Sensation & Perception Exam 2

Fixation?

The alignment of the eyes using extraocular muscles

What are the extraocular muscles?

Superior rectus, inferior rectus, medial rectus, lateral rectus, superior oblique, inferior oblique

Where are the superior & inferior rectus muscles and what do they control?

Superior is above, inferior is below, and they mostly control up/down eye movements

Where are the superior & inferior oblique muscles and what do they control?

They come in from the side and wrap around working as a stabilizer & for fine tuning (looking at the right and left corners)

Where are the lateral & medial rectus muscles and what do they control?

Lateral is outside, medial is in the middle (towards the nose), and they are used for left/right eye movements (can converge and diverge)

What is focus?

The focusing of light on the retina, specifically the fovea using intraocular muscles

Can you fixate without focusing?

Yes

Can you focus without fixating?

No

These are the types of voluntary eye movements

Saccades, Smooth Pursuit, Vergence

What are saccadic eye movements?

Quick, jerky ballistic eye movements used to quickly scan an environment

What are smooth pursuit eye movements?

Smooth, tracking eye movements that follow an object

What are (con or di) vergence eye movements?

Movements that adjust the eye to look at different distances: convergence is inward movement & divergence is outward movement

When looking at something close...

The angle is bigger

When you go from looking at your computer to the lecture screen, what eye movement(s) are occurring?

Divergence saccadic eye movements

Can you do more than one eye movement at a time? If so, which?

Yes, only vergence can happen with either saccadic or smooth pursuit

What are the involuntary eye movements?

Microsaccades, nystagmus, vestibulo-ocular reflex (VOR)

What are involuntary eye movements?

Uncontrollable eye movements

What is microsaccades?

Tiny tremors/shivering of the eyes helpful to stabilization of images

What is nystagmus?

Pattern of eye movements that has both a saccadic and smooth pursuit component

What is optokinetic nystagmus?

The rapid eye movement when following a moving object

What is end-point nystagmus?

A rhythmic eye movement that occurs when looking to the extreme side

What is a vestibulo-ocular reflex (VOR)?

A reflexive eye movement that maintains fixation with head movements

What is accommodation?

The process of changing the shape of the lens to focus on near or distant objects

How does the lens behave when seeing near objects?

The lens becomes thicker (stronger) to focus light

When looking at an object nearby...

The focal distance and point is shorter

How does the lens behave when seeing objects that are far?

The lens become thinner (weaker) to focus light

When looking at an object far...

The focal distance and point are longer

What happens to light from a distant object as it enters the eye?

The light comes in parallel, requiring only slight convergence

What percentage of refraction is done by the cornea?

2/3 (~67%)

What percentage of refraction is done by the lens?

1/3 (~33%)

How do accommodation and vergence interact when seeing near?

Eyes converge and accommodation gets stronger

How do accommodation and vergence interact when seeing far?

Eyes diverge and accommodation gets weaker

How long do accommodation and vergence drive each other?

Until light fades

What is accommodative convergence?

The accommodation system affecting/driving the convergence system

What is convergent accommodation?

The vergence system affecting/controlling the accommodation system

What is an emmetropic eye?

A normal eye with light focused properly on the retina

What is myopia and how is it corrected?

Nearsightedness, where light focuses in front of the retina corrected with concave lens

How does a concave lens correct myopia?

Slightly diverging the light moving the focal point back to the retina

What does a diopter measure?

The strength of a lens based on its focal length

What does -2 D mean?

Beyond 20 inches vision gets blurrier

What is the formula for diopter?

D = 1 / focal line in meters

If light is focused anywhere other than the retina, what is the result?

Blurry vision

What is hyperopia?

Far-sightedness where light focuses behind the retina

Can someone have some of both hyperopia and myopia?

Yes

What is astigmatism?

When the eye has multiple focal points depending on light orientation

What is presbyopia?

Elder eyes, where vision deteriorates with age

What happens to the lens as we age?

It yellows and hardens, leading to decreased flexibility

What is sclerosis?

Lens hardening which can't be corrected and has to be replaced

Cataracts

A clouding of the lens. Replaced in surgery by an artificial lens [haptic]

What is senile miosis?

A gradual decrease in pupil dilation capability with age

How many cones are in each eye?

6-8 million

How many rods are in each eye?

120-130 million

Where are cones mostly distributed?

The fovea

Where are the rods mostly distributed?

In the periphery

Do cones have high or low acuity (can see small details)?

High

Do rods have high or low acuity (can see small details)?

Low

Do cones have color vision?

Yes

Do rods have color vision?

No, they are achromatic

When are cones primarily used and are they sensitive to light?

Mainly during the day and they are not light sensitive "photopic"

When are rods primarily used and are they sensitive to light?

Mainly at night and they have high sensitivity to light "scotopic"

Where do cones hit peak wavelength?

550 nm

Rods hit their peak wavelength at?

500 nm

How long does dark adaptation take for cones?

About 5 mins

How long does dark adaptation take for rods?

About 30 mins

What is mesopic vision?

Occurs at sunrise and set involving both rods and cones

What is the Purkinje shift?

The shift in color perception as light decreases

What happens to greens and reds during the day, mesopic, and night?

Day: reds are brighter than greens, Mesopic: greens are brighter than reds, Night: greens turn to gray & red turns to black

What is a kinked function?

Where cones & rods are combined in dark adaptation

What is the golden rule of dark adaptation?

Anything below your adapted level of luminance you cannot see

What is the fraction for vision? (ex. 20/20 or 20/40)

What you see / What the average person sees at that length

What is the transduction process for vision called?

Photo-Isomerization

What is photo-isomerization?

When the chromophore changes shape and releases energy after the photon is absorbed, changing the flow of electrical current surrounding the photoreceptor (takes

What are the parts of photopigments?

A protein (opsin) and a vitamin A derivative (chromophore)

What is a chromophore?

The light catching portion of the photopigment

What is the process of transduction?

Light causes hyperpolarization in the photoreceptors which inverts the signal to depolarization in the collector cells

What vitamin deficiency causes blindness?

Vitamin A

How do photopigments function?

The protein and derivative bind, catch light, blow apart, and then slowly come back together

What are the collector cells?

Bipolar, horizontal, and amacrine

What is rhodopsin?

Photopigment in rods

As collector cells become more positive, what happens to the rods and cones?

They get more negative

What is iodopsin?

Visual pigment in cones

What are retinal ganglion cells?

Retinal cells providing the output signal from the retina (their axons physically leave the eye)

How many RGCs are in each eye?

About 1.25 million per eye

What are the two main types of RGCs?

Parvocellular and magnocellular cells

Parvocellular cells make up what percentage of RGCs?

80%

Magnocellular cells make up what percentage of RGCs?

20%

Where are parvocellular cells located?

In the central retina

Where are magnocellular cells located?

In the periphery

Do parvocellular cells have thinner or thicker axons?

Thinner axons

Do magnocellular cells have thinner or thicker axons?

Thicker axons

What do parvocellular cells excite/inhibit?

Color

What are magnocellular cells very sensitive to?

Luminance

How are parvocellular cells with motion?

Ok

How are magnocellular cells with motion?

They are good with rapid

What is the ratio for overall neural convergence in RGC processing?

(rods/cones:RGC)

130:1

What is the ratio for neural convergence near the fovea in RGC processing?

(rods/cones:RGC)

9:1

What is the ratio for neural convergence in the periphery during RGC processing?

(rods/cones:RGC)

600:1

What is lateral inhibition in RGC processing?

When neurons fire, they inhibit neighboring cells

Why is lateral inhibition important?

It helps detect edges

Why is it easier for horseshoe crabs to isolate cells fired or not?

Their retinal ganglion cells are huge