Ophalmology- physiology of eye

1. PHYSIOLOGY OF EYE

2. PHYSIOLOGY OF VISION
MECHANISMS INVOLVED:
• Phototransduction
• Processing and transmission of visual sensation
• Visual perception

3. PHOTOTRANSDUCTION
• Phenomenon of conversion of light energy into nerve impulse is
called phototransduction.
• Light falling on the retina causes photochemical changes which
can trigger a cascade of biochemical reaction leading to the
generation of electrical changes

4. PHOTOCHEMICAL CHANGES
It includes
• Rhodopsin bleaching
• Rhodopsin regeneration

6. ELECTRICAL CHANGES
Activated rhodopsin triggers biochemical reactions
Generation of receptor potential in photoreceptors
Light energy is converted to electrical energy which is further
processed and transmitted

7. PROCESSING AND TRANSMISSION OF VISUAL
IMPULSE
• Visual image is interpreted and analyzed in serial and parallel fashion.
SERIAL PROCESSING- successive cells in the visual pathway starting
from photoreceptors to the cells of lateral geniculate body are involved
in increasingly complex analysis of the image.
PARALLEL PROCESSING- two kinds of cells can be distinguished in the
visual pathway:
The large cells- magno or M cells
The small cells- parvo or P cells

9. • The magnocellular pathway carries information about form,
movement, depth, and differences in brightness; the parvocellular
pathway carries information on color and fine detail.
• Some signals move to the thalamus, which sends the visual signals to
the primary cortex. The part of the visual cortex that receives sensory
inputs from thalamus is called primary visual cortex.
• Visual signals can also travel from the retina to the superior colliculus,
where eye movements are coordinated with auditory information.
• Visual signals can move from the retina to the suprachiasmatic
nucleus (SCN), which is involved in sleep/wake patterns and annual
cycles.

10. VISUAL PERCEPTION
It is an integration of:
-light sense
-form sense
-sense of contrast
-colour sense

11. LIGHT SENSE
• It is the awareness of light.
• Light minimum- measured when eye is dark adapted for 20-30 mins.
• Visual adaptation invloves:
-dark adaptation
-light adaptation

12. • DARK ADAPTATION
ability of the eye to adapt itself to decreasing illumination.
-dark adaptation time- time taken to see in dim illumination
- Delayed dark adaptation occurs in diseases of rods- Retinitis pigmentosa,
Vitamin A deficiency
- Rods are used more in scotopic vision.
- Cones are used more in photopic vision.

13. DARK ADAPTATION CURVE

14. • LIGHT ADAPTATION
It is a process by means of which retina adapts itself to bright light.
Very quick- occurs over 5 minutes

15. FORM SENSE
• It is the ability to discriminate between shapes of objects.
• Cones play a major role. Therefore form sense is most accurate at
fovea
• Components of visual acuity are:
- minimum visible
- resolution
- recognition
- minimum discriminable

16. • MINIMUM VISIBLE: ability to determine whether the object is present or not
• RESOLUTION: Discrimination of two spatially separated targets.
Minimum resolvable is the minimum separation between two points, which can be
discriminated as two.
• RECOGNITION: individual not only discriminates the spatial characteristics of the
test pattern but also identifies the patterns with which he has had some
experience. eg: identification of faces
• MINIMUM DISCRIMINABLE: spatial distinction when the threshold is much lower
than the ordinary acuity.

17. SENSE OF CONTRAST
• Ability of the eye to perceive slight changes in the luminance
between regions which are not separated by definite borders. Loss of
contrast sensitivity results in mild fogginess of vision.
• It is affected by age, refractive errors, glaucoma, amblyopia, diabetes,
optic nerve diseases and lenticular changes.
• Can be measured by:
- Arden gratings
- Cambridge low contrast gratings
- Pelli – Robson contrast sensitivity chart
- Visitach chart
- Functional acuity contrast test

20. Sample size was calculated by referring to the article done by Priya
Gogia et al
n= z2s2 / d2
=1.962 x 2.5782/0.5242
=93
Where z= value of alpha error with 95% confidence
S= standard deviation of initial SOFA score from the reference article
=2.578
d=5% relative error of mean SOFA score from reference article