Biological Psychology Chapter 5 (Kalat)

Major Connections in the Visual System

- (a) Part of the visual input goes to the thalamus and from there to the visual cortex. Another part goes to the superior colliculus - (b) Axons from the retina maintain their relationship to one another—what we call their "retinotopic organization"—throughout their journey from the retina to the lateral geniculate and then from the lateral geniculate to the cortex.

How Infants Divided Their Attention between Faces

- A right-side-up face drew more attention than an upside-down one, regardless of whether the faces were realistic (left pair) or distorted (central pair). - They divided their attention about equally between two right-side-up faces (right pair), even though one was realistic and the other was distorted. - Evidently, a newborn's concept of face requires the eyes to be on top, but the face does not have to be realistic.

Young children vs adults on facial recognition

- According to fMRI data, young children activate more of their brain than adults do, when trying to recognize a face - Their accuracy is also lower than that of adults - Through childhood and the early teenage years, connections strengthen between the fusiform gyrus, especially in the right hemisphere, and part of the inferior occipital cortex known as the "occipital face area"

Visual processes in the brain

- According to fMRI studies as people viewed pictures, most objects do not activate one brain area more than another. - That is, the brain does not have a specialized area for seeing flowers, fish, birds, clothes, food, or rocks - However, three types of objects do produce specific responses. - The brain is amazingly adept at detecting biological motion—the kinds of motion produced by people and animals

Lens

- Adjustable - Light is focused by _____ and cornea and projected onto the retina

Human Foveal and Peripheral Vision

- Because of the distribution of rods and cones, you have good color vision in the fovea but not in the periphery - Although rods outnumber cones by about 20 to 1 in the human retina, cones provide about 90 percent of the brain's input

Columns of Neurons in the Visual Cortex

- Cells with similar properties group together in the visual cortex in columns perpendicular to the surface - For example, cells within a given column might respond to only the left eye, only the right eye, or both eyes about equally. - Also, cells within a given column respond best to lines of a single orientation.

Iris

- Colored area around pupil

Ciliary muscle

- Controls the lens

Optic nerve

- Formed by the axons of the ganglion cells - leaves the retina and travels along the lower surface of the brain - The optic nerves from the two eyes meet at the optic chiasm, where, in humans, half of the axons from each eye cross to the opposite side of the brain - information from the nasal half of each eye (the side closer to the nose) crosses to the contralateral hemisphere. - Information from the temporal half (the side toward the temporal cortex) goes to the ipsilateral hemisphere - The percentage of crossover varies from one species to another depending on the location of the eyes

Amount of Time Infants Spend Looking at Patterns

- Human newborns come into the world predisposed to pay more attention to faces than other stationary displays - Even in the first 2 days after birth, infants look more at faces than at most other stimuli. - This tendency supports the idea of a built-in face recognition module. - However, the infant's concept of face is not like an adult's.

Early Exposure to a Limited Array of Patterns

- If a kitten spends its entire early sensitive period wearing goggles with horizontal lines painted on them, nearly all its visual cortex cells become responsive only to horizontal lines - Even after months of later normal experience, the cat does not respond to vertical lines

Development of the visual cortex

- In a newborn mammal, many of the normal properties of the visual system develop normally at first, before birth - Waves of spontaneous activity sweep over the developing retina, synchronizing the activity of neighboring receptors and enabling appropriate combinations of receptors to establish connections with cells in the brain - Still, when an animal first opens its eyes, cells of the visual system show patterns of activity that are little more than random noise - Watching a visual stimulus quickly reduces the noise - A study of people who were born without eyes found that the connections from the primary visual cortex to its main targets were more or less normal - Evidently, certain axon paths develop automatically, without any need for guidance by experience. - Nevertheless, visual experience after birth modifies and fine-tunes many of the connections.

Convergence of Input onto Bipolar Cells

- In the fovea, each bipolar cell receives excitation from just one cone (and inhibition from a few surrounding cones), and relays its information to a single midget ganglion cell. (results in better acuity: sensitivity to detail) - In the periphery, input from many rods converges onto each bipolar cell, resulting in higher sensitivity to faint (dim) light and low sensitivity to spatial location.

Transformations of a Drawing

- In the inferior temporal cortex, cells that respond strongly to the original respond about the same to the contrast reversal and mirror image but not to the figure-ground reversal. - Note that the figure-ground reversal resembles the original in terms of the pattern of light and darkness, but it is not perceived as the same object.

Cells in V2

- In the secondary visual cortex (V2), just anterior to V1 in the occipital cortex, most cells are similar to V1 cells in responding to lines, edges, or sine wave gratings, except that V2 receptive fields are more elongated. - Also, some V2 cells respond best to corners, textures, or complex shapes - Areas V2 and V3 have some cells highly responsive to color, and other cells highly responsive to the disparity between what the left and right eyes see— critical information for stereoscopic depth perception

Hermann von Helmholtz

- Known as the trichromatic theory - He found that people could match any color by mixing appropriate amounts of just three wavelengths. Therefore, he concluded that three kinds of receptors—we now call them cones—are sufficient to account for human color vision.

Distribution of Cones in Human Retina

- Long- and medium-wavelength cones are far more abundant than short-wavelength (blue) cones - Consequently, it is easier to see tiny red, yellow, or green dots than blue dots - Although the short-wavelength (blue) cones are about evenly distributed across the retina, the other two kinds are distributed haphazardly, with big differences among individuals

Another interpretation for most cortical neurons in V1 area

- Many cortical neurons respond best to a particular spatial frequency and hardly at all to other frequencies. - Most visual researchers therefore believe that neurons in area V1 detect spatial frequencies rather than bars or edges. - If so, it is a feature detector for a feature that we don't perceive consciously

Opsins

- Modify the photopigments' sensitivity to different wavelengths of light

Impaired Infant Vision and Long-Term Consequences

- Most people immediately see two squares, one overlapping the other. - However, a man who lost vision from age until 43 sees this display as three objects. - At age 43, a corneal transplant enabled him to recover vision. - Immediately he could see colors and he could soon identify simple shapes. - Eventually he learned to recognize common household objects, but unlike most people who identify objects immediately, he had to think about it more carefully - Even 10 years later, he could not identify whether a face was male or female, happy or sad

Cornea

- Not adjustable - Light is focused by lens and ______ and projected onto the retina

Medial superior temporal cortex (MST)

- One of the two areas especially important for motion perception - Adjacent to the MT - receive input mostly from "magnocellular path"- respond best to more complex stimuli, such as the expansion, contraction, or rotation of a large visual scene - respond if a whole scene expands, contracts, or rotates. That is, they respond if the observer moves forward or backward or tilts his or her head. -responds best to the expansion, contraction, or rotation of a visual display

Middle temporal cortex (MT)

- One of the two areas especially important for motion perception - aka V5 - receive input mostly from "magnocellular path" - also responds to photographs that imply movement, such as a photo of people running - People who had electrical stimulation of area MT (while they were undergoing exploratory studies to find the cause of their severe epilepsy) report seeing vibrations or other hallucinated movements during the stimulation - They also become temporarily impaired at seeing something that really is moving - MT activity is apparently central to the experience of seeing motion.

Pictures of places

- One part of the parahippocampal cortex (next to the hippocampus) responds strongly to _________, and not so strongly to anything else.

Faces

- Part "fusiform gyrus" of the inferior temporal cortex, especially in the right hemisphere, responds more strongly to _______ than to anything else.

Three types of primate ganglion cells

- Parvocellular - Magnocellular - Koniocellular - The existence of so many kinds of ganglion cells implies that the visual system analyzes information in many ways from the start

Visual path within the eye

- Receptors send their messages to bipolar and horizontal cells, which in turn send messages to amacrine and ganglion cells. The axons of the ganglion cells form the optic nerve, which exits the eye at the blind spot and continues to the brain.

Cells in the Primary Visual Cortex

- Simple - Complex - End-stopped (hypercomplex)

The opposite of motion blindness

- Some people are blind except for the ability to detect which direction something is moving. - How could someone see movement without seeing the object that is moving? - Area MT gets some input directly from the lateral geniculate nucleus of the thalamus. - Therefore, even after extensive damage to area V1 (enough to produce blindness), area MT still has enough input to permit motion detection - When they say which direction something is moving, they insist they are just guessing - different areas of your brain process different kinds of visual information, and it is possible to develop many kinds of disability.

Deprived Experience in One Eye

- When a kitten opens its eyes at about age 9 days, each neuron responds to areas in the two retinas that focus on approximately the same point in space—a process necessary for binocular vision - However, innate mechanisms cannot make the connections exactly right because the exact distance between the eyes varies from one kitten to another, and the distance changes over age - experience is necessary for fine-tuning. - If an experimenter sutures one eyelid shut for a kitten's first 4 to 6 weeks of life, synapses in the visual cortex gradually become unresponsive to input from the deprived eye - After the deprived eye is opened, the kitten does not respond to it - A similar period of deprivation in older animals weakens the response to the deprived eye, but not as strongly as it does in young ones - After an eye deprived of vision in adults is reopened, cells gradually return to their previous levels of responsiveness

Deprived Experience in both Eyes

- When just one eye is open, the synapses from the open eye inhibit the synapses from the closed eye - If neither eye is active, no axon outcompetes any other - For at least 3 weeks, the kitten's cortex remains responsive to visual input, although most cells become responsive to just one eye or the other and not both - If the eyes remain shut still longer, the cortical responses start to become sluggish and lose their well-defined receptive fields - Eventually, the visual cortex starts responding to auditory and touch stimuli instead

Strabismus (strabismic amblyopia OR lazy eye)

- a condition in which the eyes do not point in the same direction - leads to fail to learn use both eyes at the same time- poor stereoscopic depth detection - A promising therapy for lazy eye is to ask a child to play three-dimensional action video games that require attention to both eyes - Good performance requires increasing attention to exactly the kind of input we want to enhance - This procedure appears to improve the use of both eyes better than patching does, although neither procedure has much effect on stereoscopic depth perception

Astigmatism

- a decreased responsiveness to one direction of line or another, caused by an asymmetric curvature of the eyes - About 70 percent of all infants have - Normal growth reduces the prevalence of astigmatism to about 10 percent in 4-year-old children.

Fovea (meaning "pit")

- a tiny area of the retina specialized for acute, detailed vision - Because blood vessels and ganglion cell axons are almost absent near the fovea, it has nearly unimpeded vision - for perceiving detail, each receptor in the fovea connects to a single bipolar cell, which in turn connects to a single ganglion cell that has an axon to the brain

Bodies

- an area close to the face area (fusiform gyrus) responds more strongly to _______ than to anything else

Motion blindness

- an impaired ability to perceive movement - being able to see objects but unable to see whether they are moving or, if so, which direction and how fast - caused by the damage to MT and MST - People with motion blindness are better at reaching for a moving object than at describing its motion, but in all aspects of dealing with visual motion, they are far behind other people.

Visual agnosia

- an inability to recognize objects despite otherwise satisfactory vision - "visual lack of knowledge" - result from damage in the temporal cortex - Someone might be able to point to visual objects and slowly describe them but fail to recognize what they are

Pupil

- an opening in the center of the iris where light enters

Blind spot

- area at the back of the retina where the optic nerve exits - it is devoid of receptors (no receptors) - it is occupied by exiting axons and blood vessels.

Primary visual cortex (area V1, or striate cortex)

- area of the cortex responsible for the first stage of visual processing - area that information from the lateral geniculate nucleus of the thalamus goes to - striped appearance - If you close your eyes and imagine seeing something, activity increases in area V1 in a pattern similar to what happens when you actually see that object - People with damage to area V1 report no conscious vision, no visual imagery, and no visual images in their dreams - In contrast, adults who lose vision because of eye damage continue to have visual imagery and visual dreams

Visual field

- area of the world that an individual can see at any time - the part of the world that you see—before you can identify the color.

Photopigments

- chemicals contained in rods and cones that release energy when struck by light - consist of 11-cis-retinal (a derivative of vitamin A) bound to proteins called "opsins" - Light converts 11-cis-retinal to all-trans-retinal, thus releasing energy that activates second messengers within the cell

Retinex theory (a combination of the words retina and cortex)

- concept that the cortex compares information from various parts of the retina to determine the brightness and color for each area - Can be the explanation for the color and bright constancy - visual perception requires reasoning and inference, not just retinal stimulation.

Visible light (what we call "lights")

- consists of electromagnetic radiation within the range from less than 400 nm (nanometer, or m) to more than 700 nm - We perceive the shortest visible wavelengths as violet. Progressively longer wavelengths are perceived as blue, green, yellow, orange, and red - Although the wavelengths vary as a continuum, we perceive distinct colors.

Magnocellular path

- detects overall patterns, including movement over large areas of the visual field - Given that the __________ is color insensitive, MT is also color insensitive.

Midget ganglion cells

- ganglion cells in the fovea of humans and other primates - each is small and responds to just a single cone (each cone in the fovea has a direct route to the brain) - Because the midget ganglion cells provide 70 percent of the input to the brain, your vision is dominated by what you see in and near the fovea

Cells of the lateral geniculate

- have receptive fields that resemble those of the ganglion cells—an excitatory or inhibitory central portion and a surrounding ring with the opposite effect - After the information reaches the cerebral cortex, the receptive fields become more complicated.

Opponent-process theory

- idea that we perceive color in terms of opposites - That is, the brain has a mechanism that perceives color on a continuum from red to green, another from yellow to blue, and another from white to black. - After you stare at one color in one location long enough, you fatigue that response and swing to the opposite

Explanation for negative color afterimage

- imagine a bipolar cell that receives excitation from a short-wavelength cone and inhibition from long- and medium-wavelength cones. It increases its activity in response to short-wavelength (blue) light and decreases it in response to yellowish light. After prolonged exposure to blue light, the fatigued cell decreases its response. Because a low level of response by that cell usually means yellow, you perceive yellow - However, that explanation cannot be the whole story

Color vision deficiency

- impaired ability to perceive color differences - some people have otherwise satisfactory vision without seeing all the color that other people do. That is, color is in the brain, not in the light or the object itself. - In contrast to our three types of cones, many birds, reptiles, and fish have four types. So far as they are concerned, all humans are color deficient. - Color deficiency results because people with certain genes fail to develop one type of cone, or develop an abnormal type of cone

Magnocellular neurons ("large celled")

- large cell bodies with large receptive fields that are distributed evenly throughout the retina including periphery - respond strongly to movement and large overall patterns, but they do not respond to color or fine details

Another finding for cortical cell

- later researchers found that a cortical cell that responds well to a single bar or line responds even more strongly to a sine wave grating of bars or lines

Cells in inferior temporal cortex

- learn to recognize meaningful objects. - A cell that responds to the sight of some object initially responds mainly when it sees that object from the same angle, but after a bit of experience it learns to respond almost equally to that object from other viewpoints. - It is responding to the object, regardless of major changes in the pattern that reaches the retina

Lateral geniculate nucleus

- part of the thalamus - thalamic nucleus that receives incoming visual information - The place most ganglion cell axons go to - A smaller number of axons go to the superior colliculus and other areas, including part of the hypothalamus that controls the waking-sleeping schedule - The lateral geniculate, in turn, sends axons to other parts of the thalamus and the visual cortex. Axons returning from the cortex to the thalamus modify thalamic activity

Predisposition of face recognition

- people are highly predisposed to see faces wherever possible. - Just draw two dots and an upward curved line below them, and people call it a "smiley face."

Inferior temporal cortex

- portion of the cortex where neurons are highly sensitive to complex aspects of the shape of visual stimuli within very large receptive fields

Amacrine cells

- receive information from bipolar cells and send it to other bipolar, amacrine, and ganglion cells - Amacrine cells refine the input to ganglion cells, enabling certain ones to respond mainly to particular shapes, directions of movement, changes in lighting, color, and other visual features

Neurons in MT and ventral part of MST

- respond briskly if something moves relative to the background, but they show little response if the object and the background both move in the same direction and speed - enable you to distinguish between the result of eye movements and the result of object movements.

Fusiform gyrus

- responds strongly to a face viewed from any angle, as well as line drawings and anything else that looks like a face - In several cases, physicians electrically stimulated the fusiform gyrus during exploratory surgery. - Varying with the intensity and duration of stimulation, the result was either a difficulty in perceiving faces or a vivid distortion of faces - One patient exclaimed, "You just turned into somebody else. Your face metamorphosed"

Occipital face area

- responds strongly to parts of a face, such as the eyes and mouth -

Negative color afterimage

- result of staring at a colored object for a prolonged length of time and then looking at a white surface - a replacement of the red you had been staring at with green, green with red, yellow and blue with each other, and black and white with each other. - The brighter the light and the longer you stare, the stronger the effect

Parvocellular neurons ("small celled")

- small cell bodies with small receptive fields in or near the fovea PCP (Phencyclidine) - well suited to detect visual details. - They also respond to color, each neuron being excited by some wavelengths and inhibited by others. - The high sensitivity to detail and color relates to the fact that parvocellular cells are located mostly in and near the fovea, which has many cones

Koniocellular neurons ("dust celled")

- small ganglion cells, similar to parvocellular neurons, but they occur throughout the retina - have several functions, and their axons terminate in several locations

Brightness Constancy

- the ability to perceive the brightness of an object by comparing it to other objects

Color constancy

- the ability to recognize colors despite changes in lighting - cannot easily be explained by trichromatic and opponent-process theory - For this reason, we should avoid talking about the color of a wavelength of light. A certain wavelength of light can appear as different colors depending on the background

Retinal disparity

- the discrepancy between what the left and right eyes see - stereoscopic depth perception requires the brain to detect retinal disparity - Experience fine-tunes binocular vision, and abnormal experience disrupts it

Prosopagnosia

- the inability to recognize faces due to damage of several brain areas - That problem can result from damage to the fusiform gyrus, or from a failure of that gyrus to develop fully - In some people the right fusiform gyrus is significantly smaller than average and has fewer than normal connections with the occipital cortex - In contrast, if you can recognize faces more easily than average, it may be that you have richer than average connections between fusiform gyrus and occipital cortex. - People with prosopagnosia can read, so visual acuity is not the problem - They recognize people's voices, so their problem is not memory - if they feel clay models of faces, they are worse than other people at determining whether two clay models are the same or different - Their problem is not vision, but something that relates specifically to faces. - they can describe each element of a face, such as brown eyes, big ears, a small nose, and so forth, but they do not recognize the face as a whole - As people learn to read, the fusiform gyrus becomes more responsive to words and (in the left hemisphere) less responsive to faces - Evidently the fusiform gyrus participates in many types of detailed visual recognition. - However, even in people with extreme levels of expertise, many cells in the fusiform gyrus respond more vigorously to faces than to anything else

Red-green color deficiency

- the most common form of color deficiency - people have trouble distinguishing red from green because their long- and medium wavelength cones have the same photopigment instead of different ones. - The gene causing this deficiency is on the X chromosome - Male > Female

Retina

- the rear surface of the eye, which is lined with visual receptors - Light from the left side of the world strikes the right half of the retina, and vice versa - Light from above strikes the bottom half of the retina, and light from below strikes the top half

Lateral inhibition

- the retina's way of sharpening contrasts to emphasize the borders of objects. - the reduction of activity in one neuron by activity in neighboring neurons - important for many functions in the nervous system - The receptors send messages to excite nearby bipolar cells and also send messages to horizontal cells that slightly inhibit those bipolar cells and the neighbors to their sides - The net result is to heighten the contrast between an illuminated area and its darker surround - Actually, light striking the rods and cones decreases their spontaneous output, and the receptors make inhibitory synapses onto the bipolar cells - Therefore, light on the rods or cones decreases their inhibitory output - A decrease in inhibition means net excitation

Trichromatic theory (Young-Helmholtz theory)

- theory that color is perceived through the relative rates (ratio) of response by three kinds of cones, each one maximally sensitive to a different set of wavelengths - When all three types of cones are equally active, we see white or gray.

Sensitive period

- time early in development when experiences have a particularly strong and enduring influence - depends on inhibitory neurons - However, even long after the sensitive period, a prolonged experience—such as a full week without visual stimulation to one eye—produces a smaller but measurable effect on the visual cortex - Cortical plasticity is greatest in early life, but it never ends.

Horizontal cells

- type of cell that receives input from receptors and delivers inhibitory input to bipolar cells - Spread widely - Local cells (no axon and no action potentials, its depolarization decays with distance) - The rods and cones of the retina make synapses with horizontal cells and bipolar cells - make inhibitory contact onto bipolar cells, which in turn make synapses onto "amacrine cells" and "ganglion cells"

Bipolar cells

- type of neuron in the retina that receives input directly from the receptors - located closer to the center of the eye - get their name from the fact that a fibrous process is attached to each end (or pole) of the neuron.

Ganglion cells

- type of neuron in the retina that receives input from the bipolar cells - located still closer to the center of the eye - The ganglion cells' axons join together and travel back to the brain

Cones

- type of retinal receptor that contributes to color perception - abundant in and near the fovea, are less active in dim light, more useful in bright light, and essential for color vision

Rods

- type of retinal receptor that detects brightness of light - abundant in the periphery of the human retina, respond to faint light but are not useful in daylight because bright light bleaches them

Complex cells

- type of visual cortex cell located in areas V1 and V2 that responds to a pattern of light in a particular orientation anywhere within its large receptive field - Most complex cells respond most strongly to a stimulus moving in a particular direction—for example, a vertical bar moving horizontally. - The best way to classify a cell as simple or complex is to present the stimulus in several locations - A cell that responds to a stimulus in only one location is a simple cell. One that responds equally throughout a large area is a complex cell

End-stopped cells/ Hypercomplex Cells

- type of visual cortex cell that resembles complex cells; - responds best to stimuli of a precisely limited type, anywhere in a large receptive field, with a strong inhibitory field at one end of its field - has a strong inhibitory area at one end of its bar-shaped receptive field - The cell responds to a bar-shaped pattern of light anywhere in its broad receptive field, provided the bar does not extend beyond a certain point - resemble complex cells

Dorsal stream

- visual path in the parietal cortex that helps the motor system locate objects - important for visually guided movements. - the action pathway - "how" pathway - People with damage to the parietal cortex see objects but they don't integrate their vision well with their arm and leg movements - They can read, recognize faces, and describe objects in detail but they cannot accurately reach out to grasp an object - Although they can describe from memory what their furniture looks like, they cannot remember where it is located in their house

Ventral stream

- visual paths in the temporal cortex that are specialized for identifying and recognizing objects - the "what" path - the perception pathway - people with temporal lobe damage can use vision to guide their actions, but they cannot identify what the objects are.

Saccades

- voluntary eye movements - You do not see your own eyes move because area MT and parts of the parietal cortex decrease their activity during voluntary eye movements - Activity does not decrease while your eyes are following a moving object. - The brain areas that monitor saccades tell area MT and the parietal cortex, "We're about to move the eye muscles, so take a rest for the next split second." - Neural activity and blood flow in MT and part of the parietal cortex begin to decrease 75 milliseconds (ms) before the eye movement and remain suppressed during the movement

They become larger because each cell's receptive field is made by inputs converging at an earlier level.

As we progress from bipolar cells to ganglion cells to later cells in the visual system, are receptive fields ordinarily larger, smaller, or the same size? Why?

The child sees well enough to identify whether two objects are the same or different, but the child doesn't understand what the visual information means. - In particular, the child cannot answer which visual display matches something the child touches. - However, understanding of vision improves with practice.

If an infant is born with dense cataracts on both eyes and they are surgically removed years later, how well does the child see at first?

It produces more excitation than inhibition for the nearest bipolar cell. For surrounding bipolar cells, it produces only inhibition. The reason is that the receptor excites a horizontal cell, which inhibits all bipolar cells in the area.

If light strikes only one receptor, what is the net effect (excitatory or inhibitory) on the nearest bipolar cell that is directly connected to that receptor? What is the effect on bipolar cells to the sides? What causes that effect?

Feature Detectors

Neurons whose responses indicate the presence of a particular feature

Law of Specific Nerve Energies

Statement that whatever excites a particular nerve always sends the same kind of information to the brain

Ability to recognize faces correlates with the strength of connections between the occipital face area and the fusiform gyrus.

The ability to recognize faces correlates with the strength of connections between which brain areas?

Receptive Field

The area in visual space that excites or inhibits any neuron

The temporal cortex has specialized areas for perceiving places, faces, and bodies, including bodies in motion.

The brain has no specialized areas for perceiving flowers, clothes, or food. For what items does it have specialized areas?

People become motion blind shortly before and during a saccade (voluntary eye movement), because of suppressed activity in area MT.

Under what circumstance does someone with an intact brain become motion blind, and what accounts for the motion blindness?

- Neurons of the parvocellular system have small cell bodies with small receptive fields, are located mostly in and near the fovea, and are specialized for detailed and color vision. - Neurons of the magnocellular system have large cell bodies with large receptive fields, are located in all parts of the retina, and are specialized for perception of large patterns and movement.

What are the differences between the parvocellular and magnocellular systems?

Astigmatism results when the eyeball is not quite spherical. As a result, the person sees one direction of lines more clearly than the other.

What causes astigmatism?

If the eye muscles cannot keep both eyes focused in the same direction, the developing brain loses the ability for any neuron in the visual cortex to respond to input from both eyes. Instead, each neuron responds to one eye or the other. Stereoscopic depth perception requires cells that compare the input from the two eyes.

What early experience would cause a kitten or human child to lose stereoscopic depth perception?

If one eye is closed during early development, the cortex becomes unresponsive to it. If both eyes are closed, cortical cells remain somewhat responsive for several weeks and then gradually become sluggish and unselective in their responses.

What is the effect of closing one eye early in life? What is the effect of closing both eyes?

The receptor excites both the bipolar cells and the horizontal cell. The horizontal cell inhibits the same bipolar cell that was excited plus additional bipolar cells in the surround.

When light strikes a receptor, does the receptor excite or inhibit the bipolar cells? What effect does it have on horizontal cells? What effect does the horizontal cell have on bipolar cells?

Neurons in areas MT and MST respond strongly when an object moves relative to the background, and not when the object and background move in the same direction and speed.

When you move your eyes, why does it not seem as if the world is moving?

It starts with the ganglion cells in the retina. Most of its axons go to the lateral geniculate nucleus of the thalamus, but some go to the hypothalamus and superior colliculus.

Where does the optic nerve start and where does it end?