Eye physiology from guyton and halls physiology Part 4

BY Muhammad Ramzan Ul Rehman Nishtar Ken 1

BY Muhammad Ramzan Ul Rehman Nishtar Ken
2

—“DEPTH
PERCEPTION”  The ability to determine
distance is called depth
perception.
 A person normally perceives
distance by three major
means:
 (1) the sizes of the images of
known objects on the retina
 (2) the phenomenon of moving
parallax,
 (3) the phenomenon of
stereopsis, binocular Vision.

 by using this mechanism of moving parallax, one can tell the relative distances of
different objects even though only one eye is used.
 It is almost entirely this moving or binocular parallax (or stereopsis) that gives a
person with two eyes far greater ability to judge relative distances when objects
are nearby than a person who has only one eye.
 However, stereopsisis virtually useless for depth perception at distances beyond
50 to 200 feet.

 An instrument to examine the inside
of eye, especially the retina and optic
disc
 It has a light source on the end.
 Eye can be magnified.
 We can look directly through the
pupil to the back of eye.

 Ability of the eye to determine
the precise shape and details
of the object is called visual
acuity
 For humans, it is 30 seconds of
an arc.

 Chart for testing eyes consists of
letters of different sizes placed 20
feet ( 6 meters) away from the person
being tested.
 Person is said to have normal vision
if he can see the letters from the
distance of 20 feet or 6 meters. so he
have vision of 20/20 or 6/6

 PHOTOCHEMICALS in cones have almost the same
composition as Rhodopsin in Rods.
 Protein portion (opsins) in cones are called Photopsins.
 The color sensitive pigments of the cones, are the
combinations of retinal and photopsins.

Cones are selectively sensitive to different
colours:
Blue
Green
Red
Only one of three types of color pigments is
present in each of the different cones
These color pigments are respectively called:
Blue-sensitive pigment
Green sensitive pigment
Red-sensitive pigment
YOUNG-HELMHOLTZ THEORY(theory of trichromatic color
vision)

 BLUE SENSITIVE PIGMENTS
Peak absorbance of light
wavelength 445nm
 GREEN SENSITIVE PIGMENTS
wavelength 535nm
 RED SENSITIVE PIGMENTS
wavelength 570nm
 RHODOPSIN
wavelength 505nm

 Human eye can detect all gradations of colors.
 Red, green & blue monochromatic light mixed in different combinations.

Orange monochromatic light
stimulates:
 Red cones-------99%.
 Green cones -----42%.
 Blue cones---------0%.
 Ratio of stimulation -------99:42:0.
 CNS interprets this ratio as
sensation of orange color.

Blue monochromatic light stimulates
 Red cones-------0%.
 Green cones -----0%.
 Blue cones---------97%.
 Ratio of stimulation -------0:0:97
 CNS interprets this ratio as
sensation of blue color.

 Ratio of 83:83:0-------------yellow.
 Ratio of 31:67:36-----------green.

• When there is Equal stimulation of all cones there is perception of white light.
• White is combination of all the wavelengths of the spectrum.

 The term Colour blindness is misleading if taken literally, because colourblind
people CAN see colors, but cannot make out the difference between some
complementary colors.

 CAUSE: Due to congenital absence of a single group of color receptive cones from
the eyes
 Person is unable to distinguish some colors from others.
 Usually…..absence of either L (Red) cones or M (Green) cones.
 People with two functional cones are called Dichromate.

 Person is unable to distinguish red from green due to missing of either of these
cones.
 Absence of M (Green)cones :Deuteranopia
 Absence of L (Red) cones:Protanopia
 Green, orange, red & yellow colors have wavelength 525 to 675nm.
 These colors are normally distinguished from one another by red & green cones.

 Genetic disorder only in males
 Photopsins are coded on X chromosomes.
 It never occurs in females ,because one of the two X Chromosomes has normal
gene for each type of Cone.
 about 8% of women are color blindness carriers
 Females are only color blindness carrier.

 Rarely blue cones are missing.
 Genetically inherited state.

Rapid method to determine color blindness.
 Charts are arranged with a confusion of spots of several
different colors.
These charts observe spectral sensitivity curves of the
different cones at same time.
 Ideally a collection of 38 plates filled with colored dots
build the base of this test. The dots are colored in different
shades and a number is hidden inside with shades of
another color.

 Ishihara Charts were made by a
Japanese ophthalmologist Shinobu
Ishihara (1879-1963).
 He was working at the Military
Medical School
 He was asked to devise a test to
screen military recruits for
abnormalities of colour vision.
 His assistant was a colourblind
physician who helped him test the
plates.

The person
with normal
color vision
reads 74,
where as the
red green
blind person
reads 21.

 Person with
normal
vision reads
42, red
blind
person
reads2,and
green blind
person
reads 4.

Eye Movements are Controlled by
Neural Pathways for Control of Eye
Movementairs of Muscles.
Superior and inferior
obliques rotate the
eyes
Medial and lateral
recti move eyes side
to side
Superior and inferior
recti move eyes up
and down
BY Muhammad Ramzan Ul Rehman Nishtar Ken Figure 51-7; Guyton & Hall 31

 Fixation Movement:
Movement of eyes to bring a
discrete portion of visual field into
focus on the fovea.
Controlled by
Frontal eye fields
Brodmann’s area 8 and 19

 Fixation movements of the eyes controlled by
two neuronal mechanisms, voluntary and
involuntary.
 Voluntary fixation movements controlled by an area in the
premotor cortex.
 Involuntary fixation mechanism causes eyes to “lock” on
object of attention found with the voluntary fixation
mechanism. Controlled by secondary visual areas of the
occipital cortex.
 Results from negative feedback mechanism controlled at the
level of the superior colliculus that prevents objects of
attention from leaving the foveal portion of the retina.

Saccadic Movement
Jumping of eyes from one object to another.
each jump is called a Saccade and the
movement is called Opticokinetic movements.
Pursuit Movement
Fixation of eyes to a moving object

When the visual scene is moving (turning the
head), the eyes fix on one highlight after
another in the visual field jumping at a rate of 2
to 3 jumps/sec. These jumps are called saccades,
and the movements are called opticokinetic
movements.
 Saccades occur very rapidly (only 10% of the
time is spent making saccades).
Vision is suppressed during a saccadic
movement.

By Muhammad Ramzan UL Rehman
BY Muhammad Ramzan Ul Rehman Nishtar Ken
38

Eye physiology from guyton and halls physiology Part 4

Recommended

Recommended

More Related Content

What's hot

What's hot (20)

Viewers also liked

Viewers also liked (19)

Similar to Eye physiology from guyton and halls physiology Part 4

Similar to Eye physiology from guyton and halls physiology Part 4 (20)

More from Muhammad Ramzan Ul Rehman

More from Muhammad Ramzan Ul Rehman (20)

Recently uploaded

Recently uploaded (20)

Eye physiology from guyton and halls physiology Part 4