Visual Pathway Flashcards
Slide Set 3 (Wk3 L1 /Wk3 L2)
What is the range of wavelengths in visible light?
400 - 700 nm
Where does light become neural pathways?
optic nerve
What is considered the “relay station”?
The LGN (lateral geniculate nucleus) in the thalamus
What are the two systems that occur once information is passed from the thalamus to the cortex?
1) The “What” System: identifying objects, understanding the different patters and structures
2) The “Where/How” System: spatial processing
What is transduction?
The process of physical energy from light being translated into neural signal
Light energy –> Chemical reaction –> Electrical signals
- mediated by receptors (cones and rods)
- “bleaching” in photopigments
When does light not move in a straight line?
When it travels through objects that bend it (REFRACTION)
e.g. done by the cornea and the lens
Why is refraction important for vision
The lens must focus and perfectly converge light onto the retina
(ACCOMODATION)
How does the lens manipulate refraction?
Through accommodation, which involves changing the shape of the lens by contracting or relaxing the ciliary muscles that are attached to the lens
(under control of the motor system in the brain)
What are the two ways that images on the retina are inverted?
1) upside down
2) left to right
What is isomerization?
- receptors contain many pigment molecules
- one photon is absorbed by a pigment molecules, causing it to become ISOMERIZED and change shape
- a chain reaction occurs where other pigment molecules are also isomerized
- leads to the receptor generating an electrical/neural signal
What is another word for isomerization?
bleaching
photopigments are bleached by photons
How many photons are needed to activate a receptor?
one photon
Why is the retina considered inverted?
Receptors are located at the very back
What structures are located at the center of our visual field?
- Fovea (point of central focus)
* Macula
What is the area outside of the central visual field called?
Periphery
Human field of view: horizontally
180º
Human field of view: binocular vs monocular
~140º binocular vision; ~ 40º monocular
Human field of view: nasal vs temporal
each eye is ~60º nasally; ~100 degree temporally
Human field of view: vertically
~60º above the horizontal meridian
~75º below the horizontal meridian
*upper visual field is restricted b/c of the eyebrows
What is the rule of thumb?
the width of our thumb occupies two degrees of visual angle
What is the visual field of the fovea?
~2º diameter of central vision
note:
- no rods, only cones
- 1% retina but ~50% of visual cortex
What is the visual field of the para-fovea?
area immediately surrounding the fovea
~4-5º around the center
What is the visual field of the periphery?
the area beyond 8º of the center
note: much fewer resources devoted to this huge portion of the visual field; this is why we must orient to our central field of view (fovea)
What is eccentricity?
a measure of how far something is from the center
What is the blind spot?
~3º area where there are no receptors, therefore we can not take in light information
*axons of many ganglion cells forming the optic nerve and leaving the eye
What are our four dark adaptation mechanisms?
1) constriction of the pupil:
changing the pupil size controls the amount of light that enters the eye
2) throwing away photons:
in bright conditions, there are too many photons
reaching the retina and bleaching the pigments –> most pigments are not available and do not respond to
photons
3) ganglion cells:
their receptive field mostly encode contrast and not overall level of light
4) the duplex system of receptors:
rods and cones enable us to operate well in both dim and bright conditions
In ganglion cells, what happens when light hits the ON portion of the receptive field?
firing is excited above baseline
In ganglion cells, what happens when light hits the OFF portion of the receptive field?
firing is inhibited below baseline
What happens if both ON and OFF are hit by stimulus (uniform light)
excitatory and inhibitory cancel each other out, and the cell remains at baseline
–> ganglion cells only care about CONTRAST (light vs dark)
How does “pooling” differ between rods and cones?
Rods: many rods tend to project to the same ganglion cell (pool)
Cones: different cones tend to project to specific ganglion cells 1:1 (more picky, no pooling)
What is the acuity-sensitivity tradeoff?
RODS
(Sensitivity): Pooling allows rods to have higher sensitivity, bc there is a higher probability of ganglion cells receiving info and being activated
(Acuity): Pooling decreases acuity in rods b/c it prevents stimuli patterns from being differentiated (the same ganglion cell is still being activated)
CONES:
(Sensitivity): Not-pooling decreases the probability of ganglion cells being activated, because cones may or may not be excited by a photon and they are directly attached to a single ganglion cell
(Acuity): Not-pooling increases acuity b/c different neurons are being activated and can be distinguished from one another
What is a scotoma?
an area of partial alteration in the field of vision consisting of a partially diminished or entirely degenerated visual acuity that is surrounded by a field of normal – or relatively well-preserved – vision
*a blindspot is a type of scotoma
–> brain uses a filling-in process
