Week 2 : Intro to the visual system

Question 1

Light…

Answer 1

• electromagnetic energy
• moved very fast (186,000 miles per second)
• amplitude/intensity (height) characterizes brightness
• wavelength (distance between adjacent peaks nm, inverse frequency) characterizes colour
• larger wavelength = shorter frequency (1/wavelength)
• frequency = number of waves per unit of time

Question 2

Wavelength & energy

Answer 2

• wavelength also related to energy
• shorter the wavelength is, the higher the energy is… wavelengths shorter than visible light such as gamma rays, X-rays and ultraviolet rays
• The longer the wavelength is, the lower the energy is… longer wavelengths than visible light such as infrared radiation, microwaves and radio waves

Question 3

Double slit experiment…

Answer 3

• beam of light projected against screen w/ 2 slits to determine what the pattern of illumination on the second screen would look like
• if the light was a stream of particles, we’d expect them to pass through the slits along a straight trajectory & create 2 bands of illumination on the 2nd screen
• if light acted more like a wave, the light passing thru the slits would project more diffusely onto the 2nd screen, creating a pattern of striped illumination (THIS WAS OBSERVED)

Question 4

The photoelectric effect…

Answer 4

• the idea that individual photons of light striking metallic surface will cause electrons to be ejected from the metallic surface & the electrons can be measured easily
• so, based on their hypothesis that light behaved as a waveform, they predicted that as they increased the amplitude of the light wave (brighter), more electrons would be ejected with higher energy than when the light amplitude was low
• they found this was wrong… however, increasing light frequency (smaller wavelength) created the emitted energy
• this would have been expected if light behaved as a particle (Einstein LOL)

Question 5

Wave particle duality theory…

Answer 5

• light is a wave in some ways and a particle in others
• Light is made-up of particles called photons that also behave in a wave-like manner

Question 6

visible light spectrum

Answer 6

• what we can see only makes up a small subsection of the electromagnetic spectrum
• the visible light spectrum ranges from red light (700nm) to violet light (400nm) and is bounded by infrared and ultraviolet
• the term visible is humancentric & other animals can see more

Question 7

Field of vision

Answer 7

• 2 eyes on front of head 6cm apart create the visual field without eye movements
• approx. 140 degrees elevation & 190 degrees horizontally
• part of this visual field is only seen by one eye… 110 degreed at centre are visible to both eyes (we need this for depth perception)
• bird of prey… eyes positioned more laterally on skulls & have much broader field of view… but area seen by both eyes way smaller & to accommodate birds can converge their eyes inward to increase depth perception

Question 8

extraocular muscles

Answer 8

• superior & inferior rectus muscles allows rotation of eyes upward & downward
• lateral & medial rectus muscles allow rotation of eyes left & right
• superior & inferior oblique muscles allow for small rotations of the eye

Question 9

Cornea

Answer 9

• the clear front surface of the eye that allows light in
• major focusing element
• begins the process of refracting/bending the light to come into focus on the retina
• rigid and can only focus clearly at one distance
• it is the transparent part of the sclera (tough membrane protecting eye/ the white of the eye)

Question 10

Anterior chamber

Answer 10

• between the Cornea and iris is the fluid filled anterior chamber

Question 11

Iris & pupil

Answer 11

• iris = coloured part (2 muscles w opening in middle)
• the only portion of the eye in which light can flow
• iris functions to regulate the size of the pupil to optimize visual function across a broad range of lighting conditions
• when the muscles of the iris contract, the pupil is constricted (undialiated) such that light entry is restricted
• when the radial muscles of the iris are contracted, the pupil is dilated which allows for more light to enter (low lighting conditions)
• this is not under conscious control known as pupillary reflex
• heterochromia = 2 different coloured irises

Question 12

Posterior chamber

Answer 12

• the space between the iris and the lens
• filled with fluid called aqueous humor

Question 13

The lens / accommodation

Answer 13

• the lens is responsible for focusing incoming light so that the image falls squarely on the retina
• located just behind iris
• lens changes shape to focus and this is called accommodation
• very rapid process… but changing from near object to far is faster than from far to near
• controlled by ciliary muscles that work in concert with the zonule fibres that connect the lens to the choroid membrane
• the increased curvature of the lens allows the eye to focus on a close object
• When you can no longer focus on an object, you have reached your near point… it gets farther away as you age (presbyopia)

Question 14

Hyperopia (lens disorder)

Answer 14

• farsightedness
• occurs when accommodation is insufficient to correct the focus behind the retina
• this results in an inability to focus on closely placed objects
• glasses (contacts)

Question 15

Myopia (lens disorder)

Answer 15

• nearsightedness
• the point of focus created by distant objects lies in front of the retina & no amount of relaxation can create the offset
• glasses (contacts)

Question 16

Presbyopia (lens disorder)

Answer 16

• age related farsightedness
• the lens begins to stiffen with age which have effects on accommodation that resemble hyperopia
• reading glasses

Question 17

Astigmatism (lens disorder)

Answer 17

• refractive disorder
• occurs when the eyeball is not perfectly round which results in an inability to focus light at a single point on the retina
• these multiple focal points result in blurred visual perception at any viewing distance

Question 18

Cataract (lens disorder)

Answer 18

• occur when the composition of the tissue that makes up the lens changes so it is no longer perfectly clear
• the result of the clouding is a random diffraction of light as it enters the eye so there is no clear image on the retina
• surgery to treat = lens is removed & replaced by a permanent contact lens

Question 19

Retina…

Answer 19

• where light is translated into a pattern of action potentials that can be interpreted by visual brain structures
• multilayered structure located at the posterior surface of the eyeball
• organization seems backward… light going through the vitreous chamber passes thru retinal ganglion cells, bipolar, horizontal & amacrine cells before reaching the photoreceptors at the very back
• the responses of these photoreceptors are then relayed back toward the retinal ganglion cells whose axons form the optic nerve

Question 20

overview of the retinal neurons…

Answer 20

• rods = night vision/light detection/greyscale vision
• cones= high visual actuary/colour vision/daytime vision
• horizontal cells = receive info from photoreceptors & other horizontal cells/cross talk across photoreceptors
• bipolar cells = receive information from photoreceptors/send signals to retinal ganglion cells
• amacrine cells = receive info from bipolar cells & other amacrine cells/cross talk function
• retinal ganglia = receive info from bipolar cells/send signals to brain via optic nerve

Question 21

Optic nerve & blindspot

Answer 21

• fovea represents a patch of the retina with exceptional resolution… the blind spot (optic disc) adjacent to the fovea is completely blind to incoming light
• this is where the axons of the retinal ganglion cells convert to form the optic nerve
• this region has no photoreceptors
• we do not perceive this blind spot cause images from both eyes fill that in

Question 22

Glaucoma (retina disorder)

Answer 22

• healthy eyes have small drainage channels that allow for the regulation of vitreous fluid volume & pressure within the eye
• glaucoma ppl have a disorder of these drainage channels that leads to buildup of pressure within the eyeball
• this squeezes the optic nerve & can ultimately lead to vision loss

Question 23

macular degeneration (retina disorder)

Answer 23

• the macula describes a region at the centre of the retina which contains the fovea
• macular degeneration occurs when this region of the eye is structurally degraded
• in majority of cases, this involves the formation of druse deposits (buildups of lipid & protein on the macula) which lead to degeneration
• in minority of cases, this involves development of abnormal vasculature

Question 24

retinitis pigmentosa (retina disorder)

Answer 24

• occurs when deposits of pigment form on the retina, preventing the sensation of light & leading to blindness

Question 25

Newborn vision

Answer 25

• lot of work has been done by Dr Daphne Maurer at Mac
• tested newborn eyes
• results = infants have a distinct preference for real faces & shows we are born ready to assign a special perceptual significance to facial stimuli
• infants have rather poor spatial acuity compared to adults
• underdeveloped colour vision as well

Question 26

Rods

Answer 26

• predominately at the periphery of the retina
• about 120m rods
• 1 type of rod
• these receptors only see in shades of grey
• no rods in the fovea, in periphery starting about 20 degrees off of the fovea

Question 27

Cones

Answer 27

• cluster in and near the fovea
• about 7 million
• 3 types of cones
• see in colour
• cones most centrally located in the fovea

Question 28

Parafoveal region

Answer 28

• where the rods and cones mix, in the outer areas of the fovea

Question 29

where exactly does light sensation occur?

Answer 29

• the discs in the outer segment of the photoreceptors
• specifically, there are molecules that straddle the disc membrane called opsin (rods) or chromodopsin (cones) which contain the light sensitive molecule retinol

Question 30

Retinal isomerization

Answer 30

• it is the retinol (rhodopsin) molecules (photopigment) that actually absorb photons of light
• this absorption causes and changes in the shape of retinol that leads to a signalling cascade within the photoreceptor cell
• once the retinol has changes shape, it can no longer capture light… so other biochemical processes reset the photopigment to its original shape so it can detect more light
• each photoreceptor may contain up to 100-200 photopigment discs

Question 31

Retinal circuit

Answer 31

• the photoreceptors initiate the flow of information within the retinal circuit which contains several different types of cells…
• photoreceptor transmits signals via bipolar cells to the retinal ganglion cells whose axons project toward forebrain nuclei
• within the circuit there is also horizontal cells that allow for communication among neighbouring photoreceptors
• amacrine cells allow for communication among neighbouring ganglion cells

Question 32

light hyperpolarizes photoreceptors

Answer 32

• within photoreceptor cells, the signalling cascade ends with the hyper polarization of the photoreceptor
• in its resting, unstimulated state, photoreceptor cells are depolarized & release a stream of neurotransmitters onto bipolar cells, many of which in turn actively inhibit the firing of retinal ganglia
• when light falls on the photoreceptors, the cell hyper polarizes, reducing its signal output to the bipolar cells & ultimately disinhibit the ganglia, leading to an increased signal to upstream structures

Question 33

fovea

Answer 33

• no rods, lots of cones
• this contributes to the exceptionally high resolution of the fovea
• the retinal ganglia & the bipolar cells are bent out of the way, so light falling on the fovea has a direct path to the light sensitive photoreceptor
• no receptors at all at the optic disc

Question 34

Convergence

Answer 34

• not all photoreceptors have a 1:1 ratio between photoreceptors & ganglion cells
• instead, convergence takes place where a number of photoreceptors report to a common ganglion cell that relays their summed activity
• there is little convergence at the centre of the retina & more on the periphery (further improving fovea high acuity)
• The process of convergence allows individual ganglion cells to encode some stimulus features like size from the process of spatial summation (but this reduces spatial acuity)

Question 35

Ganglion cell receptive fields

Answer 35

• this pattern of convergence and the horizontal & amacrine cells allow for local communication among neighbouring cells & creates complex receptive fields for ganglion cells
• this further increases their ability to resolve the location & size of visual stimuli

Question 36

On-centre & off-centre

Answer 36

• these receptive fields are constructed such that light falling at the centre has one effect on the cells firing rate, while light falling on the region immidietly surrounding that area has the opposite effect
• When light falls on both the centre and the surround of the receptive field, this leads to a simultaneous increase and decrease in firing rate, such that the effect is no change from the baseline rate of activity
• A stimulus that falls outside of the receptive field for a given ganglion cell, would have no effect on that cells response pattern

Question 37

tetra chromatic vision

Answer 37

people who have a fourth cone type

Question 38

Duplex vision

Answer 38

• the difference between rods & cone cells gives rise to 2 separable visual systems that work together providing what has been referred to as duplex vision
• rods = scotopic vision… low light/poorer acuity/peak density 20 degrees periphery + peak sensitivity 505nm
• cones = photopic vision… lit conditions/excellent visual acuity/ peak in density & resolution at fovea 550nm

Question 39

Seeing in the dark…

Answer 39

• cone cells adapt quickly to the dark (with much lower levels of sensitivity though)
• rods adapt to a very high level of sensitivity to the dark but it takes more time (30mins)
• the effect of having both rods and cones provides a 2-step curve that adapt quickly at first and to a high sensitivity level
• e.g.pirates wearing eye patches to keep one eye dark adapted at all times

Question 40

Retinal prothesis

Answer 40

In the case of retinal degradation and blindness, an electrode array can be place against the back of the eye that attempts to relay signals captured by a small camera placed at the bridge of the nose to the optic nerve for processing by visually sensitive brain regions