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Neuroscience and mental health year 2 > Structure and function of the eye part 2 > Flashcards

Structure and function of the eye part 2 Flashcards

1
Q

Essentially, what is glaucoma

A

A neurodegenerative disease of the optic nerve and retina
Specific defection of field of vision associated with structural abnormalities of the optic nerve
IOP not present- IOP is a modifiable risk factor for glaucoma- only target for therapeutic strategies
But isn’t cause of glaucoma
With macular degeneration- is the first blinding disease worldwide
Silent and asymptomatic until the very late stages- good screening and management can save vision and a lot of money for the government,

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2
Q

What is the leading cause of reversible blindness

A

Cataract- once you operate- then assuming the other structures of the eye are intact- vision is completely restored.

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3
Q

Compare the anterior segment of the eye to the posterior segment

A

Anterior Segment – Ocular structure anterior to the lens

Posterior Segment – Ocular structure posterior to the lens

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4
Q

Compare the anterior chamber to the postetior chamber

A

There are two anatomical chambers within the eye
Anterior chamber (just behind the cornea and in front of the lens)
Posterior chamber- region between the lens zonules and the back of the iris

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5
Q

What is important to remember about the posterior chamber

A

The posterior chamber consists of small space directly posterior to the iris but anterior to the lens. The posterior chamber is part of the anterior segment[1] and should not be confused with the vitreous chamber (in the posterior segment).

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6
Q

Describe the vitreous chamber

A

between the lens and the retina

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7
Q

Summarise the anterior chamber

A

– Between Cornea and Lens – Filled with Clear Aqueous Fluid – Supplies nutrients

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8
Q

Describe the production of the aqueous humour

A

Aqueous humour is produced by the ciliary epithelium of the ciliary body in the posterior chamber and flows into the anterior chamber through the pupil.

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9
Q

Summarise intra-ocular fluid

A

• CiliaryBody – Secretes aqueous fluid in the eye • Intraocular Aqueous Fluid flows anteriorly into the Anterior Chamber along the green arrow • Aqueous Fluid supplies nutrient • Trabecular Meshwork drains the fluid out of the eye • Normal Intraocular Pressure –12 21mmHg

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10
Q

Describe the drainage of the aqueous humour

A

It is drained via the trabecular meshwork into the canals of Schlemm- modified veins surrounding the trabecular network- 80-90%
The rest is drained by passive reabsorbtion (diffusion) in the uveal-scleral outflow

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11
Q

Define glaucoma

A

Optic neuropathy with characteristic structural damage to the optic nerve, associated with progressive retinal ganglion cell death, loss of nerve fibres and visual field los

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12
Q

Describe the characteristics of glaucoma

A

• Glaucoma – Medical Condition of Sustained Raised Intraocular Pressure (risk factor) – Retinal Ganglion Cell Death and Enlarged Optic Disc Cupping
–Visual Field Loss, Blindness

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13
Q

What is important to remember about IOP in glaucoma

A

It is only a risk factor- not the cause

Only 50% above the NICE threshold (24mmHg) have glaucoma- some develop it below the threshold

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14
Q

What is the most common type of glaucoma and what is it caused by?

A

Primary open angle glaucoma

It is caused by a functional blockage of the trabecular meshwork

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15
Q

State another relatively common type of glaucoma. What is it caused by?

A

• Increased pressure pushing the iris/lens complex forwards, blocking the trabecular meshwork –vicious cycle

Closed angle glaucoma
This can be acute or chronic
It is caused by the forward displacement of the iris-lens complex – narrowing the trabecular meshwork

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16
Q

Describe a risk factor for and explain the treatment of closed angle glaucoma

A

• Risk factors -small eye (hypermetropia), narrow angle at trabecular meshwork • May present with sudden painful red eye with acute drop in vision • Can be treated with peripheral laser iridotomyto create a drainage hole on the iris

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17
Q

What is the accommodation reflex mediated by

A

The accommodation reflex demonstrates that when the eye is focused on a distant object the pupil is dilated. However, if the patient is asked to suddenly focus on an object close to their face, the pupil constricts. This is mediated by cranial nerves II (optic) and III (oculomotor)

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18
Q

What are the consequences of glaucoma and how will they see objects

A

Progressive loss of peripheral vision
Blindness

Brain will fill in blind spots- according to what is around the object
This is fine when looking at a row of trees- but will be bad if a car enters the blind spot

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19
Q

Which type of cells in the eye are primary affected in glaucoma?

A

Retinal ganglion cells

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20
Q

What is the function of aqueous humour

A

Provides nutrients to the cornea and other tissues in the anterior chamber
Cornea has no blood supply- gets its oxygen from the air

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21
Q

How is a constant intra-ocular pressure maintained

A

The amount of fluid produced by the ciliary body is substantial; the entire volume of fluid in the anterior chamber is replaced about 12 times per day. Thus, the rate of aqueous humour production must be balanced by a comparable rate of drainage to ensure a constant intraocular pressure
Failure of this will lead to glaucoma- leading to reduced blood supply to the eye- which will damage the retinal neurones.

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22
Q

How can you distinguish between veins and arteries on opthalamoscopy

A

Veins- thicker and darker

Arteries- thinner and brighter

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23
Q

What can happen with retinal tears or detachment

A

Patient may see a black curtain and will report being unable to see

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24
Q

What is the optic nerve blind spot and how can we find it clinically

A

§ Where the optic nerve meets the retina there are no light sensitive cells. It is a blind spot.
§ Take a piece of paper and draw a dot and 10 cm to the left an x.
§ Close your right eye and hold the paper at arms length.
§ Look at the dot and move the paper towards you.
§ What happens to the X? § It disappears into the blind spot!
Optical disk has no photoreceptors

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25
Q

Why is the blind spot undetected with one eye closed

A

When the world is viewed monocularly, the visual system simply fills in the missing part of the scene. To observe filling in, simply notice what happens when the pencil lies across the optic disk representation, at about 5-8 degrees in diameter, remarkably, it looks complete.

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26
Q

Describe the fovea

A

Your fovea is the most sensitive part of the retina. § It has the highest concentration of cones, but a low concentration of rods. § This is why stars out of the corner of your eye are brighter than when you look at the directly. § But only your fovea has the concentration of cones to perceive in detail
But when you look at it directly- it will have greater definition, but a lower light sensitivity

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27
Q

Describe the fovea test

A

Look at the star (in the centre) and try to read the letters (in the periphery)
The letters will appear blurry as you are focussing on the star- due to the fovea

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28
Q

Describe the puncta and lateral and medial canthus

A

Lateral/Medial Canthus: corners of the eye; caruncle present in medial canthus
Puncta: medial openings that drain fluid into the lacrimal canaliculi to the tear sac

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29
Q

Describe the innervation of the cornea

A

The cornea is very sensitive and it is innervated by the ophthalmic branch of the trigeminal nerve (CN V)

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30
Q

Describe the central vision

A 
– Detail Day Vision, Colour Vision 
–Fovea has the highest concentration of cone photoreceptors 
– Reading, Facial Recognition 
– Assessed by Visual Acuity Assessment
 – Loss of FovealVision 
–Poor visual acuity
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31
Q

Describe the peripheral vision

A 
– Shape, Movement, Night Vision 
– Navigation Vision 
– Assessed by Visual Field Assessment 
– Extensive loss of Visual Field  
–unable to navigate in environment, patient may need white stick even with perfect visual acuity
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32
Q

Describe the different types of blindness

A

Patient may lose central vision and thus have impaired visual acuity- but their peripheral vision may be fine- allowing them to navigate themselves around the room easily.
Conversely, the patient’s central vision may be very good but their peripheral vision is impaired, meaning that they need a walking stick to navigate the room.

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33
Q

Where do the photoreceptors in the retina face

A

Away from the pupil

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34
Q

Why do the photoreceptors face away from the retina

A

Incoming light travels through the retina, then bounces back on the pigment epithelium and then hits the pigment epithelium.
Photons that hit the centre will be reflected back to photoreceptors in the centre, photons that hit the curves will reflect back onto the photoreceptors at the side- allowing us to see where the objects are in space (hit different portions of the retina).

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35
Q

Describe the key properties of the pigment epithelium

A

Dark- so absorbs a lot of light- so inside the eye not much refraction or reflection of light- also the reason why pupil is black- only when you shine a direct light into pupil and see the light coming back out- red reflex.

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36
Q

Why is the pupil back

A

Once the light hits the photoreceptor it is not reflected anymore and so cannot come back out of the eye

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37
Q

Summarise the structure of the retina

A

Outer Layer –Photoreceptors (1st Order Neuron) –Detection of Light
Middle Layer –Bipolar Cells (2ndOrder Neurons) –Local Signal Processing to improve contrast sensitivity, regulate sensitivity
Inner Layer –Retinal Ganglion Cells (3rd Order Neurons) –Transmission of Signal from the Eye to the Brain

Outer means away from the centre of the eye
Inner- means towards the centre of the eye

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38
Q

Describe the macula lutea and the foveal pit

A

– Macula –Macula Lutea (yellow patch), pigmented region at the centre of the retina of about 6 mm in diameter
– Fovea forms the pit at the centre of the macula due to absence of the overlying ganglion cell layer
– Fovea has the highest concentration of photoreceptors for fine vision

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39
Q

Describe ocular coherence tomography

A

Similar to ultrasound
Shine light through pupil
Receive the light that is reflected back- will be reflected differently by different structures- some light waves will be in phase, others out of phase, computer uses this to compute and image of the retina in layers (light, dark, light, dark)- clinically used to assess the macula and foveal pit- can look for build up of fluid.

40
Q

Describe the properties of rod photoreceptors

A

· Longer outer segment with photo-sensitive pigments
· 100 times more sensitive to light than cones
· Slow response to light
· Responsible for night vision (scotopic vision)
· 120 million rods

At the lowest level of illuminations, only the rods are activated (scotopic vision). The difficulty of making fine visual discriminations under very low light is a common experience. The problem is primarily the poor resolution of the rod system (and to a lesser extent, the fact that there is no perception of color because in dim light there is no significant involvement of the cones). Although cones begin to contribute to visual perception at about the level of the starlight, spatial discrimination at light level is still poor
Why we can see surroundigs relatively well at night compared to what we are actually looking at (cones in fovea not activated at night)

41
Q

Describe the properties of cone photoreceptors

A

· Less sensitive to light
· Faster response to light
· Responsible for daylight vision and vision and colour vision (photopic vision)
· 6 million cones

42
Q

Describe the range of wavelengths that the human eye can see

A

450-680nm

Blue/violet- dark red

43
Q

Describe the distribution of rods and cones across the retina.

A

Rods have the highest density just outside the macula
They decrease in density the further you move away from the macula
There are NO rods in the macula
Cones are ONLY found in the macula
The highest density of cones is in the fovea

see graph of number of receptors per square metre against the angle relative to the fovea

44
Q

Describe the roles of rods

A 
• Scotopic(Rod Vision) Vision
 -Peripheral and Night Vision 
(More Photoreceptors, More Pigment, Higher Spatial and Time Summation) 
– Peripheral and Night Vision 
– Recognizes motion
45
Q

Describe the role of cones

A

Photopic(Cone Vision) Vision
–Central and Day Vision
– Central and Day Vision
– Recognizes colour and details

46
Q

Where can one find the highest concentration of Rod Photoreceptors in the retina?

A

–D) 20-40 degrees away from Fovea

47
Q

Describe the importance of colour in vision

A

Perceiving colour allows humans and many other animals to discriminate objects on the basis of distribution of the wavelengths of light that they reflect to the eye.
While differences in luminance are often sufficient to distinguish objects, colour adds another perceptual dimension that is extremely useful when differences in light intensity are subtle or non-existent.
Colour vision is a the result of the special properties of the cone system.

48
Q

Describe the differing sensitivities of rods and cones to different colours

A

Unlike rods, which contain a single photopigment, the types of cones are defined by the photopigment they contain.
Each photopigment is differentially sensitive to light of different wavelengths, and for this reason cones are referred to as blue, green, and red or, more appropriately, (S)-SHORT, (M)-MEDIUM, (L)- LONG wavelength cones- to describe their spectral sensitivities.

49
Q

Explain how the rods and cones respond to colour

A

The nomenclature is misleading- as it seems to imply that the cones provide colour information for the wavelength of light that excites them best.
In fact, individual cones, like rods, are entirely colour-blind in that their response is simply a reflection of the number of photons they capture, regardless of the wavelength of the photon (or more properly, its vibrational energy),
It is impossible, therefore, to determine an extracts colour information from spectral stimuli.

50
Q

What are the three types of cone photopigment and which colours do they respond maximally to?

A

S-cone – short wavelength – BLUE
M-cone – medium wavelength – GREEN
L-cone – long wavelength – RED

Can use lasers to look into eye and even the brain (intrinsic imaging)
Use longer wavelengths the further you want to penetrate

51
Q

Describe the Isihara test

A

Colour Perception Test
• Ishihara Isochromatic Plates can test for red-green deficiencies only
• Consists of plates of circle of dots appearing randomly in size
• Subjects with normal red green vision will recognize the correct pattern in the form of a 2-digit number
• Patients with colou rvision deficiencies will not recognize any pattern or recognize the wrong pattern

52
Q

How can normal human colour vision be described

A

Trichromatic, based on the relative levels of activity in the three sets of cones that have different absorption sepectra. The trichromatic nature of colour vision is supported by perceptual stuidies showing that any colour stimulus can be matched to a second stimulus composed of three superimposed light sources (short, medium and long wavelength).

53
Q

Describe dichromatic colour vision

A

Only two bandwidths of light are needed to match all the colors that can be perceived.
Two most prevalent forms are:
protanopia- impairment in perception of long wavelengths
deuteranopia- impairment in perception of medium wavelengths
Both have difficulties in the perception of red and green, and for this reason dichromacy is often called red-green colour blindness.

54
Q

Describe the other form of dichromacy, tritanopia

A

Impaired perception of short wavelengths, a condition commonly called blue-yellow colour blindness.

55
Q

Describe anamolous trichromats

A

The majority of individuals with colour vision deficiencies
Three light sources are needed to make all possible colour matches, but the matches are made using intensity values different to those used by most individuals.
Protonamolous trichromats- higher intensity longer wavelength to stimulation to make colour matches
Deuteranomalous trichromats- higher intesnsity medium wavelength stimulation to make colour matches
Tritanomalous trichromats- shorter wavelength

56
Q

Explain the common colour deficiencies

A

Deuteranomaly: green-weak; sensitivity of green (M) cones shifted towards red (2.7%
Deuteranopia: green-blind; all green sensitive (M) cones are missing
Protanomaly: red-weak; red (L) cones present but have a mutation
Protanopia: red-blind; all red sensitive (L) cones are missing
Tritanomaly: blue-weak; blue (S) cones present but have a mutation
Tritanopia: blue-blind; all blue sensitive (S) cones are missing
Achromatoplasia: no colour vision whatsoever (<0.0001%)

Decrease in prevalence as you go down

57
Q

What is important to remember about individuals with achromatoplasia

A

Can still distinguish between orange (dark grey) and green (light grey)

58
Q

What is the term given to shifted peaks?

A

Anomalous trichomatism

59
Q

Summarise dark adaptation

A

Dark Adaptation
– Increase in light sensitivity in dark
– Biphasic Process • Cone adaptation 7 minutes
• Rod adaptation 30 minutes – regeneration of rhodopsin (photopigment in rods, GPCR system)- rods higher sensitivity to light- line flat after 30 mins- no need to dark adapt more

Eyes always see-even when shut- seeing black- no photons hitting the retina- photoreceptor still works and will always see 24/7.
Photoreceptors in outer part of retina- highest metabolic demand- why it is susceptible to damage- and insult

60
Q

Summarise light adaptation

A 
– Adaptation from dark to light 
– Occurs over 5 minutes 
– Bleaching of photo-pigments 
– Neuro-adaptation 
– Inhibition of Rod/Cone function
61
Q

How does retinal light change in light adaptation and what is responsible for this effect?

A

Light sensitivity decrease in dark adaptation
This suppression of light sensitivity is caused by photopigment bleaching and neuro-adaptation inhibiting rod and cone function

62
Q

Describe pupil adaptation to light

A

• Pupil Adaptation (minor) – Constriction of pupil with light

63
Q

Which is the commonest form of colour vision deficiency in humans?

A

red-green confusion

64
Q

Explain the basis of refraction

A

Refraction: light passing from one medium to another change velocity and hence angle (unless perpendicular)

65
Q

How do we calculate the refractive index

A

n= speed of light in a vacuum/ speed of light in the medium
n=c/Vm

The denominator in this case will ALWAYS be smaller and produce a unitlessvalue greater or equal to 1. This value is called the new medium’s INDEX OF REFRACTION, n.

Need to assume same humidity, temperature and pollution levels of air (vacuum)
All substances have an index of refraction and can be used to identify the material.

66
Q

What happens when you go fishing

A

The cause of this is due to the fact that light BENDS when it reaches a new medium. The object is NOT directly in a straight line path, but rather it’s image appears that way. The actual object is on either side of the image you are viewing.
Refraction Fact #2:As light goes from one medium to another, the path CHANGES!

67
Q

What exactly does light do when it reaches a new medium

A

Some of the light REFLECTS off the boundary and some of the light REFRACTS through the boundary.
Angle of incidence = Angle of Reflection
Angle of Incidence > or < the Angle of refraction depending on the direction of the light

68
Q

Describe a convex lens

A

A converging lens (Convex) takes light rays and bring them to a point
Parallel rays come into lens- rays focussed onto a focal point (distance between centre of lens and focal point is the focal length)

69
Q

Describe a concave lens

A

A diverging lens (concave) takes light rays and spreads them outward.

70
Q

Describe the applications of a convex lens

A

A camera uses a lens to focus an image on photographic film.

Lens in the eye does the same to focus an image on the retina

71
Q

Describe emmetropia

A

Adequate correlation between axial length and refractive power • Parallel light rays fall on the retina(no accommodation)

72
Q

Describe ametropia

A

Essentially a refractive error of the lens:
• Mismatch between axial length and refractive power • Parallel light rays don’t fall on the retina (no accommodation)
–Nearsightedness (Myopia)
–Farsightedness (Hyperopia)
–Astigmatism
–Presbyopia

73
Q

Describe the causes of myopia

A

• Parallel rays converge at a focal point anterior to the retina
• Etiology : not clear , genetic factor
• Causes – excessive long globe (axial myopia) : more common – excessive refractive power (refractive myopia)
Lens or cornea too convex

74
Q

Describe the symptoms of myopia

A

–Blurred distance vision
–Squint in an attempt to improve uncorrected visual acuity when gazing into the distance - squint to apply stenopeic (or pinhole glasses) hole- try to get light light coming straight- doesn’t get refracted and so will land on retina- no need for refractive power of lens- can see clearly
–Headache
Can see nearby object fine- rays are diverging upon entering the lens and not parallel- and so the focal point is shifted backwards

75
Q

How is myopia treated

A

Divergent glasses- concave lenses- to push focal point onto retina
Refractive surgery onto cornea- to flatten central bit of cornea (rather than dome shaped)
Cataract operation to remove lens without replacing it- -24 short-sighted ranking, lens 24 refractive power- so removing the lens will correct the short-sightedness- but it can damage structures in the eye

76
Q

Summarise hyperopia

A

Parallel rays converge at a focal point posterior to the retina
• Etiology : not clear , inherited
• Causes – excessive short globe (axial hyperopia) : more common – insufficient refractive power (refractive hyperopia)

Essentially, lens or cornea is flatter than normal
In certain degrees of hyperopia, accommodation can compensate and bring the focus onto the retina

77
Q

Describe the symptoms of hyperopia

A

Symptoms –visual acuity at near tends to blur relatively early
• nature of blur is vary from inability to read fine print to near vision is clear but suddenly and intermittently blur
• blurred vision is more noticeable if person is tired, printing is weak or light inadequate
–asthenopic symptoms: eye pain, headache in frontal region, burning sensation in the eyes, blepharoconjunctivitis

78
Q

Describe Ambylopia

A

–Amblyopia –uncorrected hyperopia > 5D (dioptres- ranking of long-sightedness)
One eye emmetropic, other eye hyperopic- brain learns (continues to develop to age of 4/5)- to ignore images from the ‘bad’ eye- so the other eye becomes amblyopic (lazy)

Hazardous- no physiological use for this eye- may not be able to see things in path

Objects close blurrier than further away objects- but blurry whether long or short distance.

Irreversible process

79
Q

Describe treatment for hyperopia

A

Same as hyopia

80
Q

Describe the shape of a cornea

A

Spherical- with arches (projections) that are equal in diameter
Elliptical shape that is horizontally orientated.

81
Q

Summarise astigmatism

A
  • Parallel rays come to focus in 2 focal lines rather than a single focal point
  • Etiology : heredity
  • Cause : refractive media is not spherical–>refract differently along one meridian than along meridian perpendicular to it->2 focal points ( punctiform object is represent as 2 sharply defined lines)- object can land in front of or behind the retina
82
Q

Describe the causes of astigmatism

A

Astigmatism: parallel rays focus in 2 focal lines rather than a focal point - due to non-spherical refractive media, so refract differently along one meridan compared to that perpendicular to it; treated with cylindrical lens (width and height not same)
Diameter of cornea not the same- arches unequal in diameter

83
Q

Describe the symptoms of astigmatism

A

• Symptoms –asthenopic symptoms ( headache , eyepain)
–blurred vision
–distortion of vision
–head tilting and turning
confusion
seeing images as comets during night
base of cornea not round- one meridian different length to other

84
Q

Describe the treatment for astigmatism

A

–Regular astigmatism :cylinder lenses with or without spherical lenses(convex or concave), Sx
–Irregular astigmatism : rigid CL , surgery

Meridians in vertical plane of cylinder- straight through it
Meridians on side of cyclinder (which will be concave or convex)- divereged or converged- rotate cylindrical lens until steep meridian is brought into focus

85
Q

Describe the near response triad

A

Adaptation for Near Vision
• Near Response Triad
– Pupillary Miosis(Sphincter Pupillae) to increase depth of field
– Convergence (medial recti from both eyes) to align both eyes towards a near object
– Accommodation (Circular Ciliary Muscle) to increase the refractive power of lens for near vision

86
Q

Describe presbyopia

A

• Naturally occurring loss of accommodation (focus for near objects)
• Onset from age 40 years
• Distant vision intact
• Corrected by reading glasses (convex lenses) to increase refractive power of the eye
Lens becomes stiffer

87
Q

Describe the treatment for presbyopia

A 
–convex lenses in near vision 
• Reading glasses 
• Bifocal glasses
 • Trifocal glasses 
• Progressive power glasses

Bifocals have a lens that is divided into two parts. The lower part is used to view things near to the person’s eyes whereas the upper part is used to view things that are distant. Trifocals contain a lens that has an additional region apart from the two that are already present in bifocals

88
Q

Describe spectacle lenses

A

–Monofocal lenses : spherical lenses , cylindrical lenses –Multifocal lenses

89
Q

Describe contact lenses

A

–higher quality of optical image and less influence on the size of retinal image than spectacle lenses –indication : cosmetic , athletic activities , occupational , irregular corneal astigmatism , high anisometropia , corneal disease

–disadvantages : careful daily cleaning and disinfection , expense –complication : infectious keratitis , giant papillary conjunctivitis , corneal vascularization , severe chronic conjunctivitis

90
Q

Describe intraocular lenses

A

–replacement of cataract crystalline lens –give best optical correction for aphakia , avoid significant magnification and distortion caused by spectacle lenses

91
Q

Describe surgery as a type of optical correction other than lenses

A

–Keratorefractive surgery :RK, AK, PRK, LASIK, ICR, thermokeratoplasty
–Intraocular surgery : clear lens extraction (with or without IOL), phakic IO

Laser to remove epithelium of cornea- open flap- change shape of cornea- close flap

92
Q

Describe clear lens extraction + IOL

A
  • IOL: Intra ocular lens.
  • Same as cataract extraction.
  • Implantation of artificial lens.
  • Lose accommodation (patient will need reading glasses).
93
Q

Outline how accommodation is achieved

A

• Accommodation Mechanism
– Contraction of the Circular Ciliary Muscle inside the Ciliary Body
– This relaxes the zonules that are normally stretched between the ciliary body attachment and the lens capsule attachment
– Note that zonules are passive elastic bands with no active contractile muscle
– In the absence of zonular tension, the lens returns to its natural convex shape due to its innate elasticity
– This increases the refractive power of the lens • Mediated by the efferent Third Cranial Nerve

94
Q

In accommodation, which one of the following events does not take place?

A

Relaxation of the ciliary circular muscle

95
Q

When are photoreceptors most sensitive to light

A

At lower levels of illumination
But cones have a more effective adaptation method- seen in time course in response of cones in response to light flashes- the response of a cone (even to a bright light flash)- produces the maximum change in photoreceptor current, recovers in about 200ms, more than four times faster than that of rod- response of individual cone does not saturate at high levels of steady stimulation , as the rod response does (but rods still adapt over a range of luminence values)
Rods produces a reliable response at 1 photon of light, whereas more than 100 photons of light are required to produce comparable responses in cones.

Neuroscience and mental health year 2 (24 decks)

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