Unit 1.5 Visual Acuity

Question 1

What is visual acuity?

Answer 1

Visual acuity is the smallest spatial detail that can be resolved by the visual system.

Question 2

How is visual acuity commonly measured?

Answer 2

Visual acuity is measured using distance (e.g., 20/20 or 6/6) by optometrists and by the smallest visual angle of a cycle of a grating by vision scientists.

Question 3

What does a visual acuity of 20/20 (6/6) mean?

Answer 3

It means that a person can see at 20 feet what a person with normal vision can see at 20 feet, recognizing letters at a visual angle of 5 minutes of arc.

Question 4

Who invented the method for designating visual acuity?

Answer 4

Herman Snellen

Question 5

What are Landolt Rings, and why are they used?

Answer 5

Landolt Rings are specially designed symbols used in visual acuity testing in many European countries; they do not rely on the test subjects knowing letters.

Question 6

Is 20/80 (6/24) good or bad eyesight?

Answer 6

bad

Question 7

What is spatial frequency in the context of visual acuity?

Answer 7

Spatial frequency refers to the number of cycles of a grating per unit of visual angle, usually specified in degrees.

Question 8

What is contrast sensitivity?

Answer 8

Contrast sensitivity is the ability to detect differences in illumination between a figure and its background.

Question 9

What are cycles per degree?

Answer 9

the number of dark and bright bars per degree of visual angle

Question 10

What is the Contrast Sensitivity Function (CSF)?

Answer 10

describes how contrast sensitivity varies with spatial frequency and is used to assess visual acuity.

Question 11

What is the contrast threshold?

Answer 11

the minimum difference in contrast for you to detect a pattern (lower threshold = see better)

Question 12

What is the relationship between contrast threshold and contrast sensitivity?

Answer 12

Contrast sensitivity is the inverse of contrast threshold; a lower contrast threshold indicates better contrast sensitivity.

Question 13

For photopic (daylight vision), where does the contrast sensitivity peak?

Answer 13

around 2-4 cycles per degree (sensitivity is reduced for mesopic (twilight) or scotopic (nighttime) vision)

Question 14

How does visual acuity change with age?

Answer 14

contrast sensitivity is reduced with aging, particularly for high spatial frequencies, affecting overall visual acuity.

Question 15

How does the visual system process sine wave gratings?

Answer 15

The visual system samples the grating discretely, breaking down images into components, each represented by a sine wave grating of a particular spatial frequency. This process is known as Fourier analysis.

Question 16

What is the role of phase in sine wave gratings?

Answer 16

The phase of a grating refers to its position within a receptive field. It can affect the response of ganglion cells, with maximum responses occurring when the phase aligns appropriately with the receptive field structure.

Question 17

What is the role of phase in sine wave gratings?

Answer 17

The phase of a grating refers to its position within a receptive field. It can affect the response of ganglion cells, with maximum responses occurring when the phase aligns appropriately with the receptive field structure.

Question 18

How do ganglion cells respond to different spatial frequencies of gratings?

Answer 18

Low spatial frequency gratings activate ganglion cells weakly, while medium frequencies elicit a high response, and high spatial frequency gratings produce a weak response.

Question 19

What is the role of phase in the response of ganglion cells to gratings?

Answer 19

The phase of a grating refers to its position within a receptive field. Ganglion cells have optimal responses when the grating aligns well with their receptive field structure; a shift in phase can lead to reduced or no response.

Question 20

What is the significance of center-surround receptive fields in ganglion cells?

Answer 20

Center-surround receptive fields allow ganglion cells to detect contrast. When light is presented in the center of the receptive field, the cell fires rapidly, while light in the surroundings can inhibit this response, enhancing edge detection and contrast sensitivity.