Chapter 10: The Central Visual System

what can you predict about the kind of information being processed within each layer?

*look at table in class slides*

achromatopsia

-Partial or complete colorblindness despite normal cones in the retina -associated with cortical damage in the occipital and temporal lobes without damage to V1, the LGN, or the retina, therefore there must be specialized color processing in the ventral stream

cortical module

-The unit of cerebral cortex that is necessary and sufficient to analyze one discrete point in a sensory surface -each module looks at a portion of the visual field

cytochrome oxidase

-a mitochondrial enzyme used for cell metabolism -often stained for in the striate cortex which reveals a series of pillars of staining in regular intervals in layers II, III, V, and VI

describe receptive fields of konicellular LGN neurons

-are center-surround and have either light/dark or color opponency -KEY: COLOR

how could you determine which layer of the LGN we are listening to?

-by how long the action potentials lasted response to stimulation -test different orientations of light -how long the response is

complex cells

-cell in V1 that do not have distinct on and off regions -more complex receptive fields -typically binocular and sensitive to stimulus orientation

simple cells

-cells in V1 that have an on-center or off-center with antagonistic surround (similar to LGN) -typically binocular and sensitive to stimulus orientation

optic chiasm

-combined from the optic nerves from both eyes -lies at the base of the brain, just anterior to where the pituitary gland dangles down

role of axons that directly innervate the midbrain tectum (superior colliculus)

-commands eye and head movements to bring the image of this point in space onto the fovea -orienting the eyes in response to new stimuli in the visual periphery -important for orienting fovea to objects that may be of threat

what locations do the optic tract axons form synaptic connections with?

-hypothalamus: regulation of circadian rhythms -10% with the midbrain (pretectum): reflex control of pupil and lens -most with the lateral geniculate nucleus (LGN) in the dorsal thalamus -superior colliculus: orienting the movements of head and eyes

area MT (V5) receptive fields

-large receptive fields that respond to stimulus movement in a narrow range of directions -all cells are direction sensitive (motion over structure of stimulus) -perceived motion rather than actual motion via comparison of activity across columns a full 360 degrees range of preferred directions

describe receptive fields of magnocellular LGN neurons

-like M-type ganglion cells, they have relatively large center-surround receptive fields, respond to stimulation of their receptive field centers with a transient burst of APs, and are insensitive to differences in wavelength -KEY: LOW CONTRAST STIMULI, NOT COLOR

describe receptive fields of parvocellular LGN neurons

-like P-type ganglion cells, they have relatively small center-surround receptive fields and respond to stimulation of their receptive field centers with a sustained increase in the frequency of APs, and many of them exhibit color opponency -KEY: COLOR

where do the axons of stellate cells, which are indistinguishable from dendrites, make connections? what is the one exception to this?

-local connections within the cortex -spiny stellate neurons in layer IVB that project to area V5

why does Hubel and Wiesel argue that a 2 x 2 mm chunk of striate cortex is both necessary and sufficient to analyze the image of a point in space?

-necessary because its removal would leave a blind spot for this point in the visual field -sufficient because it contains all the neural machinery required to analyze the form and color of objects viewed through either eye

direction selectivity

-neurons respond when a bar of light at the optimal orientation moves perpendicular to the orientation in one direction, but NOT in the opposite direction -in VI

what layers receive input from only one eye vs input from each eye?

-one eye: IVC -each eye: II, III, V, VI

components of the retinofugal pathway

-optic nerve -optic chiasm -optic tract

ventral stream of extrastriate cortex

-perception of the visual world and the recognition of objects -most similar to parvo-interblob and blob cells in V1 -receives some amount of input from all pathways in primary visual cortex

hierarchy of receptive fields

-photoreceptors: small patched on the retina -retinal ganglion cells: center-surround and sensitive to contrast and wavelength of light -striate cortex: simple and complex cells sensitive to orientation and binocularity -extrastriate cortical areas: cells are selectively responsive to more complex shapes, object motion, and faces

blobs

-pillars of cytochrome oxidase-rich neurons in layers II, III, V, and VI -receive direct LGN input from the koniocellular layers, as well as the parvocellular and magnocellular input from layer IVC of striate cortex

the LGN's single major synaptic target

-primary visual cortex and is located in the occipital lobe of the primate brain, surrounding the Calcarine fissure -other similar terms: Brodmann's area 17, V1, and striate cortex

optic radiation

-projection from right and left LGN to cortex -this pathway mediates conscious visual perception because its damage causes blindness

dorsal stream of extrastriate cortex

-serve the analysis of visual motion and the visual control of action -most similar to magnocellular neurons in V1 -receives some amount of input from all pathways in primary visual cortex

receptive fields in layer IVC (include alpha and beta)

-similar to the magnocellular and parvocellular LGN neurons providing their input -they are generally small monocular center-surround receptive fields -IVC alpha: neurons are insensitive to the wavelength of light -IVC beta: neurons exhibit center-surround color opponency

how do simple cells respond before and after light stimuli?

-spontaneous firing before light stimuli -on center and off surround for a bar of light in a specific orientation (orientation specific)

degrees of segregation of inputs in the thalamus, striate cortex, and higher cortical areas?

-thalamus: segregation -striate cortex: limited convergence of info -higher cortical areas: massive divergence of info

blob receptive fields

-wavelength sensitive -monocular -lack orientation and direction selectivity -some are circular -some are color-opponent center-surround -some have red-green and blue-yellow color opponency w/o surround regions -some have double-opponent cells -recent studies show they are similar to interblob meaning they are selective for both orientation and color

3 parallel pathways in V1

1. magnocellular 2. parvo-interblob 3. blob

3 important points to remember about retinotopy

1. 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 2. a discrete point of light can activate many cells in the retina, and often many more cells in the target structure, due to overlap of receptive fields 3. there are no pictures in the primary visual cortex for snapshots of the world, only arrangement of connections between the retina and V1 based on the brain's interpretation of distributed patterns of activity

3 roles of neurons with complex motion sensitive properties?

1. navigation 2. directing eye movements 3. motion perceptions

reasons why one would argue against extremely selective receptive fields tuned to each of the millions of objects we perceive?

1. no evidence that a portion of the cortex is this tuned 2. such great selectivity appears to be counter to the general principle of broad tuning that exists throughout the nervous system 3. too risky for the nervous system to rely on extreme sensitivity because damage would cause loss of recognition of only specific objects

what axons synapse at which layers of the right LGN?

2, 3, 5-right eye (ipsilateral) axons 1, 4, 6- left eye (contralateral) axons

Non-M non-P type ganglion cells

5% of cells in retina. Specialized for processing color. Project to koniocellular cell layers of the LGN

M-type ganglion cells

5% of cells in the retina. Large cell bodies, high dendritic arborization specialized for transient changes in stimulation. Projects to bottom two layers of the LGN

the neocortex in general, and the striate cortex in particular, have neuronal cell bodes arranged into about how many layers?

6

how many distinct layers of the LGN?

6 (layer 1 is the most ventral)

P-type ganglion cells

90% of cells in retina. Smaller cell bodies with more sustained firing patterns to stimulation. Specialized for processing visual details and some color processing. Projects to top 4 layers of the LGN.

orientation column

A column going down the visual cortex that contains neurons with the same orientation preference. (ex. from layer II down through layer VI)

retinotectal projection

A neural pathway that carries information from the retina to the superior colliculus

who were the first to study the physiology of the striate cortex with microelectrodes?

Hubel and Wiesel

what layer of the striate cortex do most axons of the LGN terminate?

IVC

where do magnocellular LGN neurons project to?

IVC alpha

where do parvoocellular LGN neurons project to?

IVC beta

pulvinar

In humans, the posterior portion of the thalamus. It is heavily involved in visual processing and direction of attention (ex. in cases of snakes).

where do VI pyramidal cells send their axons to?

LGN

magnocellular pathway

M-type ganglion cells of retina->magnocellular layers of the LGN->IVC alpha of striate cortex->IVB

parvo-interblob pathway

P-type ganglion cells of the retina->parvocellular layers of the LGN->IVC alpha of striate cortex->layers II and III interblob regions

another name for striate cortex and reasoning

V1 (visual area one) because it is the first cortical area to receive info from the LGN

Where does V1 project to?

V2 and V3 and V5 (MT)

area IT input

V4

which receptive fields respond to an elongated bar of light moving across the receptive field?

VI

which receptive fields respond to a specific orientation of light moving across the receptive field?

VI neurons outside layer IVC

Autostereogram

a complex two-dimensional image perceived as three-dimensional when not focused on for a period of time

stereogram

a picture that uses the principle of binocular disparity to create the perception of a three-dimensional image

fusiform face area

a region in the temporal lobe (area IT) of the brain that helps us recognize faces

what is the brain stem's activity responsible for in the LGN?

alertness and attentiveness

where do V pyramidal cells sends their axons to?

all the way down to the superior colliculus and the pons

what type of center-surround receptive fields do neurons in the LGN have?

antagonistic-small spot in the center may yield a larger response than a large spot also covering the antagonistic surround

extrastriate areas

areas in the cortex beyond the striate that have unique receptive field properties

what kind of receptive field do the striate cortex respond to?

bars of light

how does a small spot of light in the primary visual cortex activate thousands of V1 neurons?

because their receptive fields range from a fraction of a degree to several degrees across, and nearby cells have receptive fields that overlap a great deal

from which eye(s) do neurons in layers superficial to IVC respond?

binocular meaning they respond to light from both eyes

transection of the left optic nerve

blindness in the left eye

Midline transection of optic chiasm

blindness in the regions of the visual field viewed by the nasal retinas-that is, the peripheral visual fields on both sides

transection of the left optic tract

blindness in the right visual field as viewed through either eye

since interblob and blob receptive fields are indistinguishable, what is the most distinguishing element between the two types of cells?

blob neurons have distinct cytochrome oxidase labeling and fire APs at a higher rate

what type of cells are responsible for the analysis of object color?

blob pathway neurons because they are color sensitive

How are the left and right eye LGN inputs arranged when they reach the striate cortex? Do they randomly intermix or are they kept segregated?

by injecting amino acid into one eye of a monkey and using autoradiography, Hubel and Wiesel discovered that axon terminals relaying information were split up into a series of equally spaced patches in layer IV, as in a series of alternating bands

medial superior temporal (MST) function

cells selective for linear motion, radial motion, and circular motion

how would you build the receptive field of a simple cortical cell?

combine LGN receptive fields together which are already lined up and project to one simple cortical cell in V1

role of axons that directly innervate the midbrain (pretectum)

control the size of the pupil and certain types of eye movement

decussation

crossing of fiber bundle of the axons originated in the NASAL retina from one side of the brain to the other

what property does this cell display?

direction specificity

area MT (V5) physical arrangement

direction-of-motion columns analogous to the orientation columns in V1

what neurons are thought to be specialized for the analysis of object motion?

direction-selective neurons

pituitary hypertrophy

disrupts normal vision because enlargement of the pituitary causes compressing of the crossing fibers of the optic chiasm and results in the loss of peripheral vision

what animal models were first used to discover more about the central visual system?

domestic cat and rhesus monkey

2 types of stream in the extrastriate areas

dorsal and ventral stream

optic nerve

exit the left and right eyes at the optic disks, travel through the fatty tissue behind the eyes in their body orbits, then pass through holes in the floor of the skull

does the retina pass along info about patterns of light and dark that fall on it or extract info about differences in brightness and color?

extract ex. color-there is no such things as physical color, only reflected wavelengths of light that are interpreted by are cone cells and synthesized in our brain

optic tracts

form after the decussation and run under the pia along the lateral surfaces of the diencephalon

how is info processed by geniculocortical pathway?

in parallel by neurons specialized for the analysis of different stimulus attributes

which receptive fields respond to a circular spot of light matched in size to the receptive field center?

in the retina, LGN, and layer IVC

Prosopagnosia

inability to recognize faces even though vision is otherwise normal

area V4 input

input from blob and interblob regions of the striate cortex via a relay in V2

how are neurons outside layer IV organized?

into alternating bands dominated by the left and right eye

retinotopy

is an organization whereby neighboring cell in the retina feed info to neighboring places in their target structures-in this way, the 2D surface of the retina is mapped onto the 2D surface of the subsequent structures

ventral layers 1-2 of the LGN contain what kind of neurons (magnocellular LGN layers)?

larger neurons

V4 receptive fields and sensitivity

larger receptive fields than cells in the striate cortex, and many of the cells are both orientation and color selective

what two structures are involved in the pathway serving conscious visual perception?

lateral geniculate nucleus (LGN) of the thalamus and the primary visual cortex (area 17, V1, or the striate cortex)

what layer do the axons of some pyramidal cells extend collateral branches that make horizontal connections?

layer III

where does binocular vision start?

layer III

where is the retinotopic organization established?

layer IV

how are the actually 9 distinct layers of neurons in the striate cortex combined to make 6 to maintain Brodmann's convention?

layer IV=IVA, IVB, IVC layer IVC=IVC alpha and IVC beta

where do koniocellular LGN neurons project to?

layers I and III

which layers play a key role in visual processing and provide most of the information that leaves VI for other cortical areas?

layers II and III

what visual field does the right LGN receive information from?

left

Bitermporal hemianopsia

loss of peripheral vision (tunnel vision)

M-type ganglion cells project exclusively to which LGN layer?

magnocellular

what type of cells are responsible for the analysis of object motion and the guidance of motor actions?

mangnocellular pathway neurons because they are direction selective

after the axons of retinal ganglion cells leave the retina, where do the vast majority synapse first?

many of the axons of the ganglion cells that exit the eye synapse with the thalamus (NOT the optic chiasm)

the cells in VI, layer IVC are monocular or binocular?

monocular

from which eye(s) do neurons in layers IVC respond?

monocular meaning they respond to light only in one of the eyes via receiving afferents from a layer of the LGN representing one eye

what neurons are thought to be used for the analysis of object color

neurons sensitive to wavelength

binocular receptive fields

neurons that actually have two receptive fields, one in the ipsilateral eye and one in the contralateral eye

are magnocellular, parvo-interblob, and blob neurons strictly segregated and do they contain unique receptive fields?

no because signals mix and receptive fields are found across all pathways

would cells in IVC alpha layer respond to different wavelengths of light, not just white light?

no because this corresponds to magnocellular cells

does striate cortex output reflect magnocellular, parvocellular, and koniocellular segregation in the LGN?

no it has a different form of parallel processing

blob pathway

nonM-nonP types ganglion cells of the retina->koniocellular layers of the LGN->cytochrome oxidase blobs in layers II and III

LGN's response to startling stimuli?

not directly evoking APs in LGN neurons but modulation of the magnitude of LGN responses to visual stimuli, thus the LGN is more than a simple relay from the retina to the cortex, it is the first site in the ascending visual pathway where what we see is influenced by how we feel

wishing all layers of the LGN, the neurons are activated by ____ eye(s) and On-center and off-center cells are ___

one; intermixed

how does one study neural coding in a much larger scale than micro electrodes?

optical imaging through voltage sensitive dye, imaging of intrinsic signals, and in vivo two-photon calcium imaging

what neurons are thought to be specialized for the analysis of object shape?

orientation-selective neurons

where do II, III, and IVB pyramidal cells send their axons to?

other cortical areas

pyramidal cells

outside the IVC layers, these neurons are also covered with spines and are characterized by a single thick apical dendrite that branches as it ascends toward the pia mater (white matter) and by multiple basal dendrites that extend horizontally

if we don't rely on extremely selective receptive fields, how do we perceive?

parallel processing is used throughout the visual system and "division of labor"

what lobe is the medial superior temporal (MST) in?

parietal lobe

what type of cells are responsible for the analysis of fine object shape?

parvo-interblob pathway neurons because they have small orientation-selective receptive fields

P-type ganglion cells project exclusively to which LGN layer?

parvocellular

what happens to orientation selectivity as one goes tangentially through the cortex in a single layer?

preferred orientation shifts

major (80%) input into the LGN?

primary visual cortex

the neurons in the LGN give rise to axons that project to _____

primary visual cortex

area MT (V5) function

processing of object motion

an additional thalamic target besides LGN of the axons from retinal ganglion cells?

pulvinar

what type of cells send axons out of the striate cortex to form connections with other parts of the brain?

pyramidal cells

area MT (V5) input

receives retinotopically organized input from a number of other cortical areas, such as V2 and V3, and also directly innervated by cells in layer IVB of striate cortex

face specific hypothesis

region of the brain is selectively responsive to faces

where does the LGN receive input?

retina, thalamus, primary visual cortex (corticofugal feedback) and the brain stem

"partial" decussation

said because only the axons originating in the nasal retinas crossover

V4 perceives what

shape/form and color perception

spiny stellate cells

small neurons with spine-covered dendrites that radiate out from the cell body in the 2 tier of layers IVC in the striate cortex

ventral layers 3-6 of the LGN contain what kind of neurons (parvocellular LGN layers)?

smaller neurons

where are inhibitory neurons in the striate cortex and where do they send connections to?

sprinkled in all cortical layers and form only local connections

how to study the AP discharges of a geniculate neuron in response to visual stimuli and map its receptive field?

sticking a microelectrode in the LGN

what properties do complex cells respond to?

stimulus on and stimulus off initially

from which eye(s) do neurons in layers II and III respond?

strongly driven by the eye represented in layer IVC although they are binocular

how many layers of the visual cortex receive input from the LGN or sends output to a different cortical or subcortical area?

subset

role of axons that directly innervate the hypothalamus

synchronizing a variety of biological rhythms, including sleep and wakefulness, with the daily dark-light cycle

area IT output

temporal lobe structures involved in learning and memory

what do the 6 layers of the LGN indicate?

the M-type, P-type, and nonM-nonP type ganglion cells in the 2 retinas synapse on cells in the different layers

ocular dominance columns

the bands of cells extending through the thickness of the striate cortex

binocular visual field

the central portion of both visual hemifields that is viewed by both retinas

full visual field

the entire region of space (measured in degrees of visual angle) that can be seen with both eyes looking straight ahead

visual hemifield

the half of the visual field to one side of the fixation point

what visual fields are viewed by in the right LGN?

the nasal left retina and the temporal right retina

what visual fields are viewed by in the left LGN?

the nasal right retina and the temporal left retina

retinofugal projection

the neural pathway that leaves the eye, beginning with the optic nerve

orientation selectivity

the property of a cell in the visual system that responds to a limited range of stimulus orientations

why is the similarity of the LGN and ganglion receptive fields so surprising?

the retina is not the main source of synaptic input to the LGN

where is all the information about the left visual hemifield directed to?

the right side of the brain

optic tectum

the superior colliculus in nonmammalian vertebrates

how many estimated cortical modules in the striate cortex?

thousand

where do neurons in IVC beta project to?

to cells in layer III

where do neurons in IVC alpha project to?

to cells in layer IVB

neighboring cells in the retina feed in formation forward to their target structures

true

how is retinotopy preserved in binocular receptive fields?

two receptive fields of a binocular neuron are precisely placed on the retinas such that they are looking at the same point in the contralateral visual field

koniocellular LGN layers

ventral to each layer. input from nonM-nonP retinal ganglion cells and also project to the visual cortex

area IT function

visual perception and visual memory

conclusion of mapping receptive fields from LGN?

visual receptive fields of LGN neurons are almost identical to those of the ganglion cells that feed them

where does information from the left and right eye first begin to mix?

when layer IVC stellate cells project axons up to IVB and III

radial connections

where intracortical connections extend perpendicular to the cortical surface along radial lines that run across the layers, from white matter to layer I

area TI good stimulis

wide variety of colors and abstract shapes such as faces

what can happen from a lesion in the extrastriate visual cortex?

world perceived in snapshots due to difficulty perceiving motion

can a pyramidal cell in one lay have dendrites extending into other layers?

yes

do pyramidal cell axons in all layers form local connections in the cortex?

yes

does striate cortex input reflect magnocellular, parvocellular, and koniocellular segregation in the LGN?

yes

does the info projected by the retinal ganglion cells in the LGN remain segregated?

yes

would cells in IVC beta layer respond to different wavelengths of light, not just white light?

yes because this corresponds to parvocellular cells