Chapter 10: The Central Visual System

One hypothesis of the corticofugal pathway to the LGN is :

"top-down" modulation from the visual cortex to the LGN gates subsequent "bottom-up" input from the LGN back to the cortex

the layers of the LGN are layered like:

(layers numbered 1-6 : ventral most being 1)a stack of 6 pancakes, one on top of the other

in the left eye, axons synapse on cells in layers :

1, 4, and 6

optimal stimulus of inferotemporal visual cortex :

1. Face

3 roles proposed of how the visual system makes use of neurons with complex motion0sensitive properties in MST or fo the "simpler" direction-selective cells in V1, MT, and other areas :

1. Navigation : 2. Directing eye movements 3. Motion perception

The ganglion cell axons "fleeing" the retina pass through three structures before they synapse in the brain stem, these 3 structures are :

1. Optic nerve 2. Optic chains 3. Optic tract

prosopagnosia usually results from :

1. a stroke and is associated with damage to extrastriate visual cortex, perhaps including the fusiform area

prosopagnosia is characterized by :

1. difficulty recognizing faces even though vision is otherwise normal

AN influential model used to to make sense of how the brain makes sense of our complex visual world is based on the idea that there are 3 pathways within V1 that perform different functions in parallel and these pathways are :

1. magnocellula rpathway 2. the parvo0interblob pathway 3. blob pathway

the dorsal stream neurons are most similar to those of ___________ and ventral stream neurons have properties more like features of __________ . howeever each extrastriate stream receives some amunt of input from all the pathways in the primary visual cortex

1. magnocellular neurons in V! 2.parvocellular and blob cells in V1

achromotopsia is associated with cortical damage in the :

1. occipital and temporal lobes, without damage to V1, LGN, or the retina

the contributions to vision of these extrastriate areas are still being debated but it appears that there are two large-sscale corticals treams of visual processing known as :

1. one stretching dorsally from the striate cortex toward the parietal lobe 2. one stretching ventrally toward the temporal lobe

the parvo-interblob pathway :

1. originates with P-type ganglion cells of the retina, 2. which project to the parvocellular layers of the LGN 3. parvocellular LGN send axons to layer IVCalpha of striate cortex w 4. which in turn projects to layer 2 and 3 interblob regions

the blbo pathway :

1. receives input from the subset of ganglion cells that are neither M-type or P-type (nonM-nonP) 2. these nonM-nonP type cells project to the knoiocellular layers of the LGN 3. the kniocellular LGN projects directly to the cytochrome oxidase blobs in layers 2 and 3

optimal stimulus of ON-center retinal ganglion cells :

1. spot surrounded by dark annulus

Area V4 receive sinput from :

1. the blob and interblob regions of the striate cortex via a relay in V2

1st point to remember abut retinotopy :

1. 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 (there are many more neurons in the striate cortex that receive input from the central retina than from the peripheral retina

visual perception is defined as :

1. the task of identifying and assigning meaning to objects in space

In the right LGN, the right eye (ipsilateral) axons synapse on LGN cells in layers :

2, 3, and 5

2nd point to remember about retinotopy is :

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

layer IVCbeta which receives input from parvocellular LGN neurons projects mainly to layer :

3

3rd pint to rememebr about retinotopy is :

3. dont be misled byt the word "map" perception is based on the brain's interpretation of distributed patterns of activity, not literal snapshots of the world

A major output of V4 is an area in the inferior temporal lobe known as :

Area IT

The central portion of both visual hemifields is viewed by :

BOTH retinas

The region of space that is viewed by both retinas is called :

Binocular visual field

A transaction of the LEFT optic nerve would render a person :

Blind in the LEFT eye *and vice-versa

Lesions anywhere in the retinofugal projection from the eye to the LGB to the visual cortex in humans will cause :

Blindness in part or all of the visual field

A transaction of the LEFT optic tract would lead to :

Blindness in the RIGHT visual field as viewed through either eye

At the optic chiasm, the axons originating in the NASAL retinas cross from one side to the other, this process of crossing of a fiber bundle from one side of the brain to the other is called a :

Decussation

Koniocellular LGN axons make synapses primarily in layers :

I and III

the blobs are located in layer :

IV

parvocellular LGN neurons project to layer :

IVCbeta

A small number of optic tract axons peel off to form synaptic connections with cells in the hypothalamus, and another 10% or so continue past the thalamus to innervate the midbrain, but most of them innervate the :

Lateral geniculate nucleus (LGN) of the dorsal thalamus

what makes the similarity of LGN and ganglion cell receptive fields so surprising is that the retina is :

NOT the main source of synaptic input to the LGN

The extinction of the midbrain is the major target of the retinofugal projection in all:

Nonmammalian vertebrates

A midline transaction of the optic chiasm would do what :

Only affect he fibers that cross the midline * bc Therese axons originate in the nasal portions of both retinas, blindness would result in the regions of the visual field viewed by the nasal retinas (peripheral fields on both sides)

The nerves from both eyes combine to form the _______, which lies at the base of the brain, just anterior to where the pituitary gland dangles down

Optic chiasm

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

Optic nerves

The neurons in the LGN give rise to axons that project to the primary visual cortex. This projection from the LFN to the cortex is called the :

Optic radiation

In vertebrate groups, the superior colliculus is called the :

Optic tectum

Following the partial decussation at the optic chiasm, the axons of he retinofugal projections form the _______, which run under the pic along the lateral surfaces of the diencephalon

Optic tracts

Because only the axons originating in the nasal retinas cross, we say that a ______ decussation of the retinofugal projection occurs at the optic chiasm

Partial decussation

Outside of the layer IVC are many :

Pyramidal cells

The neural pathway that leaves the eye, beginning with the optic nerve, is often referred to as the :

Retinofugal projection

The projection from the retina to the superior colliculus is often called :

Retinotectal projection

About 10% of the ganglion cells in the retina project to a part of the midbrain text in called the :

Superior colliculus

Direct projections to part of the hypothalamus play an important role in :

Synchronizing w variety of biological rhythms, including sleep and wakefulness, with daily dark-light cycle

Rule of thumb: optic nerve fibers cross in the optic chiasm, such that :

The left visual hemifield is "viewed" by the right hemisphere and the right visual hemifield is "viewed" by the left hemisphere

Direct projections to part of the midbrain called PRETECTUM , control :

The size of the pupil and certain types of eye movements

Most of the ________- neurons outside layer IVC are orientation selective

V1

the primary visual cortex is also called :

V1 and the striate cortex ****

Area MT (v5) receives retinotopically organized input from a number of other cortical areas such as :

V2 and V3 and it is also directly innervated by cells in layer IVB of the striate cortex

Starting at the white matter (containing the cortical input and output fibers( , the cell layers are named by roman numerals :

VI, V, IV , III, and II

By definition, objects appearing to the left of the midline are considered to be in the left ________

Visual HEMIfield and vice-versa (right visual hemifield)

cytochrome oxidase is :

a mitochondrial enzyme used for cell metabolism, the stain is not uniformly distributed in layers 2 and 3

achromatopsia is characterized by :

a partial or complete loss of color vision despite the presence of normal functional cones in the retina

P-type retinal ganglion cells, have relatively small center-surround receptive fields and respond to stimulation of their receptive fields with :

a sustained increase in the freqeuncy of action potentials; many of them exhibit color opponency

the neocortex in general, and the striate cortex in particular, have neuronal cell bodies arranged into :

about half a dozen layers

receptive fields in primary visual cortex range from a fraction of a degree to several degrees across, and nearby cells have receptive fields that overlap a great deal. For these reasons, even a small spot of light will :

activate thousands of V1 neurons

each neuron in layers IVCalpha and IVCbeta receives :

afferents from a layer of the LGN representing the left or right eye

the LGN also receives synaptic inputs from neurons in the brain stem whose activity is related to :

alertness and attentiveness

layer V pyramidal cells send axons :

all the way down to the superior colliculus and pons

Area MT is most notable for the fact that :

almost all the cells are direction selective, unlike areas earlier in the dorsal stream or anywhere in the ventral stream

Hubel and Wisel found that many neurons in V1 respond best to :

an elongated bar of light moving across their receptive fields but the orientation of the bar is critical

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

it is generally believed that neurons sensitive to wavelength are important for the :

analysis of object color

Neurons is the LGN have ____________ center-surround receptive fields

antagonistic

the primary visual cortex is broadmann's area _____ and is located in :

area 17 and is located in the occipital lobe of the primate brain

one of the most studied areas in the ventral stream is :

area V4

the left and right eye inputs to layer IV are laid out :

as a series of alternating bands, like the stripes of a zebra

the magnocellular pathway :

begins with M-type ganglion cells of the retina 2. these cells send axons to the magnocellular layers of the LGN 3. these layers project to IVCalpha of the striate cortex 4. which in turn projects to layer IVB

neurons have _______________ meaning they actualy have 2 receptive fields, one in the ipsilateral eye and one in the contralateral eye

binocular receptive fields

most neurons in layers superficial to IVC are :

binocular, responding to light in either eye

the neurons in the interblob aareas have what properties :

binocularity, orientation selectivity, and direction selectivity

the pillars of cytochrome oxidase rich neurons haave come to be called :

blobs

the V4 area is important for :

both shape perception and color perception

Axons arising frim M-type, P-type, and nonM-nonP ganglion cells in the two retinas synapse on :

cells in different LGN layers

layer IVCalpha which receives magnocellular LGN input, projects mainly to :

cells in layer IVB

most of the neurons in the V1 that do NOT have distinct ON and OFF regions and therefore not considered simple cells but rather they are referred to a s:

complex cells because their receptive fields appeared to be more complex than those of simple cells

such a unit of braint issue has come to be called :

cortical module

the pyramidal cells in different layers innervate :

different structures

many V1 receptive fields also exhibit ____________, meaning they respond when a bar of light at the optimal orientation moves perpendicular to the orientation in one direction but in the opposite direction

direction selectivity

some blob cells have both a color-opponent center and a color-opponent surround and are called :

double-opponent cells

receptive fields of cells in the knoiocellular layers are center-surround and have:

either light-dark or color opponency

optimal stimulus of a simple cell in V1:

elongated bar of light

Within all layers of the LGN, the neurons are activated by only one :

eye (they are monocular)

neurons in the parvo-interblob pathway have small orientation-selective receptive fields, so perhaps they are involved in the analysis of :

fine object shape

scientists at MIT discovered that there is an area located on the fusiform gyrus in the human brain that is mroe responsive to faces than other stimuli, this area is calledd the :

fusiform face area (FFA)

the axons of some layer 3 pyramidal cells extend collateral branches that make :

horizontal connections within layer 3

it is possible to study the action potential discharges of a geniculate neuron in response to visual stimuli and map its receptive field by :

inserting a microelectrode into the LGN

In addition to the neurons in the 6 prinicpal layers in the LGN, it was later discovered that there are numerous tiny neurons that lie just ventral to each layer, these layers are called :

koniocellular LGN layers

the blobs receive input drectly from the :

koniocellular layers of the LGN and magnocellular and parvocellular input via layer IVC

the blobs receive direct LGN input from :

koniocellular layers, as well as parvocellular and magnocellular input from layer IVC of striate cortex

layer 1, is located just under the pia mater, and is characterized as :

largely devoid of neurons and consists almost entirely of axons and dendrites of cells in other layers

As one progresses from the retina to extrastriate visual cortex, receptive fields become :

larger and are selective for more complex shapes

A closer look at the LGN reveals that the two ventral layers (1 and 2) contain :

larger neurons than the 4 more dorsal layers

Axons from the LGN terminate in several different cortical layers, with the largest number going to :

layer IVC

Magnocellular LGN neurons project primarily to :

layer IVCalpha

pyramidal cells are found in :

layers 3, 4B, 5, and 6

the major targets of the two optic tracts are the :

left and right lateral geniculate nuclei (LGN) located in the dorsal thalamus

M-type ganglion cells in the retina project entirely to the :

magnocellular LGN

the ventral layers of the LGN are called :

magnocellular LGN layers

sensitivty to the direction of stimulus motion is a hallmark of neurons receiving input from the :

magnocellular layers of the LGN

in an area known as ____________ there are cells selective for linear motion, radial motion, and circular motion

medial superior temporal (MST)

layer IVC stellate cells project axons radially up mainly to layers IVB and 3 where for the first time, infromtaion from the elft and right eye begins to :

mix

hubel and wiesel argued that a 2 x 2 chunk of striate cortex is both necessary and sufficient to analyze the image of a point in space because :

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

pyramidal cells are :

neurons covered with spines and are characterized by a single thick apical dendrite that branches as it ascends toward the pia mater and by multiple basal dendrites that extend horizontally

Koniocellular layers (referred to as K1-K6) receive input from :

nonM-nonP types of retinal ganglion cells and also project to the visual cortex****

bilateral damage to portions of extrastriate visual cortex have a side effect of :

not being able to visually perceive motion (lif ein snapshots)

many neurons in the blobs are color selective, so they might be involved in the analysis of :

object color

in an area known as V5 or MT, strong evidence indicates that specialized processing of __________ takes place

object motion

the magnocellular pathway ay be involved in the analysis of :

object motion and the guidance of motor actions

the bands of cells extending through the thickness of the striate cortex are called :

ocular dominance columns

each corticl module contains :

ocular dominance columns, orientation columns, and cytochrome oxidase blobs to fully analyze a portion of the visual field

the ventral stream is thought to be involved in the eprception :

of the visual world and the recognition of objects

on average, due to the greater cytochrome oxidase activity in blobs, firing rates in blobs is __________ than firing rates in the interblob neurons

on average higher

all layer IVC neurons receive input fro only :

one eye, most neurons in layers II, III, V, and VI receive some amount of input from each eye

in aaddition to the siny neurons, inhibitory neurons which laack spines, are sprinkled in all cortical layers as well, but these neurons form :

only local connections

radial columns observed by wisel and hubel that suggested the orientation remains the same for all the selective neurons encountered from layer 2 down thrugh layer 6 :

orientation column

neurons eliciting a response based on a bar of light with a particular orientation are said to exhibit :

orientation selectivity

neurons int he V4 are both:

orientation selectivve and color selective

layer 2, 3, and IVB pyramidal cells send their axons to :

other cortical areas

in addition to the retina, the LGN receives input from :

other parts of the thalamus and the brain stem

P-type ganglion cells in the retina project exclussively to the :

parvocellular LGN

the dorsal layers of the LGN are called :

parvocellular LGN layers

if V4 is damaged then :

perceptual deficits involving shape and color result

Most intracortical connections extend :

perpindicular to the cortical surface along radial lines that run across the layers, from whtie matter to layer 1

for the msot part, only __________________ send axons out of thhe striate cortex to form connections with other parts of the brain

pyramidal cells

perpendicular connections to the cortical surface along radial lines that run across the layers are known as :

radial connections

in the LGN, every layer receives :

retinal afferents and sends efferents to the visual cortex

LGN neurons receive synaptic input from the :

retinal ganglion cells and most geniculate neurons project an axon to the primary visual cortex via yhe optic radiation

In the LGN, the different information deriived from the 3 categories of retinal ganglion cells from the two eyes remains largely :

segregated

At the LGN, the input from the two eyes is kept :

separate

recent studies have shown that blobs and interblob neurons both :

show selectivity for both orientation and color

interblob areas include both:

simple and complex cells

______________ are neurons that receive a converging input from LGN cells with receptive fields that are aligned along one axis

simple cells

the segregation of ON and OFF regions is a defining property of _____________, and it is because of this receptive field structure that they are orientation selective

simple cells

Spiny stellate cells are :

small neurons with spine-covered dendrites that radiate out from the cell body

the axons of _________ generally make local connections only within the cortex

stellate cells *except for stellate neurons in layer IVB which project to V5

magnocellular LGN neurons hhave relatively large center-surround receptive fields, and respond to :

stimulation of their receptive field centers with a transient burst of action potentials, and are insensitive to differences in wavelength *like M-ganglion cells

direction-selective cells in V1 are a _________ of the cells that are orientation selective

subset

the segregation of LGN neurons into layers suggests :

that different types of retinal information ar ebeing kept separate at this synaptic relay

the anatomical organization of the LGN supports the idea :

that the retina gives rise to streams of information that processed parallel

the conclusion of microelectrode studies of the LGN is :

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

rather than randomly mixing, neurons connected to the left and right eyes are as distinct in layer IV as they are in :

the LGN

in the visual cortex, only a subet of the layers receives input from :

the LGN or sends output to a different cortical or subcortical area

direction-selective neurons are thought to be specialized for :

the analysis of object motion

orientation-selective neurons are thought to be specialized for :

the analysis of object shape

Many different neuronal shapes ahve been identified in striate cortex, but the focus is on two principal types based on :

the appearance of their dendritic trees

the upper portion of V1 represents :

the bottom half of visual space

In layer 3 and 4B, an axon may form synapses with :

the dendrites of pyramidal cells of all cell layers

above centers of ocular dominance patches in layer IVC, th layer 2 and 3 neurons are mroe strongly driven by :

the eye represented in layer IVC (meaning : their response is dominated by one eye even though they are bincular)

the receptive fields of neurons in layer IVC are similar to :

the magnocellular and parvocellular LGN neurons providing their input

layer VI pyramidal cells give rise to :

the massive axonal projection back to the LGN

the LGN has a single major synaptic target :

the primary visual cortex

the major input, constituting of about 80% of the excitatory synapses, comes from :

the primary visual cortex

autoradiography is :

the process of placing a piece of film over a thin sections of tissue and later developing the film like a photograph

for the most part, each K layer gets input from :

the same eye as the overlying M or P layer Ex: K1 receives input from the contralateral eye just as layer 1 neurons do

sping stellate cells are primarily seen in :

the two tiers of IVC

the lower portion of V1 represents :

the upper half of visual space

the LGN is considered to be the agetway to :

the visual cortex and therefore to conscious visual perception

the 6 layer scheme of the striate cortex is misleading because :

there are actually 9 distinct layers of neurons but to maintain broadmann's convention, neuroanatomists combined 3 sublayers into 1 : IVA, IVB, IVC -> IV layer IVC is further divided into two tiers called IVCalpha symbol and IVC beta symbol

the anatomical segregation of neurons into layers suggests that :

there is a division of labor in the cortex, similarly to what we saw in the LGN

beyond area MT, in the parietal love, are areas with additional types of specialized movement sensitivity.

true

the striate cortex is constructed from perhaps a thousand cortical modules :

true

beyond the V1 lie another _______ distinct extrastriate areas of cortex that have unique receptive field properties

two dozen

the dorsal stream appears to serve the analysis of :

visual motion and the visual control of action

Area IT is the farthest extent of :

visual processing in the ventral stream

blob cells , unline interblob cells, are generally :

wavelength sensitive, and monocular, and they lack orientation and direction selectivity *they resemble the knoiocellular and parvocellular input from the LGN