M2 Lecture 8: Feb7 Flashcards
By combining information from several retinal ganglion cells, it is possible to detect what
the orientation of lines.
Simple cells preferentially respond to what
lines that have a certain orientation.
what is a Hypercolumn
A 1-mm block of striate cortex containing “all the machinery necessary to look after everything the visual cortex is responsible for, in a certain small part of the visual world” (Hubel, 1982).
Each hypercolumn contains cells responding to every possible orientation (0–180 degrees), with one set preferring input from the left eye and one set preferring input from the right eye.
what is Retinotopy
visual information coming from adjacent locations in the retina will project to adjacent locations in the primary visual cortex (V1).
is it possible to reconstruct the retinal image based on brain activity
theoretically– through Retinotopy
how did the experiment for mind reading/retinotopy go
- `Show hundreds of random images and shapes to a participant (10 x 10 patches).
- Use patterns of activity across thousands of voxels in V1 to predict the luminosity of smaller rectangles within the 10 x 10 patch.
- Use the predictive patterns to predict which shape the participant is seeing based solely on brain activity.
- Record activity in Higher Visual Cortex (HVC; lateral occipital complex (LOC), fusiform face are (FFA), and parahippocampal place area (PPA), and Lower Visual Cortex (LVC; V1 to V3) while participants are sleeping.
2) Wake them up in sleep stages 1 or 2, and ask them what they were dreaming about.
3) Use the patterns of activity in the ~20 sec before awekening to predict what a participant is dreaming of.
4) Use the predictive patterns to predict dream content solely based on brain activity.
is Decoding accuracy is better in higher visual cortex or lower
higher
what is Extrastriate cortex
Brain regions bordering primary visual cortex that contains other areas involved in visual processing
what is included in Extrastriate cortex
V2, V3, V4, etc
After extrastriate cortex, visual processing is split into what pathways
a dorsal “where” pathway and a ventral “what” pathway
“Where” pathway is concerned with what
the locations and shapes of objects but not their names or functions.
“What” pathway is concerned with what
the names and functions of objects regardless of location.
what is included in V1 Organisation
Parvocellular (ventral/“what”) and magnocellular (dorsal/“where”) pathways
what is Parvocellular
ventral/“what”
what is magnocellular
dorsal/“where”
Early areas (V1, V2, V3, V4 and MT) maintain what
retinotopic organization
Receptive field sizes increase from V1 to where
higher level areas.
what increases from V1 to higher level areas.
Receptive field sizes
Receptive field complexity
explain how Receptive field complexity increases from V1 to higher level areas
For instance, “boundary ownership.” Neurons in V1 would respond equally to the identical edge, but V2 neurons would respond more in “a” because the black edge is owned by the square.
Neurons in V4 respond to what
concave, convex, or straigth edges.
Neurons in V4 respond to concave, convex, or straigth edges
what are the conditions for ths
The contour has to have a precise relationship with the rest of the figure (e.g. convex countour pointing up from the figure).
Preffered location covers a large portion of what
the retinal image
aka certain neurons will be more responsive to certain edges
what is the INFEROTEMPORAL (IT) CORTEX
Part of the cerebral cortex in the lower portion of the temporal lobe, important for object recognition.
what is Visual agnosias
Failure to recognize objects in spite of the ability to see them.
what are the types of Visual agnosias
Apperceptive agnosia
Associative agnosia
Prosopagnosia
what is Apperceptive agnosia
inability to form a percept despite normal vision. The basic elements of the object are seen (lines, etc.) but cannot be integrated into a stable percept.
- lesion closer to V1 - cannot copy
what is Associative agnosia
There is a perceptual representation of the object, but the patient doesn’t know what the object is.
- lesion further away from V1 - can copy, but cannot identify - can recognize object if presented in another modality
what is Prosopagnosia
An inability to recognize the identity of faces.
what are the characteristics of Receptive field properties of IT neurons
Very large—some cover half the visual field
Don’t respond well to spots or lines
Do respond well to stimuli such as hands, faces, or objects
is Object recognition fast or slow in IT cortex
can be fast (~150 ms)
Not a lot of time for feedback from higher brain area
what is the Feed-forward process in the IT cortex
A process that carries out a computation (e.g., object recognition) one neural step after another, without the need for feedback from a later stage to an earlier stage
where are grandmother cells found
IT cortex
(IT cortex) The temporal lobe also comprises the hippocampus, a structure that is important for what
storing and retrieving memories.
Grandmother cells might therefore be important for what
our recognition of certain learned objects
Grandmother cells might therefore be important for our recognition of certain learned objects.
However, our experience of an object can’t only just depend on these types of cells, explain
For instance, when we look at an object (say a toaster), we know exactly where it is, and whether or not we are looking at an actual toaster or just the word “toaster
A truly invariant neuron wouldn’t give us information about these important details, which are essential in guiding action
what is The problem of object recognition
The pictures are just a bunch of pixels on a screen, but in each case you perceive a house.
How do you recognize the first and third images as depicting the same house, but from different viewpoints?
How does your visual system move from points of light, like pixels, to whole entities in the world, like houses?
what is Structuralism
Dots make lines -> lines make corners -> corners make shapes -> shapes are connected one to another and make objects.
How can we know for instance that house continues behind the snowman? How do we know the car is in front (not under) the house?
gestalt principles
Our perceptual system needs to make inferences based on what
rules
what is Gestalt
In German, “form” or “whole.”
what is Gestalt psychology:
“The whole is different than the sum of its parts.”
what is Gestalt grouping rules:
A set of rules that describe when 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 Occlusion:
The perception that a form (here a circle) is occluding another form can be understood using the principles of good continuation and closure.
what is Similarity:
Similar looking items tend to group (color, size, orientation, etc…)
But items have to be similar on only one dimension,
Not a complex conjunction of dimensions.
what is Proximity
Items that are near each other tend to group.
proximity can be overruled by what
grouping or connecting the dots.
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 if they are connected.
what is Texture segmentation
Carving an image into regions of common texture properties.
what does Texture grouping depend on
depends on the statistics of textures in one region versus another, i.e. the gray “chunks” in the middle don’t have, in average, the same size as in the surround.
