2- Visual Form Processing Flashcards by Anya Laila

What are the main neural response properties V1 has?

-Small receptive fields (diameter 1 degree)

Orientation (will respond well to bars that are vertically oriented in a specific way), and Spatial Frequency Tuning (will respond when best matching the receptive field e.g width of that excitatory bar/ sinusoidal gradings)

-Contrast-dependent response (responding to difference between lightest and darkest parts of that space)

-Binocularity (input layers have occular dominance colums which respond to information either from the left or right eye, but in subsequent layers of V1 these combine and respond to stimulation from both eyes

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Which pathway includes visual form processing?

Ventral stream

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Repeated presentation of a preferred stimulus … neural responsivity?

reduces

repeatingly adapting a stimulus and making the eye respond: presented a moving stimulus, the differences in contrast from the stimulus and background of photo becomes less sensitive to the eye as eye adapts

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Contrast response functions:

Following adaptation, some V1 neurons show an overall reduction in firing rate. Why does repeatingly adapting a stimulus lead to a decreased neural response over time?

neural fatigue

neurons respond less over all different contrast levels

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Contrast response functions:

Following adaptation, some V1 neurons show an overall reduction in firing rate.
However some V1 neurons change differently.

Explain this change.

Some exhibit a lateral shift in their contrast sensitivity functioning along the axis.

shifting the response range towards the adapted contrast to adjust (contrast gain control)

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What is the process when V1 cells exhibit a lateral shift in their contrast sensitivity functioning due to adaption?

Contrast gain control

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The selectivity of contrast adaptation shows similar properties to the response profiles of individual neurons
(Ie how much a neuron adapts is proportional to how well it responds to the stimulus in the first place)
Give some examples:

orientation & spatial frequency
tuning
interocular transfer will be mimicked

Orientation selectivity:
neuron will emit a big change in response if you adapt to the preferred orientation. But if you adapt on an orthogonal rating (90* rotated receptive field) there is no change

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Contrast adaption in V1:
Selectivity
Changes in V1 cells responding to a stimulus depend on what?

Depends on the response of a given neuron to the adapting stimulus

e.g How much a neuron adapts is proportional to how well it responds to the stimulus in the first place
-higher response
-higher adaption to response

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Contrast adaption in V1:
Neurons tuned to vertical orientations will have a higher or lower change in response when there is a vertical stimulus?

higher

-neurons tuned to more rotated orientations will change their response less (selective reductionism)

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When adapting people to a higher contrast rating (to measure contrast sensitivity):
over the duration of the experiment you can change the contrast grating and find an individuals sensitivity threshold.
What is an individuals contrast sensitivity threshold?

The lowest contrast where they can just detect the stimulus

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What would researchers be measuring if they had an adaption stimulus that was flickering with vertical lines (black and white)?

Contrast adaptation

instead of using a static stimulus, this avoids negative afterimages (as the average luminance isn’t changing over time)

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When researchers measure contrast sensitivity:
Prolonged exposure to a given visual stimulus selectively alters contrast sensitivity

What is the effect of this on our ability to detect patterns similar/ not similar to the adapting stimulus?

ability to detect patterns similar is impaired

but no change in ability to detect different patterns

(mimicking the neural adaption to neural response process)

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Perceptual contrast adaptation effects:
Mirroring the response properties of individual neurons in V1, adaptation-induced changes in contrast sensitivity exhibit which main 4 traits?

(the properties of those perceptual effects in experiments match up with if those effects are being produced by adaptation in V1)

1- Location specificity (specific)
2- Orientation tuning
3- Spatial Frequency tuning
4- Partial Inter-Ocular transfer

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V1 properties match up depending on
Orientation tuning explain this

Now changes/ elevation in contrast threshold falls off as the difference between adapted and tested orientations is increased

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V1 properties match up depending on
Location specificity (specific) explain this

Adapting at one location doesn’t alter sensitivity to stimuli presented at a distant location

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V1 properties match up depending on
Spatial Frequency tuning explain this

Now changes/ elevation in contrast threshold falls off as the difference between adapted and tested spatial frequency is increased

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V1 properties match up depending on
Partial Inter-Ocular transfer explain this

Adapting to stimuli presented to one eye will affect contrast sensitivity measured in the other eye (albeit to a lesser extent)
partly transfers with sensitivity with both eyes due to binocular and monocular responses of neurons of V1

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Contrasts that are well above our thresholds are known as?

supra-threshold stimuli

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Adaption has an affect on our perception when the stimuli reaches the supra-threshold as it distorts the appearance of objects that are visible to us.

Give 2 examples of aftereffects that occur:

Tilt-after effect
Spatial frequency aftereffect

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What is the Tilt-after effect?

Adaptation to a particular orientation produces a repulsive bias in the perception of nearby orientations

e.g- P tested on vertical lines, adaption test is rotated, P will view original test stimuli as slightly rotated also, distorting our perception of the original straight lines

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What is the Spatial frequency aftereffect?

Adaptation to a given spatial frequency produces repulsive biases in the perception of nearby spatial frequencies

e.g- P tested on clear separate lines, adaption test shows blurred lines, P will view original test stimuli as slightly blurred, distorting our perception of the original straight lines

Perceptual consequences - The mechanisms of aftereffects process

Think about the orientation of stimulus is coded and represented in V1:

Present P with vertical perceptual stimulus (black and white vertical bars)

Cells in V1 B/M respond to that orientation and either change their response rate (vertically tuned completely B or partially M), or don’t change at all.

What type of response is this known as?

Population response (population coding model)
-notifies the brain that majority of vertical tuned cells in V1 are responding to stimulus, thus the image must have vertical lines

When P is presented with an adapting stimulus (slightly tilted), this produces
selective reduction in symmetry of neural responsivity
(e.g the ones tuned for vertical lines lower their response towards the adaptive stimuli, but the cells tuned not for vertical identification do not change as they are more further away)

Now when we go back to viewing the original vertical stimulus:
We have an asymmetrical reduction in response (image is rotated slightly in opposite direction counter-clockwise)
The cells tuned to the original orientation have changed

Overall shifting/ skewing the neural population response away from the adapted value

What is the main function of contrast adaption?

Less clear, theories suggest

-Neurons change their response to remain consistent with Contrast Gain Control

Function of contrast adaption:
Name 2 reasons why contrast gain control (adaptation) may be useful (adaptation shifts steepest part of contrast response function towards prevailing contrast level)?

1- should allow observers to better discriminate differences in contrast around this level when viewing stimulus

But, variability in research findings that there is selective improvement in contrast discrimination

2- acts to improve the efficiency of neural representations (saves energy by not allowing neurons to respond to stimuli that isn’t changing in environment)
this reduces the metabolic cost of continued responses to regular features of the environment

so, the neurons average their response activation over time, adaptation equalizes the response level across populations of neurons over time

As we move through visual pathways of the brain, what 2 changes are most apparent?

receptive fields sizes increase (expand on amount of incoming info they can process over a wider range) repetitive change in the selectivity of neurons (respond better to more complex features e.g edges, facial recognition)

Adaptation to subjective contours: What are subjective contours?

Lines or edges perceived in an image where there is no luminance or colour difference (also known as illusory contours) -neurons in V1 respond only to real contours -40% of neurons in V2 respond to subjective contours

True of false, neurons in V1 will respond to subjective contours?

False -40% of neurons in V2 respond to subjective contours and neurons in V1 respond only to real contours

Adaptation to subjective contours: Adaptation to subjective contours induces ... which closely resemble those produced obtained by luminance contours

tilt aftereffects

Shape is picked up by neurons in V...?

Neurons in V4

Is there an adaptation to shape or is it just multiple local tilt aftereffects? Yes, there is some evidence of distinctive tuning properties: Suzuki (2005) demonstrated aftereffects following adaptation to a variety of shape properties, such as convexity and aspect ratio

-some tolerance to changes in size/position -occur for very brief adaptation periods -broad orientation tuning (aftereffect drops off quite systematically as you change the orientation) these effects may be more tolerant to changes in orientation

Adaptation to Glass patterns containing concentric or radial structure (made up of dot stimuli which are presented in a particular orientation which follows the consensus of all the other dots) -results in the perception of structure in completely random test patterns Explain this:

studies show that observers will perceive the test 'randomly orientated dot' pattern in a structure of opposing ends to the adapted stimulus they were previously shown

Which neurons would respond to these glass patterns containing concentric or radial structure (made up of dot stimuli which are presented in a particular orientation which follows the consensus of all the other dots)?

V4 neurons (respond to perceived shape orientation)

Adaptation to faces: Adaptation to a distorted face horizontally compressed (squished inwards) makes the original face look distorted in the opposite direction which would be?

horizontally expanded (squished outwards)

Adaptation to faces: What can Face Morphing Technology be useful for indentifying?

Used to investigate a range of face aftereffects -take sets of photos from one individual to extract an average face (emotional aftereffect) -average from sets of photos from 2 genders to blend and make 3 faces of male, neutral, female adapting to that female face will make the neutral face look more male

Research has been done on if the adaptation is happening in face selective areas of the visual brain or not. What are the properties of face aftereffects? Face aftereffects depend on...

1- Size tolerance 2- Orientation tolerance 3- Object specificity

Face aftereffects depend on certain properties: Explain Size tolerance

Manipulating the relative size of adapting and test REDUCES face distortion aftereffects But does NOT abolish

Face aftereffects depend on certain properties: Explain Orientation tolerance

Inverting the test face reduces magnitude of face distortion aftereffects

Face aftereffects depend on certain properties: Explain Object specificity

Adaptation to distorted faces alters the appearance of other objects (e.g. clocks), though not to the same extent as other faces

Multidimensional face space: it has been proposed that coding for faces happens in a different way in contrast to how basic image properties are coded (e.g. orientation, spatial frequency). Faces are coded through...

Norm-based coding

Which process involves each individual face coded by how it deviates from a prototype or average face on multiple dimensions?

Norm-based coding -every other face is then coded based on how it deviates from that 'norm' face (prototype theory)

What can be produced by extrapolating the identity axis beyond the norm (having the opposite properties to a given face)?

anti-faces

what are Identity afteraffects?

When we update our schema prototype of that average face, constantly keeping track of faces we see and updating the prototype with new information -adaptation to a anti-face makes average face look like original face -our ability to recognise the original face is improved, but recognition of other identities impaired

Identity afteraffects improves our ability to ... but distorts our ability to ...?

recognise the original face but recognise other identities -adaptation changes the appearance of the adapted face, making it look less distinctive -adaptation to the neutral face does not alter the appearance of a distorted face

Contrast adaptation reduces the responsivity of neurons in V1, either by reducing overall ... rate or shifting the ...

firing, contrast response function (contrast gain control)

These adaptation-induced changes display similar tuning properties to the neurons themselves such as...

position specificity, orientation and sf tuning, partial interocular transfer

In human observers, adaptation selectively reduces ... sensitivity and induces repulsive ... in perceived orientation and spatial frequency.

contrast, biases

The properties of these perceptual effects mirror those seen at the neuronal level, and can be explained via ... models incorporating selective adaptation.

simple population coding

Tilt-aftereffects demonstrated with illusory contours suggest adaptation at a site beyond ...

Adaptation to simple shapes and global form patterns produce ... which are somewhat tolerant to changes in low-level stimulus attributes (e.g. position, size, spatial frequency).

aftereffects

Face aftereffects are more tolerant to low-level stimulus attributes and behave differently to ... (renormalisation rather than a repulsive aftereffect).

repulsive aftereffects