The Central VIsual System Flashcards
Retinofugal projection
neural pathway that leaves the eye
Pathway from retina to V1
Retina ->optic disk->forms optic nerve->optic chiasm where the fiber bundles cross->optic tract->Lateral geniculate nuclei LGN in dorsal thalamus (10% innervate midbrain, small amount to hypothalamus)->axons to primary visual cortex, striate cortex
optic chiasm where the fiber bundles cross is called
decussation
projection from the LGN to the cortex is called
the optic radiation
How do we know the optic radiation is the pathway of conscious visual perception?
because the lesions cause blindness in all or part visual field
binocular visual field
region of space viewed with both retinas
What are the analysis of stimulus attributes that are processed in parallel by specialized neurons in striate cortex?
binocularity, orientation selectivity, direction selectivity
retinotectal projection
Pathway from optic tract to superior colliculus
Lateral geniculate nucleus, structure and division of the layers? From what ganglion cell types does the input com from?
LGN neurons are separated in six layers. 1,2 ventral layers contain larger neurons and dorsal 3-6 contain smaller cells. Ventral to all layers are Kaniocellular LGN layers
Ventral cells: mangocellular LGN layers (from M-type ganglion cells)
Dorsal cells: parvocellular LGN layers (from P-type ganglion cells)
Kaniocellular LGN layers, ventral to all layers, receive input from nonM-nonP ganglion
Receptive fields of LGN cells
- magnocellular LGN neurons have relatively large center-surround receptive fields, like M-type
- parvocellular LGN cells, like P-type retinal ganglion cells, have relatively small center-surround receptive fields
- Receptive fields of cells in the koniocellular layers are center-surround and have either light/dark or color opponency.
Waht are the nonretinal inputs to the LGN?
- LGN receives inputs from other parts of the thalamus and the brain stem:
- 80% from primary visual cortex; corticofugal feedback pathway
- receives synaptic inputs from neurons in the brain stem whose activity is related to alertness and attentiveness
What are the nonretinal inputs to the LGN?
- LGN receives inputs from other parts of the thalamus and the brain stem:
- 80% from primary visual cortex; corticofugal feedback pathway
- receives synaptic inputs from neurons in the brain stem whose activity is related to alertness and attentiveness
Retinotopy
neighboring cells in the retina feed information to neighboring places in their target structures—in this case, the LGN and striate cortex
Three important thing to remember of retinotopy:
- the mapping of the visual field onto a retinotopically organized structure is often distorted because visual space is not sampled uniformly by the cells in the retina.
- discrete point of light can activate many cells in the retina, and often many more cells in the target structure, due to the overlap of receptive fields
- Mapping between retina and V1, perception is based on the brain’s interpretation of distributed patterns of activity, not literal snapshots of the world.
Lamination of the striate cortex, what are the sublayers?
- at least 9 distinct layers of neurons
- Three sublayers of IV, IVA, IVAB, IVC: IVCalfa, IVCbeta
- above the white matter
Types of cells in different layers of striate cortex and where do they form connections to?
- stellate cells: in IVC, local connections only within the cortex
- Pyramidal cells: layers III, IVB, V, and VI, and spiny stellate cells are found in layer IVC. only pyramidal cells send axons out of the striate cortex to form connections with other parts of the brain.
- inhibitory neurons, which lack spines, are sprinkled in all cortical layers as well. These neurons form only local connections.
Input of striate cortex, from where to which layer? Do the neurons receive information from one or two eyes?
IVC contains two overlapping retinotopic maps:
- Magnocellular LGN neurons project primarily to layer IVCalfa->layer IVB
- Parvocellular LGN neurons project to layer IVCbeta. -> layer III
- in IVB and III, for the first time, information from the left eye and right eye begins to mix
- Koniocellular LGN axons follow a different path, making synapses primarily in layers I and III.
- Whereas all layer IVC neurons receive input from only one eye, most neurons in layers II, III, V, and VI receive some amount of input from each eye
- input to a neuron can be dominated by left or right eye
ocular dominance columns
Alternating patches of left eye and right eye input reaching layer IV an the overall radial projections, neurons outside layer IV are organized into alternating bands (like zebra stripes) dominated by the left and right eye. Bands of cells extending through the thickenss of the striate cortex.
Output of striate cortex, pathways?
- the pyramidal cells send axons out of the striate cortex into the white matter
- > Layer II, III, and IVB pyramidal cells send their axons to other cortical areas
- > Layer V pyramidal cells send axons all the way down to the superior colliculus and pons.
- > Layer VI pyramidal cells give rise to the massive axonal projection back to the LGN
- IVC gives output to Cytchrome Oxidase blobs
->Pyramidal cell axons in all layers also branch and form local connections in the cortex.
What are Cytchrome Oxidase blobs?
- a series of pillars of cytochrome oxidase–rich neurons at regular intervals, running the full thickness of layers II and III and also in layers V and VI
- receive direct LGN input from the koniocellular layers, as well as parvocellular and magnocellular input from layer IVC of striate cortex
Receptive fields of IVC
- Small monocular center-surround receptive fields
- IVCalfa the neurons are insensitive to the wavelength of light
- IVCbeta the neurons exhibit center-surround color opponency.
- Outside layer IVC (and somewhat within), new receptive field characteristics, not observed in the retina or LGN, are present.
The features of binocularity of neurons in striate cortex? (what layers)
- most neurons in layers superficial to IVC are binocular, responding to light in either eye
- II and III neurons are more strongly driven by the eye represented in layer IVC (i.e., their response is dominated by one eye even though they are binocular).
- the neurons have binocular receptive fields, meaning that they actually have two receptive fields, one in the ipsilateral eye and one in the contralateral eye
How are the neurons in V1 orientation selective?
- respond best to an elongated (pidennetty, venytetty) bar of light moving across their receptive fields.
- > The greatest response is given to a bar with a particular orientation; bars perpendicular to the optimal orientation generally elicit much weaker responses
orientation column
the preferred orientation remains the same for all the selective neurons encountered from layer II down through layer VI
How are the neurons in V1 direction selective?
they respond when a bar of light at the optimal orientation moves perpendicular to the orientation in one direction but not in the opposite direction. (kohtisuoraan)
Simple cell
converging input from LGN cells with receptive fields that are aligned along one axis. E.g. The middle location gives an ON response and the two flanking locations give OFF responses.
Complex cell
-do not have distinct ON
and OFF regions
-constructed from the input of several like-oriented simple cells
Features of simple and complex cells
Both are typically binocular and sensitive to stimulus orientation. Different neurons show a range of sensitivities to color and direction of motion.
Blob cells features
- blobs receive input directly from the koniocellular layers of the LGN and magnocellular and parvocellular input via layer IVC.
-wavelength sensitive and monocular, and they lack orientation and direction selectivity.
-Receptivefields:
color-opponent center-surround organization, -red–green or blue–yellow color opponency in the center of their receptive field, with no surround regions at all
What is a hypothetical model of parallel pathways in primary visual cortex? What might these pathways be involved in?
Three pathways:
-the magnocellular pathway, the parvo-interblob pathway, and the blob pathway
Mangnocellular pathway= M-type ganglion cells in retina-> mangocellular in LGN -> Layer IVCalfa in V1-> Blob and layer IVB-> Extrastriate cortical areas
-might be involved in the analysis of object motion and the guidance of motor actions.
Blob pathway -> nonM-nonP ganglion cells in retina->Koniocellular in LGN->Blob in II and III->Extrastriate cortical ares
-might be involved in the analysis of object color
Parco-interblob-pathway->P-type ganglion cells in retina->Parvocellular in LGN->Layer IVCeta->Interblob or blob>Extrastriate cortical areas
- involved in the analysis of fine object shape .
NOTE! Research has shown that the three proposed pathways do not keep magnocellular, parvocellular, and koniocellular signals separate; instead they mix. Also, receptive field properties such as orientation and color tuning are found across the proposed pathways. Thus, it is not the case that magnocellular, parvo-interblob, and blob neurons are strictly segregated and have entirely unique receptive field properties.
A cortical module
Each cortical module contains ocular dominance columns, orientation columns, and cytochrome oxidase blobs to fully analyze a portion of the visual field. cortex is constructed from perhaps a thousand cortical modules
What is beyond the striate cortex?
-Beyond V1 lie another two dozen distinct extrastriate areas of cortex that have unique receptive field properties.
There appear to be two major pathways handling different types of visual information, one extending dorsally toward the parietal lobe that deals with motion and the other involved with color and form extending ventrally toward the temporal lobe.
The dorsal stream. Pathway, feature and task?
-Area MT receives retinotopically organized input:
Projections V1->V2->V3->MT
Or V1(IVB)->MT->MST
- large receptive fields, transient(ohimenevä) responses to light, and direction selectivity.
Beyond area MT, in the parietal lobe, are areas with additional types of specialized movement sensitivity:
- Medial superior temporal MST: cells selective for linear motion, radial motion, and circular motion. Visual system might use these for : navigation, directing eye movements, motion perception
- lesions in this: seeing world in snapshots
The ventral stream. Pathway, tasks, inputs from?
V1->V2->V3->V4 ->IT
or V1->V2->V4->IT
AREA V4
- receives input from the blob and interblob regions
- a V4 have larger receptive fields than cells in the striate cortex, and many of the cells are both orientation selective and color selective.
- important for both shape perception and color perception
AreaIT
- farthest extend of visual processing in the ventral stream
- A wide variety of colors and abstract shapes have been found to be good stimuli for cells in IT
- output from area IT is sent to temporal lobe structures involved in learning and memory; IT itself may be important for both visual perception and visual memory
achromatopsia
A rare clinical syndrome in humans known as achromatopsia is characterized by a partial or complete loss of color vision despite the presence of normal functional cones in the retina.
prosopagnosia
difficulty recognizing faces even though vision is otherwise normal
fusiform face area
an area in the human brain that is more responsive to faces than to other stimuli and located in fusiform gyrus
Receptive Field Hierarchy and Perception
- Photoreceptors
- Ganglion cells, center-surround structure contrast and wavelength of light
- Striate cortex, simple and complex receptive fields, orientation selectivity and binocularity.
- extrastriate cortical areas, cells are selectively responsive to more complex shapes, object motion, and even faces
- >receptive fields become increasingly larger and more complex moving away from V1
Following a bicycle accident, you are disturbed to find that you cannot see anything in your left visual field. Where has the retinofugal pathway been damaged?
-In the right optic tract, left homonymount heminopia
What is the source of most of the input to the left LGN and from where to what layer?
From right visual field, nasal right retina and temporal left retina, in left LGN left eye(ipsilateral) axons synapse in layers 2,3,5 and right eye(contralateral) axons synapse in layers 1,4,6
A worm has eaten part of one lateral geniculate nucleus. You can no longer perceive motion in the right visual field of your right eye. What layer(s) of which LGN have most likely been damaged?
Layers 1,4,6 in left LGN
List the chain of connections that link a cone in the retina to a blob cell in the striate cortex. Is there more than one path by which cones connect to the blob cell?
Cone->Bipolar cell
->M-type ganglion cell->magnocellular layer->IVCalfa->Blob
OR
->P-type ganglion cell->Parvocellular layer->IVCbeta->Blob
What is meant by the statement that there is a map of the visual world in the striate cortex?
Retinotopy is an organization whereby neighboring
cells in the retina feed information to neighboring places in their target
structures—in this case, the LGN and striate cortex. In this way, the two dimensional surface of the retina is mapped onto the two-dimensional
surface of the subsequent(myöhempi) structures.
The lower portion of V1 represents the
top half of visual space, and the upper portion of V1 represents the bottom half
of visual space. Notice also that the map is distorted, with more tissue devoted
to analysis of the central visual field.
What is parallel processing in the visual system? Give two examples.
- IVB projects to cortical areas that are thought to be involved in motion perception.
- Magnocellular pathway might be involved in the analysis of object motion and the guidance of motor actions.
- Parco-interblob-pathway might be involved in the analysis of fine object shape .
If a child is born cross-eyed and the condition is not corrected before the age of 10 years, binocular depth perception will be lost forever. This is explained by a modification in the circuitry of the visual system. From your knowledge of the central visual system, where do you think the circuitry has been modified?
In the striate cortex where the information of two eyes mix in the layer IV and III. ??
What layers of the striate cortex send efferents to other visual cortex areas?
IVCalfa->IVB
ICVbeta->III
What new receptive field properties are found in the striate cortex and other cortical areas that are not seen in the retina or LGN?
Direction selectivity, binocularity, and orientation selectivity
What sort of experiment might you perform to investigate the relationship between visual perception and neural activity in the visual cortex?
Show certain kind of photos to a subject in fMRI??
