The Visual Cortex Flashcards
What are two ways we can record electrical activity in the brain?
EEG
Intra/extracellular recordings
What is an EEG?
Amplifies evoked potentials produced by large numbers of neurons
What are intra/extracellular recordings?
Measure activity of a single neuron, using a microelectrode
What do both PET and fMRI show?
Metabolic activity
How does PET work?
Takes a radioactive form of glucose
X-rays cause positron to be emitted
Where do optic nerves from the retinal ganglion cells meet?
The optic chiasm
Where are images from the left visual field processed?
In the right hemisphere
Where are images from the right visual field processed?
In the left hemisphere
Where do optic tracts from the optic chiasm project to?
The lateral geniculate nucleus (LGN) in the thalamus (90%)
The superior colliculus (10%)
Where do visual signals from the LGN travel?
The occipital lobe, the visual receiving area
What is the visual receiving area also called?
The striate cortex due to its striped appearance
What cells are present in the LGN?
Parvocellular and magnocellular
What is the cell body size, number, conduction speed, response type, receptive field size, percepts, and colour of p-cells?
Cell body = small
Number = many
Conduction = slow
Response = sustained
Receptive field = small
Percepts = high spatial detail
Colour = colour
What is the cell body size, number, conduction speed, response type, receptive field size, percepts, and colour of m-cells?
Cell body = large
Number = few
Conduction = rapid
Response - transient
Receptive field = large
Percepts = motion sensitive
Colour = black and white
What is the geniculostriate pathway?
P-cells and some m-cells go to the LGN
Then project to the primary (V1) and secondary (V2) cortex in the occipital lobe
What are the layers of the LGN?
6 layers, each with a retinotopic map
Magno -> 4Calpha, Parvo -> 4Cbeta
What is a retinotopic map?
Adjacent neurons correspond to spatially related points on the retina
What is the tectopulvinar pathway?
Remaining m-cells project to the superior colliculi of the tectum (part of brainstem, guides visual attention)
Then projects to thalamus: pulvinar and lateral posterior nuclei
Then to V2 and beyond
What does the tectopulvinar path control?
Eye movements, fixations, detection/orientation to visual stimuli, motion and location
What do layers 2 and 3 of the striate cortex contain?
Blobs = sensitive to wavelength but not orientation
Interblobs = area between blobs, sensitive to orientation but not wavelength (parvo only)
What did Hubel and Wiesel do?
Found cells in V1 that responded well to certain stimuli
What are simple cells?
Layer 4
Respond to a bar or line in a particular location on the retina in a specific location
What are complex cells?
Layers 2/3
Responds to a line or bar in a particular location on the retina that has a specific orientation and is moving in a certain direction
What are hypercomplex (end-stopped) cells?
Beyond V1
Responds to a bar, corner, or angle having a certain length or width in a particular location on the retina that has a specific orientation and is moving in a certain direction
What is a location column?
Cells respond to stimuli from the same retinal location
What is an ocular dominance column?
Cells respond to stimuli presented to one eye only
What is an orientation column?
Cells respond to line stimuli having the same orientation
-orientation columns differ in orientation selectivity by 10°)
What is a hypercolumn?
A region containing a single location column, which contains left and right ocular dominance columns, which contain the set of orientation columns from 0° to 180°
What is V3 sensitive to?
Moving edges of a certain orientation?
What is V3 believed to handle?
Perception of forms and local motion
Where does V3 project?
To the temporal lobe
What do cells in V4 respond to?
Perceived colour of a surface but not the wavelength
Where does V4 project?
To the temporal lobe
What is cerebral achromatopsia?
Unable to see colour in the right half of the visual field (V4)
What is the inferior temporal cortex involved in?
Identifying stimuli
What did Gross, Rocha-Miranda, and Bender discover?
Cells that responded best to a silhouette of a monkey’s paw when waved goodbye to an unresponsive cell
What do primary cells in the IT respond to?
To simple stimuli like dots, squares
What do elaborate cells in the IT respond to?
Complex shapes or shapes with colour or texture
What is prosopagnosia?
Inability to recognize faces due to IT damage in the fusiform gyrus
What is the medial temporal cortex sensitive to?
Overall motion and direction of an object but not colour
Where does the medial temporal cortex project to?
The parietal lobe
What is cerebral akinetopsia?
Unable to see objects in motion
What is the ventral (temporal) pathway?
Parvo -> V1 -> V2 -> V4 -> IT
What is the ventral (temporal) pathway concerned with?
Object recognition and identification
What system
What is the dorsal (parietal) pathway?
Magno -> V1 -> V2 -> V3 -> V4 -> MT (V5) -> parietal lobe
What is the dorsal (parietal) pathway concerned with?
Locating objects, motion, spatial relationships, depth
Where system
What were Pohl’s lesion studies?
Object discrimination = shown object, presented choice task, if target was selected then reward
Landmark discrimination = object presented, food hidden in well closest to object
Monkeys trained then task was reversed, after learning lesion on temporal or parietal lobe
What are the effects of a lesion on an object task?
Temporal = a lot of impairment
Parietal = minimal impairment
What pathway affected
What are the effects of a lesion on a landmark task?
Unoperated = no impairment
Temporal = minimal impairment
Parietal = a lot of impairment
Where pathway affected
How did Kohler’s neuroimaging provide evidence for what-where distinctions?
Objects were presented in two frames, either changed objects or changed locations
Object task = IT cortex region
Spatial task = parietal lobe
What is visual agnosia?
Failure or deficit in perceiving or recognizing visual objects
What is apperceptive agnosia (visual form agnosia?
Deficit in perception of whole objects
Can’t recognize, discriminate, or copy complex visual forms
Can grasp objects but can’t identify them
What is the neuropathy of apperceptive agnosia?
Caused by widespread damage to the posterior occipital complex due to CO or mercury poisoning
Failure of binding features together in early visual processing
What is associative agnosia (visual object agnosia)
Deficit in associating percept with meaning
Cannot draw from memory
Can copy pictures but can’t identify them
What is the neuropathy of associative agnosia?
Not consistent
Whole object is not identified due to damage in late visual processing
What is category-specific agnosia?
Inability to identify objects from specific categories
What is orientation agnosia?
Able to recognize drawings of objects rotated but impaired in recognizing the picture’s orientation
Often copies drawings in perpendicular orientation
What is simultanagnosia?
Inability to perceive more than one aspect of a visual stimulus and integrate details into coherent whole
What is dorsal simultanagnosia?
Can only perceive one of a number of overlapping objects
What is vental simultanagnosia?
Can perceive more than one object at a time but cannot identify more than one
What is pure alexia?
Cannot read words, must do a letter-by-letter reading
May be the same as ventral simultanagnosia
What is topographic agnosia?
Impaired recognition of scenes and landmarks
Gets lost easily
What is a double dissociation study?
In one case study, one ability is working while another is not and vice-versa in another case study
What is Balint’s syndrome?
Patient had bilateral damage to the superior posterior parietal lobes
Optic ataxia
Optic apraxia
Simultanagnosia
Can recognize objects but not tell where they are located
Intact what but damaged where system
What is optic ataxia?
Inability to reach for and grasp objects in field of view
What is optic apraxia?
Inability to guide eye movements or change visual fixations
What was wrong with Milner and Goodale’s patient DF?
Had CO poisoning, damage in occipitotemporal region
Visual form agnosia
Can perceive colour and texture
Can accurately reach for and grasp objects
What was DF tested on?
Orientation matching - hold the card at the same slant as slot, did poorly
Visuomotor posting - put the card into the slot, did well
Damaged what system, intact where
What was wrong with Weiskrantz’s patient DB?
Had right striate cortex removed
Stimuli can’t be detected in right visual field
Could identify light even if he said he couldn’t see it
Blindsight
What is blindsight associated with?
V1 damage
How can people with blindsight “see”?
Due to tectopulvinar pathway bypassing V1
Implies that some visual information is processed at an unconscious level
What is an orientation tuning curve?
Indicates relationship between orientation and firing, determined by measuring the responses of a simple cortical cell to bars with different orientations
What is selective adaptation?
Firing causes neurons to become fatigued which results in decreased firing rate and the neuron will fire less when presented with the stimuli immediately again
What is selective rearing?
If an animal is raised in an environment that contains only certain types of stimuli, then neurons that respond to those stimuli will be more prevalent
What is cortical magnification?
Apportioning a large area on the cortex to the small fovea
What is cortical magnification factor?
The size of the magnification
What are hypercolumns well suited for?
Processing information from a small area in the visual field
What is tiling?
Columns working together to cover the entire visual field
What areas are in the extrastriate column?
V2, V3, V4, V5
What happens as we move from V1 to higher-level extrastriate areas?
The receptive field sizes gradually increase
What is ablation?
Destruction or removal of tissue in the nervous system
What is the how (action) stream?
Determines how a person carries out an action
We aren’t aware in daily behaviour as the what and how streams work together seamlessly
What area of the brain has the largest receptive fields?
The inferotemporal cortex (IT)
Encompasses entire receptive field
