Extrastriate Processing Flashcards
Visual info that occurs in the retina, LGN, and striate cortex undergo further analysis and synthesis in visual areas outside the striate cortex referred to as
Extrastriate cortex
Extrastriate cortex
- 20 different distinct visual areas
- each area contains retinotopic map, a map of the visual field
- each area (specialized modules) plays a different role in processing visual information
Map of the visual field in the 20 visual areas of the extrastriate
Retinotopic map
Widespread distribution of information beyond striate cortex
Projections to various extrastriate cortical areas
- these in turn projecto to numerous higher level cortical centers
- synthesized info from the different senses, integrating it with memory
Feed forward
Feed back info in the extrastriate cortex
From higher cortical areas to lower areas including hire striate cortex
Via reciprocal connections
Central stream
What pathway
-ID objects
Dorsal stream
Where
Motion perception
Localization in visual space
Action organization
Dorsal stream components
Dorsal-dorsal stream
- processes action information
- damage two this area may result in optic ataxia
- inability to point to or reach for a visual target
Central-dorsal stream
-includes extrastriate area MT/V5
Cortical streams as extension of the retinogeniculate pathways
- the parvo pathway does have input to the ventral processing stream
- magno pathway inputs to the dorsal stream
- both retinogenicualte pathways may provide input to both cortical processing streams
- they communicate with each other
Higher visual areas and modules
Analyze specific attributes of the visual world
Area 4 (V4)
Abundance of cells withchromatic sensitivities
- makes this area well adapted to color perception
- color
Cells in the inferotemporal cortex (IT)
Respond to complex forms, including faces
-indicates a role in form perception
These are considered to be part of the ventral processing stream
V4 and IT
Cells in area MT/V5 are well suited for
Encoding motion
-dorsal processing stream
PET
Radioactive tracer used to observe changes in blood flow, indicative of increases cortical metabolism
MRI
Better resolution than PET
Reveals corticala activity by detecting levels of oxygenation
-BOLD;blood oxygen level dependent
Use of MRI and PET scan in brain
Used to monitor brain activity while a subject performs a task, such as looking at a moving object
Imaging studies reveal the task determines chip area of the brain
Is most active
If a human subject views an array of various colored, moving objects and is asked to attend to a green object
Areas in the ventral stream may show elevated activity
If the shape (or form) of the object becomes the focus of attention the ventral stream may again show activity . The subject no views the same array but asked to attend to the movement of the objects
Dorsal stream may manifest increased activity
Binding problem
-to perceive a red car moving down highway
-motio nand position info processed along he dorsal
-color and form information processed along the ventral stream
-information from these two streams is combined with memory
-information form the various cortical areas must be synthesized to result in an integrated perception
-
Binding problem coordinated by
The prefrontal cortex
-this area has long been though to play a role in cognition
Cells in _______ are capable of analyzing motion information
MT-V5
- the aperture problem
- this stimulus, which is perceived as a plaid moving in the direction indicated, is composed of two drifting gratings as illustrated
- whereas each of the two component drifting grating would appear to move in the direction indicated, the plaid (an integrated stimulus composed of two components) appears as a single object moving in an aintermediate direction
Selective neurons in extracellular recordings
Respond strongly to the movement of the individual grating components of the plaid
Respond weakly to the movement of the plaid itself
Certain cells in the areas MT/V5 respond best tp the
Movement of the plaid as a whole
-these neurons encode what we perceive rather than the individual grating components
Global motion perception in area MT/V5 can be demonstrated with
Dot kinematograms
-extracellular recordings show that certain neurons constituting this region manifest coherence thresholds very similar to those measured psychophysically in the same animal
Human PET and fMRI studies of random dot kinematograms and areas MT/V5
It is more active when a human subject views a moving object than when viewing a stationary object
Motion after effect
Motion illusion that occurs in the absence of motion
Waterfall illusion MAE
- induced by staring at downward rushing water for several minutes
- when gaze is changes from the waterfall to the surruonding landscape, the landscape appears to rise
Result from the adaptation of direction-specific motion detectors
MAEs
- is adapting stimulus reduces sensitivity to its direction of movement
- causes subsequently viewed stationary stimuli to appear to move in the opposite direction
Motion detectors mediating MAE are suspected to be located in the
Cortex
- first site of substantial interaction between the two eyes
- adaptation of one eye elicits an MAE in the other eye
Human viewing concentric rings moving outward and suddenly stop
- moving rings activate Mt/V5
- when stopped, individual Eilene experience an MAE
- stationary rings appear to move inward
- fMRI shows that MT/V5 is active even though rings are no longer moving
- provides additional evidence for MT/V5 role in motion perception
The receptive fields of cells in IT
Large
- provides the basis to integrate information over an extensive area and analyze complex patterns
- cells in IT respond best to comparatively sophisticated shapes
- compared to cells ins trade cortex, which respond to bars, edges, and gratings
Mikey cortex is called IT probably equivalent to the lateral occipital complex in humans
-responds well to
Objects, but not to scrambled objects
-LOC responds to all objects, not showing selectivity for a particular type of object
FFA (fusiform face area)
Responds well to faces but not other objects
PPA (parahippocampal place area)
Strongly activate by objects and places, but not by faces
0percieving senses
Prospagnosia
Inability to recognize faces
- FFA and V4 are close to each other
- may also manifest cerebral achromatopisa
- inability to distinguish hues
Top-down attention
Endogenous attention
- volitional, you do not need to attend to these words, and could choose to turn tour attention elsewhere
- choosing to study pages of a book with other stimuli around you
Bottom up (Exogenous) attention
Studying and a loud crashing sound causes you to look up
Attention as a spotlight
- Directed toward the object of interest
- when first go into coffee shop, its broad and then narrrwos as you look for seat
Most of the time, the spotlight of attention is directed at the same object the ______
Eyes are fixating
- an object that has captured ones attends ncan usually best be investigated using fovea vision
- the spotlight of attention does not need to coincide with fixation
- the ability to fixate one object but attend to another has been demonstrated both for top down and bottom up attention
Serial processing during visual attention
Looking at all of the tables in the coffee shop to find an empty one
-we direct our spotlight of attention on each object of interest to see if its available
Preattentive processing
If there is a color assigned to empty tables, you would be Abel to find an empty table much quickly
-number of unavailable tables and distractions do not matter in this case
Change blindness
When looking at two pictures that have very subtle differences simultaneously, it is hard to see the changes between the two.
-phenomenon in which we cannot readily see the differences between the two images
How to fix change blindness
Rapidly change between the two pictures
Red/green bar activity: monkey fixating on X, make him respond to red in peripheral
Neuron repsonsds vigorously
Monkeys responds to green
Response to red is diminished
Top down visual attention is where
Prefrontal cortex
Top down attention and neuronal activity
Cells in striate cortex apparently do not demonstrate top down attentional influences
- suggests there may be a filtering of visual information from striate cortex V4
- at least partially dependent on the stimulus to which the animal is attending
Lesions in striate cortex
Simple blind spots (simple scotoma)
Lesions in extrastriate areas
Visual agnosoia
- inability to recognize objects
- this condition can take many different forms depending on the area off the brain compromised
Object agnosia
Can’t recognize objects
Agnosia for drawings
Can’t recognize drawn objects
Prosopagnosia
Can’t recognize faces
Simultagnosia
Can’t perceive more than one object
Color agnosia
Can’t association of Colton’s with objects
Color anomia
Can’t name Colton
Cerebral chtomatopsia
Can’t distinguish hues
Visual spatial agnosia
Stereoscopic vision, topographical relations
Cerebral akinetopsia
Motion perception problems
Defect in the perception of motion
Akinetipisa
Damage to the dorsal processing stream
Akinetopsia
What is normal in someone with akinetopsia
VF, VA, and color vision
Cones in someone with achtomatopsia
Normal complement of cones, but is unable to distinguish hies secondary to an extrastriate lesion
Patient experience with cerebral achromatopsia
- experiences color vision prior to the lesion
- loss of color is quite stark
- visual world is seen as black and white
- this is unlike the cone-based color anomalies where patients have impaired color discrimination but claim to see color
Certain lesions can lead to visual neglec: synthesthesia
Patient with this condition mayshave only the left side of the face and ignore the right side
-lesions in the superior temporal lobe
Simultagnosia
- may not be able to perceive more than one object at a time
- may be present in Balint’s syndrome
- bilateral damage to the parietal lobes
Balint’s syndrome
Condition in which there is typically bilateral damage tot he parietal lobes
-simultagnosia
The presentation of a letter or number results in the perception of a color
Synesthesia
- the patient may report the perception of green whenever presented with the number 6 and red presented with 2.
- 5+1=green
Charles bonnet syndrome
Visual phantom
- 15% of visually impaired patients
- complex hallucinations that take a form of a person
- very vivid
- patients most likely to experience it hours to days following visual loss
- recognized by the patients by not being real
- this is how it differs from schizo or Dementia
- most patients not bothered by the hallucinations
Phantom vision is thought to result from the loss f
Normal affront input to the visual cortex
-may explain the relatively high incidence of CBS in patients with ARMD, wherein the major representation to striate cortex is destroyed
The content of the visual hallucinations usually do not disturb CBS patients, however
They may be concerned they have something more serious neuro going on
Grandmother cell?
- Not confined to higher cortical areas
- occurs in the lower centers including the striate cortex too
- the visualization of a remembered object results in a pattern of activity similar to that elicited by actually viewing object
- hippocampus encoding memories
Perceptual learning
Psychophysical studies reveal the visual performance of adults can be improved with practice
- stereoscopic
- orientation
- peripheral reading speed
- and vernier tasks
Learning of certain tasks appear to be
Selective
-suggest is neural learning occurs early in the visual system
Does A person need to be aware they are learning for vision to be trained?
No
Necker cube
Ambiguous or bistable figure
-looks like its going in or out of the page depending on your perspective
Cortex manifests a significant degree of
Specialization
Central processing stream areas,, including
V4 and IT
- object recogniztion
- what
Dorsal stream
MT/V5
- spatial
- where
Prefrontal cortex
Coordinate info from the ventral and dorsal
