Topic 7: visual processing Flashcards
where is the retina and what does it contain?
back of the eye, contains photoreceptors specialised to convert light energy into neural activity
describe the properties of light
-electromagnetic radiation visible to our eyes
-wave of energy, has:
…wavelength –> distance between successive peaks and troughs,
…and has frequencies –> the number of waves per second
…and amplitude, the difference between wave trough and peak
the energy content of electromagnetic radiation is proportional to its…
frequency
radiation emitted at a high frequency (short wavelength) has the lowest energy content, true or false?
False, it has the highest energy content, examples are gamma radiation emitted by some radioactive materials, wavelengths less than 1nm
radiation emitted at lower frequencies (longer wavelengths) has less energy, true or false?
True, examples are radar and radio waves, wavelengths greater than 1nm
What is the part of the electromagnetic spectrum that is detectable by our visual system?
wavelengths of 400-700nm
What is the pupil?
the opening that allows light to enter the eye and reach the retina, appears dark because of light-absorbing pigments in the retina
What is the iris?
surrounds the pupil, pigmentation provides what we call the eye’s colour, the iris contains 2 muscles that can vary the size of the pupil, one makes it smaller when contracts the other larger
What is the cornea?
covers pupil and iris, glassy transparent external surface of the eye, continuous with the sclera
What is the sclera?
the “white of the eye”, which forms the tough wall of the eyeball
where does the eye ball sit?
sits in a bony eye socket in the skull, also called the eye’s orbit
What are the extraocular muscles?
-inserted into the sclera, move the eyeball in the orbit, not visible because they lie behind the conjunctiva
What is the conjunctiva?
membrane that folds back from the inside of the eyelids and attaches to the sclera
What is the optic nerve?
Carry axons from the retina, exits the back of the eye, passes through the orbit, and reaches the base of the brain near the pituitary gland
What is the ophthalmoscope?
a device that enables one to peer into the eye through the pupil to the retina, can see blood vessels on its surface
where do blood vessels in the eye originate?
from a pale circular region called the optic disc, which is also where the optic nerve fibres exit the retina
what doesnt occur at the optic disk and why?
sensation of light, because there are no photoreceptors there
What doesnt occur at the place where large blood vessel exit the eyes?
sensation of light, because vessels cast shadows on the retina
What is the macula?
middle of each retina, dark-coloured region with a yellow-ish hue, part of the retina for central vision (as opposed to peripheral vision). distinguished also by the relative absence of large blood vessels
What is the fovea?
a dark spot about 2mm in diameter, retina thinner in fovea than elsewhere, centre of retina
What is aqueous humour?
nourishment by fluid for the cornea is provided by the aqueous humour (lies between cornea and lens)
what is the lens?
located behind the iris, suspended by ligaments (called zonule fibres), attached to ciliary muscle, which forms rings inside the eye
What is vitreous humour?
serves to keep eyeball spherical, lies between the lens and retina
What light does the eye collect? and how does it focus images?
-The light rays emitted by or reflected off objects in the environment, and focuses them onto the retina to form images
-bringing objects into focus involves the combined refractive powers of the cornea and lens –> cornea is site where most refractive power of the eye is
What does the eye use in order to see a single point?
refraction to focus the light to one point of the retina, light passes into a medium where its speed is slower and it will bend toward a line that is perpendicular to the border, or interface, between the media
What is the focal distance?
The distance from the refractive surface to the point where parallel light rays converge
What does the focal distance depend on in the eye?
curvature of the cornea, the tighter the curve, the shorter the focal distance
What is the refractive power?
reciprocal of the focal point, measure in measurements called “diopter” (cornea has refractive power of roughly 42 diopters)
What is the refractive power of the lens and cornea?
lens = roughly 6 diopters
cornea = roughly 42 diopters
what is the lens more involved with?
-in forming crisp mages of objects located closer than 9m from the eye
-With near objects, the light rays originating at a point are no longer parallel. Rather, these rays diverge from a light source or a point on an object, and greater refractive power is required to bring them into focus on the retina.
-This additional focusing power is provided by changing the shape of the lens, a process known as accommodation
What happens during accommodation?
-During accommodation, the ciliary muscle contracts and swells in size, thereby making the area inside the ring smaller and decreasing the tension in the suspensory ligaments.
-Consequently, the lens becomes rounder and thicker because of its natural elasticity. This rounding increases the curvature of the lens surfaces, thereby increasing their refractive power.
-Conversely, relaxation of the ciliary muscle increases the tension in the suspensory ligaments, and the lens is stretched into a flatter shape.
what is the pupillary light reflex?
-involves connections between the retina and neurons in the brainstem that control the muscles that constrict the pupils –> when lights increases
-this is consensual, shining light on one eye causes constriction of both pupils
What is the benefits of the pupillary light reflex?
increase in depth of focus
Describe the visual field
-The left visual field is images on the right side of the retina and the right visual field on the left side of the retina
-similarly upper visual field is image on bottom of retina and vice versa
What is visual acuity?
The ability of the eye to distinguish two points near each other
What does visual acuity depend on?
several factors but especially on spacing of photoreceptors in retina and precision of eye’s refraction
What is the viusal angle?
Distance across the retina, ability of eye to revolve points that are separated by a certain number of degree
What is the pathway of visual information before exiting the eye?
photoreceptors to bipolar cells to ganglion cells
what do photoreceptors do?
respond to light, they influence membrane potential of the bipolar cells connected to them
What do the ganglion cells do?
fire action potential in response to light, and these impulses propagate along the optic nerve to the rest of the brain
what are the horizontal cells?
assists in further retina processing, receives input from photoreceptors and projects neurites laterally to influence surround bipolar cells and photoreceptors
What are amacrine cells?
assists in further retina processing, generally receive input from bipolar cells and project laterally to influence surrounding ganglion cells, bipolar cells and other amacrine cells
what are the cells that are light-sensitive and the cells that are influence by light only via direct and indirect synaptic interactions?
only rod and cone photoreceptors are light sensitive, all the other cells are influenced
What is the source of output from the retina?
only the ganglion cells
what fires action potential in the retinal?
with exception of certain amacrine cells, only ganglion cells fire action potentials, other cells depolarise or hyperpolarize but do not fire AP
What organization does the retina have?
laminar organization, in an “inside-out” arrangement, i.e., light must pass from the vitreous humour through the ganglion cells and bipolar cells before reaches photoreceptors
what is the order of cell layers of the retina? and what does each layer consist of?
direction of nose to back of head, ganglions cell layer –>inner plexiform layer –> inner nuclear layer –>outer plexiform layer –>outer nuclear layer –> layer of photoreceptors –> pigmented epithelium
-ganglion cell layer contains ganglion cells
-inner plexiform layer contains synaptic contacts between bipolar, amacrine and ganglion cells
-inner nuclear layer contains cell bodies of bipolar cells
-outer plexiform layer where photoreceptors make synaptic contact with bipolar and horizontal cells
-outer nuclear layer contain cell bodies of photoreceptors
-layer of photoreceptors, photoreceptors
What are the regions of photoreceptor?
outer segment, inner segment, cell body and synaptic terminal
Describe the outer segment of photoreceptors
-contains a stack of membranous disks, light sensitive photopigments in the disk membranes absorb light, thereby triggering changes in the photoreceptor membrane potential
-rod and cones distinguished by shapes of outer segment –> rods have long cylindrical outer segment containing many discs –> cones have shorter, tapering outer segments with few membranous disks
-rods 1000 times more sensitive to light because more disk (about 5 million cones and 92 million rods)
What is duplex retina?
two complementary systems in one eye
besides the outer segments how do rods and cones differ and relate?
-all rods have same pigment but 3 types of cones containing different pigment –>variations among pigments make the different cones sensitive to different wavelengths of light
- only cones responsible for seeing colour
-most of 5 million cones in fovea and proportion diminishes substantially in retinal periphery
-no rods in central fovea
-many more rods than cones in peripheral retina
in a cross section, the fovea appears as a pit in retina, what’s the reason for this pit?
-due to a lateral displacement of cells above the photoreceptors, allowing light to strike the photoreceptors without passing through the other retinal cell layers. maximises visual acuity at the fovea by pushing aside other cells that might scatter the light ad blur the image.
describe transduction in Rods
-in photoreceptors, light stimulation of photopigment activated G-proteins, which in turn activates an effector enzyme that changes the cytoplasmic concentration of a second messenger molecule. This change causes a membrane ion channel to close, and the membrane potential is thereby altered
-in darkness rod outer segment is about -30mV –> depolarisation cause by influx of Na+ through special channels in outer segment of membrane
describe movement of charge across membrane in the dark of rod
-movement of positive charge across membrane, occurs in dark, is called dark current –> sodium channels stimulate to open by intracellular second messenger called cyclic guanosine monophosphate (cGMP) (produced in photoreceptor by enzyme guanylyl cyclase), keeping the Na + channels open. Light reduces cGMP, causing the Na + channels to close, and the membrane potential becomes more negative. Thus, photoreceptors hyperpolarize in response to light
what is the pigment responsible for absorption of electromagnetic radiation in hyperpolarising response in rods? and receptor protein for it
in the stacked disks of rod outer segment, pigment called rhodopsin, receptor protein called opsin, opsin has seven transmembrane alpha helices typical of G-protein-coupled receptors
What happens when absorption of light occurs?
change in conformation of retinal so that activates the opsin –> process known as bleaching because changes the wavelengths absorbed by the rhodopsin (photopigment literally changes colour from purple to yellow).
What does bleaching of rhodopsin do?
stimulates a G-protein called transducin in disk membrane, which in turn activates the effector enzyme phosphodiesterase (PDE), which breaks down cGMP that is normally present in cytoplasm of the rod (in the dark). The reduction in cGMP causes the Na+ channels to close and the membrane to hyperpolarize
summarise steps in transduction of light by rods
- light activates (bleaches) rhodopsin
- transducin, the G-protein, is stimulated
- phosphodiesterase (PDE), the effector enzyme, is activated
- PDE activity reduced the cGMP level
- Na+ channels close, and the cell membrane hyperpolarizes
In bright light, cGMP levels in rods fall to the point where the response to light becomes saturated ; increasing the light level causes no additional hyperpolarization. Thus, vision during the day depends entirely on the cones, whose photopigments require more energy to become bleached. True or false?
True
what is the differences and similarities of transduction in cones and rods
-virtually the same, only major difference is in the type of opsin in the membranous disks of the cone of the outer segment
-the different opsin related to different spectral sensitiveies
–>short wavelength or “blue” cones, about 430nm
–>medium wavelength or “green” cones, about 530nm
–>long wavelength or “red” cones, about 560nm
there is overlap with these opsin not just activated at these wavelengths
what determines colour?
relative contributions of short- (blue), medium- (green) and long- (red) wavelength cones to the retinal signal –> theory of all colours made up of blue, green and red (of the colours not wavelengths of cones) called Yong-helmholtz trichromacy theory –> not to be confused to red wavelengths only perceiving red colour and etc.
what is dark adaption?
-The transition from all-cone daytime vision to all-rod nighttime vision is not instantaneous; depending on how high the initial light level is, it can take minutes to nearly an hour to reach the greatest light sensitivity in the dark. (aka getting used to the dark),
-sensitivity to light increases a millionfold or more during this period
how is dark adaption explained?
-one way is dilation of the pupils, allows more light to enter eye (diameter of pupil ranges from 2-8mm) –>changes to sizes increase sensitivity to light by only a factor of 10.
-another way is regeneration of unbleached rhodopsin and adjustment of the functional circuitry of the retina so that information from more rods is available to each ganglion cell
What is light adaption and how does it occur?
-adaption of light, reversing the changes in the retina that accompanied dark adaption
What does the light-dark adaption gives us?
-the light-dark adaption in the duplex of retina gives our visual system the ability to operate in light intensities ranging from moonless to midnight to bright high noon
describe calcium’s role in light adaption
-ability of eye to adapt to changes in light level relies on changes in calcium concentration within cones
-constriction of the pupil helps a bit in reducing the light, however, most important change is the gradual depolarisation of membrane back to about -35mV
- reason this happens stems from cGMP-gate sodium channels also admit calcium
-Ca 2+ enters the cones and has an inhibitory effect on the enzyme (guanylyl cyclase) that synthesizes cGMP. When the cGMP-gated channels close, the flow of Ca 2+ into the photoreceptor is curtailed along with the flow of Na + ; as a result, more cGMP is synthesized (because the synthetic enzyme is less inhibited), thereby allowing the cGMP-gated channels to open again. Stated more simply, when the channels close, a process is initiated that gradually reopens them even if the light level does not change. Calcium also appears to affect photopigments and phosphodiesterase in ways that decrease their response to light. These calcium-based mechanisms ensure that the photoreceptors are always able to register relative changes in light level, though information about the absolute level is lost.
at each synaptic relay what are the responses modified by in retina?
by lateral connections of horizontal cells and amacrine cells
What is the neurotransmitter released by photoreceptor when depolarised? when does this occur and when does hyperpolarization occur
amino acid glutamate when depolarised, depolarisation in the dark, hyperpolarization by light –> less transmitters release in light
what do the horizontal cells feed information to?
laterally in the outer plexiform layer to influence activity of neighbouring bipolar cells and photoreceptors
What is the receptive field in retina?
-light that is applied to small portion of retina that changes firing rate of neuron
What can bipolar and receptive fields be categorised into?
-two classes ON and OFF –> based on the response to the glutamate release by photoreceptors
What is the circuitry the gives rise to bipolar receptive fields?
-consists of direct input from photoreceptors and indirect photoreceptor input relayed by horizontal cells
What are OFF bipolar cells?
light shined onto a cone will hyperpolarize some bipolar cell –> because light will effectively turn them off
what are ON bipolar cells?
light shined onto a cone will depolarise other bipolar cells –> these cells are “turned on” by light
The cone-to-bipolar synapse inverts the signal from the cone. The cone hyperpolarises to light, but the ON bipolar cell depolarises. True or False?
true
How can different bipolar cells give opposites responses to direct cone input?
-there are 2 kind of receptors that receive glutamate release by the photoreceptors
Describe the OFF bipolar cells
-have ionotropic glutamate receptors, and these glutamate-gated channels mediate a classical depolarising excitatory postsynaptic potential from the influx of Na+. hyperpolarised of cone causes less transmitter to be release, resulting in a more hyperpolarised bipolar cell
Describe ON bipolar cells
have G-protein-couples (metabotropic) receptors and respond to glutamate by hyperpolarising
Each bipolar cell receives direct synaptic input from a cluster of photoreceptors. The number of photoreceptors in this cluster ranges from one at the centre of the fovea to thousands in the peripheral retina. true or false?
true
describe the synaptic interactions of photoreceptors, horizontal cells and bipolar cells
-First, when a photoreceptor hyperpolarizes in response to light, output is sent to horizontal cells that also hyperpolarize.
-Second, the effect of horizontal cell hyperpolarization is to counteract the effect of light on neighbouring photoreceptors.
-light is shined onto two photoreceptors connected through horizontal cells to a central photoreceptor and bipolar cell. The effect of this indirect path input is to depolarize the central photoreceptor, counteracting the hyperpolarizing effect of light shined directly on it.
what are the 2 parts of the receptive field of bipolar cell? and what does this means?
-a circular area of retina providing direct photoreceptor input, the receptive field centre;
and a surrounding area of retina providing input via horizontal cells, the receptive field surround
-the response of bipolar cell’s membrane potential to light in the receptive field centre is opposite to that of light in the surround –> thus these calls are said to have antagonistic centre-surround receptive fields
the centre-surround receptive field organisation is passed on from what to what?
from bipolar cells to ganglion cells via synapses in the inner plexiform layer
What do ON-centre and OFF centre ganglion receive inputs from?
from corresponding type of bipolar cell
what is an important difference in centre-surround receptive field organisation in bipolar and ganglion cells
-ganglion cells fire action potentials –> they fire whether or not they are exposed to light, and increases or decreases in relations to the receptive field centre or surround
describe the OFF ganglion cell action
-will fire fewer AP when small spot of light is projected to the centre of its receptive field, it will fire more AP if small dark spot covers receptive field centre
in both ON and OFF types of cell the response to stimulation of centre is cancelled by what?
response to stimulation of the surround
What are the 2 major types of ganglion cells in retina?
large M-type ganglion cells (about 5% of population) and smaller P-type ganglion cells (about 90% of population)
what happens when there is a generation of centre response of ganglion cells receptive fields?
-When on-centre bipolar cell is
depolarized it increases
transmitter release onto on-centre ganglion cell dendrites
-On centre ganglion cell is
depolarized and fires faster
Off-centre bipolar cell is
hyperpolarized causing
decreased transmitter
release.
-Off centre ganglion cell
hyperpolarizes and fires more
slowly
describe the generation of antagonistic surround of ganglion cell RFs by horizontal cells
-Photoreceptors in surround
hyperpolarize and release less
glutamate onto horizontal cells
-Horizontal cells hyperpolarize and so release less neurotransmitter
-Horizontal cell neurotransmitter
(GABA) hyperpolarizes photoreceptor terminals (like light in centre of RF)
-Thus less transmitter causes
depolarization of photoreceptor
terminals in RF centre increasing
glutamate release onto bipolar cells in RF centre
Describe colour opponency in P-type ganglion cells
P type ganglion cells have colour opponent RFs Red ON centre, green OFF surround shown Blue – yellow opponency also found
describe the pathway of visual perception
-originates in retina –> lateral geniculate nucleus (thalamus) –> primary visual cortex –> higher order visual areas in occipital, temporal, and parietal lobes
describe receptive fields of LGN neurons
almost identical to the ganglion cells that feed them
Describe magnocellular LGN neurons receptive fields
large centre-surround receptive field with transient response
describe parvocellular LGN cells receptive field
Small centre-surround receptive fields with sustained response
Describe the retinotopy
-Map of the visual field onto a target structure (retina, LGN, superior colliculus, striate cortex)
-Central visual field (fovea) overrepresented in map
-Discrete point of light can activate many cells in the target structure due to overlapping receptive fields.
-Perception is based on the brain’s interpretation of distributed patterns of activity—not literal map
describe the monocular receptive fields
–Layer IVC: similar to LGN cells
–Layer IVCalpha: insensitive to the wavelength
–Layer IVCbeta: centre-surround colour opponency
decribe the binocular receptive fields
–Most neurons in layers superficial to IVC are binocular.
–Two receptive fields—one for each eye
describe parallel processing
-Simultaneous input from two eye
–> Input from eyes compared in cortex –> Determines depth and distance of object
-Information about light and dark: ON-center and OFF-centre ganglion cells
-Different receptive fields and response properties of retinal ganglion cells: M and P cells, and nonM–nonP cells
describe parallel processing and perception
Hierarchy of complex receptive fields:
-In retinal ganglion cells: center-surround structure, sensitive to contrast and wavelength of light
–In striate cortex (v1): orientation selectivity, direction selectivity, and binocularity
–Extrastriate cortical areas: selective responsive to complex shapes (faces) and motion
Rods and cones both hyperpolarize in light, true or false
True
