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Summary 2.4 - Perception
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Problem 1: a keen eyeSource -1: Blake
How do the eyes move?
- Unlike animals we need extra effort to see but we are granted ‘extraocular muscles’ in order to
move eyes to a certain extent
- The muscles are like strings attached to the eyeball it contracts
- Movement is proportionate to the strength (so if muscles are strong then movement is sharp)
- Direction depends on the placement of the muscle pulling the eye
- Types of rectus muscles: essentially, only a contraction to the direction in which the muscle is
placed. The rear of the eyeball has an immovable end and the contracting muscles are attached
toward the front
Medial rectus: close to the nose so rotates to nose
Lateral rectus: towards the ear
Superior rectus: top of the eyeball (toward eyebrow)
Inferior rectus: bottom of the eye
- Types of conjugate eye movement: where both eyes are in the same direction
Convergence inward
Divergence outward
How should the eyes be protected?
Orbital fat: serves as a cushion and buffer for any trauma
Eyelids: to keep eyes moist, a reflex against intruders
Blinks do not have a black out effect i.e. like being in a dark room and suddenly there’s light. When
eye blinks, the brain sends signals to suppress vision (just before and during each blink) dimming is
harder to detect during a blink bc the brain sends signals
Blinking and tears have behavioral outputs. crying is moistens eyes and the evaporated tears = snot
Older we get the dryer our eyes = no crying
The structure of the human eye
The spherical eyeball has 3 layers:
1) Outermost layer protective: fibrous tunic
Sclera: hi pressure, packed with fibers, white, and hard.
Cornea: bulge on the front, allows for light, transparent bc no blood supply.
2) Middle layer nourishment: vascular tunic
Choroid: source of blood vessels and capillaries for the photoreceptors. No light comes through
here
The anterior chamber ciliary body: spongy network of tissues aqueous humor: fluid made by
the ciliary body, it takes care of the lens and cornea’s needs (i.e oxygen, nourishment and hygiene
through homeostasis) if overflowed then “intraocular pressure” potentially glaucoma
Iris: if the outer layer has high pigment then eye will be brown is not then light. The inner layer
has blood vessels and mix of the overlapping colors is what makes the characteristic
Pupil: have muscles that control amt. of light that enters. Inner set (circular) shrinks/contracts vs.
outer set (radial) large/dilates
When light is high then the pupils contract
When ANS is stimulated excited = pupil’s dilate
The lens/crystallize lens: found behind the iris and has 3 parts:
Elastic covering/capsule: regulates flow of aqueous humor in the lens and molds the shape of
the lens “accommodation”
Epithelial layer: lies within the capsule. Constantly produces fibers and the most recent ones
are outside. The older ones can lead to sclerosis
, Lens: hi transparency for light to focus onto the retina. If not then it’s a cataract (cataracts
usually due to diabetes)
The vitreous chamber
Vitreous fluid: egg like not renewed like aqueous humor so can be a floater or retinal
detachment
3) Inner layer signals: retina convert light into neural signals to the brain
Outer segment: photoreceptors against light and toward eye to get nourishment from the
choroidal layer
Bipolar amacrine + horizontal cells: collects and combines the signal to pass onto RGC
Retinal ganglion: receives the signals sends it to the brain
Retinal landmarks and blood supply
- Using an ophthalmoscope we find the blood supply location within the retina (these are two spots
near the retina):
Macula: dark center that receives the image
Optic disk: at the far right from the macula, has no RGC’s and has a cluster of optic fibers hence
a blind spot in the retina
- The retina is the MVP and so needs great metabolic support
Choroidal circular system: the choroidal layer for the photoreceptors
Retinal circular system: cells in the retina transport with the blood supply existing in front of
the retina.
- Pigment epithelium: outermost sheath containing 1 layer of cells
A barrier where choroidal blood must pass through in order to nourish the photoreceptors
(Vit. A as it helps with pigment)
The eye as an optical instrument
- The retina needs to mimick the distribution of light that it receives onto the brain. It isn’t passive
with the light however it can only do so if the light is strong enough
Image formation in the human eye
Optic power: power to bend or refract light increases visual acuity (lens)
- the eyeball size also helps with visual acuity & a good crystallize lens however optical power
changes when the eyeball shape is different (cornea and lens = optics)
Divergent waves: cannot form a cohesive photo with divergent waves. Ideally it needs to be reversed
through the convex lens converged waves onto the retina
- distance (grade of the lens) depends on the optic power and degree of light divergence
- strong diverging light when it is close to lens vs. weak diverging light when it is farthest
Emmetropic: when the eye is shaped just right distant/near object focuses exactly on the retina
Hyperopia: far sighted. When image is close it is hard to accommodate
- the problem: the image is formed behind the retina. Because the eyeball is too short natural
accommodation is difficult and effortful
- the fix: convex lens to increase the optical power
myopia: when the eyeball is too long
- the problem: the image is formed infront of the retina “near sighted”
- the fix: concave lens (we do not want to enhance the power rather to regulate it)
- genetically common in some countries and a nurture type thing (i.e sailors)
presbyopia: natural old sight
- with age the accommodation skills start to suck usually 40’s to 70’s
- Why: sclerosis and low elasticity of the lens capsule (shape of the lens and for accommodation)
- Convex lens is needed (when problems with accommodation = convex lens for the win)
Astigmatism: when the cornea, which allows light to enter, is misshapen
, - Not about accommodating but more about refracting of light. Needs lasik
A sideway look at the retina
Fovea: a layer that thins out from the macula and turns into a pit (there are no rods here)
Thin layer to not absorb light or else photoreceptors will not get them
Photoreceptors: humans have rods and cones unlike some species
- High concentration of cones in the center and more rods in the periphery
- bc the photoreceptors aren’t evenly distributed = img wont appear as sharp (Hermann’s grid)
- Are positioned like mosaics with their own neural firing rates depending on the light = this gives
accurate representation to the brain
- Babies and albinos have low foveal pit = less photoreceptors and more sensitivity
- The magnitude of neural response is a product of how the cell alters it
- The photoreceptors take in tiny lights and combine them into a whole in the brain
Image formation = source of light object reflects this sends the sculpture to the brain
Retinal ganglion cells (RGCs): process neural info received via the photoreceptors
- Send signals to the brain via action potentials (img. is made up of transduced signals)
- Collects the separate photoreceptor outputs and summarize the important parts
- Used microelectrodes on monkey optic nerve while showing pics = firing confirms their
functions even when no image has been shown yet
- RGCs has a trigger before accepting light from photoreceptors receptive field
If only in the center then “on” if only in the side “off” if light is overlapping between the 2 then
firing is mixed
Receptive field corresponds to visual space so if we move the subject it also moves from our
retina and it also corresponds to an RGC
Receptive field acts as a checklist (bouncer) of orientation before RGC can let light enter
Lateral inhibition: when 2 antagonistic regions in the retina have a standoff
Neural architecture of ‘on and ‘off regions’
Horizontal cells: laterally spread and interconnected with any one of the photoreceptors
- They attenuate weak signals so the more active ones end up being more active
- They accentuate a more active clusters of active neurons
- The signals are then passed onto the RGC’s via bipolar cells
Bipolar cells: they are attached to the horizontal cells sometimes but on most occasions mainly attached
to photoreceptors
- Amt. of glutamate received is due to light received electrical signals of bipolar cells is
proportionate to the glutamate received
- Photoreceptors produce glutamate at an inverse proportion to the light they receive
- Therefore Have 2 varieties: 1) responds positively in an increase in glutamate (a decrease in light)
2) responds positively with a decrease in glutamate. (and increase in light)
- Bipolar cells encodes the photoreceptor glutamate into on and off components
Amacrine cells: influences how vigourous the RGC’s perform over time
- controls whether the RGC should get the signal or not bc they are outnumbered by the
photoreceptors
3 types of ganglion cells:
Magnocellular - Is thick so sends faster to the brain
- Bigger compared to P
- Responds better to subtle changes in light
- Responds better to rapidly moving objects
- Doesn’t need color to see unlike P
Parvocellular - Has a smaller receptive field size = better
response to small objects
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