99HUB103 Vision

1
The Physical Stimulus Spatial pattern
(This is a poorly-generated approximation to a
sine wave)
2
The Physical Stimulus Spatial pattern
Why are we interested in the visual systems
response to sine waves? When you know the
response to individual sine waves you can, if the
system is linear, determine the response to
the complete pattern. Remember the Fourier
analysis tutorial. We can think of the limbs
of the letter E as being a square wave - and,
as you know, this can be generated by adding
together a series of sine waves.
3
The Physical Stimulus Spatial pattern
photometer
4
(contrast)
Position
N.B. The (spatial) frequency 1/period For
vision, this is measured in cycles per degree
5
Spatial (Threshold foveal Performance)
Pattern not seen
1 Modulation
Visual Acuity.
(i.e.reciprocal contrast)
Pattern seen
(100)
Spatial Frequency
6
The following slide allows you to view your
own contrast sensitivity function. Try viewing
it at different distances, and youll see the
appearance change. Also, you may see
inconsistencies at the extreme right of the
figure. This is termed aliasing and occurs
because the pixel nature of the screen does not
allow perfect reproduction of the picture.
7
Seeing your own CSF
Spatial Frequency
Contrast
8
The Measurement of Spatial (foveal) Visual
Performance
Contrast sensitivity is a relatively new way of
assessing vision The more traditional way is to
use an Optometrists test chart, which contains
letters of different sizes.
9
H VF ETP NUFCOK UTFEVNP
60 36 24 12 6
Letter Size (explained on the next slide)
10
The Measurement of Spatial (foveal) Visual
Performance
The letter size is specified by the distance at
which the complete letter subtends 5 mins of arc.
The detail subtends 1 min arc.
.
6 metres
12 metres
11
The Measurement of Spatial (foveal) Visual
Performance
6 6 Vision - the ability to read the 6 metre
letter at a distance of 6
metres (USA - 20 feet) hence 20
20 Minimum angle of
resolution (MAR) here is 1 minute of arc
12
The Measurement of Spatial (foveal) Visual
Performance
Minimum angle of resolution (MAR) here is 1
minute of arc A recent development is to use as
a metric the logarithm of this angle - logMAR
scale (Bailey and Lovie) 6/6 1
minute arc gt log 1 0 6/12
2 minutes arc gt log 2
0.3 6/60 10 minutes arc gt log 10
1 Measured this way, the logMAR scale is,
arguably, at an interval level - hence one can
perform parametric statistical analysis on data
obtained using this test.
13
H V F E T P N U F C O K F V U P L N
P N P U L B E V

The following are letter charts which have
decreasing contrast.
1 2 3 4 5 6
14
E H F E T P N U F C O K P L N F V
U L L B E V N P
U
1 2 3 4 5 6
15
P H E V E T P N U F C O K E V U T F N
P E V N L B P
U
1 2 3 4 5 6
16
P H E V E T P N U F C O K E V U T F N
P E V N L B P
U
1 2 3 4 5 6
This is a high-contrast version of the previous
chart.
17
Compare E E
smaller
higher
Letter Size Spatial
Frequency
18
The Optometrists chart will generally only
contain high-contrast letters. These provide
maximum acuity. As you will see from the
following graph, one would predict that lower
contrast letters would produce a lesser
visual performance.
19
Contrast Sensitivity
0.1
Pattern not seen
1
Pattern seen
Contrast
10
100
Spatial Frequency
Cut-off