Measuring Visual Acuity

Question 1

What is the Visual angle?

Answer 1

This is the angle that an image subtends at the eye by the height of the image viewed from a distance

*image size is typically presented in terms of visual angle

Question 2

What does the image size depend on?

Answer 2

Whatever the absolute size of the image, it will depend upon viewing distance

If you move right up close to the image, it is going to give a large visual angle

From a distance, it will be a small visual angle

Question 3

What limits the resolving power of the eye, assuming vision is perfect (20/20)?

Answer 3

Spacing of cones

Question 4

What is the relationship between the Visual angle and the size of the retinal image?

Answer 4

The visual angle is going to be the same with respect to the image that is projected onto the retina

• two alpha angles are going to be the same
• use trigonometry to measure the height of the alpha angle on the retina, working out the height of the retinal image
• if you know the visual angle, you can work out what size that will be on the retina
• diameter of the eyeball is constant at about 24-25mm

Question 5

What is spatial frequency?

Answer 5

the relative width of the bands in a sine-wave grating, measured in cycles per degree of visual angle
-number of cycles per degree

The more cycles per degree there are

• the higher the spatial frequency
• the higher the resolving power
• the finer the detail

Question 6

What do higher spatial frequencies require?

Answer 6

The higher the spatial frequency, the more detail there is.

Highest spatial frequencies require high contrast and small receptive fields.

Question 7

What is the finest detail we can see?

Answer 7

30 cycles per degree

-we can only do this at high contrast levels

Question 8

How do we use gratings?

Answer 8

Typically, in a grating, rather than using abrupt on/off steps, you would smooth it in a sine-wave format, to represent the transition between the white and black. The reason for this is because the RGC receptive field and their centre-surround organisation are set up to detect changes, and so you want to avoid very high contrast edges.

But, if you do have very high contrast edges, it would be a specific hyper-stimulus potentially to retinal ganglion cells.

Question 9

What sets the upper limit on spatial frequency?

Answer 9

Cone spacing in central fovea (160k per mm*2)

Question 10

Which cones in the retina dominate the central fovea? And what are they involved in?

Answer 10

Red and green cones dominate the central fovea, and they are involved in high-resolution vision

Question 11

What is contrast?

Answer 11

the difference between minimum and maximum luminance

*luminance is the amount of light coming off a stimulus

Question 12

What is contrast sensitivity?

Answer 12

the ability to detect differences in light and dark areas

Question 13

What is contrast threshold?

Answer 13

The minimum contrast required for detection of a stimulus

*reciprocal of contrast sensitivity

Question 14

How can we calculate contrast?

Answer 14

Maximum luminance (Im) - Minimum luminance (In) / Maximum luminance (Im) + Minimum luminance (In)

Question 15

What is the relationship between Contrast sensitivity and spatial frequency?

Answer 15

Contrast sensitivity varies with spatial frequency

Question 16

How do we measure contrast sensitivity?

Answer 16

Contrast sensitivity has traditionally been measured using sinusoidal gratings of different spatial frequency

Essentially, you can do psychophysical experiments where you can vary the spatial frequency and measure the contrast sensitivity

Question 17

How is contrast sensitivity determined?

Answer 17

Contrast sensitivity is determined by measuring contrast threshold at a given spatial frequency (say it was a contrast of 0.01), then taking the reciprocal (contrast sensitivity of 100)

This allows increasing sensitivity to be displayed in the positive direction

Question 18

At intermediate spatial frequencies, contrast sensitivity is…

Answer 18

reasonably constant

this captures what is intuitively obvious, that if you are trying to resolve fine detail, you would need brighter lighting (more contrast)

• because, the higher the spatial frequency
• higher resolving power
• finer detail

Question 19

What are orientation columns?

Answer 19

Organised regions of neurones located in the primary visual cortex (V1), spanning multiple cortical layers, that are excited by visual line stimuli of varying angles

The geometry of the orientation columns are arranged in slabs that are perpendicular to the surface of the primary visual cortex

Question 20

What are orientation columns tuned to?

Answer 20

Particular orientations

Spots of light testing the LGN and RGC neurones don’t work very well, but elongated visual stimuli such as bars of light or gratings do work well

Question 21

What is the tilt aftereffect?

Answer 21

The tilt aftereffect (TAE) is a visual illusion in which the perceived orientation of a line/grating is changed after prolonged adaptation to another oriented line/grating

This is because after prolonged inspection of a line/grating, you adapt to that orientation, and therefore when you are presented with a new orientation, the de-coding mechanism is biased because of the previous adaptation

Question 22

Describe the response to orientations Considering orientation tuning of neurones.

Answer 22

· Orientation-sensitive neurones do not respond to just a single orientation (they respond to a range → broadly tuned)

· Tuning curve describes the range and selectivity with which a neurone responds to orientations

Question 23

How specific are orientation-sensitive cells?

Answer 23

We don’t have an orientation sensitive cell for every possible orientation. Therefore, we compare the signals across cells with different orientation tuning.

The system does not need an orientation sensitive cell for every possible orientation. Orientation could be estimated by averaging cells with similar preferences.

Question 24

Explain the tilt-after effect.

Answer 24

Errors in estimation of orientation would occur if prolonged stimulation is applied.

If a cell is adapted to one orientation, the response is decreased to this preferred orientation or similar orientations due to adaptation. Switching to a different orientation- adaptations take seconds to ‘fade away’. New orientation is therefore interpreted in the opposite direction due to adaptation, slightly off (due to asymmetry from reduced response). Therefore this effect only works if the difference in orientation is such that there is an overlap within the tuning curve

The strength of the illusion will decline as the difference in orientation increases

Question 25

What does the tilt-after effect imply?

Answer 25

Population coding of orientation
· adapt to grating, then measure threshold contrast required to detect vertical test grating
· plot as function of adapting grating orientation
· effect declines and disappears as difference in adapting orientation increases