L1 Contrast Flashcards
What are the 4 terms we use to represent information in the brain?
Pathways
Attributes
Maps
Cells
What do we mean by ‘processing as a computation?
The brain uses ‘algorithms’ to process information like a computer
Is the brain bi-lateral or uni-lateral?
(same on both sides or unique on both sides)
Bi-lateral
Roughly how long does processing information in the brain lag behind what is actually happening in the world?
80ms
What is the linking proposition?
The between state of brain and physiology.
Wiki: “Linking propositions are statements that relate perceptual states to physiological states, and as such are one of the fundamental building blocks of visual science.”
What is the lowest level of resolution for visual input?
Photo-receptors on the retina
What is the second level of representation of visual input?
Spatial representation
(used to make maps of space)
What is the third level of resolution for visual input
Specialisations: Attributes and Pathways
What is the fourth level of resolution of visual input
How it actually occurs
(e.g. computation analogy)
What is the fifth level of visual input
The linking proposition
To link what is happening in the brain to physiology
Are individual part of the brain specific for one function?
To some extent, however they multi task
Where is contrast processed in the brain?
Retina
LGN (Lateral Geniculate Nucleus)
Cortex
In psychology, what is luminance
A measure of energy emitted, or reflected by, a light source
How is luminance measured?
Candelas per square metre of surface
What is the ‘Michelson Contrast’?
What is the formula?
Measurement of the relative difference in luminance levels over a given area.
(maximum luminance - minimum luminance) / (the minimum luminance / the sum of the max and min luminance)
What do difference calculation of the ‘Michelson Contrast’ depend on?
The spatial structure of the scene
Which one of these is high contrast and which one is low contrast?
Why?
Black = High
Grey = Low
High luminance in black bars, grey is less contrast because it is similar.
What does the Pelli-Robson contrast sensitivity chart measure?
Quantifies the difference in peoples contrast capability.
Why is it hard for us to see things with little contrast?
Our brains are interested in changes in the environment (predators moving, different objects etc.)
We have limited processing capacity and an infinite amount of possible sensory information so it makes sense computationally that it wouldn’t take in information with little change.
What type of contrast do cells respond to?
Edge contrast
but not much to mean level of light (because of adaptation)
How does luminance and contrast levels give us information regarding forms and shapes in the environment?
By helping to outline shapes through differing luminance it helps identify form.
How does contrast relate to where we look?
When looking at an image, our fixation point focuses on the differences in contrast.
Image explanation: similar luminance but focusing on a wide range of contrast
Are humans equally sensitive to all contrasts?
No, we are sensitive to a certain range of contrast
What is sensitivity in perception?
The inverse of threshold
What is the resolution limit in regard to spatial frequency?
With high spatial frequency where we can no longer detect contrast.
How is contrast coded?
a) Stimulus -> Receptor -> travels along the axon of the presynaptic neuron to the postsynaptic neuron
What are action potentials?
The messages that neurons send each other
What is this diagram showing us?
A diagram of an experimental setup for single-cell recording
Orientation sensitive single cell, only responds in a certain orientation
What is a receptive field?
The area of space where the sensory neuron will activate if a stimulus enters the area.
When light falls on the retina and gets coded what kind of responses can this lead to?
Leads to excitation or inhibition of neuronal responses.
Describe what a linear receptive field is and how it helps us measure cell activity.
You have a linear receptive field in certain areas of the cortex that apply a set of weights to image processing.
The weighted sum of the output is linear, this can be used to predict cellular activity from a specific image.
What is the grey circle problem with the linear response?
What does it suggest?
Two grey circles look different, one on the left looks darker than one on the right due to contrast.
It suggests that context in the spatial structure is important.
We don’t see things as they actually are (i.e. grey being the same) but instead we take in the spatial contrast.
What mechanisms help explain these luminance interactions (from the grey circle problem)?
Lateral inhibition
How does lateral inhibition occur?
As a function of the receptive fields properties of retinal ganglion cells
What type of receptor field does the retinal ganglia have?
ON centre - OFF surround
What does ON center - OFF surround mean?
If you put a stimulus in the centre you get an increase in response. If you put a stimulus on the periphery (- area) you get a decrease in neuronal response.
Why are they called linear receptive fields?
Because you are summing the inputs (sum of ON centre - OFF surround inputs)
Do you only see ON centre - OFF surround receptor cells in vision?
No, also in touch (and other sensory modalities)
Which one of these has a larger response if they are ON-centre OFF-surround cells?
Why?
What explains the Herman Grid illusion?
ON centre - OFF surround
As you move your eyes across you are moving your ON centre - OFF surround receptive fields which creates the grey blackouts on the grid.
Where are linear receptive fields reported?
Retina, lateral geniculate nucleus (LGN) and V1
Are all receptive fields linear?
No, there are multiple ways that receptive fields are coded particularly as we move up the cortical structure and depending on the spatial structure.
What is the early levels of processing in the LGN characterised by
Contrast control mechanisms
What is cortical processing characterised by?
Nonlinear processing
