Visual System III Flashcards
Larger cell body means what?
Larger cell body, larger axon = faster conducting of action potentials (even faster if myelinated)
Which pathway (magno/parvocellular) is this correspondent to:
“Transient response to sustained illumination”
Magnocellular
this basically means that it responds to transient changes in environment eg when your life is at risk (therefore also needs to be fast)
How does light come onto the retina?
Upside down and backwards
How to left and right eye axons organize themselves through the optic chiasm and the optic tracts?
50% of the axons cross the midline of the optic chiasm.
The composition of info in the tract is completely diff than in the nerve (nerve has whole optic field of one, chiasm has two copies of the left optic field and two of the right visual field, and vice versa for the right optic tract)
What are 4 main targets of retinal ganglion cells?
- Lateral geniculate nucleus:
- Part of the thalamus, the biggest target of the retinal ganglion cells (if you cut this youre blind)
- Has fibers branching out to primary visual cortex (aka striate cortex or broadman area 17) - Part of the thalamus that regulates circadian rhythyms (dont need to know name)
- The pretectum, where the key visual input goes in to regulate pupil size in response to light. Lights off = pupils big, lights on = pupils small.
- The superior colliculus: controls eyes and head when you wanna look around at visual stimuli
What is a function of the pretectum
key visual input goes in to regulate pupil size in response to light.
Lights off = pupils big,
Lights on = pupils small
What is one function of the superior colliculus?
Controls eyes and head when you wanna look around at visual stimuli
What is the pupillary light reflex?
Shining bright light into the eye, which activates the retinal ganglion cells, including some group that goes to the pretectum, then it goes to Edinger-Westphal, etc, then it send a message to pupil to adjust its size.
If there is any pressure on the circuit, the pupils won’t respond normally
If doctor is shining the light on one eye, they are expecting BOTH pupils to change sizes, and if they don’t the doctor knows there is probably bleeding or something
What is the thalamus?
Main relay center for most info (visual, auditory, somatosensory, etc) headed to cerebral cortex
Thalamus is composed of many relay nuclei. INfomration is relayed through the thalamus to the cortex, not much processing integration.
Lateral geniculate nucleus (LGN) is the main relay nucleus for primary visual cortex
- Retinal ganglion cells project to LGN, and LGN project to primary visual cortex
What is the main relay nucleus for the primary visual cortex, and what is the pathway of projection?
Lateral geniculate nucleus (LGN) is the main relay nucleus for primary visual cortex
- Retinal ganglion cells project to LGN, and LGN projects to primary visual cortex
Describe the segregation of information through the layers of the LGN (a relay nucleus in the thalamus).
The segregation of input is as follows:
Layers 1, 4, 6 = contralateral eye
Layers 2, 3, 5 = ipsilateral eye
Layers 1,2 = magnocellular
Layers 3-6 = parvocellular
LGN neurons have center-surround receptive fields.
- Thalamus is a relay nucleus, not a processing hub. ie complicated visual processing does not occur here. LGN neurons here have central surround receptive fields just like the retinal ganglion cells, nothing changes
How does the LGN organize its input and output.
Input stays segregated through the layers of the LGN.
The LGN projects up to primary visual cortex, and the input from the two eyes is also kept separate here.
What is the macula?
- macula is the fovea basically
- it takes up half the visual cortex, and the other half of the visual field is everything else.
- Therefore half the processing is done for just the fovea, shows how much this system has evolved to prioritize the fovea
What is the sulcus in between the top and bottom of the primary visual cortex called? What is the significance of its size?
The calcarine sulcus (deep)
Amount of SA of this brain area is enormous, the calcarine sulcus is one of the deepest in the brain, so there is a lot of cortex in here
How are upper and lower visual fields mapped onto the cortex?
Upper and lower visual fields are mapped onto opposite sides of the cortex
Describe how the optic radiations (geniculostriate fibers) are arranged. Why are they arranged like this?
The fibers representing the superior retinal quadrants (inferior visual field) travel under the PARIETAL CORTEX
The fibers representing the inferior retinal quadrants (superior visual field) travel under the TEMPORAL CORTEX
They are travelling from the LGN to the visual cortex, but the lateral ventricles are in the way.
- Therefore the axons going to the upper bank vs lower bank take completely different pathways, upper through parietal cortex, lower through temporal cortex
- So if you have a big stroke that is just in parietal cortex could knock out lower visual field, and if its in just the lower temporal cortex it could knock out upper visual field (this is assuming it doesn’t affect the visual pathways already)
What would happen if you had a big stroke in just the parietal cortex (in terms of the visual field)?
What about if the stroke occured in the lower temporal cortex?
If you have a big stroke that is just in parietal cortex could knock out lower visual field,
If its in just the lower temporal cortex it could knock out upper visual field (this is assuming it doesn’t affect the visual pathways already)
What are the lateral ventricles?
Lateral ventricle converts blood to cerebrospinal fluid, all the neurons grow from the lateral ventricle as you develop
Describe the layers of the neocortex and their function.
Layer 4: main thalamic input to cerebral cortex
- where LGN projects for example
Layers 2/3: Pyramidal neurons that project to other regions of cerebral cortex (corticocortical)
Layer 6: Pyramidal neurons that project back to thalamic nucleus (corticothalamic) provide feedback control to thalamic relay.
- Project to LGN for example
Layer 5: Pyramidal neurons that project to subcortical structures (project out of the cortex to all subcortical structures)
- (corticospinal) to spinal cord
- (corticobulbar) to reticular formation
- (corticostriatal) to striatum (caudate/putamen) of basal ganglia
- (corticotectal) to superior colliculus
Describe how the size of the layers of neocortex changes depending on what kind of cortex it is.
Layer 5 is largest in the motor cortex because it has a big output function (projects from the cortex to all subcortical structures)
Layer 4 and 2/3 are largest for sensory cortex
Describe how the orientation of a line segments affects the cells in the cortex, and how this changes horizontally/vertically down columns of the cells.
All the neurons see the same point in the receptive field, but they do very diff things w the stimuli that are in this receptive field
All the cells preferred the crack in the same organization in the vertical electrode
But horizontally, the orientation of the line segment the cells preferred was changing
Describe the model for orientation tuning in V1
Receptive fields line up so that any time there’s a light spot in the area (any time any of 1,2, or 3 area activated) but optimally activated by all three at the same time
Describe the organization of the primary visual cortex.
All the neurons in a chunk have the same spatial receptive field, but along one dimension we have orientation columns where the neurons rely on the line segments, and some preference for left vs right eye
T or F: Input from the left and right eye is mixed in the primary visual cortex.
FALSE. Input from the left and right eye is NOT MIXED, kept separate into the primary visual cortex.
