23: Entopic Phenomenon

Question 1

effects of cataracts

Answer 1

affects visual performance, but who’s shape is not visible to the owner

Question 2

effects of vitreous floaters

Answer 2

often visible, sometimes annoying, but rarely a source of poor vision

Question 3

if a point source is placed at the first principal focal point of the eye, what occurs?

Answer 3

then a collimated pencil of rays passes through the vitreous, forming a large patch of light on the retina (imperfections at any point in the ocular media are easily recognized as areas of different brightness in the patch)

Question 4

what is the motion parallax used for?

Answer 4

may be used with a point source at the first focal plane of the eye to determine the approximate depth of an opacity within the eye

Question 5

what are classical halos called

Answer 5

coronas

Question 6

what are halos called spoke patterns produced by

Answer 6

by diffraction within the eye (by light diffraction around very small, regularly-repetitive cellular structures within the eye)

Question 7

rainbow edges around the halos are NOT due to ___

Answer 7

NOT due to : chromatic abberation

Question 8

why do rainbow edges around the halos form?

Answer 8

because the diffractive effect is wavelength dependent

Question 9

what is an example of non-diffractive halos?

Answer 9

from cataracts (scatter) or high spherical aberration

Question 10

angular diameter for normal halo in:

corneal epithelium

Answer 10

3.0°

Question 11

angular diameter for normal halo in:

corneal epithelium or endothelium

Answer 11

4.5°

Question 12

angular diameter for normal halo in:

crystalline lens fibers

Answer 12

6.0°

prominent spoke pattern due to radial arrangement of spoke patterns

Question 13

angular diameter for normal halo in:

second order diffraction from sources of the 4.5° halo

Answer 13

9.0°

Question 14

what are Purkinje’s trees a result of

Answer 14

vasculature of the central retinal artery and central retinal vein, which lie in the inner retinal layers, and cast a shadow on the photoreceptors which usually cast a stabilized retinal image e(because they do not move) and photoreceptors dark adapt

Question 15

why does the vasculature usually go undetected?

Answer 15

photoreceptors in the shadow quickly dark adapt (become more sensitive) canceling out the lower brightness in the shadow and the vessels cannot be detected

Question 16

how can the vessels be made visibly entopitcally

Answer 16

using a moving light source

Question 17

flying spots of the retina are caused by what?

Answer 17

they are small tumbling discs following fixed curve paths in the perofovea which are caused by individual blood corpuscles darting through retinal capillaries one at a time

Question 18

what can the “flying spots of the retina” be used to test?

Answer 18

clinical test for estimating visual acuity before cataract surgery and investigating macular blood flow

Question 19

the blue arc of the retina follows what pattern?

Answer 19

follows the trajectory of the optic nerve fibers in the retina between the horizontal raphe and the optic disc

Question 20

explanation for the blue arcs of the retina

Answer 20

conduction of activity in the non-myelinated fibers from the site of light stimulation to the optic nerve head generates some activity in the retina along its curved pathway

Question 21

2 carotenoids that make up the macular pigment

Answer 21

lutein and zeaxanthin

Question 22

macular pigment absorbed maximally in:

Answer 22

short wavelengths (400-520nm)

Question 23

what does macular pigment do?

Answer 23

filters the light before it can reach the photoreceptors

Question 24

what conditions do you need to see maxwell’s spot

Answer 24

occurs when viewing an evenly illuminated surface with alternating blue and yellow filters- spot is a dark region around the fovea which fades

Question 25

maxwell’s spot is a result of:

Answer 25

impregnation of the macular region with a carotenoid pigment related to xanophylls which is absent or at least attenuated in the center of the macula

Question 26

Haidinger’s brushes are due to:

Answer 26

an interaction between the Henle fibers of the fovea and the macular pigment

Question 27

what are the Henle fibers

Answer 27

the photoreceptor axons in the outer plexiform layer that now pass away from the foveola to reach the inner nuclear layer

Question 28

where is the macular pigment in relation to the Henle fibers?

Answer 28

Henle fibers are neatly arranged next to each other and in a radial pattern, and in between them lies the macular pigment

Question 29

the interaction of macular pigment and Henle fibers causes what?

Answer 29

acts as a linear polarizing filter (the light is transmitted through the Henle fibers in some parts of the foveola and blocked by the Henle fibers in other parts of the foveola)

Question 30

what is the Haidinger’s brushes perceived as?

Answer 30

a small dark “propeller”

which rotates if the light source is rotating

Question 31

what test if based on the Haidinger’s brushes?

Answer 31

basis of a test of eccentric fixation (the propeller is an accurate marker for the anatomical dove)

Question 32

what occurs when someone with eccentric fixation viewing the polaroid light monocularly

Answer 32

they see the propeller spinning at a location away from their fixation point (because they do not use their anatomical fovea as their fixation point in monocular viewing)

Question 33

what is a phosphene?

Answer 33

a visual sensation of light caused by physical manipulation of the retina which can be due to mechanical pressure, electrical stimulation, and even cosmic rays during space flight

Question 34

the stiles-crawford effect refers to the observation that:

Answer 34

light rays passing through the edge of the pupil are generally less effective for stimulating vision than those passing through the middle

Question 35

SC effect is due to what property

Answer 35

waveguide properties of receptors