Lecture 31: Visual System II, Central Visual Pathways Flashcards
How can seeing be defined?
Using optical input to the two eyes to know what things are, where they are and to guide action based on this knowledge
What are the central projections of the optic nerve?
- Superior colliculus
- pretectal complex (midbrain)
- Accessory optic nuclei (midbrain)
- suprachiasmatic nucleus (hypothalamus)
- Lateral geniculate nucleus (Palmer does researchhere)
What is the function of Suprachiasmatic nucleus (SCN)?
Optic nerve input synchronizes circadian rhythms to the light dark cycle
1% RGCs project here
What is function of superior colliculus?
Generates orienting head and eye movements
Also gets extensive inputs from visual and other cortices
Mostly “M-cell” (movement) input from retina
What is the function of pretectal nuclei?
Efferents to Edinger-Westphal for parasympathetic fibers
-pretectum is part of tectum in midbrain
What is function of accessory optic nucleus?
Reflex following movement.
Inputs from other category
What is the Retino-geniculo-cortical (striate) pathway?
Photoreceptors LGN primary visual cortex (striate) in the occipital lobe
What is the Optic nerve
distal to optic chasm (most anterior)
What is the Optic tract
proximal to optic chasm (more posterior)
What are the hemifields that each eye have?
- Nasal hemifield
2. Temporal hemifield
What will pathology in optic tract involve?
Both eyes because optic tracts contains optic fibers from ipsilateral temporal hemifield and contralateral nasal hemifield
What will pathology in optic nerve involve?
Complete blindness in only one eye
Where is the right visual field represented in the cortex?
In the left (contralateral) cerebral hemisphere
What axons cross in the optic chiasm and what axons remain ipsilateral?
Nasal hemifield cells cross
Temporal hemifield cells remain ipsilateral
If you view something from the right side, where does the light hit the eye?
On the temporal hemifield of the left eye
And
On the nasal hemifield of the right eye
What is a homomynous defect?
When you have a defect on the same side of both eyes (nasal on one eye and temporal hemifield in the other)
What is a hemiopsia?
When you lose sight on one hemifield
Where do the two visual fields (R and L) overlap?
From -60 to 60 degrees
What is function of LGN?
Specific thalamic relay nucleus for the primary visual cortex
Main pathway for seeing
Inputs for M and P cells
How many layers are in the LGN? How are they organized?
6 layers total (lamina)
Organized according to the type of RGC afferents making synapse at each one
Where do M cells terminate in LGN? P cells? Contralatearl vs. ipsilateral eye?
1 and 2 (magnocellular layer)
3-6 (parvocellular layer)
Contralateral and ipsilateral layers are SEPARATE (1,4,6 and 2,3,5 respectively)
Where do the on/off center cells segregate?
In the parvocellular layers
If you see layers 1, 4 and 6 lesioned on the right side of LGN, what eye is lesioned?
The contralateral or the left one
How is the LGN organized?
Lateral geniculate nucleus (LGN) is organized in a retinotopic map
Translation: two eyes project into LGN in such a way that adjacent points on the retina are represented by adjacent points in LGN.
Much more space is given to the fovea and central visual field (presumably to focus more on acuity)
What is the role of the brainstem in waking vs. sleeping state?
When awake, brainstem sends signals to keep resting membrane potential
When sleeping, brainstem no longer sends signals and allow thalamicocortical cells (TC) to hyperpolarize
What is the striate cortex? Where is it located?
V1, primary visual cortex
It is the largest functional area of the cerebral cortex
-controls visuotopy
It is located on the upper and lower banks of the CALCARINE FISSURE
In the striate cortex, how are the upper and lower visual fields represented?
The upper visual fields are represented on the lower bank of the calcarine fissure
Lower visual fields are represented on the upper bank of the calcarine fissure
What does it mean to call thalamus a gate?
It means thalamus controls waking and sleeping state…so it dictates whether to allow external stimuli to activate your cortex or not
Where do M and P cells terminate in the striate cortex?
M and P cells terminate in layer IV (out of VI in cortex)
But they terminate in NON overlapping layers WITHIN layer IV
M cells send shit up to areas II and III
-then those neurons send shit to Areas 18 and MT
-also projects to V and VI
P cells terminate in IV C (beta)
-go up to layers 2 and 3
-go to layers V and VI (which then go to pons, LGN respectively)
-also go to areas 18 and V4
What is the significance of layer IVB of the striate?
- orientation selective
- direction selective
- stereoscopically selective
How are receptive fields represented in visual cortical cells (visual cortical cells)?
They are organized into columns and segregated from bright and dark
On-center and Off-center receptive fields in IVCalpha are segregated
(can be tilted 45 degrees or can be alternating columnar…off-center – on-center – off –center)
-different IVB cells are stimulated better by different orientations of stimuli
What types of modular organization is present in visual cortex?
i. orientation selectivity
ii. direction selectivity
iii. ocular dominance
iv. color
What is the ocular dominance (OD) modular organization
-OD bands in cortex project in alternating zones
-some lamina are represented by ipsilateral eye
-some laminae are represented by contralateral eye
-depending on where axons come from in the eye, there are also alternate inputs into the striate cortex
Significance: it means cells of thalamus or cortex were driven ONLY by cells coming from one eye
-each is ISOLATED in its representation…no overlap between the two
What is the color modular organization?
Different blobs in striate control for color (separated in spaced blobs)
Located in layers 2 and 3 of striate
The ones that stain positive cytochromoxidase as a blob, means that’s where the color is being processed
What is the relationship between blobs (color) and ocular dominance modular organization?
If you lift up layers 2 and 3, you will see the OD organization in layer 4
What is organization selectivity modular organization?
It is characterized by “pinwheels” (the colored pinwheels, remember?)
- orientation is around the clock (around the pinwheel)
- it means these are the organization of how all possible representation of visual field are reflected in the striate
When you take the OD bands, CO blobs and pinwheels are put together? Composite modular organization?
The color blobs are always sin the middle of the OD bands
- centers of the pinwheels are always on the CO blobs…so always in the center of the OD bands)
- this organization (color blobs in middle of OD bands in middle of pinwheels) is repeated again and again
What is the function of lateral connection in striate?
Allows the different modalities of the cortices (orientation, color, edges) to interact with one another to form tangible precepts of reality
What are the 3 functions of lateral connection in striate?
i. excitatory
Significance: interneurons can excite neurons whose receptive fields do NOT overlap at all with the receptive field of the cortical area that was excited
ii. like orientations
iii. between columns whose receptive fields are non-overlapping space
Example: allows brain to see a triangle in “kaniza triangle” because the cells that do see a contrast on the edges of triangle are able to relay that signal to make an illusory edge of a triangle
Production of a “precept” of an edge
Things to remember about Retino-GENICULO-strate pathway
-retinotopically organized
-simple map with central magnification (fovea)
M&P cells in separate layers
Ipsi and contralateral eyes in separate layers
Gating by extraretinal inputs
Thing sto remember about Retino-geniculo-STRIATE Pathway
-retinotopically organized
LGN axons terminate in layer IVC with M and P inputs and LE/RE inputs separate
Molecular: orientation, ocular dominance, color
Binocular cells (stereopsis) – Layer IVb
Diverse outputs: superior colliculous, pontine nuclei, LGN and other cortical areas
M and P largely separate all the way
Long range connections modify responses elicited from the receptive field
