Task 4: On Columns & Pathways Flashcards
What is the optic nerve composed of?
the axons of retinal ganglion cells
Where do the axons of the retinal ganglion cells cross?
partly at the optic chiasm
What forms the optic tract?
optic nerve
Where is the optic tract found?
wrapped around the midbrain and axons then continue to the LGN in the thalamus where they synapse
Where do LGN axons go?
expand via white matter as optic radiations (which carry information) where they travel to V1
Where do axons in the optic tract terminate?
most terminate in the LGN and some in the superior colliculus
The left primary visual cortex gets input from which eyes and which visual field?
The right primary visual cortex gets input from which eyes and which visual field?
The left primary visual cortex gets input from both eyes but only the right visual field
The right primary visual cortex gets input from both eyes but only the left visual field
Define LGN
structure in thalamus, part of midbrain, receives input from retinal ganglion cells and has input and output connections to visual cortex
What does each cerebral hemisphere act as?
relay stations on the way from retina to cortex
What kind of receptive fields do LGN neurons have?
concentric receptive fields like retinal ganglion cells therefore respond to same patterns and provide input
LGN neurons respond to
a) both eyes
b) one eye
b
Why does the LGN have 6 layers?
the visual system splits input from image into different types of information
What are koniocellular cells/layers?
split the layers of the LGN between magno and parvo cellular layers
-> each koniocellular layer is specialized e.g. one is for relaying signals from the S-cones
Define magnocellular layer
- 2 bottom layers of LGN
- Cells are physically larger than those in parvocellular layers
- input from M retinal ganglion cells
- respond to large fast-moving objects
Define parvocellular layers
- top 4 layers of LGN
- cells are physically smaller than those in the top 2 layers
- receives input from P retinal ganglion cells
- responsible for processing details of stationary targets
Topographical mapping
the orderly mapping of the word in the LGN and visual cortex
-> provides a neural basis for knowing where things are in space
Give 2 alternative names of topographical mapping
- electronic map
2. retinotopic map
Left LGN receives projections from ___ side of retina in both eyes
Right LGN receives projections from ___ side of retina in both eyes
left, right
Define contralateral
layers 1, 4 and 6 of right LGN receive input from left eye
layers 1, 4 and 6 of left LGN receive input from right eye
Define ipsilateral
layers 2, 3 and 5 of right LGN get input from right eye
layers 2, 3 and 5 of left LGN get input from left eye
Define contralateral neglect
cannot see whats either on the left or right (draw numbers of a clock on half a clock)
There are more feedback connections from visual cortex to LGN than vice-versa
True or false?
true
Give 2 alternative names for V1
- striate cortex
2. primary visual cortex
Define V1
area of cerebral cortex that receives direct inputs from LGN and feedback from other brain areas
Explain topographical mapping in relation to the visual cortex
image in our right is mapped onto regions corresponding to layer 3 and 4 in striate cortex that tells our visual system that image must be in positions 3 and 4 of visual field
Explain magnification in relation to the visual cortex
information is scaled from different parts of visual field, thus, objects imaged near fovea are processed by neruons in large part of striate cortex and objects imaged in far right/left periphery are given tiny portion of it
Define cortical magnification and its consequence
amount of cortical area devoted to specific region in visual field
consequence - acuity declines in orderly fashion with eccentricity (distance from fovea)
Why is foveal representation in cortex so magnified?
Since high resolution requires great number of resources, in order to see the entire visual field with such high resolution we would need larger eyes and brain. Therefore, we evolved a visual system that provides high resolution in center and lower resolution in periphery
Define the receptive field of a neuron
the region in space in which the presence of a stimulus alters neuron’s firing rate
Describe the receptive field of striate cortex neruons
elongated therefore respond more vigorously to bars, lines, edges and gratings than to round spots of light
Discuss how selective responsiveness is a property of receptive fields
individual neurons don’t respond equivalently to any stripe in its receptive field
Define orientation tuning
tendency of neurons (complex cells) in striate cortex to respond optimally to certain orientations and less to others
-> more cells are responsive to horizontal and vertical orientations than to obliques= humans have lower visual acuity and contrast sensitivity for oblique targets than for horizontal and vertical ones
Explain how striate cortex neurons are “filters”
for a portion of image that excites the cell, each striate cortex neuron functions as a filter - electrical, or optic neruon that allows the passage of some range of parameters and blocks the passage of others
Discuss cortical cells and spatial frequencies
each cortical cell is tuned to a particular spatial frequency which corresponds to a particular line width
-> cortical cells respond to a smaller range of spatial frequencies than retinal ganglion cells
Define ocular dominance
property of receptive fields of striate cortex neurons by which they demonstrate a preference, responding more rapidly when stimulus is presented in one eye than when it is in other
Name the 2 types of cortical neurons and explain what is meant by end stopped or not
- simple
- complex
-> each one is end-stopped or not: process by which a cortical cell first increases its firing rate as bar length increases to fill up its receptive field and then decreases its firing rate as the bar is lengthened further, play a role in the ability to detect luminance boundaries and discontinuities
Define simple cortical cell
cortical neuron whose receptive field has clearly defined excitatory a inhibitory regions
What is meant by simple cells being phase-sensitive?
might respond only if stripe is presented in center of receptive field
Discuss simple cells and edge detectors
highly excited when there is light on one side of its receptive field and darkness on the other
Discuss stripe detectors and simple cells
responds best to line of light that has particular width surronded on both sides by darkness
Define complex cell
cortical neuron whose receptive field does not have clearly defined excitatory and inhibitory regions
What is meant by complex cells being phase-insensitive?
respond regardless of where the stripe is presented, as long as it is within cell’s receptive field
-> subunits of receptive fields give these cells spatial frequency and orientation tuning but complex pooling operation makes this cell insensitive to precise position of stimulus within its receptive field
Both simple and complex cells show ___ preference
ocular
Define column
neurons with similar receptive fields and orientation preferences arranged in columns that extend vertically through the cortex
How is the striate cortex organised?
in location and orientation columns
Define location column
perpendicular to cortex surface, all neurons within a location column have their receptive fields at the same location on retina
Define orientation column
each column contains cells that respond best to a particular orientation
Whats another name for end stopped cells?
hypercomplex cells
Define hypercolumn
block of striate cortex containing 1 location and 1 orientation column, each covering every possible orientation (0-180 degrees) with one set preferring from the left eye and one set preferring input from the right eye (ocular dominance)
Why don’t all hypercolumns see the world at the same level of detail?
cortical magnification
What is the hypothesized function of blobs in striate cortex?
they do something with colour and movement
Define ventral pathway i.e. what pathway and magnocellular system
pathway reaching temporal lobe (inferotemporal cortex), responsible for determining object’s identity
What is a result of lesions to the inferotemporal cortex?
disrupt circuit specialized for identifying objects (visual agnosia)
Describe the neurons of the ventral stream
they are very sensitive to form, pattern and colour - high resolution selectivity
Define dorsal pathway i.e. where/how/action pathway and parvocellular system
pathway leading from striate cortex to parietal lobe (posterior parietal region) which is responsible for determining object’s location, sensorimotor transformation for visually guided actions directed at objects
What is a result of lesions to the posterior parietal region?
interfere with neural mechanisms underlying spatial perception (optic agnosia)
Are the ventral and dorsal pathways entirely seperated or do they have connections?
there are connections between them
Do signals in both pathways
a) flow up
b) flow back
c) both
c
Why are the pathways also connected to the frontal cortex?
because sometimes our lower order thinking cannot identify
Define double dissociation
helps determine whether 2 functions operate independently from one another, involves 2 people:
1: damage to brain area causes function A to be absent while B is present
2: damage to another area of brain causes function B to be absent while A is present
Define single dissociation
allows one to infer that function X and Y are independent of each other in some way, involves 1 person
1: person has lesion to structure A disrupting function X but not Y
Define module
a structure specialized to process information about particular types of stimuli, areas for stimulus e.g. face, places, bodies go through ventral pathway
What area is responsible for detecting faces and where is it located?
fusiform face area, located in fusiform gyrus on underside of brain, directly below inferior temporal cortex
What area is responsible for places and where is it located?
parahippocampal place area, located in inferior temporal cortex
What area is responsible for body and where is it located?
extrastriate body area, located in inferior temporal cortex
What area is responsible for identifying contrasting object images with scrambled versions and where is it located?
lateral occipital complex, location from lateral to ventral cortex
What area is responsible for identifying words?
visual word form area
Define visual agnosia
unable to recognize or describe common objects, faces, pictures etc however can navigate through them everday
-> brain damage to occipitotemporal region (mediates object but not spatial vision)
Define optic ataxia
unable to reach accurately towards visual targets that they have no difficulty recognizing, cannot position hands when reaching toward objects oriented in different angles, cannot grasp
-> damage to posterior parietal region
Define Balint’s syndrome
disorders of spatial attention, gaze and visually guided reaching
-> damage to bilateral parietal region
