adaptation

Question 1

what different attributes in the world do we adapt to

Answer 1

light level
colour balance
orientation of gratings
perceived eye gaze
body orientation
viewpoints
blur
motion
mean colour
colour constancy 
pin-cushion distortions
facial identity

Question 2

what different effects help us understand adaptation

Answer 2

mechanisms of sensitivity
adjustment to light level
types of neural coding
efficient allocation of neural resources

Question 3

what is the process of adaptation

Answer 3

the relationship between stimulus and response is not fixed, rather the response to a stimulus depends on the level of prior exposure to that stimulus

response often diminishes with extended exposure

encompasses a range of underlying that occur over many timescales in all perceptual systems

when timescales are long it is difficult to separate adaptation from neural learning and other types of plasticity

Question 4

what general principles of neural coding does adaptation reveal

Answer 4

the way in which a response to a stimulus changes with exposure/ levels of the stimulus helps us to predict how different schemes of neural coding (e.g rate/place) are implemented

Question 5

what is luminance

Answer 5

a measure of intensity that is weighted by the eyes’ sensitivity to different wavelengths (log cd m^2)
i.e the energy of different wavelengths weighted

Question 6

how do environmental light levels change

Answer 6

vary over a large range in photopic luminance -6 to 8 log cd m^2

scotopic - rod vision functions alone, range of c. 10^3 (-6 to -3)

mesopic - rod and cone vision function together, range of c. 10^3 (-3 to 1)

photopic - cone vision functions alone, range>10^6 (1 to 8)

stimulus changes by 10 log units

neurons have a limited range of responses (2 log units, factor of 100)

Question 7

how can we investigate light adaptation in rods

Answer 7

choose an area of the retina dense with rod cells
choose the stimulus wavelength that maximally exploits the difference in rod and cone spectral sensitivity

e.g green light flashing against red background (which suppresses L and M cones preventing them from being sensitive, revealing rod sensitivity to light)

the intensity of the flash is measured at the point where the flash is just visible

record increment threshold (the amount of extra light needed to just see the test light)

Question 8

how do we plot a single sensitivity curve

Answer 8

plot increment threshold as a function of the intensity of the background to produce a TvI curve

Question 9

what is the relationship between background intensity and incremental threshold

Answer 9

as the intensity of the background increases, the increment threshold increases

the amount of light needed to detect an increment divided by the background intensity is a constant

k= I/ΔI

Question 10

what is the gradient of a single sensitivity curve

Answer 10

if weber’s law holds,
rearranging to ΔI = KI and taking the logarithm of both sides
shows logΔI= log I + c = 1
such that the gradient = 1

Question 11

what is weber’s law

Answer 11

the change in a stimulus that will be just noticeable is a constant ratio of the original stimulus

holds for 4-5 log units but not for extremes of stimulation

Question 12

what is rod saturation a failure of adaptation

Answer 12

when the background light becomes to intense, the rods are no longer able to signal additional light - this is the point of rod saturation

this demonstrates a failure to manage to the range of external intensities and to maintain the neural system

Question 13

how is response saturation avoided

Answer 13

spatial and temporal vision changes as mean light level changes
the visual system uses this to deal with saturation
makes highly effective adjustments to the mean light level and to the relative activations of the cone classes

Question 14

what is spatial frequency

Answer 14

the variation in light intensity (number of cycles of variation per degree of visual angle)

Question 15

what is temporal frequency

Answer 15

variation in intensity over time (cycles of flicker in a second, Hz)

Question 16

what do we find from looking at full data sets

Answer 16

sensitivity is the reciprocal of threshold (power of -1)

Question 17

what is the contrast sensitivity function (CSF)

Answer 17

sensitivity to contrast at different frequencies

the ease with which people are able to detect objects of various sizes and perceive the structural detail, such as texture, of those objects.

Question 18

how can we measure CSF of visual system

Answer 18

using grating stimuli that vary in spatial frequency
adjust contrast until a threshold is found
change mean luminance to test the effect of change in mean level of sensitivity to contrast at different frequencies

Question 19

what is the troland

Answer 19

units of the amount of luminance that reaches the retina

Question 20

what is the troland candela per meter squared

Answer 20

measure of luminance where light energy is weighed by the sensitivity of the eye to different wavelengths

Question 21

what are the parameters of the CSF curves used

Answer 21

CSF measured at different light levels
measured in the troland candela per meter squared
modified to account for the size of the pupil

Question 22

what do spatial CSF show about adaptation

Answer 22

systematic change with light level
as we adapt to light with increasing light level, CSF becomes more sensitive to contrast

the shape of the CSF changes so that the particular spatial frequencies we are most sensitive to change with light adaptation/ mean light level

At high light levels, the function is bandpass (peaked shape), at low light levels it is low pass (most sensitive to lower spatial frequencies)

this change in shape indicates a shift from dominance of spaitally opponent processing to simple summation

visual system summing light over increasingly smaller areas

Question 23

what does the temporal CSF show about light adaptation

Answer 23

similar pattern whereby the system sums light over shorter and shorter periods

analogous to how changing the shutter speed of a camera manages exposure to prevent saturation

Question 24

how do we adapt to orientation

Answer 24

bias in perception after exposure to a tilted stimulus
vertical bars no longer appear straight
tilt after-effect: adaptation to the orientation of a grating
repulsion after effect - in opposite direction to what we are adapted to

Question 25

how does coding by place explain orientation adaptation

Answer 25

spatial frequency channels have a peak sensitivity to a particular SF
we have a bank of many overlapping spatially tuned SF
a grating with a specific SF will produce a characteristic distribution of activity across SF channels

when we have adapted to a SF, the sensitivity of these channels is reduced by the amount they have been activated i.e the inverse proportion to the stimulus

subsequent presentations of reference stimuli post adaptation shows reduced channel output from most adapted channels so perceived SF is shifted

Question 26

what is place coding

Answer 26

coding by the relative activation across the population of SF channels

Question 27

what is the distance paradox

Answer 27

there must be some difference between the adapting frequency and the test frequency to get an effect

when the effect is symmetrical there is no perceived bias in the spatial frequency that we see

this would be produced when we adapt to a low spatial frequency and then test with low spatial frequency

the effect of adaptation is most noticeable at SFs removed from the adaptation frequency

these effects are common in many perceptual attributes such as perceived eye gaze, body orientation, and viewpoint

Question 28

what is MONUC

Answer 28

common principle in perception whereby many dimensions in our perceptual experience are coded by multiple overlapping narrowly-tuned, univariant channels

Question 29

what implicit assumptions do we make about individual channels

Answer 29

narrowly-tuned compared to the range of stimuli in the world

follow the principle of univariance (changes in stimulus affect the magnitude of the response but not form of response)

Question 30

what are perceptual norms

Answer 30

some stimulus dimensions have special null points
these are norms which are not accounted for by MONUC
motion: a balance between left and right appears stationary

blur: a balance between blurred and too sharp appears focused
colour: a balance between yellow/blue and red/green appears achromatic or white

adaptation can change the norm (lens?)

Question 31

how do we adapt to blur (Webster, 2011)

Answer 31

pre-adaptation norm: participants set their perceived perfect focus

adapt to a stimulus which is either blurred or sharpened relative to their norm

post adaptation must readjust the stimulus to appear focussed

norm is pulled towards the adaptation stimulus

Question 32

how does webster (2011) explain perceptual norms with two broadly tuned channels

Answer 32

channel A and B with opposite sensitivity across the stimulus dimensions
cross at the point of the perceptual norms

sensitivity adjustments are inversely proportionate to the amount the two channels have been activated

this has a stronger effect of reducing sensitivity on one of the two channels depending on the position of the stimulus dimension

adaptation shifts the norm towards the adaptation level

Question 33

what is opponent coding

Answer 33

difference between two channels
neurons explicitly code in terms of the difference between these two channels

adaptation at pre-opponent site changes the balance
shifts the norm towards the adaptation level

adaptation at post-opponent site (contrast) changes sensitivity without shifting the norm

Question 34

what opponent-like processes are observed for encoding faces (MacLeod & Webster, 2011)

Answer 34

stimulus dimensions have a clear norm

implies that the visual system encodes relative responses

may be explicit opponent-like processes

however there are inconsistencies e.g contrast-adaptation for faces has little effect (what is the norm for faces? symmetry?)

general configure information seems to be adapted but not a low level attribute like SF channels

Question 35

what are the benefits of adaptation

Answer 35

the visual system is able to integrate light over increasingly small time scales and areas as light intensity increases

adaptation maintains our neurons in an appropriate operating range so they do not saturate from the variation in the environment

natural signals in the environment have peaked distributions of likely stimulus levels

the mean and variance of these distributions can vary widely

must be encoded by neurons with limited dynamic range

adaptation fits neurosn into the dynamic range available

able to adjust to the mean and contrast, matching the variation in the stimulus

enables us to separate colours out in the scene to better identify salient objects

prevents response saturation

maximises the neural resources that can respond to the signals that are most common

Question 36

summary

Answer 36

adaptation is a ubiquitous characteristic of sensory and perceptual systems

crucial in maintaining efficient coding of the rangs of environmental stimuli

light adaptation is not rescaling - to maintain sensitivity the visual systems changes the spatial areas and temporal durations over which it sums light

sensitivity changes that depend on the spatial and temporal frequencies in the stimulus

patterns of adaptation can provide information about the underlying encoding of a stimulus dimension

different models of encoding make different predictions about the effects of adaptation

A consequence of adaptation is that neural resources are directed to the most prevalent stimuli in the environment.