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Visual: Adaption > 1-Light and Dark Adaptation > Flashcards

1-Light and Dark Adaptation Flashcards

1
Q

Which process describes the visual system alters its operating properties in response to recent changes in the environment?

A

The process of adaption

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2
Q

Adaption is the widespread…

A

widespread (or even ubiquitous) property of neural sensory systems.

It can influence individual cells in the visual brain to shaping our perception of the world

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3
Q

What does it mean when adaption is passive?

A

Happens unconsciously, continuously throughout the environment.

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3
Q

Most of the things we look at are just reflecting light from another source.

The luminance of a retinal image typically depends on which two factors?

A
  1. Surface illumination
  2. Relative surface reflectance (albedo)
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4
Q

What is albedo also known as?

A

Relative surface reflectance

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5
Q

What part of the eye, takes light from the back of our eyes and signals the characteristics of the light throughout the rest of the brain?

A

The retina

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5
Q

Surface illumination is involved when judging the luminance of a retinal image.

What is Surface illumination?

A

the amount of light falling onto an object

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6
Q

Relative surface reflectance is involved when judging the luminance of a retinal image.

What is Relative surface reflectance?

A

the proportion of light that is reflected back from the surface

(how much light is absorbed by the object and how much is reflected onto our eyes)

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7
Q

Which part of the eye is tasked with converting this image (of Surface illumination and Relative surface reflectance) into a useable neural signal?

A

The retina
-mediates the different light levels in order to help us read text and see objects

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8
Q

The surface of illumination can change in different environments.

Which one represents indoor lighting?

A

Fluorescent tube (1-100)

-lumination is much higher outside in higher light conditions (sunlight)

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9
Q

What range of luminance is 100-10000 cd/m2?

A

full sunlight, high light conditions

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9
Q

What luminance range is 0.001-0.00001 cd/m2?

A

Moonless night

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10
Q

The luminance of a piece of white paper is 1,000,000,000 times higher in outdoor sunlight than on a moonless night
What is this in magnitude?

A

109 = 9 orders of magnitude

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11
Q

The output of the retina has a limited response range.
The RGC’s signal the input depending on how much they spike.
How much is this aprox per second?

A

varies approximately 1-300 spikes per second
(cannot have numbers in-between)

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12
Q

When changing locations from inside and outside, what must the retina accommodate for and maintain?

A
  1. Accommodate large changes in mean luminance (109 range)
  2. Maintain sensitivity to differences in luminance within a scene (our ability to see and discriminate details in an image)
    (typically less than 103 range)
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13
Q

State the main ways the retina accommodates large changes in mean luminance (109 range) while still maintaining sensitivity to differences in luminance within a scene:

A
  1. Pupil Dilation and constriction
    (to regulate how much light enters the eye)
  2. Duplex Function
  3. Dark adaptation
  4. Light adaptation
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14
Q

What is the absence of eye movements, objects in the periphery appear to fade and can disappear altogether?

A

Troxler fading

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15
Q

What is the result when steady viewing of a visual pattern differentially adapts portions of the retina?

A

Negative afterimages

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15
Q

What reveal opponent coding in chromatic pathways?

A

Coloured afterimages

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16
Q

Coloured afterimages are revealed in which type of pathways?

A

chromatic pathways

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17
Q

What is the form of gaining control which also dictates that the visual system maintains a constant sensitivity to differences in luminance across a wide range of background luminance levels?

A

Weber’s law

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18
Q

What is the photopic range/ adaptation shifts the entire response function of photoreceptors, resulting in responses that represent contrast rather than absolute intensity?

A

Lightness constancy

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19
Q

A main way the retina accommodates large changes in mean luminance (109 range) while still maintaining sensitivity to differences in luminance within a scene is by Pupil changes.

Explain this:

A

The overall amount of light entering the eye is regulated by controlling the size of the circular pupil.

Pupillary light reflex

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20
Q

What is the term used to describe how much light comes in to the pupil?

A

Pupillary light reflex

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21
Finish the sentence: In very bright conditions (high intensities) light causes the pupil to become ... (allowing ... light in)
smaller, less
22
In low intensity light conditions, light causes the pupil to become ... (allowing ... light in).
larger, more
23
The overall amount of light entering the eye is regulated by controlling the size of the circular pupil. However, in humans this accounts for only a 10-fold to 20-fold change in the intensity of light on the retina. What does this mean?
10 to the power of 9 = the range over which light can vary. Our eyes can only restrict this to about 10 to the power of 1(still 8 orders of magnitude to account for)
24
Vertical slit pupils found in other species are slightly more effective, e.g. 135-fold change in which animal)?
the cat -the can change the area of their pupil at larger ranges
25
What is the operating range of the retina which is increased by having distinct receptor systems which are specialised for different lighting conditions.
Duplex Function
26
How is the operating range of the retina is increased?
By having distinct receptor systems which are specialised for different lighting conditions
27
What type of vision refers to lower light conditions?
Scotopic vision
28
What type of vision refers to higher light conditions?
Photopic vision
29
Name 2 properties of Scotopic vision:
1- operates under low light conditions 2- driven exclusively by rods (no colour)
30
Name 2 properties of Photopic vision:
1-operates under well lit conditions 2-mediated by cones (detects colour)
31
What is the term used to describe the luminance range which overlaps between scotopic and photopic vision?
Mesopic vision
32
Duplex function: What is highly light sensitive (can respond to a single photon of light)?
Rods
33
Duplex function: What has high convergence (average of 120 ... to one retinal ganglion cell)? This means how they relay information in the brain
Rods
34
Duplex function: What saturates under daylight conditions?
Rods
35
Duplex function: What has low convergence (average of 6 ... to one retinal ganglion cell, but one-to-one relationship in fovea)?
Cones
35
Duplex function: What are less light sensitive (require 100s of photons to respond)?
Cones
36
Duplex function: What continues to respond in high light conditions?
Cones
37
Duplex function: Cones have a 1-1 mapping, why is this useful?
important for high spatial acuity (the ability to detect the shape of a test object) we know exactly where the light has come from
38
Rods have higher sensitivity to light, why is his useful?
To monitor a large amount of space However, spatial resolution is lost (hard for RGC to tell where in the branch the light has come from when it fires)
39
Cones have low sensitivity to light, why is this useful?
meaning they can keep working well in high light conditions
40
which process helps maintain visual sensitivity across a wide range of luminance levels?
Light and dark adaptation
41
Which process involves Retinal neurons, including bipolar and ganglion cells, adjusting their response properties based on the mean luminance.
Neural adaptation -involves changes in synaptic gain and helps the retina maintain sensitivity to contrast within a scene
41
Which process involves the retina adjusting to changes in the variability of light intensity within a scene?
Contrast adaptation -helps in distinguishing objects and details even when the overall luminance changes
42
Dark adaptation: In viewing conditions for dark adaptation experiments, why does the researcher not point a faint test light (just until the individual can detect it) directly into the eye point subjects are fixating on?
To stimulate part of the retina that has both cones and rods - the middle of the eye (fovea) has a high concentration of cones so to point it directly would not capture the rods aswell (which are key in dark adaptation) - also have to make sure that we do not aim the light into the blind spot (optic nerve leaves the back of the eye/ no photoreceptors) - so we move to the periphery a little
42
Changing rod and cone sensitivity (when adapting to dark light conditions: Via Bleaching/regeneration of photopigment
The concentration of the active pigment within the photoreceptors (used to detect light as it enters the eye) -helps convert light energy into a neural response
43
What is the name of the rod photopigment used in dark adaptation?
Rhodopsin
44
What 2 parts are Rhodopsin molecules made up of?
1-Opsin 2-Retinal
44
In everyday life: You enter a cinema room, it is very dark and your eyes (rods) have to adjust to see things more clearly until your sensitivity improves. How can we measure this in experimental conditions?
1-expose subject to a uniform adapting light source Once sat in a dark room: 2-flash a very dim light with increasing intensity until the subject can just detect it (to measure their sensitivity) = to find a threshold intensity Repeat process at regular time intervals (minutes) to see how the threshold changes 3-plot the subject’s light detection threshold as a function of time in the dark 4-map out this dark adaptation curve
44
Bleaching/regeneration of photopigment: Explain the Rhodopsin cycle
1- Rhodopsin molecules are made up of a light sensitive protein (opsin) 2- Also made up of a chromophore (retinal) 3- When light hits Rhodopsin molecule, it triggers a change in the form of the retina (changes shape) 4- Known as glitching (2 parts break away from one another) 5- triggers other changes, leading to hyperpolarisation of the photoreceptor leading to firing ATP of the retinal ganglion cells 6- Retinal and Opsin are reassembled to form Rhodopsin Recovery of rod photopigment (rhodopsin) is slower than the 3 types of cone photopigments
45
What is the light sensitive protein which forms Rhodopsin?
Opsin
46
What is the chromophore (a type of vitamin A) which forms Rhodopsin?
Retinal
47
Dark adaptation: Which part in the process of Bleaching/regeneration of photopigment triggers light energy into a neural response?
Glitching
48
The rhodopsin cycle includes bleaching and regeneration which refers to what?
Separation and reforming molecule
49
Rhodopsin cycle: Bleaching/regeneration of photopigment After a photopigment molecule is bleached (used to detect a photon of light), the molecule must be ... before it can be used again.
regenerated It takes increasing amounts of time for this regeneration to happen (reason for why rods take 20-30 min to fully adapt to low light settings) As this regeneration occurs more frequently in the cycle, there is a greater amount of concentration of R molecules which activate This leads to you being more sensitive to detect light when it enters the eye Recovery of rod photopigment (rhodopsin) is slower than the 3 types of cone photopigments (s,m.l)
49
During dark adaptation, name the ways in which rod and cone sensitivity change?
1-Regeneration of photopigments 2-Negative feedback
50
Mechanisms within the retina change our photoreceptors' sensitivity What is Negative feedback?
Horizontal calls can mediate sensitivity in the photoreceptors when light levels change. This is feedback from horizontal cells onto the photoreceptors also changes their sensitivity If horizontal cells respond strongly they signal the photoreceptors to turn it down. If there is not much light the horizontal cells release the inhibition (sensitivity of receptors)
51
Light adaptation: Everyday life when you exit a cinema into normal outside light settings. What is happening in the eye?
The regeneration of all our photopigments Inhibition is turned down from the horizontal cells Then incoming daylight bleaches the plentiful rod and cone photopigments, leading to a massive firing in the retinal ganglion cells. At first everything seems glaringly bright and ‘washed out’, but things return to normal after a minute or so.
51
Describe the mechanisms of light adaptation:
1-The pupil constricts to reduce light input 2-shift to photopic system (cones) 3-reduction in photoreceptor sensitivity (e.g. rapid bleaching of photopigments)
52
Why do star-gazers use red torches?
To avoid bleaching rod system once dark adapted -allows them to see less brighter stars without undoing all the dark adaptation
53
Why do some fighter pilots wear red glasses before a night flight?
Rods are not sensitive to red light (L), making it possible to dark adapt prior to entering dark conditions -these glasses block out all the apart from the red (L) wavelength at night
54
Name the consequences of light/dark adaptation (other than we would not be able to see)?
1-Lightness constancy 2-Negative afterimages 3-Troxler fading
55
Which process changes the sensitivity of the retina allowing us to perceive the reflectance of an object’s surface as constant, regardless of changes in illumination (indoor or outdoor light levels)?
Lightness constancy (luminance contrast)
56
What is the difference between an object’s luminance and the background?
Luminance contrast
57
Lightness constancy (luminance contrast): Why does black paper look black indoors and outside?
Perception of lightness does not simply reflect the absolute luminance of an object. More important is luminance contrast which is the difference between an object’s luminance and the background
58
Lightness constancy: In the photopic range, the entire operating range of retinal ganglion cells shifts according to the mean light level. This is known as ... and results in responses that represent ... rather than absolute intensity.
gain control, contrast
59
Gain control (matching the output to the input especially when controlling for contrast levels of stimuli outside and indoor light) in the retina ensures that the visual system maintains what?
Sensitivity to contrast despite large changes in mean luminance
60
Gain control reflects which law?
Weber’s Law
61
Which type of images are a consequence of light/dark adaptation?
Negative afterimages -(inverted versions of stimuli) these are by-products of shifts in sensitivities Colour afterimages -chromatic afterimages (building up different sensitivities of chromatic pathways in the retina) e.g. adaptation to red causes a reduction in the sensitivity of long wavelength cones, creating an imbalance in the inputs to red/green opponent retinal ganglion cells
62
A colour afterimage showing: Red Blue Yellow Green will adapt to which colour due to a firing response from RGC's once the image has disappeared?
Red> green Blue> yellow Yellow> blue Green> red Some colour of the afterimage in the central portion of each shape changes depending on the orientation of the test shape. (eg. if the images are changing shape)
63
Which effect involves an optical illusion where a stationary visual stimulus fades and disappears when you fixate on a particular point for a prolonged period (e.g. stare at a fixed point in the center of an image, the surrounding colors or patterns may gradually disappear)?
Troxler effect -occurs mostly in peripheral vision, because receptive fields in the periphery are considerably larger than our fixational eye movements
64
The visual system must continue to respond in vastly different light levels, whilst maintaining sensitivity to differences in ... within the scene
luminance (i.e. contrast) -The visual system also needs to ensure that our perception of lightness remains constant across conditions
65
The mean luminance of natural scenes can vary by how many orders of magnitude (a factor of a billion)
9 orders of magnitude
66
Which type of adaptation is this?
Light
67
Which type of adaptation is this?
Dark

Visual: Adaption (15 decks)

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