lecture 23 - the visual system Flashcards
briefly explain the general organization principle of the sensory systems
- physical stimulus interacts with its appropriate sensory receptor
- transduction
- graded receptor potential represents the stimulus and leads to action potentials in the sensory neuron
- delivers signal to the CNS
visual system: stimulus?
light
visual system: organ?
retina
visual system: receptor?
rod and cone cells
visual system: mapping?
retinotopy
what is visible light?
electromagnetic energy between 380 and 760 nm
does light have wave properties?
yes
does light have particle properties?
yes
what is the function of the iris?
to control the amount of light that reaches the retinas
what is the function of the pupil?
the pupil is a hole in the iris where light enters
describe the pupil with respect to different intensities of light?
constricts in bright light, dilates in dim light
how does the pupil pass light? what does this mean for image projection on the retina?
like a pin hole camera - the image is projected on the retina upside-down and mirror reversed backwards)
what is binocular disparity
different position of each eye produces a disparity of the visual angle (slightly different image is received by each retina, brain compares and fuses both if they are similar enough to give what we actually see)
what is binocular disparity important for?
depth perception
what is the function of the cornea?
protective outer layer
what is the function of the ciliary body?
ring shaped muscle that changes the focus of the lens
what is the function of the fovea?
- small depression in the retina of the eye
- visual acuity is highest
- the center of the field of vision is focused in this region
- high [ ] of retinal cones
what is the optic disc?
- round spot on the retina from axons of retinal ganglion cells
- these transfer signals from photoreceptors of the eye to the optic nerve
what is the function of the sclera?
“white” of the eye - support
what is the function of the choroid?
vascularized
together, the choroid and the retinal pigmented epithelium do what?
reduce light scattering around the light sensitive portion of photoreceptors
what is myopia?
far back objects look blurry because the image is focused before the retina (‘near-sighted’)
what is hyperopia?
near objects look blurry because the image is focused behind the retina (‘far-sighted’)
what is more common myopia or hyperopia?
myopia, but hyperopia becomes more common later in life
describe the cellular structure of the retina
- photoreceptor layer
- bipolar cell layer –> horizontal and amacrine cells
- ganglion cell layer
- fovea
describe the photoreceptor layer
- contains rods and cones
- both have folded membranes that contain light sensitive pigments - breakdown products lead to a reduction in the intracellular Na leakage
- leads to hyperpolarization of the receptor resting potential
- action potential passed along to bipolar, then ganglion cell layer
rods are sensitive to what?
dim light
cones are sensitive to what?
color
describe the bipolar cell layer
projection neurons that collect and integrate photoreceptor signals
describe amacrine and horizontal cells
interneurons that receive input from the other cells (photoreceptor, bipolar, ganglion) and provide information laterally to the surrounding circuitry
describe the ganglion cell layer
axons from this cell layer leave the retina at the optic disc and form the optic nerve
describe the fovea at the cellular level
area of highest acuity (modified for high resolution vision), because of:
- few bvs
- increased [cones]
- most bipolar and ganglion cells per photoreceptor, meaning there’s a low convergence of optic signals with the cones here
what is the blind spot?
optic disc (no photoreceptor cells here)
as we move away from the fovea, what happens to the concentration of rods/cones
- concentration of cones decreases
- concentration of rods increases
compare the sensitivity of cone and rod vision
cones: low
rods: high
compare the temporal resolution of cone and rod vision
cones: high
rods: low
define transduction
conversion of one form of energy into another
what is the first step of visual transduction?
conversion of light to neural signals by the rods and cones
describe the neural circuitry of the retina (propogation of a signal through the cellular layers)
- photoreceptor membrane contains sodium channels that are open in the dark
- when light strikes, rhodopsin changes causing sodium channels to close
- this causes hyperpolarization of the neuron, reducing glutamate release to the bipolar cell
- this causes the bipolar cell to depolarize, depolarizing the ganglion cell
compare the convergence of cones and rods
- 1 cone, 1 bipolar cell, 1 ganglion cell
- 8 rods, 4 bipolar cells, 1 ganglion cell
describe the visual field
- aka foveal field
- two views of the world (from 2 eyes)
- each projected as a retinal image
describe the visual pathway
- left visual field of both eyes projects to right retina
- retina then relays to right thalamus and from there to the right visual cortex
- opposite for the right: projects to left retina, to left thalamus to left visual cortex
do the nasal retinas cross? do the temporal retinas cross?
- yes
- no
describe thalamocortical projection of the visual pathway
- upper visual field of both eyes is projected via Meyer’s loop to the lower bank of the calcarine sulcus
- the lower visual field of both eyes projects to the upper bank of the calcarine sulcus
describe the thalamus with respect to vision
- each nuclei is 6 layered
- receives temporal half of ipsilateral retina (same)
- receives nasal half of contralateral retina (opp)
what do neurons at the thalamic level process with respect to vision?
basic retinal information of borders
what does the thalamus respond to with respect to vision?
contrast and movement in the visual field
what is the geniculocalcarine tract (optic radiation)?
projection of axons from the lateral geniculate nucleus
describe the optic radiation when carrying information form the upper retina
passes around lateral ventricles, curving posteriorly, ending in the upper lip of the calcarin sulcus
describe the optic radiation when carrying information form the lower retina
passes forward around the inferior horn and back to the lower lip of the calcarine sulcus
the left lower visual field of both eyes projects to:
the right upper cortex
give an overview of the visual pathway
-retinal image is inverted and reversed compared to the visual field
(-photoreceptor–>bipolar cell–>ganglion cell)
-axons of retinal ganglion cells form the optic nerves
-at the optic chiasm, axons from the temporal halves of each retina continue into the optic tract on the same side, while axons from the nasal halves cross to the optic tracts on the opposite sides
-most axons in the optic tract terminate in the lateral geniculate nucleus
-some axons terminate in the superior colliculus
-axons are sent back and forth between the lateral geniculate and the cortex via the optic radiations
-left primary visual cortex gets input from both eyes, but only from the right visual field, and vice versa
describe the projection of the fovea in the visual cortex
- projects to 1/3 of it
- disproportionally represented because cortical areas are proportional to the number of sensory cells (retinotopically organized)
where is the visual cortex located?
medially, surrounding the calcarine sulcus
describe convergence during transduction
inputs from both retinas converge on the same cortical neurons - important for depth perception
what would be the result of a bilateral lesion to the visual cortex?
blindness, some visual reflexes may remain intact (below)
what would be the result of a lesion to the left optic nerve?
blindness in the left eye (e.g., by aneurism)
what would result from a lesion to the left optic tract?
visual field deficits (e.g., by interruption in blood supply to optic tract)
what would result from a lesion to the left optic nerve at its junction with the optic chiasm?
visual field deficits (upper field deficit in right eye) (e.g., pituitary tumor)
what would be the result of a lesion to the optic radiation?
visual field deficit (e.g., by lesions to Meyer’s loop)
what are the visual association cortices?
- prestriate cortex
- interotemporal cortex
- posterior parietal cortex
where is the prestriate cortex?
in the occipital lobe, excluding the primary visual cortex
where does the prestriate cortex receive information from?
primary visual cortex, pulvinar nucleus (thalamus), other associated nuclei
what would be the result of a lesion to one of the association cortices?
visual agnosia - objects are seen but their significance is not understood
what is the role of the interotemporal cortex?
higher processing: object and face recognition (WHAT)
what is the role of the posterior parietal cortex?
higher processing: appreciation of space (WHERE)
describe the dorsal stream of visual processing
- primary visual cortex –> prestriate cortex –> posterior parietal cortex
- where pathway
describe the ventral stream of visual processing
- primary visual cortex –> interotemporal cortex
- what pahtway
describe generally, how visual information is processed
- parallel: simultaneously along multiple pathways
- functionally segregated: structures specialized in diff kinds of analysis
- hierarchical: increasing neuroanatomical and functional complexity
apperceptive visual agnosias?
- failure to perceive objects even though visual aciuty is normal
- cannot draw or copy other people’s drawings
- deficit in perception not reception - injury along ventral stream
prosopagnosia?
- inability to recognize faces
- caused by damage to human visual association cortex
- e.g., lesion to right temporal lobe
where is the visual attention centre?
right parietal lobe
what is the pupillary reflex useful for?
demonstrating functional midbrain and CN III
when can the pupillary reflex be damaged?
when the brain swells, so the midbrain pushes against the edge of the tentorial notch
what is the pupillary reflex?
controls diameter of the pupil - thus, controls how much light reaches the retina
what is the near (association) relfex?
- when an object approaches
- eyes converge slightly
- pupils constrict to reduce blurring
- lenses accomodate (thicken) to focus on the close object
how does the lens accomodate?
- ciliary fibers attach to the lens
- when ciliary muscle contracts around the ciliary fibers, they lengthen and go slack
- this allows the lens to naturally bulge, increasing the light refraction, improving sight for closer objects
