Vision(1)

Question 1

Can any animal see in complete darkness?

Answer 1

No. All vision depends on light; some animals can see under very dim light, but no animal can see in complete darkness.

Question 2

Can other animals see wavelengths humans cannot? Give an example.

Answer 2

Yes. Example: rattlesnakes can see infrared waves (too long for humans), allowing them to detect warm-blooded prey in dim conditions.

Question 3

What two properties of light are important for human perception?

Answer 3

Wavelength: relates to color perception

Intensity: relates to brightness perception.

Question 4

Are color and brightness properties of light itself or perception?

Answer 4

They are perceptions created by the visual system.

For example, ‘bright red light’ refers to perception, not the physical property itself.

Question 5

What structure controls the amount of light entering the eye?

Answer 5

The pupil, a hole in the iris, regulates light entry. The iris contains contractile tissue that adjusts pupil size.

Question 6

How does pupil size affect sensitivity and acuity?

Answer 6

Small/constricted pupils: sharper image, greater depth of focus, less light (used in bright conditions).

Large/dilated pupils: more light for dim conditions, but reduced acuity and depth of focus.

Question 7

What is the function of the lens in the eye?

Answer 7

The lens focuses incoming light onto the retina by bending (refracting) it.

Question 8

How does the lens adjust for near and distant objects?

Answer 8

Near objects: lens becomes more cylindrical (ciliary muscles adjust tension), increases refraction, sharp focus.

Distant objects: lens flattens, less refraction. This process is called accommodation.

Question 9

Why do vertebrates usually have two eyes?

Answer 9

Two eyes allow vertebrates to see from left and right sides, giving a nearly 360° visual field without moving the head.

Question 10

Why do some vertebrates (including humans) have forward-facing eyes?

Answer 10

Forward-facing eyes allow both eyes to view the same object simultaneously, enabling depth perception and three-dimensional vision.

Question 11

What is binocular disparity?

Answer 11

The difference in the position of the same image on the two retinas. It is greater for close objects and helps the brain construct 3D perception from 2D retinal images.

Question 12

When is convergence of the eyes greatest?

Answer 12

When inspecting close objects, because the eyes must turn slightly inward to align images on corresponding retinal points.

Question 13

What is the primary function of the retina?

Answer 13

The retina converts light into neural signals, conducts them toward the CNS, and participates in processing those signals.

Question 14

What are the five main types of retinal neurons?

Answer 14

Receptors, horizontal cells, bipolar cells, amacrine cells, retinal ganglion cells.

Question 15

What is the role of horizontal and amacrine cells in the retina?

Answer 15

They are specialized for lateral communication, processing signals across the major channels of sensory input.

Question 16

How do retinal neurons communicate?

Answer 16

They communicate chemically via synapses and electrically via gap junctions.

Question 17

What is unique about the retina’s structure?

Answer 17

The retina is inside-out: light passes through other layers before reaching the receptors, and signals travel back out to the retinal ganglion cells.

Question 18

What visual problem is created by the exit of retinal ganglion cell axons?

Answer 18

The blind spot, a gap in the receptor layer where axons exit the eye.

Question 19

How does the fovea help reduce visual distortion?

Answer 19

The fovea reduces visual distortion because light hits the photoreceptors directly, without passing through many other cells. This gives us clear, sharp vision in the center of what we see.

Question 20

What is completion (filling in) in vision?

Answer 20

The visual system uses information from receptors around the blind spot (or edges of objects) to fill in missing information, creating a continuous perception of objects.

Question 21

What is surface interpolation?

Answer 21

A process where the visual system infers the appearance of large surfaces (like color and brightness) from information about edges, rather than directly from the retina.

Question 22

Why is completion and surface interpolation important?

Answer 22

It shows that the visual system does more than copy the external world; it actively constructs perception from partial information.

Question 23

What are the two types of photoreceptors in the human retina?

Answer 23

Cones (cone-shaped) and rods (rod-shaped).

Question 24

What does the duplexity theory of vision state?

Answer 24

Cones and rods mediate different kinds of vision:

Photopic vision (cones): high-acuity, color vision in bright light.

Scotopic vision (rods): sensitive vision in dim light, but low acuity and no color.

Question 25

Where are cones and rods distributed in the retina?

Answer 25

Fovea: only cones, no rods. Boundary of fovea: cones decrease, rods increase. Max rod density: 20° from fovea. Nasal hemiretina: more rods than temporal hemiretina.

Question 26

What is the main difference in wiring between the photopic and scotopic systems?

Answer 26

Scotopic system: hundreds of rods converge on one retinal ganglion cell → high sensitivity, low acuity. Photopic system: few cones converge on one ganglion cell → lower sensitivity, high acuity.

Question 27

How does convergence affect vision in dim light?

Answer 27

In dim light, rod outputs summate on ganglion cells → detectable signal. Cones cannot summate as effectively, so dim light may not be perceived by cones.

Question 28

What are spectral sensitivity curves?

Answer 28

Graphs showing the relative brightness of lights of different wavelengths at the same intensity. Humans have two curves: photopic (cones) and scotopic (rods).

Question 29

At what wavelengths are humans most sensitive under photopic and scotopic conditions?

Answer 29

Photopic (cones): ~560 nm → yellow-green light. Scotopic (rods): ~500 nm → blue-green light.

Question 30

What is the Purkinje effect?

Answer 30

A shift in relative brightness of colors during the transition from daylight (photopic) to dim light (scotopic). Red/yellow appear bright in daylight, but blue/green appear brighter at dusk.

Question 31

Why does the Purkinje effect occur?

Answer 31

Because rods and cones have different spectral sensitivities: Cones: peak at 560 nm → see yellow/red best in daylight. Rods: peak at 500 nm → see blue/green better in dim light.