The central visual system

Question 1

What is the neural pathway, beginning with the optic nerve, that leaves the eye called?

Answer 1

retinofugal projection

Question 2

Name three structures he ganglion cell axons “fleeing” the retina pass through before they form synapses in the brain stem

Answer 2

The components of this retinofugal projection are, in order, the optic nerve, the optic chiasm, and the optic tract.

The optic nerves 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.

The optic nerves from both eyes combine to form the optic chiasm (named for the X shape of the Greek letter chi), which lies at the base of the brain, just anterior to where the pituitary gland dangles down.

Following the partial decussation at the optic chiasm, the axons of the retinofugal projections form the optic tracts, which run just under the pia along the lateral surfaces of the diencephalon.

Question 3

What is meant by the binocular visual field?

Answer 3

Fix your gaze on a point straight ahead. Now imagine a vertical line passing through the fixation point, dividing the visual field into left and right halves. By definition, objects appearing to the left of the midline are in the left visual hemifield, and objects appearing to the right of the midline are in the right visual hemifield.

By looking straight ahead with both eyes open and then alternately closing one eye and then the other, you will see that the central portion of both visual hemifields is viewed by both retinas. This region of space is therefore called the binocular visual field.

Question 4

Where do the optic tract axons go from the optic chiasm?

Answer 4

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.

Question 5

Where does the LGN project to from here and what is this projection called?

Answer 5

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

Question 6

How would lesions in the left optic tract and left optic nerve differ in how it is experienced?

Answer 6

While a transection of the left optic nerve would render a person blind in the left eye only, a transection of the left optic tract would lead to blindness in the right visual field as viewed through either eye.

Question 7

What would be experienced from a midline transection of the optic chiasm

Answer 7

A midline transection of the optic chiasm would affect only the fibers that cross the midline. Because these fibers originate in the nasal portions of both retinas, blindness would result in the regions of the visual field viewed by the nasal retinas—that is, the peripheral visual fields on both sides

Question 8

What function may projections to part of the hypothalamus have? Name 2 other structures which receive retinal projections in this manner

Answer 8

Direct projections to part of the hypothalamus play an important role in synchronizing a variety of biological rhythms, including sleep and wake- fulness, with the daily dark–light cycle

Direct projections to part of the midbrain, called the pretectum, control the size of the pupil and certain types of eye movement. And about 10% of the ganglion cells in the retina project to a part of the midbrain tectum called the superior colliculus

Question 9

In what type of species does this tectum play a huge role? What name is given to the superior colliculus and this projection in other species?

Answer 9

The tectum of the midbrain is the major target of the retinofugal projection in all nonmammalian vertebrates (fish, amphibians, birds, and reptiles). In these vertebrate groups, the superior colliculus is called the optic tectum. This is why the projection from the retina to the superior colliculus is often called the retinotectal projection, even in mammals.

Question 10

What is the role of the S.C?

Answer 10

In the superior colliculus, a patch of neurons activated by a point of light, via indirect connections with motor neurons in the brain stem, commands eye and head movements to bring the image of this point in space onto the fovea. This branch of the retinofugal projection is thereby involved in orienting the eyes in response to new stimuli in the visual periphery.

Question 11

Describe the composition of the LGN

Answer 11

Viewed in cross section, each LGN appears to be arranged in six distinct layers of cells. By convention, the layers are numbered 1 through 6, starting with the most ventral layer, layer 1. In three dimensions, the layers of the LGN are arranged like a stack of six pancakes, one on top of the other. The pancakes do not lie flat, however; they are bent around the optic tract.

Question 12

LGN neurons are almost identical to those of the ganglion cells that feed them. For example, magnocellular LGN neurons have relatively large center-surround receptive fields, respond to stimulation of their receptive field centers with a transient burst of action potentials, and are insensitive to differences in wavelength.

What makes this so surprising?

Answer 12

The retina is not the main source of synaptic input to the LGN. In addition to the retina, the LGN receives inputs from other parts of the thalamus and the brain stem. The major input, constituting about 80% of the excitatory synapses, comes from primary visual cortex.

Question 13

Thus, one might reasonably expect that this corticofugal feedback pathway would significantly alter the qualities of the visual responses recorded in the LGN. So far, however, a role for this massive input has not been clearly identified.

Describe a theory for the role of this massive input from the visual cortex

Answer 13

One hypothesis is that “top–down” modulation from the visual cortex to the LGN gates subsequent “bottom-up” input from the LGN back to the cortex. For example, if we want to selectively pay attention to a portion of our visual field, we might be able to suppress inputs coming from outside the attended area.

Question 14

The LGN also receives synaptic inputs from neurons in the brain stem. What activity is associated with the areasproviding synaptic input?

Answer 14

alertness and attentiveness

Question 15

What is meant by retinotopy?

Answer 15

The projection starting in the retina and extending to the LGN and V1 illustrates a general organizational feature of the central visual system called retinotopy. 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 structures.

Neighbouring locations on the retina project to neighboring locations in the LGN. This retinotopic representation is preserved in the LGN projection to V1.

Question 16

Why is the mapping of the visual field onto a retinotopically organized structure often distorted?

Answer 16

Because visual space is not sampled uniformly by the cells in the retina. Recall that there are many more ganglion cells with receptive fields in or near the fovea than in the periphery.

Corresponding to this, the representation of the visual field is distorted in the striate cortex: The central few degrees of the visual field are overrepresented, or magnified, in the retinotopic map

Question 17

Describe activation in the striate cortex in response to a single dot of light

Answer 17

A 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. The image of a point of light on the retina actually activates a large population of cortical neurons; every neuron that contains that point in its receptive field is potentially activated. Thus, when the retina is stimulated by a point of light, the activity in the striate cortex is a broad distribution with a peak at the corresponding retinotopic location.

Question 18

In which layer of the striate cortex are the receptive fields of neurons similar to magnocellular and parvocellular LGN neurons providing their input?

What does this mean?

Answer 18

By and large, the receptive fields of neurons in layer IVC are similar to the magnocellular and parvocellular LGN neurons providing their input. This means they are generally small monocular center-surround receptive fields.

Question 19

Are the neurons in layer IV sensitive to different wavelengths of light?

Answer 19

In layer IVCa the neurons are insensitive to the wavelength of light, whereas in layer IVCB the neurons exhibit center-surround colour opponency. Outside layer IVC (and somewhat within), new receptive field characteristics, not observed in the retina or LGN, are present.

Question 20

Comment on the binocularity of layer IV

Answer 20

Each neuron in layers IVCa and IVCB receives afferents from a layer of the LGN representing the left or right eye. Axons leaving layer IVC diverge and innervate more superficial cortical layers, mixing the inputs from the two eyes. Microelectrode recordings confirm this anatomical fact; most neurons in layers superficial to IVC are bin- ocular, responding to light in either eye.

Question 21

How are the ocular dominance columns demonstrated with autoradiography reflected in the responses of V1 neurons?

Answer 21

Above the centers of ocular dominance patches in layer IVC, the layer 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). In areas where there is more equal mixing of left eye and right eye projections from layer IVC, the superficial layer neurons respond about the same to light in either eye.

Question 22

Do neurons in general have monocular or binocular receptive fields?

Answer 22

We say that 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.

Question 23

Why is retinotopy preserved in these binocular receptive fields?

Answer 23

Retinotopy is preserved because the two receptive fields of a binocular neuron are precisely placed on the retinas such that they are “looking” at the same point in in the contralateral visual field.

Question 24

Why is this construction of binocular receptive fields essential in binocular animals such as humans?

Answer 24

The construction of binocular receptive fields is essential in binocular animals, such as humans. Without them, we would probably be unable to use the inputs from both eyes to form a single image of the world around us and perform fine motor tasks that require stereoscopic vision, such as threading a needle.

Question 25

What shape do most of the receptive fields in the retina, LGN, and layer IVC take?
What do they give their greatest response to?

Answer 25

Circular and give their greatest response to a spot of light matched in size to the receptive field center.

Question 26

How does sensitivity of receptive fields outside layer IVC in area V1 differ from this? (2)

Answer 26

While small spots can elicit a response from many cortical neurons, it is usually possible to produce a much greater response with other stimuli. 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. Most of the V1 neurons outside layer IVC (and some within) are orientation selective.

Many V1 receptive fields also exhibit direction selectivity; 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

Question 27

Are the orientation selectivity of nearby neurons related or unrelated?

Answer 27

Yes. As a microelectrode is advanced radially (perpendicular to the surface) from one layer to the next, the preferred orientation remains the same for all the selective neurons encountered from layer II down through layer VI. Hubel and Wiesel called such a radial column of cells an orientation column.

As an electrode passes tangentially (parallel to the surface) through the cortex in a single layer, the preferred orientation progressively shifts. We now know, from the use of a technique called optical imaging, that there is a mosaic-like pattern of optimal orientations in the striate cortex

Question 28

Are there more orientation selective or direction selective cells?

Answer 28

Orientation because direction-selective cells in V1 are a subset of the cells that are orientation selective.

Question 29

What is sensitivity to the direction of stimulus motion seen as a hallmark of?

Answer 29

neurons receiving input from the magnocellular layers of the LGN. Direction-selective neurons are thought to be specialised for the analysis of object motion.

Question 30

Contrast the proposed functions of the dorsal and ventral stream of vision

Answer 30

The dorsal stream appears to serve the analysis of visual motion and the visual control of action.

The ventral stream is thought to be involved in the perception of the visual world and the recognition of objects.

Question 31

Which of these streams are reflective of properties of the parvocellular neurons and which is reflective of the magnocellular? To what extent is this reflective of the input into these areas?

Answer 31

The properties of dorsal stream neurons are most similar to those of magnocellular neurons in V1, and ventral stream neurons have properties more like features of parvo-interblob and blob cells in V1.

However, each extrastriate stream receives some amount of input from all the pathways in the primary visual cortex.

Question 32

Which stream does area MT belong and from where does it receive input?

Answer 32

Dorsal stream: Area MT receives retinotopically organized input from a number of other cortical areas, such as V2 and V3, and it also is directly innervated by cells in layer IVB of the striate cortex.

Question 33

Why is it relevant that area MT is directly innervated by cells in layer IVB of the striate cortex?

Answer 33

Recall that in layer IVB, the cells have relatively large receptive fields, transient responses to light, and direction selectivity. Neurons in area MT have large receptive fields that respond to stimulus movement in a narrow range of directions. 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.

Question 34

Give two other reasons for why MT is associated with motion rather than being more object selective

Answer 34

The neurons in MT also respond to types of motion, such as drifting spots of light, that are not good stimuli for cells in other areas.

Further specialization for motion processing is evident in the organization of MT. This cortical area is arranged into direction-of-motion columns analogous to the orientation columns in V1.

Question 35

Is area MT indicative just how you perceive the motion? if so, how do we know this, if not, what area more fits this description?

Answer 35

Perhaps you have seen illusory motion in paintings or optical illusions; MT has been shown to be activated by some of these images too, suggesting that its neurons tell us what motion we perceive, not necessarily what motion is present.

William Newsome and his colleagues at Stanford University have shown that weak electrical stimulation in area MT of the macaque mon- key appears to alter the direction in which small dots of light are perceived to move.

Question 36

Name another dorsal area sensitive to motion and neurons in that area are sensitive to

Answer 36

In an area known as medial superior temporal (MST), there are cells selective for linear motion (as in MT), radial motion (either in- ward or outward from a central point), and circular motion (either clock- wise or counterclockwise).

Question 37

Although we don’t know, what three roles have been proposed for how the visual system makes use of neurons with complex motion-sensitive properties in MST?

Answer 37

1. Navigation: As we move through our environment, objects stream past our eyes, and the direction and speed of objects in our peripheral vision provide valuable information that can be used for navigation.
2. Directing eye movements: Our ability to sense and analyse motion must also be used when we follow objects with our eyes and when we quickly move our eyes to objects in our peripheral vision that catch our attention.
3. Motion perception: We live in a world filled with motion, and survival sometimes depends on our interpretation of moving objects.

Question 38

What two brain areas in the ventral stream does the book mention?

Answer 38

Area V4 and Area IT

Question 39

What does V4 seem to be important for? (2)

Answer 39

both shape perception and colour perception.

Question 40

What disorder is associated with damage to V4? What could this suggest about the ventral stream?

Answer 40

achromatopsia is characterized by a partial or complete loss of color vision despite the presence of normal functional cones in the retina. Achromatopsia is usually accompanied by deficits in form perception.

Because achromatopsia is associated with cortical damage in the occipital and temporal lobes, without damage to V1, the LGN, or the retina, the syndrome suggests that there is specialized color processing in the ventral stream.

Question 41

Why is area IT of particular interest in regards to its location?

Answer 41

One reason this area is of particular interest is that it appears to be the farthest extent of visual processing in the ventral stream. A wide variety of colours and abstract shapes have been found to be good stimuli for cells in IT.

Question 42

What function may Area IT have, as suggested by its outward connections?

Answer 42

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 vi- sual memory. Recognizing an object clearly involves a combination or com- parison of incoming sensory information with stored information.

Question 43

Describe on of the most intriguing findings regarding area IT in monkeys

Answer 43

a small percentage of IT neurons in monkeys respond strongly to surprisingly complex objects such as pictures of faces. These cells may also respond to stimuli other than faces, but faces produce a particularly vigorous response, and some faces are more effective stimuli than others.

Question 44

How do these findings regarding faces in monkeys translate to studies with humans?

Answer 44

there is an area in the human brain that is more responsive to faces than to other stimuli. This area is located on the fusiform gyrus and has come to be called the fusiform face area.

More recent experiments have revealed that there are actually about half a dozen patches of cortex within and near IT that are particularly sensitive to faces, and neurons in each patch have different degrees of sensitivity to face identity (Mary versus Sue) and other attributes such as the side of the head viewed (left, right, front, back)