Brief description only, Please to references for details.

1. ANATOMICAL & PHYSIOLOGICAL
BASIS OF VISUAL ACUITY
Ashim Chhusya
B. optometry, TIO,NAMS

2. REFERENCES
1. Biological Psychology, 10th ed. P152
2. http://people.eku.edu/ritchisong/birdbrain2.html
3. Anatomy & Physiology of Eye, A.K. Khurana 3rd ed. P259
4. https://www.youtube.com/watch?v=pw3L-m55Orw
5. https://www.insightvisioncenter.com/human-vision-vs-eagle-vision/
6. Clinical optics: Troy E. Fannin, O.D., Theodore Grosvenor, O.D., Ph.D.
7. https://www.livescience.com/51787-why-cats-have-vertical-pupils.html
8. https://en.wikipedia.org/wiki/Stiles%E2%80%93Crawford_effect
9. Visual perception, 4th ed.: Steven H, Schwartz p36
10. Binocular Vision and Ocular Motility, Gunter K. von Noorden p114
11. ADLER’S PHYSIOLOGY OF THE EYE: p01

3. CONTENTS
• Definition
• Component & types of Visual Acuity (VA)
• Anatomical basis of VA
• Physiological basis of VA

4. WHAT IS VISUAL ACUITY ?
• ‘acuitas’ (Latin) meaning- sharpness
• Measurement of resolving power of eye i.e. spatial limit
(threshold) of visual discrimination

5. VISION
WHAT ARE THE COMPONENTS VISION ?
4. Colour Sense
1. Light Sense
2. Contrast Sense
3. Form Sense

6. TYPES (OR COMPONENTS) OF VISUAL
ACUITY:
1. Detection Acuity (Minimum Visible detection of a feature)
2. Resolution acuity (resolution of two features, ordinary VA)
3. Recognition Acuity (identification of a feature)
4. Localization Acuity (Hyperacuity discrimination of a change in a feature)

7. DETECTION ACUITY (MINIMUM VISIBLE)
. .
. .

8. RESOLUTION ACUITY (MINIMUM SEPARABLE,
ORDINARY VISUAL ACUITY)
1. Receptor theory:
25”

9. 2. RESOLUTION THEORY

11. 3. NYQUIST THEOREM:

12. RECOGNITION ACUITY
(MINIMUM LEGIBLE)

13. LOCALIZATION ACUITY (MINIMUM
DISCRIMINABLE OR HYPERACUITY)
a

15. ANATOMICAL BASIS OF VISUAL ACUITY

16. TEAR FILM
&
BLINKING OF
EYELIDS
• Protects & lubricates
the major refracting
surface of eye, cornea
• Smoothing the
surface for refraction

17. TEAR FILM FUNCTION :
• Keep moist & lubricate the corneal surface
• Remove sloughed epithelium & debris
• Provide O2 to cornea
• Smoothens the refractive surface cornea
• Protection form microbes
• Contains various growth factors & peptides that enhance the corneal
wound repair

18. CORNEA
• Provides 2
3
𝑟𝑑
refractive power in the eye (~ +
43.00 𝐷)
• Aspheric peripherally to lower the optical
aberration in the eye
• Transparent
• (290 nm-3000 nm wavelength with partial absorption)6
• Any decrease in transparency compromise the VA
• µ=1.376

19. GENERAL KNOWLEDGE:
• Unlike humans, some birds like eagles accommodate through
both lens & cornea
• Hence enable them fast switching for better focus on near and
far objects from hundreds of feet up in the air enhance their
hunting
5

20. AQUEOUS HUMOR
• Transparent fluid chamber
• 290 nm -3000 nm with partial absorption
• Nutritious value for surrounding
structures
• µ=1.336
• AC depth by 1 mm  Myopia of 1.39 D

21. PUPIL
• Quality of vision depends upon the pupil
size
• Smaller the size of pupil, increase in depth
the of focus (pinhole effect)  hence
better the VA
• But below 2.5 mm  diffraction
• Above 6 mm  aberration
Widens the point spread function
3

22. DIFFERENT PUPIL SHAPES:
What impact does VA have if pupil is other than round?
1. Horizontal, rectangular pupils:
• Found in for e.g. goats that likely help them to scan the horizon
(panoramic view) for predators — and watch the terrain when
sprinting from danger.
7
2. Vertical-slit pupils:
• Found in for e.g. cat, a nocturnal predator ,
• Provide the best steroptic view for ambush attack.

23. LENS
• Transparent structure for
• Accommodation reflex
• Places the point image or circle of
least confusion at retina (if possible)
• Different indices at different
layers to reduce optical
aberration.
• µ =1.386 cortex
• =1.406 nucleus
• Transparent to 315 nm - 2500 nm wavelength with partial absorption
6

24. VISUAL ANGLE AT NODAL POINT
When the physical dimension of two targets makes at least a minute of angle
 resolvable as separate

25. LENS TRANSPARENCY

26. VITREOUS
• Viscous fluid in the eye
• µ=1.336
• Transparent
• 290 nm-1600 nm with partial absorption
6

27. RETINA

28. RPE
• Highest density at macula
(becomes thinner & taller)
• Higher amount of melanin
pigments towards center of
macula  prevents light
scattering & glare  clear
vision

29. RPE FUNCTION: OTHER
• Photoreceptor renewal
• Maintains integrity of SRF
• Transport of nutrients & metabolites
• Phagocytosis
• Mechanical support
• Manufacture of pigments
• Regeneration & repair function
• Electrical homeostasis
• Anti-inflammatory function

30. VISUAL RECEPTORS: RODS AND CONES
• Rods : cones = 20:1 in the human retina
• However, cones provide about 90% visual input to the brain
• The more light energy is absorbed, the more signals transferred to cortex
• In the evolutional species:
• The ratio is much higher in species that are active at night.
• For example: american oilbirds, have about rods to cone ratio
15,000:1 which live in caves and emerge only at night .1
• Falcons have eyesight eight times sharper than that of ourselves this is
because the retina of their eyes have more visual cells (1,500,000 vs 200,000
in human) at the fovea .2

31. WHY CAT CAN SEE 6-TIME DIMMER LIGHT
THAN HUMAN ?
• Presence of Tapetum lucidum, membrane under the retina
better vision

32. FOVEA
• Site of highest cells density 
• 115,000-225000 cones/𝑚𝑚2 9
• Site of Lowest light barriers 
• Absence of blood vessels (FAZ) and ganglion cell axons near the fovea
Highest visual resolution

33. One to one
connection at
fovea
Photoreceptor
Bipolar
Ganglion cell
registers the
exact location of
the input

34. AT PERIPHERY
• More and more receptors converge onto bipolar and ganglion
cells.
• The brain cannot detect the exact location or shape of a
peripheral light sour

35. PHYSIOLOGICAL BASIS OF VISUAL ACUITY

36. STILE-CRAWFORD EFFECT.

37. 1. Directional sensitivity of the cone photoreceptors
the peak photoreceptor sensitivity light rays entering eye
approximately 0.2–0.5 mm towards the nasal side
8
2. Re-directing towards the light ray propagation
3. Waveguide properties of cone (mitochondria)

38. MINIATURE EYE MOVEMENT
• Involuntary movements of eye even during strenuous steady
fixation
10
1. These involuntary eye drifts and small jerks keep the retinal image “fresh” by
allowing retinal receptors to scan the contours of an object enhancing the
neuronal activity, on which visual acuity depends
• Evolved to counteract image fading (troxler effect)
10

39. RETINAL ECCENTRICITY
• Optimum VA at fovea
• Decreases sharply with an increase in distance of the image
of an object from the fovea
• On the nasal side of the fovea
10
• VA  6/12 at 2.5 mm away
• And  6/30 at 10 mm away
• On the temporal side va decreases somewhat more rapidly
• Least VA, at superior or inferior eccentric fixation.

40. Visual acuity of the retinal
periphery.
Continuous black lines  points
of equal VA.
Note that the gradient of visual
acuity is steepest in the upper
half of the retina. The decline in
acuity with eccentricity is least on
the temporal side.
The broken line indicates the
peripheral limits of the visual field

41. LUMINANCE AND STATE OF ADAPTATION
10
• Parafoveal or peripheral rod receptors predominate in
scotopic condition.
• Foveal fixation with no rod cells fails to resolve target.
• Even though sensitivity is maximum, VA is poor
AT SCOTOPIC CONDITION:
Luminance
 Standard luminance for acuity chart is 411.1 ft. lamberts

42. AT PHOTOPIC CONDITION:
• Level of intensity is raised
• Thresholds of the cone receptors are met or exceeded
• Acuity rises steeply. With a further increase in intensity,
• Acuity is maintained over a wide range of intensities.
• Very high luminance causes unexplained reduction in
acuity.

43. CONTRAST
• The visual acuity decreases with reduction in
objective contrast
 At low degrees of contrast  visual acuity ↓
markedly
 At higher contrast levels little effect on visual
acuity
10
• Low contrast  troxler phenomenon  may
↓ VA

44. • The objective or photometric contrast refers to differences in
luminance of adjacent fields or objects
• The subjective or physiologic contrast refers to such subjective
phenomena as the change in apparent brightness of objects of a given
luminance, which depends on the luminance of the surround.
• The effect of subjective contrast is of greatest importance for vision.
The borderline between a bright and dark surface produces a blurred,
unfocused retinal image caused by the optical aberrations of the eye
and the veiling effect of stray light. These effects are offset by
subjective contrast. The image of a white field appears whiter, he
images of a dark field darker, and the borderline sharp when the two
borders on each other.

45. CONTOUR INTERACTION
(CROWDING PHENOMENON)
• The spatial arrangement of additional contours in the field
of vision  visual acuity ↓
• A universal phenomenon but prominent in amblyopics.
 E.g. Black bars on Landolt C.
• Below a critical separation between the bars and the C,
the probability of seeing the C decreased rapidly.
• The critical separation was
• Normal eyes  between 1.9’ and 3.8’ minutes of arc for
10
• Amblyopic  and between 8.4’ and 23.3’ minutes of arc

46. OPTICAL QUALITY OF THE EYE
• The rayleigh limit determined by
1. Wavelength of light ray
2. Size of the pupil
• Modulation transfer function (MTF)
𝑀𝑇𝐹 𝑜𝑓 𝑖𝑛𝑡𝑒𝑟𝑓𝑎𝑐𝑒 =
𝑐𝑜𝑛𝑡𝑟𝑎𝑠𝑡 𝐴′
𝑐𝑜𝑛𝑡𝑟𝑎𝑠𝑡 𝐴
Contrast A
Contrast A’
interface

47. • Larger the pupil diameter  poorer the modulation transfer
function in retina
• Larger the pupil diameter  larger the aberration (spherical)
 Chromatic aberration (in case
of heterochromatic ray)
• Refractive error & optical defocus
• Optical defocus ~ poor MTF

48. VISIBILITY DURATION: 𝟖𝟎 𝒎𝒔 𝒓𝒖𝒑𝒆𝒔𝒉 𝒔𝒊𝒓
Reducing the object visibility below a “critical” duration
reduces the number of threshold quanta from object light
↓ VA

49. DEVELOPMENTAL ASPECTS/AGE
• Retinal image of the normal infant eye is about 3
4 𝑡ℎ the size
of the adult’s image  hence less fine detail
• nerve fibers at the various levels within the brain, are
poorly myelinated
• Neonate’s visual appreciation for fine detail 1
13 𝑡ℎ (20/600) or
approx. 3 %, of the adult.
• The age of a year, the infant’s va is 25 % (20/80) of optimal
adult visual VA
• By the age of 5 years, the child usually has 20/20 vision.

50. ANY QUERY?
THANK YOU ^_^