Lecture 23-32 PSYCH 2E03

Light

electromagnetic radiation that is visible to our eyes

concentrated in small packets of energy, photons, which travel through space in a wave-like manner

Dual theory of light

light is both a wave and a stream of particles

it propagates like a wave and is absorbed like a particle of energy

Properties of light

wavelength, frequency, amplitude

What does light wavelength correlate with?

perception of colour/hue

What colour is longer wavelengths?

red

What colour is shorter wavelength

blue

eg. ultraviolet is shorter than light, and its close to blue

Sclera

white part around outside of eye; protective thick tissue

What are the two main refractive components of the eye?

cornea and lens

cornea performs most of the refraction and lens is adjustable, making up the rest

Cornea

continuation of sclera; transparent, contains sensory endings; REFRACTIVE

Lens

elastic, transparent, REFRACTIVE

Optic nerve

bundle of axons that transmits visual information to the brain

Zonules of zin

pull lens to make it thin and flat

Emmetropic eye

an eye with perfect vision, also known as 20/20 vision, where light entering the eye focuses perfectly on the retina, producing a crisp, clear image

Accomadation

a change in the curvature of the lens in response to changing object distances

Limit of accommodation

~10 cm in young people; lens can only fatten so much

How does lens accommodate for near objects?

contracting ciliary muscle (ring) and relaxing the zonules of Zinn, making lens fatter

Focal distnace

distance from the refractive surface and point where paralel rays converge

cor

Diopter

unit of measurement of the optical power of a lens

Lens power equation (P)

P = 1/f

Refractive power of the cornea

1/0.024 = ~42 diopters

Would a fat lens have more or less refractive power?

A fatter lens bends light more so it has a higher refractive power

Nodal point

imaginary point near back surface of the lens though which all light rays pass

How does the retinal image compare to the visual scene?

flipped horizontally and vertically

a little blurred around the edges

Hyperopia

farsightedness

focal point is behind the retina, as the lens lack sufficient refractive power

etiehr cornea is not curved enough or eyeball is too shrot

treat with convex lens - add refractive lenses

Myopia

nearsightedness

focal point is in front of retina; lens cannot flatten enough

cornea is too curved, or eyeball is too long

treat with concave lens; reduce refraction

Presbyopia

age-related hardening of the lens and reducation in elasticity of the capsule that encircles the lens

Astigmatism

lack of symmetry in the curvature of the cornea, causes blurred retinal image along the affected direction only

treat with lenses that correct refractive deficit along a particular orientation

Retinal pigment epithelium

absorb any light that isn’t absorbed by the photoreceptors - make sure light ins’t reflected/scattered inside eye

Retinal organization (outside to in)

sclera - pigment epithemium - photoreceptors - bipolar cells - retinal canglion cells

Why are photoreceptors the outermost layer of retina?

to be near retinal pigment epithelium, because is helps regenerate photopigment

What are the only neurons whose axons leave the eye?

retinal ganglion cells

Where do neurons leave the eye?

Optic nerve

properties of photoreceptors

outer segment - store photopigment

inner segment - make photopigment

cell body - nucleus

synaptic terminal

Distance between cell body and synaptic terminal of photoreceptors?

very short - no action potential

Rods

• Cylindrical outer segment

• 90 million

• Periphery of retina

• One photopigment (colour-blind)

• Specialized for night vision

Cones

• Conical outer segment

• 4-5 million

• Mostly in fovea

• Specialized for day vision

• Fine visual acuity = more sensitive

• Three photopigments (colour vision)

Distibution of rods and cones in the retina

Cone density is higher in the fovea

Rod density is highest in the periphery

RGC axons leave the eye at the optic disk

Consequence of the fact that there are no rods in the fovea?

under dim illumination, we are effectively blind in the centre 1 deg of our visual field

Rule of thumb (vision)

thumb at arm length is ~2 deg of visual field

Scotpic

Rods are more efficient than cones at converting photon absorption to neural signals  thus rods and not cones are active at low light levels

Mesopic

Cones and rods activated

Rods are only active at low light levels; above this level, photopigment cannot be activated any more  bleaching

~approx. moonlight

Photopic

• Only cones

• Cones have mechanisms to prevent bleaching at high light intensities

How does the visual system adjust to changes in illumination?

1. rods and cones have different ranges

2. photopigment must be regenerated

3. pupil size is adjustable irt light

4. ganglion cells respond best to contrast, not diffuse light

Photopigment bleaching/regeneration

When a photopigment is bleaches (used up), the molecule must be regenerated again; thus, not all photons are captured

Dark adaptation curve

initially mediated by cones’ fast recover but rods take over (recover further; lower threshold)

Photopigment

made in inner segment and stored in outer segment

consists of a protein plus a chromophore

Opsin

photopigment protein, strucutre determines which wavelengths of light the pigment molecule absorbs

Retinal (chromoshore)

absorbs light, changes conformation when bleached

What wavelength of light does rhodopsin absorb?

preferentially absorbs light at about 500nm (green-blue)

Spectrophotometry

measures how much of the incoming light is absorbed by a protein

flash light, record how much is not absorbed

What wavelength is most sensitive in dim conditions?

peak sensitivty is at 500nm

Absorption spectrum of cone opsins

short = blue = 440nm

medium = green = 530nm

long = red= 560nm

Spectral sensitivity of photopic vision

Brain combines input from 3 types of cones to create colour vision, peak is about 550nm

Distribution of cone photopigments

5-10% are blue (S-cones)

30% green (M-cones)

60% Red (L-cones)

Red:green cones

2:1 ratio

Photochromatic interval

difference between just seeing light and being able to tell its colour

Photopic vs scotpic sensitivity

Photopic sensitivity is only higher at very long wavelengths

Othewise, scotpic is typically higher

Purkinje shift

difference in perceived brightness of objects due to spectral shift

Objects of a given colour will appear to shift brightness when you switch between scotopic and photopic systems

What happens when you gradually increase the intensity of a subthreshold light at 450nm?

• Scotopic system detects it first – rods have higher sensitivity

• At some brightness, you eventually activate the cone system = now we can tell what colour the light is

Dark current

Rhodopsin is inactive, maintaining sensitivity of rods

1. In the dark, a molecule called cyclic GMP (cGMP) binds to ion channels permeable to Na+ and Ca2+

   1. Keeps them open

   2. Dark current  flow of cations (positive charge) into the outer segment in the dark

2. K+ leaves the cell though K+ leak channels in the inner segment

3. The Na+/K+ pump maintains the concentrations of Na+ and K+ inside and outside the cell

Results of the dark current…

membrane potential of a photoreceptor is ~-40mV, whereas most have -70mV resting potential

so neurotransmitter glutamate is constantly being released IN DARK

Phototransduciton in the presensce of light

1. Absorption of light by retinal

2. Rhodopsin changes conformation à activated

3. Activated rhodopsin activates a G-protein called transducing

4. G-protein activates an enzyme called PDE

5. PDE breaks down cGMP à GMP

6. cGMP-gated channels close

What happens to the membrane potential when cGMP-gated ion channels close (phototransduction)?

K+ still leaving but its not being balanced by Na+ coming in, so membrane potential gets more negative (DEPOLARIZED)

How does phototransduction compare to the receptor potential generated in other sensory systems?

In other systems, a signal produces hyperpolarization, in phototransudction, a signal produces depolarization

How does phototransduction affect neurotransmitter release?

less glutamate is released

Photopigment

protein (opsin) + chromophore (retinal)

Bipolar cells (vision)

synapse with either rods of cones and pass signals onto retinal ganglion cells (RGCs)

Retinal ganglion cells

• only neurons whose axons leave the eye

• the only retinal neurons that generate action potentials

• part of parallel visual streams

# of photoreceptors vs RGCs

100 million photoreceptors vs 1.25 million ganglion cells

Signals from multiple photoreceptors must converge onto a single RGC

How does photoreceptor-RGC convergence relate to the size of RGC receptive fields

If a neuron response to visual input in larger part of the retina = larger part of the visual field

Midget bipolar cells

few photoreceptors converge onto a single midget cell

in fovea, ratio is 1:1

Diffuse bipolar cells

converge info from many photoreceptors

Visual acuity

measure of the finest detial taht cna be resolved by the eyes

Compare the sensitivity of foveal RGCs to the sensitivity of peripheral RGC in dim diffuse light

Periphery responds better to stimulus 1, because it receives input from many different photoreceptors, so it all adds up to activate the one ganglion cell

Whereas near fovea, dim light only weakly activates the 3 cones so none of them get activated

Compare the ability of the RGCs on the fovea and periphery to discriminate between 3 separate light spots

Near fovea, we can tell when 1 and 3 are activated but not 3, vs periphery the receptive field is large so we can’t discriminate between points

Why/how does foveal vision allow us to see fine details?

1. retinal neurons (except cones) are shifted to one side to allow light unimpeded access to cones

2. cones are tightly packed

3. cones and bipolar cells are connected to each-other in a 1:1 ratio

2 types of bipolar cells

OFF - hyperpolarized by light just like photoreceptors

ON- depolarized by light

Which RGC receives input from midget bipolar cells?

P-ganglion cells

Which RGC receives input from diffuse bipolar cells?

M-ganglion cells

Which RGC projects to parvocellular LGN?

P-ganglion cells

Which RGC projects to magnocellular LGN?

M-ganglion cells

Which RGC has small size, dendritic field?

P-ganglion

Which RGC has large size, dendritic field?

M-ganglion

Which RGC is most numerous? 70% of all RGCs

P-ganglion

Which RGC is less numerous? 10% of all RGCs

M-ganglion

Which RGC is more in fovea?

P-ganglion

Which RGC is found more in periphery?

M-ganglion = more convergence

4 types of RGC

ON - P

OFF - P

ON - M

OFF - M

How do RGCs behave?

• All rgcs have a certain level of spontantneous activity (action potential firing)

• Firing rate can either increase or decrease in response to light

• Retina doesn’t just detect light, rather it detects differences in light in adjacent parts of the retina (spots of light on screen)

RGC receptive field

2 concentric zones: excitatory + inhibitory = center surround antagonism

Why does diffuse light have no effect on firing rate of RGCs?

Because the same amount of inhibition and excitation, so they’re cancelling eachotehr out completely and not producing any change in the firing rate

How to maximize firing rate of RGC?

light up as much excitation region and as little of inhibitory region as possible

How do RGCs filter stimuli by size?

respond best to stimuli that tare the right size to only stimulate excitatory region without hitting inhibitory region

How are RGCs optimized for detecting contrast?

RGCs are most sensitive to differences in the intensity of light in the centre and its surround

Perceptual consequences of centre-surround effects

1. lightness contrast

2. lightness constantcy

3. mach bands

Centre-surround effect - lightness contrast

appearance of one patch of light is affected by other light patches nearby

Centre-surround effects - lightness constancy

An overall change in the ambient illumination affects both objects and the surround in an equal manner resulting in relative constancy of lightness perception

i.e. you realize that the colour of something diddn’t change just because the lighting changed

Mach bands

False impression of a narrow bark band and a narrow light band immediately to boundary when intensity changes abruptly

causes by responses of RGCs that overlap boundary

e.g. cell fires above baseline because entire excitatory region (center) is lit but only part of inhibitory region (surround) is lit - overlaps with darker band

Effect of bipolar to RGC convergence

1. how we see spatial detail (resolution / acuity)

2. how little light we need to see (sensitivity)

Usually a trade-off between resolution and sensitivity