Section 3 Flashcards
TRUE/FALSE
Mapping of the visual field is preserved in the primary visual cortex
TRUE
What type of vision has the most cortical space?
Central vision
What is cortical magnification?
The idea that about 50% of V1 is dedicated to 1% of our visual field - the central region of our visual field
What is a consequence of cortical magnification?
visual crowding
what is visual crowding?
Because not as much cortical space is dedicated to the periphery, the reduced visual acuity produces a clutter effect where it becomes more difficult to distinguish one object from another
Which neurons perceive line orientation?
Neurons in V1
How are V1 cells spatially organized?
They organize their center-surround inputs into lines to display an elongated receptive field of a specific orientiation
Where are simple cells located?
Layer 4B of area V1
What layer of V1 do simple cells receive input from?
4C
What determines the firing rate of a simple cell in area V1?
How aligned the light is with the orientation of its receptive field
What is selective adaptation?
When you are exposed to a certain line orientation for an extended period of time, cells whose receptive field align with it will first fire rapidly but with time will adapt and fire less
What is population coding?
the idea that there is a large population of neurons in V1 that will determine your subjective perception of line orientation
Why is it that if you selectively adapt to a 45 degree line orientation you start seeing the line in the opposite direction?
The selective adaptation of neurons will depend on how close their preferred line orientation is to 45 degrees. Overall, this will affect the firing rates across the distribution but more so to those neurons closer to 45 degrees. Neurons beyond 90 degrees are much less affected, so their firing rate remains almost the same while neurons below that are damped, hence we start seeing the line in the opposite orientation
Besides line orientation, what else do neurons in V1 respond to?
directional motion, binocularity, and color sensitivity
Complex cells respond to what?
Motion
Which layers of area V1 have complex cells?
2,3,5,6
TRUE/FALSE
Complex cells do not have a preferential motion orientation
FALSE
How does the receptive field of complex cells compare to that of simple cells?
Complex cells have much larger receptive fields
What are end-stopped cells?
respond to moving light bars of a specific length
What happens if the length of the light bar is longer than the receptive field of the end-stopped cell?
The cell will not fire
What is the main function of end-stopped cells?
Size perception
What does it mean for a V1 neuron to be perfectly binocular?
Will show twice the activity when both eyes are exposed to light compared to only one eye being exposed
Type 1 neurons in V1 are driven exclusively by _________
the contralateral eye
Type 7 neurons in V1 are driven exclusively by _______
the ipsilateral eye
Types 2-6 neurons in V1 display _________
a gradient of ocular dominance
Type 4 neurons in V1 show _______
equal preference to both eyes
Which neuron type allows for depth perception?
Binocular neurons
How are cells typically organized in the visual cortex?
They are grouped together based on their degree of ocular dominance
If you insert an electrode ________ to the cortex you will record neurons of the same ocular dominance, but if you insert it ________ you will encounter neurons of different degrees of ocular dominance
perpendicular, oblique
How are cells organized within an ocular dominance column?
Cells selective to a particular orientation are organized into smaller columns, but each ocular dominance column has all the possible orientations represented
What are blobs and interblobs important for?
Blobs = color perception interblobs = orientation selectivity
What is a hypercolumn?
A 1 mm block of area V1 that contains all the machinery necessary for the perception of all the features of a stimulus in a specific area of the visual field
What is the extrastriate cortex?
brain regions bordering the PVC that contains other areas involved in visual processing (V2, V3, V4)
How is visual processing split after the extrastriate cortex?
into the “what” and “where” pathways
Describe the “Where” pathway
Concerned with the locations and shapes of objects, but not their names or functions
Describe the “What” pathway
Concerned with the names and functions of objects regardless of location
The dorsal “where” pathway is associated with information coming from ________ cells
magnocellular
The ventral “what” pathway is associated with information coming from _________ cells
Parvocellular
How does receptive field size and complexity change across the visual cortex?
From V1 to higher areas, receptive fields get larger and more complex
What is boundary ownership?
Being able to tell that an edge belongs to a specific object in the image
Neurons in which area of V1 can detect boundary ownership?
V2 neurons
What do neurons in V4 respond to?
Concave, convex, or straight edges
Are V4 neurons selective to the shape they will respond to?
Yes, their response will be along a gradient
What is area MT involved in?
Motion perception
What is the inferotemporal cortex important for?
Object recognition
What does the fusiform face area of the IT cortex respond to?
Faces
What does the parahippocampal place area of the IT cortex respond to?
places in the external world (houses, buildings)
What is visual agnosia?
Failure to recognize objects in spite of being able to see them
What is apperceptive agnosia?
inability to form a percept despite normal vision. The basic elements of the object is seen but cannot be integrated into a stable percept
What is associative agnosia?
there is a perceptual representation of the object, but the patient does not know what the object is
How is semantic knowledge of objects affected in associative agnosia?
It is preserved - if the patient is presented the object with a different modality they can recognize it
What is prosopagnosia?
The inability to recognize the identity of faces
Prosopagnosia only affects recognition of faces - what does this tell us about the brain?
The brain has specific modules that is dedicated to recognizing faces
What is the face inversion effect?
The problem humans have at recognizing faces when they have been inverted
List three properties of IT neurons
- receptive field is very large
- do not respond well to spots or lines
- do respond well to stimuli such as hands, faces, or objects
What is feed-forward processing?
A process that carries out a computation one neural step after another, without the need for feedback from a later stage to an earlier stage
What are grandmother cells?
Cells that are very specific to one type of object
Why does it not make sense for grandmother cells to be the sole contributor to object perception?
When we look at an object, we know many more details about that object than just what it is, so an invariant neuron should not be able to allow us to know all the specific details
What does object perception likely rely on?
Population coding
Can you perceive details of an object simply from feed-forward processes?
Not likely. You probably need feedback from higher to lower visual areas
What is object substitution masking? What does it tell us about object recognition?
Object substitution masking is when an object manages to preoccupy your memory such that it prevents you from being able to recognize other objects in the image. It tells us that to identify an image you need more than simple bottom up feedforward processing
What is structuralism?
The idea that objects are built up from smaller parts such as lines and then corners and then shapes
What are the Gestalt grouping rules?
They are a set of rules that describe what elements in an image will appear to group together
What is good continuation?
Two elements will tend to group together if they lie on the same contour
What is closure?
A closed contour is preferred to an open contour
What is occlusion?
The perception that a form is occluding another form, and can be understood using the principles of good continuation and closure
What is similarity?
similar-looking items tend to group (color, size, orientation, etc.)
What is a limitation of the similarity rule?
Objects can only be similar on one dimension
What is proximity?
Items that are near each other tend to group together
How can proximity be overruled?
By grouping or connecting the objects
What is common region?
Items will group if they appear to be part of the same larger region
What is connectedness?
Items will tend to group together if they are connected
What is texture segmentation?
Carving an image into regions of common texture properties
What are the five committee rules for perceptual commitees?
- put together what goes together
- avoid accidents
- honor physics
- reach a consensus (cannot hold more than one perception at the same time)
- separate what should be separated
What is figure-ground assignment?
The process of determining that some regions of an image belong to a foreground object (figure) and other regions are part of the background (ground)
What are the five Gestalt figure-ground assignment principles?
- surroundedness
- size
- symmetry
- parallelism
- extremal edges
What is the face superiority effect?
The concept where we tend to see faces in everything
TRUE/FALSE:
The visual system tends to be biased towards perceiving whole objects rather than parts
true
What is the global superiority effect?
the properties of the whole object take precedence over the properties of parts of an object
What is the naïve template theory?
The proposal that the visual system recognizes objects by matching the neural representation of the image with a second stored representation of the same “shape” in the brain
What is a criticism of the naïve template theory?
There are too many versions of an object that do not fit just one template
What is the structural description theory?
What we recognize is a particular organization of simpler features
What is the idea of objects being made up of simpler geons?
Each geon can come together in specific ways to make up an object, and small structural differences will not affect the object description. If, however, you change the relationship between two geons, you may change the perception of the whole object
What is viewpoint variance?
The issue where if you change the angle of an image, it will take longer to recognize it compared to the template
Color is a (primary/secondary) quality
secondary
What are the three steps to color perception?
- detection
- discrimination
- appearance
What is the process of detection?
the process by which our retina detects the wavelength of light which activates the appropriate cone receptors
What is the process of discrimination?
Being able to tell the difference between one wavelength from another
Why does a single photoreceptor show different responses to light of different wavelengths, but the same intensity?
Cones have an inverted U shape response function
What is the principle of univariance?
An infinite set of wavelength x intensity combinations can elicit the same response from a single type of photoreceptor
What is a consequence of univariance?
One type of photoreceptor cannot discriminate colors solely based on wavelength
Why are we able to tell the difference between light of different wavelengths using all three cone types?
For every wavelength, there is a particular combination of activities across the three types of cones that remains constant across different intensities
State which cone type each form of color blindness is associated with: deuteranope, protanope, and tritanope
Deuteranope: absence of M cones
protanope: absence of L-cones
Tritanope: absence of S-cones
What is the trichromacy theory?
The theory that the color of any light is defined in our visual system by the relationship between the three types of cone outputs
What are metamers?
Different mixtures of wavelengths that look identical
What is additive color mixing?
When two types of light are reflected from a surface to the eye, the perception of color results from the addition of the two colors
Based on the additive color mixing concept, what color does blue and yellow make?
White
What is subtractive color mixing?
If pigment A and B mix, some of the light shining on the surface will be subtracted by A and some by B - only the remainder contributes to the perception of color
What are non spectral hues?
Hues that can arise only from mixtures of wavelengths (e.g. purple)
What is the opponent color theory?
The theory that perception of color depends on the output of three mechanisms, each of them based on an opponency between two colors
Describe the hue cancellation experiments
You start with a color that is a mixture of two colors and then to eliminate one of those colors you shine a light whose color is opposite the color you want to eliminate on the color wheel
What is a unique hue?
A color that can be described with only a single term, and thus cannot be cancelled out via the hue cancellation experiment
Why is the absolute activity of a particular cone uninformative?
Cones are very sensitive to the intensity of light for a wide range of wavelengths
What is an afterimage?
A visual image seen after a stimulus has been removed
what causes afterimages?
Habituation of activated cones
What is a negative afterimage?
An afterimage whose polarity is the opposite of the original stimulus
What is color constancy?
The tendency of a surface to appear the same color under a fairly wide range of illuminants
What are the two main ways for achieving color constancy?
- discounting the illuminant
2. deduction
How can you discount the illuminant?
Determine the illuminant spectral pattern and substract it from the pattern of cone activity
