optometry education, Ophthalmology, optometrist, vision science

1. Visual Science

2. Spatial Vision
• Objects with different luminance profile in visual field are considered as spatial target and the perception is
called spatial vision.

3. Four attributes we should define while
specifying a detail on as spatial target:
i) Frequency:
- Frequency simply means how close the spatial targets lie
or how close are two light or dark bands.
- Frequency is measured in cycles/degree.
- More cycles in a degree means more frequency.
ii) Contrast:
𝑪𝒐𝒏𝒕𝒓𝒂𝒔𝒕 =
𝑰𝒎𝒂𝒙 − 𝑰𝒎𝒊𝒏
𝑰𝒎𝒂𝒙 + 𝑰𝒎𝒊𝒏
= ∆𝑰/𝑰𝒂𝒗𝒈

4. iii) Phase:
- Position of a sine wave grating with respect to
another grating is known as phase.
- If two grating are 180 degree out of phase two
light/dark grating overlap with each other.
iv) Orientation:
- Orientation refers to the angle made by
grating with respect to a reference, such as
horizontal.

5. Fourier Wave transformation:
• Sine waves of proper frequency, contrast, phase
and orientation can be used to construct other
more complex spatial stimuli.
• Add one trough/dark with another trough/dark.
• Keep on adding the wave and the last final wave
obtained will be a square wave with 100 %
blackness or whiteness.

10. Spatial Modulation Transfer Function:
• It tells us about how perfectly a lens or lens system
transfers information about the object.
• No lens is perfect, there is always degradation in
quality of image formed.
• There is reduction in contrast and image clarity is
affected even more by inherent aberration within
lens.
• This problem remains unnoticed most of the time at
lower spatial frequency than at higher spatial
frequency.

11. Human Contrast Sensitivity Function
• Consider three spatial frequency gratings at 100
% contrast.
• One with lower, other with medium and last
one with higher spatial frequency.
• Keep on decreasing the contrast of all three
grating until all grating will start to fade slowly.
• Keep on decreasing the contrast until the last
just one that is visible remains.
• Typical human contrast sensitivity can be
plotted in a graph. It is found that human
contrast sensitivity function peaks at spatial
frequency of 4cycles/degree.

12. CSF High Frequency Cut Off
• CSF shows reduction in sensitivity at high
spatial frequency.
• As spatial grating frequency is kept on
increasing, a point is reached where
grating can no longer resolved at even at
100% contrast.
• For normal adult, its value is 60
cycle/degree.
• By measuring Visual acuity of a person we
are measuring the high frequency cut-off
value for him/her
High frequency cut-off value is limited by inherent
optical aberration of human lens system and
photoreceptor packing Density

13. Low Frequency Drop Off CSF
• Receptive field of typical ganglion cell manifests centre-
surround region that respond in opposing fashion to
illumination.
• Light falling on centre causes excitation, while light falling on
surrounding causes inhibition.
• Cell will be optimally activated when bright bar falls on centre
of its receptive field and dark bar on its surrounding.
• when Both centre and surround is activated by single iso-
luminance grating the excitation and inhibition occurs
simultaneously; thus reducing the visibility of a target.
• Though the target is seen it is not perceived as the part of a
single grating system.

14. Mach bands:
• As seen in the grating where there is abrupt change in
luminance profile from dark to light or vice versa.
• Human eye is capable of separating the junction between two
luminance profile.
• But in practical life situation transition between dark and light
region is gradual.
• Human eye will only perceive drastic luminance change zone
but can’t perceive the zone with gradual change
• This invisible zone of gradual luminance change is called Mach
Bands.

15. Temporal vision
• Insert time into a spatial target and we get a temporal
target.
• Temporal target is the target with change in luminance
profile over time.
• Even here an ideal target for describing temporal
vision is a spatial grating modulated along with time.
• Ability of our visual system to detect time related
change in the property of a spatial target is called
temporal vision
• That blinking rate of the spatial target is temporal
frequency.

16. Temporal Frequency:
• Flickering light bulb is an example of temporally
modulated stimulus at high frequency.
• Hour hand of a clock moves at very slow temporal
frequency. which makes it impossible to for our visual
system to detect any temporal changes.
• Temporal target can also be explained with wave
function.
• For light off situation its trough and for light on situation
its crest
• Presentation of spatial target can be represented by
change in sinusoidal wave over time.

17. Depth of Modulation
• Target refers to the amplitude of change in luminance over time.
• 𝑃𝑒𝑟𝑐𝑒𝑛𝑡𝑎𝑔𝑒 𝑀𝑜𝑑𝑢𝑙𝑎𝑡𝑖𝑜𝑛 =
𝐴
𝐼𝑎𝑣𝑔
𝑥100
• A =amplitude of modulation
• lave = time-averaged luminance

18. Temporal Modulation Transfer Functions:
• Consider a light flickering at low frequency. Increase
the frequency of the light flickering until no flickering
is seen.
• Change the depth of the modulation maintaining the
same temporal frequency; the light will be seen
flickering again.
• Keep on increasing temporal frequency such that
further depth modulation doesn’t cause any
flickering.
• The resulting temporal frequency is called High
Frequency Cut-off value.
• Repeat the process but in reverse order.
• Decrease the temporal frequency and depth
of modulation until light is seen steady or
change in illumination over time is not
perceived.
• This low frequency where light is seen no
more flickering is called Low Frequency
Drop-off.
Hence, for every percentage of modulation it has its own high frequency cut-off and low frequency drop-off
value.

19. Critical Flicker Fusion Frequency (CFF):
• Temporal Frequency where flicker can no longer be
resolved is called Critcal Flicker Frequency.
• High temporal resolution limit of the visual system for
a given depth of modulation.
• Typically given in Hertz (Hz) i.e. 1 Hz = one
cycle/second.

20. Broca–Sulzer Effect:
• Suprathreshold flashes of light with a duration
on the order of 50 to 100 ms appear brighter
than stimuli of either shorter or longer
durations, a phenomena referred to as the
Broca–Sulzer effect.

21. Brucke-Bartley Effect:
• Light presented at approximately 10 Hz, which is seen as flickering, appears brighter
than a steady light of the same average luminance.
• Brücke–Bartley effect is a manifestation of the Broca–Sulzer effect.

22. Talbot-Plateau Law:
• Brightness of a temporally modulated stimulus, when fused, is equal to the
brightness of a steady light with the same time-averaged luminance.
• The temporally modulated light must be presented at a rate beyond the CFF.

23. Color Measurement System
• Three color making attributes are Hue, Brightness and Saturation.
a) Hue: Hue is that perception which is most closely associated with wavelength.
b) Saturation: Desaturated color appears as though it has been mixed with white.
- A monochromatic stimulus, which by definition has no white light added to it, is said to have a
colorimetric
purity of 1. Colorimetric purity, a physical property of the stimulus, is given, by the following formula:
Where 𝑝 =
𝐿ʎ
𝐿ʎ + 𝐿w
p=colorimetric purity
Lʎ= luminance of the monochromatic light
Lw =luminance of the white light that is combined with the
monochromatic light
c) Brightness: Brightness sensation closely follows the photopic luminance function

24. Bezold–Brücke phenomenon:
• Most monochromatic stimuli slightly change hue as their
intensity is adjusted.
• Consider a greenish yellow test wavelength of 550 nm. As
the intensity of this wavelength is increased, it appears to
be of a longer wavelength (i.e., it appears more yellowish).
• To maintain the initial hue appearance (greenish yellow), it
is necessary to reduce its wavelength.
• This is indicated in Figure by the line that starts at 550 nm
and tilts toward shorter wavelengths as the intensity
increases.
• These wavelengths, 478, 503,and 578 nm, are referred to
as invariant wavelengths or invariant points.
• The hues associated with these wavelengths—blue at 478
nm, green at 503 nm, and yellow at 578 nm—are called
unique hues.

25. Color constancy:
• Color constancy refers to the approximately constant color
appearance of objects as lighting conditions change.
• Color constancy assists us in identifying objects as lighting
conditions vary.
• Red apple appears approximately the same color both indoors and
outdoors and at various times of the day.

26. Physiological Basis of Color Vision:
• There are two main theories that explains color vision:
i) Trichromatic Theory
ii) Color opponent theory

27. Trichromatic Theory (Young-Helmholtz theory)
• According to this theory there are three cones in Retina that are responsible for color vision. Namely
red, green and blue cone.
• Mixture occurs by differential excitation and absorption of various wavelength which are derivative
of these primary colors.
• Once a quantum of light is absorbed, all information regarding its wavelength is lost, a principle
referred to as univariance.
• Helmholtz used color-matching experiments where participants would alter the amounts of three
different wavelengths of light to match a test color.
• Participants could not match the colors if they used only two wavelengths but could match any color
in the spectrum if they used three.
• The theory became known as the Young-Helmholtz theory of color vision.

28. Color opponent theory (Hering’s Opponent-Colors
Theory)
• The opponent-process theory states that the cone
photoreceptors are linked together to form three
opposing color pairs: blue/yellow, red/green, and
black/white.
• Activation of one member of the pair inhibits activity in
the other.
• A color perception is never described as reddish-green
or yellowish-blue.
• Besides the cones, which detect light entering the eye,
the biological basis of the opponent theory involves two
other types of cells: bipolar cells, and ganglion cells.

30. Color measurement System:
• This measurement is necessary because we have
to accurately connect an external light or surface
with an internal color sensation.
a) Munsell Color measurement
system
- Munsell color system is a color space that specifies
colors based on three properties of color: hue,
value (lightness), and chroma (color purity).
- Munsell divided into five principal hues: Red,
Yellow, Green, Blue, and Purple, along with 5
intermediate hues.

31. Maxwell Color Triangle
• A color triangle is an arrangement of colors
within a triangle, based on the additive
combination of three primary colors at its
corners.
• Scottish physicist James Clerk Maxwell
(1831-1879), He demonstrated that most
colors could be created by combinations of
three "primary" colors.

32. CIE color Space
• Commission Internationale de l'éclairage or International
Commission on Illumination.
• CIE 1931 color spaces were the first defined quantitative links
between distributions of wavelengths in the electromagnetic visible
spectrum, and physiologically perceived colors in human color
vision.
• Essential tools for color management, important when dealing with
color inks, illuminated displays, and recording devices such as digital
cameras.
• Relative amounts of primaries are denoted by x,y, & z. So their
sum, x+y+z=1. The tristimulus values are given by uppercase
letters, whereas the chromaticity coordinates are given in
lowercase letters.

33. • Two wavelengths can be mixed at any
proportion. Amount of proportion of two
wavelengths can be represented with
coordinates joining those two wavelengths
• Length of line connecting white and mixture
coordinate point and white and perimeter hue
point gives the value of calorimetric purity (in
CIE Excitation Purity).
Those two wavelengths which can be mixed
at equal proportions to give a white color are
called Complementary Colors.

34. Motion Perception
• Perception of object or target moving is know as motion perception.
• Motion perception is due to change in spatial distribution of light over time.
• Motion can be either real motion where target or subject actually moves or it could be illusory motion
where spatial targets are doesn’t move but we perceive motion.
• Middle temporal area (MT or V5) and medial superior temporal area (MST or V5a) involved in motion
perception
Real Motion:
-Real motion occurs when target actually moves. When target moves the spatial distribution of target image
changes across retina.

35. Apparent motion/Illusory motion:
a) Stroboscopic movement
b) Induced movement
c) Autokinetic movement
d) Movement after-effect

36. Stroboscopic movement:
Movement between two lights, when they are switched on and off alternately.
The nature of movement between the flashing lights depend upon both time and distance between them.
• Simultaneous Perception: if the time interval between the two flashes is less than 30 msec, they are perceived to flash
on and off simultaneously.
• Partial perception- as the interval is increased to about 30 msec, partial movement is perceived.
• Optimal movement- at an interval of about 60 msec, the light appears to move continuously. Looks like real movement.
Film frame projection rate.
• Phi-movement- at an interval of around 60-200 msec, type of movement perceived is objectless. Although movement
appears to occur but it is difficult to perceive the object

39. Induced movement:
- Light surrounded by larger objects move so that the light appears to move in opposite direction.
E.g. perception of moon racing through the clouds while it is the clouds that are moving.
Autokinetic movement/effect
- When a small stationary spot of light is observed in a dark room, the spot of light appears to move.
- It can be demonstrated by looking at a lit cigarette across a dark room.
Movement after-effect
- Moving stripes viewed prior to light.
- Light appears to move in opposite direction to the motion of stripes.
- Also called waterfall illusions- if one stares at a waterfall about 30-60 secs and looks away towards
something else, he will see the scenery move upwards

40. Autokinetic Movement after effect

41. Order of Motion
1st-order motion : First order motion, which is what we encounter in our daily lives, can be easily detected by
finding the peak in the Fourier energy distribution.
- This is motion due to luminance change
2nd-order motion
- Second-order motion has been defined as motion in which the moving contour is defined by contrast,
texture, flicker or some other quality

42. • The smallest percentage of coherence level that
gives perception of motion is called Coherence
threshold.
• The minimum distance dots have to move in
given direction to elicit the perception of motion
is called Minimum Displacement threshold.
• The maximum separation allowed for still to
perceive motion is called Maximum
displacement threshold.
This temporary suppression of movement of visual world
during saccades or pusuits is called saccadic suppression.

43. Threshold and Signal Detection Theory
• Any stimulus that can be detected by our visual system
for 50% time of the presentation is called the threshold
of the stimulus.
• Range of stimulus intensities, from dim to intense, and
the percentage of stimuli detected is plotted as a
function of stimulus intensity to produce a frequency of
seeing (FOS) curve, also referred to as a psychometric
function.
For ideal observer

44. Determination of threshold
Method of Ascending Limits
ii) Method of Descending Limits
iii) Staircase Method
iv) Method of Constant Stimuli
v) Method of Adjustment
vi) Forced Choice Method

45. Method of Ascending Limits:
- Stimulus is initially presented below threshold.
- The stimulus intensity is increased systematically until the observer reports that it is
visible.
- Several trials may be performed and observations are averaged.
Eg: Dark Adaptometry

46. Method of Descending Limits:
• A trial commences with a clearly visible stimulus (i.e., the stimulus is above
threshold)
• The visibility is decreased systematically until it can no longer be seen.
• The method of descending limits may be contaminated by observer anticipation.
E.g.: Visual Acuity test

47. Staircase Method:
• Combination of ascending and descending limits
• Increase visibility of subthreshold stimulus until
observer reports seeing the stimulus.
• Reverse visibility of the stimulus is reduced until
the observer reports that it cannot be detected.
• Threshold is taken to be the stimulus intensity at
one of the reversals, for example, the fourth
reversal.
Eg: Full threshold Strategy of Automatic Visual
field

48. Method of Constant Stimuli
• Stimulus visibility is varied randomly from presentation
to presentation.
• Because the observer is typically asked whether or
not he or she sees the stimulus.
• Sometimes referred to a “yes–no” procedure
• A FOS curve is plotted and 50% visibility is taken as
the threshold.
e.g. Supra threshold Strategy in Visual field

49. Method of Adjustment
• Subject adjusts the stimulus intensity until it is barely visible (or invisible).
• Anticipation and variations in the observer’s threshold criterion

50. Forced Choice Method
• Observer is forced to choose between
several alternative choices, one of which
contains the stimulus.
• If the experiment forces the observer to
choose between two alternatives, it is
referred to as a two alternative forced
choice (2AFC) experiment
• AFC for 4 alternatives as in figure below.
e.g. Forced choice preferential test

51. SIGNAL DETECTION THEORY
• The theory assumes that within the visual system there is a randomly fluctuating level of
background neural activity—the so-called noise.
• The observer’s task is to differentiate the signal and noise combination from the background noise
alone.
• Influenced by a number of factors, including decision criteria, attention, motivation, and internal
neural noise.
Eg; Muliple optotypes target letter visibility is affectd by nearby letters (crowding Phenomena)

53. Effect of Observer Criterion
• Hit
• Miss
• False Positive
• True Negative (correct Rejects)
Types of observer/ Criteria:
a) Strict Criteria: Responds only when complete confirmation of stimulus. More False Negatives.
b) Lax criteria: Responds even when there is not stimulus (eg. Beep sound in AVF). More False Positives.
Trigger Happy Patients.
False Negative

54. Receiver Operating Characteristic Curves
• A receiver operating characteristic
(ROC) curve, which shows the
probability of a hit as a function of the
probability of a false positive.
• Allows us to predict the effect of the
observer criteria for a given detectability
(dʹ).
L: Lax S: Strict M: average

56. WEBER’S LAW
• Just noticeable difference (JND) or Difference Limen
(DL):
10 kg--------------11 kg X
10 kg---------------12 kg √ (JND: 2 kg)
50 kg --------------- ??? kg
Ratio: ∆w/w = 2/10 = 0.2
For Particular JND ratio is always same
e.g. Ratio = ∆w/w
0.2 = ∆w/ 50
∆w = 10 kg
Hence next weight is: (50+10= 60kg)

57. • This process—sensitivity regulation—results in a constant contrast
threshold regardless of the background brightness.
• For scotopic vision, this contrast threshold is 0.14 (or 14%); for photopic
vision it is 0.015 (or 1.5%).