Lecture 10 - Vision (Part 2)

Question 1

What is a receptive field?

Answer 1

A receptive field of a neuron refers to the specific region of sensory space (like an area of the retina, for visual neurons) where a stimulus can trigger the neuron’s activity. For visual neurons, this includes the location and specific properties of light (such as brightness, color, or direction of movement) that will cause the neuron to fire. In essence, it defines what a neuron “responds to” in its sensory environment.

Question 2

How is a receptive field defined in visual processing?

Answer 2

A receptive field is the specific area in the visual field where a stimulus (such as light or color) can activate a response in a particular sensory neuron. It’s defined in relation to the fixation point—the spot where the observer is focusing. The receptive field includes the portion of the visual field that affects how neurons respond when the observer looks at that spot.

Question 3

What are the steps involved in identifying a cell’s receptive field in visual processing?

Answer 3

1. Record activity while the animal focuses on a spot: Monitor the cell’s activity to establish a baseline response to visual stimuli.
2. Expose light to different areas: Systematically shine light on different areas of the visual field to identify the cell’s receptive field.
3. Assess color and pattern sensitivity: After locating the receptive field, test the cell’s sensitivity to specific colors and patterns.

Question 4

What happens to a neuron’s activity when light is presented in its receptive field?

Answer 4

When light hits a neuron’s receptive field, its response depends on the type of neuron and the light’s position.

• On-center neurons fire more when light hits the center and less when it hits the surround.
• Off-center neurons fire more when light hits the surround and less when it hits the center.

This change in firing rate helps the brain process visual information based on light location and intensity.

Question 5

How do photoreceptor cells differ from most retinal cells in terms of action potentials?

Answer 5

Photoreceptor cells differ from most retinal cells because they do not generate action potentials. Instead, they respond to light by changing their membrane potential, which leads to a decrease in the release of the neurotransmitter glutamate. This graded release of glutamate allows them to communicate the intensity of light rather than producing an all-or-nothing action potential.

Question 6

What occurs in photoreceptor cells in the dark?

Answer 6

In darkness, photoreceptor cells have a resting membrane potential of -40 mV and continuously release glutamate.

Question 7

What happens to photoreceptor cells when they are activated by light?

Answer 7

When activated by light, photoreceptor cells hyperpolarize to -70 mV and stop releasing glutamate.

Question 8

If a neuron’s receptive field is found to be sensitive to specific colors, what does this indicate about the cell?

Answer 8

This means that the neuron is specialized to detect and respond to different wavelengths of light, allowing it to differentiate between colors. The cell’s response varies depending on the color of light within its receptive field, indicating its role in processing color information.

Question 9

In a hypothetical experiment, a researcher finds that a cell’s activity increases when exposed to blue light but decreases with red light. What can be inferred about this cell’s receptive field?

Answer 9

It can be inferred that the cell has a receptive field that is specifically responsive to blue light, indicating color sensitivity.

Question 10

What are sodium ion channels in photoreceptor cells?

Answer 10

Sodium ion channels in photoreceptor cells are unique “leak” channels that remain open in darkness, allowing sodium ions to flow into the cell.

Question 11

What is the dark current?

Answer 11

The dark current is the influx of sodium ions through open channels in photoreceptor cells, which depolarizes the cell (makes it more positive) and maintains a resting membrane potential around -40 mV in the dark. This constant depolarization enables continuous glutamate release to the downstream neurons.

Question 12

How does light affect the sodium channels in photoreceptor cells?

Answer 12

Light activates an opsin protein, triggering a signaling cascade that closes sodium channels, causing hyperpolarization to -70 mV and stopping glutamate release.

Question 13

What type of receptors are opsin proteins responsible for conscious vision perception?

Answer 13

Opsin proteins are inhibitory metabotropic receptors that hyperpolarize photoreceptor cells when activated by light, stopping glutamate release.

Question 14

How do bipolar cells respond to changes in membrane potential?

Answer 14

Bipolar cells release glutamate in a graded manner based on membrane potential; they do not generate action potentials.

Question 15

What distinguishes OFF bipolar cells from ON bipolar cells?

Answer 15

OFF bipolar cells express excitatory ionotropic glutamate receptors and mirror the activity of connected photoreceptor cells, while ON bipolar cells express inhibitory metabotropic glutamate receptors and exhibit the opposite response pattern.

Question 16

How do OFF bipolar cells behave in darkness?

Answer 16

In darkness, when photoreceptors are depolarized at -40 mV and releasing glutamate, OFF bipolar cells are also depolarized and release glutamate.

Question 17

What happens to OFF bipolar cells in light?

Answer 17

In light, when photoreceptors hyperpolarize to -70 mV and stop releasing glutamate, OFF bipolar cells hyperpolarize and cease glutamate release.

Question 18

How do ON bipolar cells behave in darkness?

Answer 18

In darkness, ON bipolar cells hyperpolarize and do not release glutamate because they express inhibitory glutamate receptors.

Question 19

What occurs in ON bipolar cells when light is present?

Answer 19

When photoreceptors hyperpolarize in light, ON bipolar cells depolarize and release glutamate.

Question 20

What are retinal ganglion cells (RGCs)?

Answer 20

Retinal ganglion cells are typical neurons that generate action potentials and express excitatory ionotropic glutamate receptors.

Question 21

How does the activity of photoreceptor cells affect retinal ganglion cells?

Answer 21

Photoreceptors detect light and adjust their release of glutamate accordingly. When light is present, photoreceptors release less glutamate, exciting ON bipolar cells and inhibiting OFF bipolar cells. This modulation of bipolar cell activity influences the retinal ganglion cells, either increasing or decreasing their firing. The retinal ganglion cells then transmit these signals to the brain via the optic nerve for visual processing. The photoreceptors’ glutamate release controls the activity of bipolar cells, which in turn affects the retinal ganglion cells, ultimately shaping the visual information sent to the brain.

Question 22

What is the function of horizontal cells?

Answer 22

• Horizontal cells are neurons in the retina.
• They connect neighboring photoreceptors (rods and cones).
• They regulate the release of glutamate via lateral inhibition (suppression of nearby photoreceptor activity)
• Horizontal cells help fine-tune signals from photoreceptors, enhancing contrast and sharpening the overall visual image.
• Refined signals are passed to bipolar cells for further processing.

Question 23

How do horizontal cells enhance the difference in activity between photoreceptors?

Answer 23

Horizontal cells enhance contrast by comparing the activity of neighboring photoreceptors. They inhibit dimly lit photoreceptors more strongly, amplifying the difference in activity between bright and dark areas, which improves brightness perception and contrast sensitivity.

Question 24

What happens to a center photoreceptor cell when it senses dim light?

Answer 24

The center photoreceptor cell slightly hyperpolarizes when it senses dim light.

Question 25

How do neighboring photoreceptors respond to bright light in relation to horizontal cells?

Answer 25

Neighboring photoreceptors activated by bright light become more hyperpolarized, which horizontal cells use to regulate the response of the center photoreceptor.

Question 26

What is the role of glutamate release in horizontal cells?

Answer 26

Horizontal cells do not release glutamate; instead, they release GABA, which inhibits nearby photoreceptor and bipolar cells, helping to fine-tune visual signals through lateral inhibition. Horizontal cells, like photoreceptors and bipolar cells, communicate in a graded manner based on their membrane potential and do not generate action potentials.

Question 27

What is the membrane potential when ON bipolar cells release more neurotransmitter?

Answer 27

More neurotransmitter is released from ON bipolar cells when the membrane potential is less negative (more positive than -45 mV).

Question 28

How does the membrane potential of ON bipolar cells change in response to light?

Answer 28

When the upstream photoreceptor cell detects light, the membrane potential of ON bipolar cells approaches around -45 mV.

Question 29

What is the membrane potential of ON bipolar cells in darkness?

Answer 29

In darkness, ON bipolar cells are depolarized to approximately -60 mV due to the constant release of glutamate from photoreceptors.

Question 30

How do neighboring brightly lit photoreceptors affect the membrane potential of ON bipolar cells?

Answer 30

When the center photoreceptor is in darkness and the neighboring surround photoreceptors are brightly lit, the surround ON bipolar cells become hyperpolarized, leading to less neurotransmitter release from these cells. Meanwhile, the center ON bipolar cells will depolarize if the center photoreceptor is illuminated.

Question 31

If a person is in a dimly lit room and suddenly steps into bright light, how would horizontal cells respond?

Answer 31

Horizontal cells would enhance the difference in activity by counteracting the hyperpolarization in dimly lit photoreceptors while depolarizing axon terminals of photoreceptors exposed to bright light.

Question 32

What is the role of OFF bipolar cells in the visual pathway?

Answer 32

OFF bipolar cells detect decreases in light intensity and respond with changes in neurotransmitter release, influencing the activity of downstream retinal ganglion cells.

Question 33

How does neurotransmitter release in OFF bipolar cells change with membrane potential?

Answer 33

More neurotransmitter is released when the membrane potential is less negative (more positive than -45 mV), similar to ON bipolar cells.

Question 34

What happens to the membrane potential of OFF bipolar cells in darkness?

Answer 34

In darkness, the resting potential of OFF bipolar cells is approximately -45 mV.

Question 35

Describe the response of OFF bipolar cells when the upstream photoreceptor cell detects light.

Answer 35

When the upstream photoreceptor cell detects light, the membrane potential hyperpolarizes to around -75 mV, resulting in less neurotransmitter release.

Question 36

What is the “centre-surround” organization in bipolar cell receptive fields?

Answer 36

The “centre-surround” organization is created by the influence of horizontal cells, allowing bipolar cells to respond differently to light in the center versus the surrounding area of their receptive fields.

Question 37

How do ON retinal ganglion cells respond to light in their receptive field?

Answer 37

ON retinal ganglion cells increase their spiking rate when light is in the center of their receptive field and decrease their spiking rate when light is brighter in the surrounding area.

Question 38

How do OFF retinal ganglion cells react to light in their receptive field?

Answer 38

OFF retinal ganglion cells decrease their spiking rate when light is in the center of their receptive field and increase their spiking rate when light is brighter in the surrounding area.

Question 39

In a scenario where a person is looking at a bright light in the center of their vision, how would OFF retinal ganglion cells behave?

Answer 39

OFF retinal ganglion cells would decrease their spiking rate because the light is in the center of their receptive field.

Question 40

What occurs when the entire receptive field is illuminated?

Answer 40

When the entire receptive field of a retinal ganglion cell is lit up, the response becomes less sensitive to differences in light, as the activation of both the center and surround of the receptive field cancel each other out. This makes it harder to detect contrasts or edges, causing a reduced ability to see details. In real life, this might look like a scene that is evenly lit, where objects appear less distinct and harder to tell apart, similar to how everything looks blurry or flat on an overcast day.

Question 41

How does foveal processing in retinal ganglion cells differ from peripheral processing?

Answer 41

In the fovea, retinal ganglion cells process color information and receive input from fewer bipolar cells, resulting in smaller receptive fields. This leads to less compression of visual information and allows for higher visual acuity, meaning the ability to see fine details. In contrast, retinal ganglion cells in the periphery often integrate input from multiple bipolar cells, leading to larger receptive fields and more compression of the visual information. While this reduces visual acuity, it increases sensitivity to light and motion, which is important for detecting movement and seeing in low-light conditions.

Question 42

What are photoreceptor cells?

Answer 42

Photoreceptor cells are specialized cells in the retina, including rods and cones, that detect light and convert it into electrical signals.

Question 43

How do rods and cones respond to light?

Answer 43

Rods and cones hyperpolarize and release less glutamate when they detect the appropriate wavelength of light within their receptive field.

Question 44

What defines the receptive fields of photoreceptor cells?

Answer 44

The receptive fields of photoreceptor cells are defined by a specific location in space and the wavelength of light.

Question 45

How do bipolar cell receptive fields differ from those of individual photoreceptors?

Answer 45

Bipolar cell receptive fields are larger because they sum the receptive fields of multiple photoreceptor cells that provide input.

Question 46

What is the significance of ON and OFF responses in bipolar cells?

Answer 46

ON and OFF responses in bipolar cells refer to how they react to light: ON cells are activated by light, while OFF cells are activated by darkness.

Question 47

Why do bipolar cells in the fovea have smaller receptive fields?

Answer 47

Bipolar cells in the fovea have smaller receptive fields because each bipolar cell is connected to just one photoreceptor (like a cone). This direct connection allows for more precise processing of visual information, giving a sharper and more detailed image, which is essential for tasks like reading or recognizing faces.

Question 48

What is the role of retinal ganglion cells (RGCs)?

Answer 48

RGCs transmit visual information from the retina to the brain and exhibit receptive fields similar to those of bipolar cells with centre-surround structures.

Question 49

How do receptive fields of thalamic neurons compare to those of retinal ganglion cells?

Answer 49

The receptive fields of thalamic neurons are similar to those of retinal ganglion cells, as both process visual information. However, thalamic neurons refine and relay these signals to the primary visual cortex for further processing.

Question 50

What is the function of simple cells in the primary visual cortex (V1)?

Answer 50

Simple cells are considered feature detectors in the visual system. They detect specific features, like the orientation of edges and lines, in the visual field. This ability to respond to particular angles or edges helps the brain build a detailed map of the shapes and structures in what we see.

Question 51

How does orientation preference vary among V1 neurons?

Answer 51

Neurons in V1 have varying orientation preferences; some respond best to vertical lines, others to horizontal lines, and some to intermediate angles.

Question 52

What is a cortical column in the primary visual cortex?

Answer 52

Cortical columns are groups of neurons in the primary visual cortex (V1) that are specialized to process information from a specific area of the visual field. Each column processes various aspects of that area, like orientation, movement, or color, and integrates the information into a broader understanding of the visual scene.

Question 53

How do sharp transitions in contrast or color contribute to object recognition?

Answer 53

Sharp transitions in contrast or color highlight the borders, edges, and corners of objects, helping the visual system detect distinct shapes and boundaries. These visual cues make it easier for the brain to separate objects from the background and recognize them accurately.

Question 54

Imagine a person looking at a landscape with distinct edges between the sky and mountains. Which visual processing structures are involved in detecting these edges?

Answer 54

The photoreceptor cells detect the light, bipolar cells integrate the signals, retinal ganglion cells transmit the information to the thalamus, and neurons in the primary visual cortex analyze the edges for object recognition.

Question 55

What is the visual association cortex?

Answer 55

The visual association cortex refers to the entire occipital lobe, excluding the primary visual cortex, and is responsible for processing visual information.

Question 56

What percentage of the cerebral cortex is dedicated to processing visual information?

Answer 56

The cerebral cortex is the outer layer of the brain involved in complex functions like sensory perception, cognition, and decision-making. Over 25% of the cerebral cortex is dedicated to processing visual information.

Question 57

What is the striate cortex, and what is its significance?

Answer 57

The striate cortex, also known as the primary visual cortex (Area V1), is the region where initial visual processing occurs before information is relayed to the visual association cortex.

Question 58

What is the extrastriate cortex?

Answer 58

The extrastriate cortex is another term for the visual association cortex, which includes areas V2, V3, V4, etc., responsible for further processing of visual information.

Question 59

Describe the dorsal stream and its primary function.

Answer 59

The dorsal stream originates in the primary visual cortex and extends to the posterior parietal lobe, primarily involved in identifying spatial locations, including where objects are situated and their movement.

Question 60

Describe the ventral stream and its primary function.

Answer 60

The ventral stream starts in the primary visual cortex and extends to the inferior temporal lobe, focusing on identifying forms (shapes) and encoding what the object is, including its color.

Question 61

How do the dorsal and ventral streams differ in terms of visual processing?

Answer 61

The dorsal stream processes "where" information related to spatial location and movement, while the ventral stream processes "what" information related to object identity and form.

Question 62

Imagine you are trying to catch a ball. Which visual stream is primarily involved in helping you locate the ball and determine its trajectory?

Answer 62

The dorsal stream is primarily involved, as it helps identify the spatial location and movement of the ball.

Question 63

If you were asked to describe the color and shape of an object, which visual stream would you be utilizing?

Answer 63

You would be utilizing the ventral stream, as it focuses on identifying the form and color of the object.

Question 64

Why is it important for the visual association cortex to have different regions sensitive to various visual features?

Answer 64

Different regions of the visual association cortex are specialized to process specific visual features, like shapes, colors, and movement. This specialization helps the brain to interpret and understand visual information, which is important for recognizing objects and interacting with the environment.

Question 65

What is depth perception?

Answer 65

Depth perception is the ability to perceive the distance and three-dimensionality of objects in the environment.

Question 66

How does the dorsal stream contribute to depth perception?

Answer 66

The dorsal stream, known as the "where" pathway, is involved in processing spatial information and depth perception, located in the parietal lobe.

Question 67

What is monocular vision?

Answer 67

Monocular vision refers to depth perception that can occur with input from just one eye, where some V1 neurons respond exclusively to visual input from that eye.

Question 68

What is binocular vision?

Answer 68

Binocular vision occurs when most V1 neurons respond to visual input from both eyes, enhancing depth perception through the combination of images.

Question 69

What are depth cues?

Answer 69

Depth cues help us judge how far away objects are. Examples include: 1. Relative size: Closer objects look larger, farther ones smaller. 2. Amount of detail: Farther objects appear blurrier. 3. Relative motion: Closer objects move faster than distant ones when we move. These cues allow us to estimate depth and distance in our environment.

Question 70

How does relative size serve as a depth cue?

Answer 70

Relative size serves as a depth cue by indicating that larger objects appear closer to the observer than smaller objects.

Question 71

What role does the amount of detail play in depth perception?

Answer 71

The amount of detail seen in objects helps gauge their distance; closer objects exhibit more detail than those that are farther away.

Question 72

How does relative movement contribute to depth perception?

Answer 72

Relative movement is a depth cue where objects appear to move at different speeds depending on their distance from the observer. Closer objects seem to move faster across the field of view, while farther objects appear to move more slowly. This difference in motion helps us judge how far away something is.

Question 73

What is stereopsis?

Answer 73

Stereopsis is the perception of depth created by combining two slightly different images received from each retina, enhancing depth perception accuracy.

Question 74

What is retinal disparity?

Answer 74

Retinal disparity refers to the slight difference between the images seen by each eye due to their horizontal separation, contributing to depth perception.

Question 75

How does stereopsis benefit activities like sports?

Answer 75

Stereopsis improves depth perception accuracy, which is crucial for activities requiring precise judgment of distance, such as sports and pouring liquids.

Question 76

What is agnosia?

Answer 76

Agnosia is a neurological disorder where a person has difficulty recognizing or interpreting sensory information, despite having intact sensory abilities (such as sight, hearing, or touch) and no significant memory loss. For example, someone with visual agnosia may be able to see objects but not recognize them, while someone with auditory agnosia may hear sounds but struggle to identify them. This impairment is specific to processing sensory information and not due to a general cognitive or memory problem. Agnosia is often caused by strokes, brain damage, or neurodegenerative conditions like Alzheimer's.

Question 77

How is agnosia linked to the sensory cortex?

Answer 77

Agnosia is caused by damage to the sensory association cortex (the part of the brain that processes and interprets sensory information from the primary sensory areas like the visual or auditory cortex). This impairs the brain's ability to process and interpret sensory information, even though primary sensory areas remain intact. This results in an inability to recognize or understand sensory input despite normal sensory function.

Question 78

How does visual agnosia differ from general blindness?

Answer 78

Visual agnosia arises from damage to the visual association cortex, while general blindness can result from damage anywhere from the eye to the primary visual cortex.

Question 79

What is akinetopsia?

Answer 79

Akinetopsia is a specific type of visual agnosia characterized by the inability to perceive movement, resulting from damage to the dorsal visual stream in the parietal lobe.

Question 80

What is cerebral achromatopsia?

Answer 80

Cerebral achromatopsia is a visual agnosia caused by damage to the ventral visual stream in the cerebral cortex, leading to a complete lack of color perception, with individuals reporting only "shades of grey."

Question 81

How does cerebral achromatopsia differ from regular achromatopsia?

Answer 81

Unlike regular achromatopsia, which is total color blindness due to defective cone opsin signaling, cerebral achromatopsia involves awareness of color but a complete inability to perceive it due to brain damage.

Question 82

What kind of experience do individuals with cerebral achromatopsia report?

Answer 82

Individuals with cerebral achromatopsia report seeing only "shades of grey" and express an awareness of color, unlike those born with regular achromatopsia who lack any concept of color.

Question 83

What causes prosopagnosia?

Answer 83

Prosopagnosia is a visual agnosia that results in the inability to recognize familiar faces, caused by damage to the fusiform gyrus (fusiform face area) in the ventral visual stream.

Question 84

How does damage to the ventral visual stream affect visual processing?

Answer 84

Damage to the ventral visual stream disrupts the processing of visual information related to object recognition and color perception, leading to conditions like cerebral achromatopsia and prosopagnosia.

Question 85

What is the role of the fusiform gyrus in visual processing?

Answer 85

The fusiform gyrus is involved in the recognition of faces, and damage to this area can lead to prosopagnosia, impairing the ability to recognize familiar faces.

Question 86

Imagine a person can see objects but cannot recognize their friend's face in a photo. What condition might this person have?

Answer 86

This person might have prosopagnosia, a condition that impairs the ability to recognize familiar faces while still allowing for visual perception of objects.

Question 87

If someone who has experienced a stroke can see colors but cannot identify what those colors represent (e.g., they can see a red apple but do not recognize it as an apple), what might they be experiencing?

Answer 87

They might be experiencing visual object agnosia, where there is impaired object recognition due to damage in the ventral stream, despite intact color perception.