Visual Systems Flashcards
What is white light?
All the wavelengths of the visible light spectrum together
What are Purkinje images?
Reflections of objects off the structure of the eye. Can be used in eye tracking.
In what part of the eye do axons leave and blood vessels enter?
The optic disk, also known as the blind spot
What are zonule fibres?
They are fibres that are extended around the eyeballs. Connected to ciliary muscles.
What are the ciliary muscles?
Donut shaped muscles that are connected to zonule fibres. It can change the shape of the lens by contracting and relaxing, for viewing objects at different differences. They also regulates the flow of aqueous humour into Schlemm’s canal.
What are the sphincter pupillae?
The iris of the eye. It can contract and dilate the pupils.
What is refraction power and what are its units and equation?
is the degree to which a lens, mirror, or other optical system converges or diverges light. It is in diopters and can be calculated by the reciprocal (1/x) of focal distance in inverse metre m−1, which is the diopter unit.
What is the far point?
The distance where the lens (flattened) begins to have a role in focussing.
What is the near point?
The distance within which accommodation of lens begins to not be good enough. It is as round as it can get. Aging degenerates the ability to make the lens ‘fat.’
What is emmetropia?
The normal healthy state of refraction in the eye.
What is hyperopia? How can it be corrected?
A developmental problem where eyeball is too short. The cornea is shape for normal eye, which means it focuses light beyond the retina. It can be fixed with convex lenses. and is common. (farsightedness)
What is myopia?
Nearsightedness. The eyeball is too long, refraction of cornea is too strong, focuses picture in front of the retina.
What is the binocular visual field? Which type of animals have a wide field and which have a narrow one?
An area of overlap between two fields. ANimals with frontal facing eyes (like humans) have narrow fields, animals with side facing eyes have a wider field (like rabbits). An advantage to having a narrow binocular field of view is the ability of stereopsis (being able to see depth)
Which cells send action potentials in the retina (the only ones)?
Ganglion cells.
What type of potentials do non-ganglion cells in the retina make?
Gradient potentials
What are the inner and outer nuclear layer?
They contain all the nuclei of the photoreceptors (in outer) and the nuclei of bipolar, horizontal and amazcrine cells in the inner nuclear layer
How much more sensitive to light are rods than cones? Why?
About 1000 times, because they have many more disks and photopigment
What is the ratio between photoreceptors and ganglion cells in the fovea and in the periphery?
About 1:1 in the fovea, many photoreceptors to 1 ganglion in the periphery (leading to greater sensitivity to light in the periphery.
Where is the greatest number of rods found?
Just outside the fovea, tapers off in numbers to the edge of the periphery
How are does rod to ganglion information about light become ambiguous?
Multiple rods triggering one ganglion creates ambiguity.
Where is there the greatest absolute density of photoreceptors on the retina? What is the consequence of this?
The fovea. Good two point accuity.
What 3 structural features of the fovea increase two point accuity?
1:1 ratio of photoreceptor to ganglion cells
Absolute density of photoreceptors
Fovea ‘pit’ that provides a less obstructed path for light
When does a rod photoreceptor secrete neurotransmitter? From what? What is the rods response to light? What is its membrane potential in light?
In the dark. From an influx of sodium causing a receptor potential from depolarization. A rod hyperpolarizes in the presence of light to about -60 mV.
The shifting concentration of _____ in light/dark are responsible for the depolarization and hyperpolarization of rods.
cGMP
What are the components of rhodopsin?
opsin protein and retinal vitamin (retinal)
What happens to rhodopsin in light?
The retinal molecule changes shape. Its tail straightens out and bleaches from purple to yellow. This conformational change activates the opsin, which causes cGMP to be converted to the inactive GMP, leading to closed sodium channels.
What is the first molecule that rhodopsin interacts with in light/dark conditions? What activates this molecule and what does this molecule activate?
A transducin G protein. The G protein is activated when rhodopsin is activated, and then the G protein activates phosphodiesterase. Phosphodiesterase convertes cGMP to GMP. The decrease in cGMP concentration results in sodium channels closing.
What is the molecule that directly opens the sodium channels in photoreceptors? What converts this molecule to its inactive form?
cGMP. Converted to GMP by phosphodiesterase.
What wavelength of light do red cones absorb?
560 nm
What wavelength of light do green cones absorb?
530 nm
What wavelength of light do blue cones absorb?
430 nm.
What wavelength of light are rods most sensitive to?
498 nm and around
How long does it take to switch from photopic to scotopic vision?
About 25 minutes
How long does it take to switch between scotopic to photopic vision?
5 minutes
Why are you colour blind at night?
Because there is not enough light to activate cones.
What type of cell is responsible for the surround receptive field being different from centre?
Horizontal cells reverse ON/OFF signal
ON bipolar cells are ___tropic
ionotropic
OFF bipolar cells are ___tropic
metabotropic
What are the characteristics of a receptive field for an OFF-centre ganglion cell?
The centre is inhibited by light and excited by dark.
What are the characteristics of a receptive field for an ON-centre
The centre is excited by light and inhibited by dark.
The surround is inhibited by light and excited by dark.
What are the characteristics of Magno ganglion cells? The consequence?
Magno ganglion cells have large dendritic fields and therefore larger receptive fields.
What types of stimuli are magno ganglion cells sensitive to?
They are sensitive to motion, but not sensitive to colour.
What type of stimuli are parvo ganglion cells sensitive to?
Parvo cells tend to be colour coded and exhibit centre-surround receptive fields to colour opponency
Describe the receptive field of a colour opponent parvo ganglion cell with a green ON centre and red off surround.
Green light on the centre of the field would send many impulses, but green light on the whole field would result in some red cones being partially activated by wavelength overlap in the red cones resulting in inhibition from the red-off surround.
What would be the response of a colour opponent ON-centre parvo ganglion cell to white light?
It would be completely inactivated as the ON centre and OFF surround would be equally stimulated
What are the visual hemifields?
They are the right and left side of vision. Each side goes to the opposite hemisphere in the cerebral cortex. Each eye can see both hemifields, but only the left hemisphere can see the right hemifield and same for right.
What is Brodmann’s area for the primary visual cortex?
Brodmann’s area 17
What receives axons form the retina and controls eye movement to visual and auditory stimuli?
The superior colliculi
What part of the thalamus has layering and has axons from the retina innervating it.
The lateral genicular nucleus (LGN)
What is the name of the bundle of axons that flows form the LGN to the primary visual cortex?
The optic radiation
How many layera are there in the LGN? What is a characteristic of these layers that makes them different from neocortex layers?
- There are spaces ventral to each P or M layer with very small neurons or nuclei (koniocellular layers). The P and M pathways are still kept seperate in the LGN
In the LGN, layers that receive axons from the contralateral eye are?
Layer 1, 4 and 6
In the LGN, layers that receive axons form the ipsilateral eye are?
Layers 2, 3 and 5
What layers in the LGN receive information from m-ganglion?
Layers 1 and 2 (magnocellular LGN layers)
What layers in the LGN receive axons from p-ganglion?
Layers 3-6 (Parvocellular LGN layers)
What layer of the LGN are nonM-nonP ganglion cells projecting to?
Koniocellular layers of the LGN, which are between the other layers.
What structure in the brain contains brodmann area 17?
Brodmann’s area 17 (primary visual cortex) is on the banks of the banks of the calcarine fissue
What makes possible the upside down projection of an image on the striate visual cortex?
small receptive fields size and topographic connections make possible the upside down projection of an image on the striate visual cortex.
What layer of the striate nucleus (V1) is split into A B and C, which later then is split into α and β?
Layer IV is split into IVA, IVB and IVC. Layer IVC is then split into IVCα and IVCβ
What does layer IVC of V1 mostly deal with?
Stellate input
Which layer of V1 has very big pyramidal cells?
Layer IVB
What layer of the V1 sends axons to the superior colliculi?
Layer V
What layer of V1 sends axons to the LGN?
Layer IV
What layer of the V1 do magnocellular inputs innervate?
Layer IVCα
What layer of the V1 does parvocellular axons innvervate?
Layer IVCβ
Which layer of the V1 has interblob and blobs?
Layer IVCβ, Koniocellular layers only have blobs.
What layer of the V1 does V1 layer IVCα project to? What stream runs beyond this?
Layer IVCα projects to IVB and then from there axons are send through the dorsal stream on the MT and MST pathways.
What function do cells on the magnocellular pathway have?
Space
Motion
Depth
What functions do cells on the parvocellular pathway have?
Shape
What functions do cells on the nonM-nonP koniocellular pathways have? What are some retinotopic proporties of cells in this pathway?
Colour sensitivity.
Circular fields
monocular vision
non-orientation
What are ocular dominance columns and where are they found?
Ocular dominance columns are seperation of the right and left eye on layer IV of the striate cortex.
How does autoradiography work?
Injection of something into something (eg. radioactive proline into eye). Then an autoradiograph was made with silver grains to show where the radioactive prolines are incorporated into cells
What is the width of most ocular dominance columns?
.5 mm in input layer IVC
What is different about blobs and interblobs?
Afferent connectivity to blobs and interblobs is different. Blobs have high concentration of cytochrome oxidase, a mitochondrial enzyme for cell metabolism.
What layers are blobs found in?
Layers II to VI (but not IV) of the V1
What type of receptive fields to simple LGN cells have?
Seperate ON/OFF regions of the fields. Other (complex) LGN cells have scattered ON and OFF regions. All LGN cells have both ON and OFF regions.
What is the lowest level of processing stream at which cells are sensitive to retinal disparity?
Layer IVB is the lowest level of processing at which cells are sensitive to retinal disparity and begin to process depth perception.
What is the consequence of complex LGN cells having scattered ON/OFF regions of receptive fields?
Sometimes they fire a while after stimulus is removed. Many properties can emerge from this, such as length of light stimulus
Where do axons from nonM-nonP ganglion cells go in the V1?
The bypass layer IV altogether and go directly to blobs in layers II, III, V and VI
What are the three pathways in vision?
Magnocellular pathway
Blob pathway
Parvo-interblob pathway`
What is a hypercolumn?
Complete set of cortical processing for any single point in visual world. Zillions of these for every point in the visual field. Independtly responsible for processing of visual world. Losing a hypercolumn from cell death means that point won’t have a cortical representation.
What is a loss of cortical representation called?
Scotoma
Every mammal has a scotoma in their visual field. What is it?
The blind spot (optic disk)
What is the area responsible for processing of motion processing? What stream is it in and from what layer of the V1 do axons going to this area originate.
The MT (medial temporal) area, innervated by cells from layer IVB. Area MT is in the dorsal stream and part of magnocellualr pathway.
What is the area in the parietal lobe that is specialized for precise movement? (linear, side to side, circular etc.). What three things does this area aid us with?
The MST area has cells selective for certain types of motion
Navigation as we move through our environment (avoiding collisions)
Directing eye movements
Motion perception
Why is the gransmother cell theory false?
Not enough neurons for this
What is the consequence of losing area V4?
Colour blindness from cortical damage called cerebral achromatopsia
What is colour blindness from cortical damage called?
cerebral achromatopsia
What is the ventral stream?
A progression of areas from V1 to V4 that run towards the temporal lobe from the occipital lobe ventrally. They all give images characteristics like colour and that sort of thing
What type of connections innvervates area V4 of the ventral stream?
Axons from blobs and interblobs are relayed through V2 to V4.
What is area IT?
An area in the ventral stream that is important in visual perception and visual memory. Contains such cells as those specialized for face recognition.
What happens if face recognition cells in area IT are damaged? What is the name of the affliction?
The inability to recognize faces, prosopagnosia occurs.
