7. Vision Flashcards
3 layers of eye
(outer –> inner)
- FIBROUS layer: sclera & cornea (outermost)
- VASCULAR layer: choroid (uvea), ciliary body, iris
- RETINAL layer: pigmented & neural/light-sensitive
which is the light-sensitive layer of the eye?
innermost retinal layer;
which is pigmented and neural
path of light (thru eye)
light from environment –>
thru cornea –>
pupil –>
lens
line through fovea separates which 2 sections?
NASAL (hemi)retina and the TEMPORAL (hemi)retina
which aspect of the eye is the NASAL hemiretina found?
medial aspect
which aspect of the eye is the TEMPORAL hemiretina found?
LATERAL aspect
in which hemi-retina is the optic disc/ optic nerve found?
in the NASAL (hemi)retina
list the retinal layers
(innermost –> outermost, which is the path of information after light hits the retina)
- PE - pigement epithelium
- POS - photoreceptor outer segments
- ONL - cell bodies of photoreceptors
- OPL - outer plexiform layer
- INL - inner nuclear layer
- IPL - inner plexiform layer
- GCL - ganglion cell layer
- NFL - nerve fiber layer
in which retinal layer are the outer segments of photoreceptors found?
photoreceptor outer segments
in which retinal layer are the cell bodies of photoreceptors found?
outer nuclear layer
in which retinal layer do the photoreceptor cells and bipolar cells synapse?
outer plexiform layer
in which retinal layer are the cell bodies of bipolar cells found?
inner nuclear layer
in which retinal layer is the synapse between bipolar cells and retinal ganglion cells?
inner plexiform layer
what is the fovea?
shallow depression on the surface of retina;
w/in macula lutea
what is the region of highest visual acuity in the eye, and why?
the fovea
very high density of photoreceptors (cones) in the center of fovea; w/ retinal ganglion cells in rest of fovea
what is found in the CENTER of the fovea?
- very high density of cone photoreceptors in center
- no retinal ganglion cells, nor rod cells
- few inner nuclear layer cells
principle behind high density of cones –> high visual acuity of fovea?
- high visual acuity results from ratio of 1 cone in central fovea : 1 bipolar cell: 1 retinal ganglion cell/ RGC (1:1:1); vs.
- outside the fovea –> where many rods converse to one biopolar cell –> and many bipolar cells converge to one RGC
key layers of fovea vs. rest of retina
is fovea vascular or avascular?
- Fovea has pigment epithelium and outer nuclear layer;
- it is avascular
distribution of ROD photoreceptors
- high density throughout retina
- SHARP DECLINE IN FOVEA; (very few to none in fovea)
distribution of CONE photoreceptors
- LOW density throughout retina
- SHARP PEAK IN FOVEA
in which direction do the total # of photoreceptors decrease, relative to the fovea?
as you move away from the fovea, the total number of photoreceptors decreases
where/what is the blind spot?
how does the brain compensate?
- no vision is possible when light is on optic nerve head (disc)
- occurs bc there are NO CELL LAYERS, ONLY the optic nerve fiber layer
- brain “fills in” missing image
two streams of visual information processing?
- magnocellular stream (location/ movement)
- parvocellular stream (form/ color)
key characteristics of MAGNOCELLULAR (“where”) stream
- location/movement
- M type retinal ganglion cells
- LARGE cell bodies & axons
- LARGE dendritic cells & receptive fields
- mostly ROD INPUT, color insensitive
- predominates in RETINAL PERIPHERY
key characteristics of PARVOCELLULAR (“what”) stream
- form/color
- P type retinal ganglion cells
- small cell bodies & axons
- small dendritic cells & receptive fields
- mostly CONE INPUT, color sensitive
- predominates in PARAFOVEAL region
which stream of visual information processing predominates in the parafoveal region?
Parvocellular (Form/color)
which stream of visual information processing predominates in the retinal periphery?
magnocellular (where) stream; w/ location and movement
which stream of visual information processing is the basis of visual acuity?
- parvocellular stream;
- input mainly from cones, and is color SENSITIVE
- provides info on form and color
pathway of magnocellular stream
- DORSAL pathway –> POSTERIOR PARIETAL cortex
- (includes V2 and MT)
pathway of parvocellular stream
- VENTRAL pathway –> INFERIOR TEMPORAL cortex
- (includes V2 and V4)
from which areas of the retinas are the following positions in the visual field (A-D) formed?
- monocular at “A” from LEFT NASAL RETINA
- binocular at “B” from RIGHT TEMPORAL and LEFT NASAL retinas
- binocular at “C” from RIGHT NASAL and LEFT TEMPORAL RETINAS
- monocular at “D” from RIGHT NASAL RETINA
from which areas of the retinas is the RIGHT visual hemifield formed?
RIGHT NASAL and LEFT TEMPORAL
which visual axons cross the midline?
NASAL ganglion cells axons cross the midline and go to opposite retinal tract
on which side of the brain is the RIGHT hemifield processed?
on the LEFT side of the brain
(the visual hemifield is processed by the OPPOSITE brain hemisphere)
optic chiasm
where the ganglion cell axons of the NASAL retinal cells DECUSSATE, whereas the temporal retinal cells continue towards the back of the same respective hemisphere
what are the central targets of the Retinal ganglion cells?
- hypothalamus, pretectum, superior colliculus
- lateral geniculate nucleus of thalamus (main target to relay information to the cortex; critical to be consciously aware of this_
central projections to thalamus
- optic tract:
- retinal ganglion cell axons project mainly to the lateral geniculate nucleus (LGN)
- –> then those neurons project to visual cortex
- (Called optic radiation; includes sublenticular fibers forming “meyer’s loop)
right visual field: inputs/pathway
- right nasal retina
- left temporal retina
- left optic tract
- left LGN
central projections to cortex
- optic radiations:
- left LGN axons project to the LEFT primary visual cortex
images in the upper right visual field
upper right visual field: –>
L. sublenticular fibers (Meyer’s loop) –>
L. lingula of the visual cortex
Images in the lower right visual field:
Images in the lower right visual field: –>
L. retrolenticular fibers –>
L. cuneus area of the visual cortex
primary visual (striate) cortex
also known as area 17 or V1
- cuneus: upper bank of calcarine sulcus
- calcarine sulcus separates the cuneus and lingula
- lingula: lower bank of calcarine sulcus
visual field projection to both retina and primary cortex:
track inferior visual field
inferior field –>
superior retina & to the superior bank/ cuneus
what is the macular region of the retina?
represented most posteriorly in V1;
containing the fovea;
large volume of cortex is devoted to macular representation
projections from LGN to primary visual cortex?
each LGN receives input from opposite visual hemifield, but input from each eye is segregated in individual layers;
input from two eyes in 1st integrated in area 17, which allows for 3D vision & stereopsis (depth perception)
when is the information separated in projections from LGN to primary visual cortex?
only when the information, reaches the cortex that the information –>
allows for depth perception and 3D vision
(a person has monocular vision will have to use other cues in order to compensate and have depth perception)
anopsia:
location of lesion; and visual disturbance
if you have a lesion on the optic nerve, you will lose vision in that eye
(loses vision in the one eye)
bitemporal (heteronymous) hemianopsia:
location of lesion; and visual disturbance
- lesion at midpoint/ crossing point –> loses the nasal retinal crossing fibers which process images at the periphery of the visual
- results in tunnel vision (bitemporal vision loss –> losing temporal field, but diff’t hemifields in each eye/ heteronymous)
contralateral (homonymous) hemianopsia:
location of lesion; and visual disturbance
- lesion eliminates activity coming from right temporal and left nasal (originate from left hemifield) –> right optic tract –> eliminates anything that would be seen with the left hemifield
- lesion on the right optic tract (contralateral) results in left field on both sides missing (homonymous)
contralateral superior quadrantanopsia:
location of lesion; and visual disturbance
- Lesion at the optic radiation–>
- lose contralateral visual field, specific quadrant based on where lesion is in the optic radiation
contralateral hemianopsia with macular sparing:
location of lesion; and visual disturbance
- if losing the entire optic radiation –>
- you would lost entire opposite visual field; there may be some “macula sparing” (due to anastomoses bc the area is so big –> so some retinal sparing occurs)
accommodation or near reflex (triad):
process?
occurs when shifting gaze from distant –> near object
- convergence of eyes
- accomonation of lens
- pupillary constriction
how does convergence of the eyes occur during the accommodation or near reflex?
- involves left and right medial recti muscles
- left and right oculomotor nuclei and nerves
- accommodation center in midbrain
which nerves are involved in accommodation of lens and the pupillary constriction?
parasympathetics in oculomotor nerve allow both the accommodation of lens and the pupillary constriction to occur
what reflex is lost in oculomotor nerve lesion?
entire accommodation or near reflex (triad) results after oculomotor nerve lesion
pupillary light reflex
and principle behind it
constriction of pupil in response to increased illumination; pupils DO NOT OPERATE INDEPENDENTLY –> so what one does (direct), the other should also (consensual)
- direct reflex response in ipsilateral eye (pupil constricts)
- consensual reflex response in contralateral eye
pupillary light reflex pathway
- pretectum on each side projects BILATERALLY to –>
- Edinger-Westphal nucleus (EW nucleus) –>
- EW neurons project to ciliary ganglion (preganglionic parasympathetics)
lesion of motor limb of pupillary light reflex pathway:
lesion to L oculomotor nerve –>
- increased light level is detected normally by either eye
- BUT the Left pupil cannot respond
