barely seem to move at all)
50
evidence for motion parallax
Rogers and Graham (1979)
manipulated a series of dots on a screen so that they imitated motion parallax cues. Participants, instead of seeing moving dots on a flat screen, perceived a set of stationary dots on a screen with depth.
51
Static, pictorial cues
* Height in a scene
* aerial perspective
* Shading
* Shadows
* Perspective
* Interposition
* relative size (‘texture’ cues),
* assumed size/familiar size.
52
What heuristics help us with some static depth cues?
(1) light comes from above
| (2) faces are convex
53
How do we perceive motion?
Object's image moves smoothly across the retina, detected by receptors.
54
Who first discovered motion receptors?
• Reichardt (1969) in the eyes of flies
• Reichardt motion detectors (OR a delay-and-compare detector)
○ use pairs of receptors which detect motion in a specific direction at a specific speed.
○ If a light input reaches receptor A first, there will be a delay before this initial EPSP signal is transmitted to receptor B. If this EPSP coincides with when the light input elicits a separate SPSP response in receptor B, the neuron will reach the threshold (summed excitation).
• NB: Can also have detectors where both receptors see a delay, and they work to compare their signals by subtracting one from the other to make is positive or negative
55
But what about motion detectors for other directions?
We have a collection of motion detectors for all different directions.
56
Evidence that can motion cues be adapted?
movement after-effect (i.e, ‘waterfall illusion’)
57
What is happening to cells in waterfall illusion
Sutherland (1961) accounted for this in terms of spatial pattern coding of motion, where cells with receptive fields at the same location code many different possible directions of motion.
When one direction of motion is then adapted, the cells coding that direction become less responsive (the cell’s firing rate drops below its resting level), such that when a static stimulus is subsequently viewed there is a net bias in motion- responsive cells in the opposite direction to that adapted and motion is seen in that direction
58
What happens to cells when we see a stimulus as static?
They are signalling all directions of motion equally
59
What do we do if motion is ambiguous?
(1) Inertia: motion is assumed to continue in the same direction unless there is evidence to the contrary
Demonstrated by Ramachandran & Anstis (1986) with the ambigous cross rotation illusion (i.e. wagon wheel effect)
(2) Rigidity: points moving relative to one another are often assumed to remain the same distance in space from one another, but to be moving in depth relative to the observer.
Demonstrated by Johannson (1950) with pairs of dots
60
evidence for the fact that after-effects are caused by cells in V1
1) cells in V1 are binocular - after-effects transfer by a factor of about 70%
2) exposure to high-contrast grating causes cells in V1 to fire less over time, whereas LGN and retina cells don't show adaptation
61
What is interocular transfer?
When a cell can be excited by information from either the left eye or the right eye, or indeed both eyes.
These cells are called binocular cells.
62
Where do different examples of adaption happen?
- Retina and LGN (monocular)
- V1 (largely binocular)
- Extrastriate cortex
63
what is macula luta
a yellowish region in the central retina, near the centre of which lies a pit (the fovea)
64
How does light -> retina?
Light then passes though the lens, which assists the cornea in producing a focused image on the retina.
65
what is lightness
brightness = light intensity
lightness = how reflective something is
66
emmert's law
perceived size on retina should be scaled up in proportion to its perceived distance
67
misapplied size constancy
corridor
titchener illusion
ponzo illusion
68
retinal input
illumination x lightness
therefore, input is ambiguous
69
perceptual constancy
having things stay the same
70
how to work out lightness?
lateral antagonism - an object will probs have the same illumination as its surround
71
support for lateral antagonism explaining lightness input?
Wallach (1948) - when asked to match the lightness of two circles, subjects set them so they had identical contrast ratios with their surround
72
examples of illusions which can't be explained by lightness constancy
White's illusion
Benary's cross
Adelson chequered shadow
73
evidence that our interpretation of the object affects our lightness interpretation
Gelb (1929) effect
-black surface appears white when it is brightly illuminated by a narrowly focused spotlight (and viewed in front of a dimly illuminated background) but not if a white surface is illuminated in front of it.
Gilchrist (1980) finding that depth perception can influence lightness perception
Dark room and poorly illuminated OR light room and well illuminated.
If appeared to be in dark room -> object judged as relatively light
If appeared to be in light room -> object judged as relatively dark
74
filling-in
single cell recordings found neurons that fire more when presented with a blindspot
BUT this is a process of normal vision too (e.g. modal completion of partially camoflaged objects)
75
where does vision go from the retinal ganglion cells
- lateral geniculate nucleus
| - primary visual cortex (V1)
76
Hubel and Weisel
places a recording electrode perpendicular to the cortical surface
"orientation columns" with preferred orientations 15 degrees apart
"hypercolumns" may be the basic unit of cortical processing
ALSO SAW EFFECT WITH PREFERRED SPATIAL FREQUENCIES
77
Blobs
Wong-Riley (1979)
stained cortex to reveal patterns of CYTOCHROME OXIDASE (blobs) within hypercolumns
cells within these blobs (bloc cells) primarily code colour info (not orientation/SF)
cells between these blobs primarily code high spatial frequencies and colour edges
78
simple cells vs complex cells
simple cells - response specific for size, orientation and position of a stimulus
complex cells - direction of motion (reichardt etc)
hypercomplex - end-stopped, respond to corners or line-ends
79
extrastriate visual areas
V2 - codes visual surfaces (e.g. Kanizsa type illusory figures)
V5 - high-level processing (patient with akinetopsia, motion blindness, after damage here) (TMS imaging(
V4/V8 - neurons in visual cortex here code responses more sophistically than V1, helps with colour constancy.
V8 - could be region for high colour coding, but celebral achromatopsia (cortical colour blindness) is the whole V4/V8 area - inferior occipitotemporal cortex. fMRI supports this.
80
to what extent can we process all the objects in the visual scene at once?
pop out study - efficient parallel search,
search at the same time
