23: Entopic Phenomenon Flashcards
effects of cataracts
affects visual performance, but who’s shape is not visible to the owner
effects of vitreous floaters
often visible, sometimes annoying, but rarely a source of poor vision
if a point source is placed at the first principal focal point of the eye, what occurs?
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)
what is the motion parallax used for?
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
what are classical halos called
coronas
what are halos called spoke patterns produced by
by diffraction within the eye (by light diffraction around very small, regularly-repetitive cellular structures within the eye)
rainbow edges around the halos are NOT due to ___
NOT due to : chromatic abberation
why do rainbow edges around the halos form?
because the diffractive effect is wavelength dependent
what is an example of non-diffractive halos?
from cataracts (scatter) or high spherical aberration
angular diameter for normal halo in:
corneal epithelium
3.0°
angular diameter for normal halo in:
corneal epithelium or endothelium
4.5°
angular diameter for normal halo in:
crystalline lens fibers
6.0°
prominent spoke pattern due to radial arrangement of spoke patterns
angular diameter for normal halo in:
second order diffraction from sources of the 4.5° halo
9.0°
what are Purkinje’s trees a result of
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
why does the vasculature usually go undetected?
photoreceptors in the shadow quickly dark adapt (become more sensitive) canceling out the lower brightness in the shadow and the vessels cannot be detected
how can the vessels be made visibly entopitcally
using a moving light source
flying spots of the retina are caused by what?
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
what can the “flying spots of the retina” be used to test?
clinical test for estimating visual acuity before cataract surgery and investigating macular blood flow
the blue arc of the retina follows what pattern?
follows the trajectory of the optic nerve fibers in the retina between the horizontal raphe and the optic disc
explanation for the blue arcs of the retina
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
2 carotenoids that make up the macular pigment
lutein and zeaxanthin
macular pigment absorbed maximally in:
short wavelengths (400-520nm)
what does macular pigment do?
filters the light before it can reach the photoreceptors
what conditions do you need to see maxwell’s spot
occurs when viewing an evenly illuminated surface with alternating blue and yellow filters- spot is a dark region around the fovea which fades
maxwell’s spot is a result of:
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
Haidinger’s brushes are due to:
an interaction between the Henle fibers of the fovea and the macular pigment
what are the Henle fibers
the photoreceptor axons in the outer plexiform layer that now pass away from the foveola to reach the inner nuclear layer
where is the macular pigment in relation to the Henle fibers?
Henle fibers are neatly arranged next to each other and in a radial pattern, and in between them lies the macular pigment
the interaction of macular pigment and Henle fibers causes what?
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)
what is the Haidinger’s brushes perceived as?
a small dark “propeller”
which rotates if the light source is rotating
what test if based on the Haidinger’s brushes?
basis of a test of eccentric fixation (the propeller is an accurate marker for the anatomical dove)
what occurs when someone with eccentric fixation viewing the polaroid light monocularly
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)
what is a phosphene?
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
the stiles-crawford effect refers to the observation that:
light rays passing through the edge of the pupil are generally less effective for stimulating vision than those passing through the middle
SC effect is due to what property
waveguide properties of receptors
