topic 5: spatial vision

Question 1

Differentiate detection acuity, resolution acuity, localization acuity and recognition acuity

Answer 1

Detection
Determine whether a stimulus or aspect of a stimulus is absent or present

Recognition
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Resolution
Detect a gap in a stimulus

Localisation
Discriminate differences in spatial position of parts of a stimulus (Vernier acuity)

Question 2

Explain hyperacuity. what are 2 types of hyperacuity tasks?

Answer 2

acuity task that arises from cortical processing instead of cone density

1.Vernier acuity: ability of the visual system to sense direction
Is the line exactly vertical or slightly angled?
Is the picture hung slanted on the wall or perfectly straight?

2.Stereoacuity: ability to perceive disparity (3D)
Sharpness or smoothness of an edge: ability to detect edge or line ‘roughness’•

Question 3

Explain the diffraction limit of the human eye

Answer 3

The Rayleigh criterion for the diffraction limit to resolution states that two images are just resolvable when the center of the diffraction pattern of one is directly over the first minimum of the diffraction pattern of the other.

Question 4

Describe the construction and scoring of logMAR charts

Answer 4

• They have the same number of letters in every row
• There is equal reduction in letter size from row to row. (unlike Snellen which have huge jumps in letter size especially at the larger sized letters)

In logMAR, the smaller the value the better the VA.
Each row is 0.1 logMAR. As there are 5 letters per row, each letter is 0.1/5=0.02 logMAR.
(Miss a letter, add 0.02. Read more letter, subtract 0.02 per letter)

Question 5

Demonstrate the methods of measuring CSF

Answer 5

1. Present a grating of a specific spatial frequency
2. Slowly increase the contrast of the grating from non-seeing to seeing (method of adjustment)
3. The sensitivity is the reciprocal of threshold (sensitivity = 1/threshold)
4. Repeat this for a range of spatial frequencies

Question 6

explain the physiological reasons behind the shape of the CSF

Answer 6

Lower sensitivity at low spatial frequency due to lateral inhibition (see next section on “receptive fields and the CSF”)

CS peaks at middle spatial frequencies as the grating matches receptive field size

The reduction of sensitivity at high spatial frequencies is due to the optical limits of the visual system to resolve detail (aberrations)

High spatial frequency cut off is at 30-60 cycles/degree, which corresponds to the high contrast Snellen BCVA,

It is related to the packing density of cone photoreceptors at the fovea. The high packing density allows light parts of the sine grating to fall on alternate rows of photoreceptors, thereby leading to better resolution (ability to discern detail)

Question 7

Describe how scotopic presentation, peripheral presentation, ametropia, cataract and diseases can affect the CSF

Answer 7

scotopic presentation:
contrast sensitivity declines with luminance, slowly losing the peak in the middle and high spatial frequencies. At even dimmer conditions the contrast sensitivity is reduced across all spatial frequencies.

peripheral presentation:
reduced sensitivity with increasing retinal eccentricity is believed to be due to the lower cortical representation of the visual field for peripheral retina vs central retina. (i.e. more of the visual cortex in the brain is dedicated for electrical signals from the central retina than the peripheral retina)

Question 8

Discuss the relationship between CSF and resolution acuity.

Answer 8

At the end of the CSF where y=0 and x= 30 cycles/degree, that is equivalent to 6/6 VA on a 100% contrast chart such as Snellen.

60 cycles/degree is equivalent to 6/3 VA