Visual Input (Dr. Bianco) Flashcards
What is stereopsis ?
The perception of depth produced by the reception in the brain of visual stimuli from both eyes in combination; binocular vision.
Are the blindspot and fovea located more on the nasal side or temporal side of the retina ?
The nasal side.
What are the two most common eyes mvnts ?
What is their purpose ?
Smooht pursuit eye mvnts (left-right) and convergence eyes mvnts (forward-backward).
Both are voluntary eye mvnts that maintain the image of a moving target on the fovea of both eyes.
What are saccades ?
Why are they useful ?
Saccades are v fast eye mvnts that redirect gaze. These mvnts of different amplitudes occur several times per second. They enable fast foveal sampling of interesting/important object features and prevent image stabilization of the retina (stabilized images fade rapidly).
Does the retina detect:
- the absolute value of of light intensity ?
- the differences in light intensity ?
DIFFERENCES !
Like most processes, the nervous system is good at detecting change, not absolute things.
What imp features can the retina detect ?
Wavelength = color
Contrast = edges
Spatial frequency = size
Motion (more complicated)
What is spatial frequency ?
Spatial frequency refers to the level of detail present in a stimulus per degree of visual angle. A scene with small details and sharp edges contains more high spatial frequency information than one composed of large coarse stimuli.
What is contrast ?
The state of being (strikingly) different from something else in juxtaposition or close association.
What is parallel processing ?
For each location in the visual space, a wide range of features are detected and extracted by subset of retinal neurons w/ different response properties.
Parallel processing is the simultaneous detection and transmission of multiple types of info from the same image.
How is the retina structured ?
It is a layered structure: pigment epithelium, photoreceptor (PR) outer segment (rods and cones outer segments), outer nuclear layer (inner segments of rods and cones), outer plexiform layer (PR-BP cell synapse or PR-HC synapse), inner nuclear layer (BP cells), inner plexiform layer (BP cell-GC synapse or AC-BP synapse), GC layer and nerve fiber layer
What are the dimensions and properties of the human retina ?
It is 0.5mm thick and transparent to light.
What is the fovea ?
The fovea is the center most part of the macula. This tiny area is concentrated w/ cones and covers about 2 degrees of the visual field. This is the only sport containing no blood vessels.
Where is the blind spot ?
In the area where the optic nerve connects to the retina in the back of each eye, which known as the optic disk (optic nerve head). There is a total absence of cones and rods in this area, and, consequently, each eye is completely blind in this spot, hence the name blind spot.
What is retinas pigmentosa ?
The loss of rods followed by progressive loss of cones, which leads to night blindness and loss of peripheral vision
What is age-related macular degeneration ?
Cone degeneration in fovea which leads to loss of central vision.
What is the limit to the maximal spatial acuity of the retina ?
The number of PRs (like the pixels of a camera).
How are PRs connected to BP in the fovea ?
In the peripheral retina ?
In the fovea, densely-packed cone photoreceptors transmit signals from very small areas and can have a one-to-one relationship with ganglion cells.
In the periphery, signals from multiple photoreceptors are ‘pooled’ onto single ganglion cells so although sensitivity is high, the spatial acuity (or resolution) is lower in comparison to the fovea and responses are slower.
How are the PRs structured ?
PR outer segments are tightly packed with membranous discs (rods) or lamellae (in cones) containing the visual pigments (either rhodopsin or one cone opsin type) at about 25,000 molecules/μm2.
Of what large family of receptors are the PRs part of ?
GPCRs.
What process kicks off phototransduction ?
How does this process work ?
Isomerisation of retinal: when the 11-cis-retinal molecule absorbs a photon, it flips to the all-trans-retinal form.
This causes a conformational change in the opsin molecule, unfolding the C-terminal region to reveal G protein (transducin) binding sites.
What are the 3 types of cones and what do the types distinguish ?
Short = blue (419) Medium = green (531) Long = red (559)
How is wavelength sensitivity determined ?
By differences in opsin amino acid sequences.
At what wavelengths are rods more sensitive ?
490nm
This explains why blue objects appear brighter at night.
What are the principal steps of phototransduction ?
a) one rhodopsin molecule absorbs one photon
b) 500 transducin molecules are activated
c) 10E5 cGMP molecules are hydrolysed by PDE (posphodiesterase)
d) 250 ion channels close
e) rod cell membrane is hyperpolarised by 1 mV
f) less Glu release w/ light
