Vison (2)

Question 1

What are the three types of involuntary fixational eye movements?

Answer 1

Tremor, drifts, and saccades (small jerky movements).

Question 2

What happens when eye movements are blocked?

Answer 2

Visual objects begin to fade and disappear because most visual neurons respond only to changing images; stabilized retinal images are not detected effectively.

Question 3

Why do involuntary eye movements matter for vision?

Answer 3

They keep images moving on the retina, allowing the visual system to detect and perceive objects during fixation.

Question 4

What is visual transduction?

Answer 4

The conversion of light into neural signals by visual receptors (rods and cones).

Question 5

What is rhodopsin and where is it found?

Answer 5

A red pigment in rods that absorbs light and initiates rod-mediated vision.

Question 6

What happens to rhodopsin when exposed to intense light?

Answer 6

It becomes bleached (loses color) and temporarily loses its ability to absorb light, but recovers in darkness.

Question 7

How does rhodopsin transduction work in rods?

Answer 7

In darkness: sodium channels are partially open → rods depolarized → steady glutamate release. In light: rhodopsin bleaching triggers a cascade → sodium channels close → rods hyperpolarized → glutamate release decreases.

Question 8

What type of receptor is rhodopsin?

Answer 8

A G-protein–coupled receptor that responds to light instead of neurotransmitters.

Question 9

How does rhodopsin’s absorption spectrum relate to human vision?

Answer 9

The absorption spectrum of rhodopsin closely matches the scotopic spectral sensitivity curve, explaining rod sensitivity in dim light.

Question 10

What percentage of retinal ganglion cell axons form the retina-geniculate-striate pathway?

Answer 10

About 90% of retinal ganglion cell axons.

Question 11

How are signals from the left visual field processed in the brain?

Answer 11

Signals from the left visual field go to the right primary visual cortex, either ipsilaterally from the right eye’s temporal hemiretina or contralaterally via the optic chiasm from the left eye’s nasal hemiretina.

Question 12

How many layers does each lateral geniculate nucleus (LGN) have, and how are they organized?

Answer 12

Six layers; three layers receive input from one eye, and three from the other eye. Each LGN receives input from the contralateral visual field only.

Question 13

Where do most LGN neurons project in the primary visual cortex?

Answer 13

To the lower part of cortical layer IV, producing striation, which is why V1 is called striate cortex.

Question 14

What does retinotopic organization mean in the retina-geniculate-striate system?

Answer 14

Adjacent areas of the retina excite adjacent neurons at all levels of the system, forming a map of the retina.

Question 15

Why is the fovea disproportionately represented in the primary visual cortex?

Answer 15

Although the fovea is a small part of the retina, about 25% of V1 is dedicated to processing its input because it mediates high-acuity vision.

Question 16

What are the M and P channels in the LGN?

Answer 16

P (parvocellular) layers: top 4 layers; small neurons; responsive to color, fine details, stationary/slow objects; mainly receive cone input. M (magnocellular) layers: bottom 2 layers; large neurons; responsive to motion; mainly receive rod input.

Question 17

How do M and P channels project to the striate cortex?

Answer 17

They project to different areas of lower layer IV, which then send signals to different areas of the visual cortex.

Question 18

Why are edges considered important in visual perception?

Answer 18

Edges define the extent and position of objects and are the most informative features of a visual display.

Question 19

What is a visual edge?

Answer 19

The boundary where two different areas of a visual image meet, perceived as contrast between adjacent areas.

Question 20

What is contrast enhancement?

Answer 20

A mechanism where edges appear more pronounced: brighter areas look brighter and darker areas look darker than they physically are, creating Mach bands.

Question 21

What is a receptive field?

Answer 21

The area of the visual field within which a stimulus can influence the firing of a neuron.

Question 22

How did Hubel & Wiesel map receptive fields?

Answer 22

1. Place a microelectrode near a single neuron.
2. Paralyze eye movements and focus the image on the retina.
3. Identify the receptive field by testing stimuli that influence firing.
4. Characterize the neuron’s response to various stimuli.
5. Move to the next neuron and repeat.

Question 23

Which neurons did Hubel & Wiesel study in the retina-geniculate-striate system?

Answer 23

Retinal ganglion cells, lateral geniculate neurons, and lower layer IV striate neurons.

Question 24

What were four common properties of receptive fields across these neurons?

Answer 24

1. Foveal receptive fields are smaller than peripheral fields.
2. Receptive fields are circular.
3. Neurons are monocular (one eye only).
4. Many receptive fields have excitatory center and inhibitory surround, producing “on” and “off” responses.

Question 25

Describe the response pattern of on-center cells.

Answer 25

Light in the center → “on” firing. Light in the periphery → inhibition, followed by “off” firing when light is turned off.

Question 26

Describe the response pattern of off-center cells.

Answer 26

Light in the center → inhibition and “off” firing when light is turned off. Light in the periphery → “on” firing.

Question 27

What do on-center and off-center cells respond best to?

Answer 27

Contrast between the center and periphery of their receptive fields.

Question 28

What happens if the entire receptive field is uniformly illuminated?

Answer 28

There is little effect on firing; neurons respond primarily to contrast, not uniform light.

Question 29

Are visual neurons active even without visual input?

Answer 29

Yes, most neurons show spontaneous activity, and responses to stimuli use only a small fraction of the energy.