2.4 Perception - Summary of Everything you Need for the Course Exam

The Visual System

1) Light

-each sensory organ is maximally sensitive to a certain physical stimulation -> sight (eye) to electromagnetic
radiation => particular form of radiation producing a visual response = light

-Newton: light = stream of particles, which are travelling in a straight line (early theory)

-each particle of light = photon -> intensity of light can be measured by counting the number of photons
=> NOTE: smallest intensity of light, which activates a receptor cell in the eye = 1 photon

-unlike Newton, Maxwell claimed that light acts as if it were a stream of WAVES
-> wavelength defines physical distance between peaks of photon waves (can vary between multiple kilometres to a
trillion of a centimetre) => HOWEVER: very short/long wavelengths are not visible

1) short wavelengths include gamma rays, X rays & ultraviolet rays
2) long wavelengths include electricity & broadcasting wavelengths associated with TV & radio

 visible spectrum ranges from 380-760 nanometres (billionth of a meter) -> normal eyes see…
1) 400 nm as violet; 2) 500nm as blue-green; 3) 600nm as yellow-orange; 4) 700nm as red

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2) Structure of Eye

-due to limited field of vision, humans can turn their heads & eyes
with great speed & accuracy => enabled by cooperative interaction
of extraocular muscles (6 per eye)

-eye is not made of rigid materials => maintains its shape due to
fluid pressure from within  protected by orbital fat & eyelids
within its location (the orbit = depression within the skull)
 blinking = prevents eye from drying out & keeps eye front clean

-hypothesis for why we don’t recognize that we can’t see while blinking:
-> when brain initiates blinking, it also prevents vision for a bit

-3 different layers of eye (from outermost to innermost): 1) fibrous tunic; 2) vascular tunic; 3) retina

OUTERMOST, FIBROUS TUNIC

-sclera = membrane, which is outer covering (seen as the white) of the eye

-cornea = region, where the sclera bulges forward (1 st optically active element in the eye) -> simple, fixed lens that
gathers & concentrates light => due to forward-extension -> allows reception of light slightly behind obs.
=> anything that decreases its transparency reduces quality of images

MIDDLE, VASCULAR TUNIC

-mostly (2/3) consists of pigmented, spongy structure (choroid) -> main task: nourishes photoreceptors in retina

-toward front of eye, the middle layer curls away from wall of the eyeball -> forms slender structure (ciliary body)
=> manufactures aqueous humor = watery fluid filling anterior chamber (behind cornea)
 provides lens & cornea with nourishment so that it doesn’t require own supply  enables complete transparency

NOTE: aqueous fluid also helps to maintain shape of eye -> too little/much fluid => too much intraocular pressure
 permanent increased intraocular pressure = glaucoma (most frequent cause of blinding eye disease)

-iris = colour membrane surrounding the central hole (actual colour varies from blue to black & is determined by
genetic makeup) -> controls amount of light entering

,-pupil = hole in the iris, through which the light enters -> its size is controlled by light reflex (Whytt’s reflex):
very small when light is bright (2mm) & very wide when light is dim (8mm)
-> adv of small pupil: 1) imperfections in the lens produce fewer distortions; 2) depth of focus (range of distances
over which objects are simultaneously in focus) is increased
=> NOTE: pupil size changes in response to emotional/attentional variables (greater pupil when highly interested)

-crystalline lens is located directly behind pupillary aperture + bc its curvature determines the amount of binding of
light, its shape is critical in bringing an image into focus at the rear of the eye (MAIN FUNCTION)
=> process of lens varying its focus = accommodation (changing focus by changing own shape):
 normal shape = spherical (near objects in focus), BUT when ciliary muscles relax, the fluidal pressure flattens the
lens (brings far objects into focus)

*outer layer of lens never stops growing (always producing new protein fibres)  they become more densely
packed in the centre of the lens with increasing age  leading to sclerosis

*anything that reduces transparency of lens = cataract -> some are not impactful & others can lead to blindness

*lens is yellowy pigmented & thus not completely transparent -> density of yellow pigment increases with age
=> function of yellow pigment: screening out some of ultraviolet rays entering the eye + screening out some blue
(which often leads to discussion of people of different ages arguing for an object being green/blue)

-large chamber of the eye is filled with jelly-like substance (vitreous), which is generally clear = vitreous humor

INNERMOST, RETINA

-retina = screen of neural elements at the back of the eye (done by photoreceptors)
-> image formed by optical system is focused => ADDITIONALLY: light is changed/transduced into neural response

1) in daylight-active animals: retina is backed up light-absorbing dark layer (pigment epithelium)
-> PURPOSE: reducing amount of reflected/scattered light that could blur the final image
2) in nightlight-active animals: light that penetrates retina is reflected back through retina by shiny surface
(reflecting tapetum) => REASON: detection of light is more important than image clarity  light passes through
retina twice, which doubles its intensity, BUT lowers image’s clarity

-3 major layers of neural tissue within the retina:

1) outermost layer (closest to scleral wall) -> contains photoreceptors => 2 types:
*long, thin, cylindrical cells = rods & *shorter, thicker, more tapered cells = cones
-> contain pigments that absorb light & start visual process (light reception)

=> 3 types of cells lay between photoreceptors & ganglion cells:

2) middle layer -> contains bipolar cells = neurons with 2 long-
extended processes
*1st making synapses with photoreceptors; *2 nd making
synapses with large ganglion cells (from 3rd layer)

=> function: transmission of modified signals from
photoreceptor cells to ganglion cells (the more glutamate is
produced in photoreceptors, the stronger the signal is sent
through bipolar cells)

 NOTE: photoreceptors release glutamate inversely:
> low glutamate-release when receiving much light & vice versa

,-additionally, there’re 2 types of cells in the retina, which have lateral connections:

1) horizontal cells -> have short dendrites & connect with multiple adjacent photoreceptors
=> function: modifying the strength of the signals generated by neighbouring photoreceptors & giving feedback
about where signals may be stronger/weaker

2) amacrine cells -> interact with spatially adjacent ganglion cells
=> function: it is speculated that they receive & modify inputs from bipolar cells + have “switching function” in
controlling whether ganglion cells receive bipolar signals from cones or rods

-upside-down organization of retina (inverted orientation of cones & rods) forces light to pass retina first before
reaching photoreceptors => seems counterproductive, BUT is required because photoreceptors need oxygen supply,
which is guaranteed through being close to all the blood vessels

-in short:
1) neural signals are generated by photoreceptors
2) these neural signals are passed through a network of bipolar, amacrine & horizontal cells (that collect &
recombine the signals)
3) transformed signals are passed on to retinal ganglion cells (info on distribution of light is extracted & recoded)
4) this info is transmitted to the brain

FOVEA (SUB-PART OF RETINA)

-most important part of retina for perception is located around optic axis -> imaginary line from centre of retina to
centre of pupil -> beginning of optic axis is yellowish = macula lutea = yellow spot

-small, circular depression at the centre of the macula = fovea centralis -> critical in visual perception
=> when looking directly at an object, your eyes are rotated so that the image falls into the foveal region

-fovea has a very unique structure: photoreceptors are densely packed & there are only cones, which are longer &
thinner than in other regions => outside of fovea, the number of cones rapidly decreases & rods increase in number

RODS & CONES (DETAILED – CAN ALSO BE FOUND IN RETINA)

-main function of both: specifying light intensity falling on a particular photoreceptor
=> together with horizontal & amacrine cells all independent info are integrated and passed on to ganglion cells

-rods & cones have different functions -> based on finding that night-active animals have only rods & day-active
animals have only cones & humans (active during night and day) have both => proposed duplex retina theory

 theory: there’re 2 separate visual systems
1) dependent on rods & responsible for vision under dim light conditions => scotopic vision
 night blindness results from having no rods in retina
2) dependent on cones & responsible for vision under bright light conditions => photopic vision
 day blindness results from having no cones in retina (looks completely different though: individual finds normal
levels of day-light painful, lacks colour vision & has poor visual acuity)

-additional findings:
*cones are highly concentrated at fovea (no rods found there) & are responsible for colour vision => 6 million
*rods more sensitive to low intensity-levels of light => 120 million

-rods & cones must interact with light first before they can signal its presence to the brain
=> they must absorb 1/more photons  substance that absorbs light = called pigment
 both rods & cones have visual pigments on the outside -> HOWEVER: pigmentation is different for both:

,1) rod pigments: rhodopsin = first isolated visual pigment -> consists of 2 parts:
*retinal = complex, organic molecule derived from vitamin A
*opsin = protein with large molecules that can potentially act as enzymes
=> when rhodopsin absorbs a photon of light, it changes shape & splits into its 2 parts
 several enzymes are activated, which opens sodium channels
 bc sodium ions flow into rod, rod cell undergoes hyperpolarization (negative charge becomes more negative)
 to maintain sufficient rhodopsin supply, the process must be reversed: retinal & opsin regenerate rhodopsin
with the help of vitamin A & some enzymes

2) cone pigments: iodopsin = first isolated visual pigment -> breaks down to retinal (same as in rods) & photopsin
(different form of opsin in rods) when exposed to light => hyperpolarization (similar as before) stimulates bipolar
cells, which then in turn stimulate ganglion cells + complex interaction between both cell kinds occur, resulting in
axons of ganglion cells carrying neural signals towards the brain

GANGLION CELLS

-transmission of visual info from eye to brain: axons of retinal ganglion cells gather together & exit from eye through
a hole in retina & scleral wall => this resulting bundle of axons = optic nerve
-> blood vessels flow through centre of optic nerve & sustain metabolic needs of the eye

-because optic nerve must pass through retina, there are no photoreceptors in this region
-> no visual response possible at this spot (blind spot) => absence of vison
 hole in visual field is automatically filled with information, fitting to its surrounding (visual completion)

-because neural axons form circular pattern, when leaving the eye -> this region is called optic disk

-nerve impulses transmitted to the brain via ganglion cell axons are already processed by the retina -> no raw data
=> integration of info from millions of adjacent photoreceptors by amacrine & horizontal cells

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3) The Eye as an Optical Instrument

-to see, our eyes must capture light reflected from objects around us + pattern of light reaching the retina (retinal
image) must mirror the distribution of actual light distribution
=> THUS: quality of vision is dependent on how well light gets through to retina

-reflected light from the surface of objects is what our eyes pick up -> originally stems from emitted light (coming
from light bulb/sun) => objects’ surface usually absorbs large parts of the light, BUT reflect also parts of it
 objects, which have high reflectance, appear light & objects, which have low reflectance, appear dark

-light’s way into our eyes: Cornea  Anterior chamber  Pupil and the lens  the vitreous chamber  Fovea
 Retina (Inverted) [passes everything first & then reaches Photoreceptors  horizontal cells  bipolar cells
 amacrine cells  ganglion cells]  optic nerve
 NOTE: image is portrayed 180 degrees turned (left-right & top-bottom are reversed)  later turned back by brain

-3 conditions of an object being seen must be fulfilled:
1) light must be sufficiently intense -> around 50% never reaches retina
2) distribution of light imaged on retina must be properly focused
3) spatial aspect must be preserved in the retinal image

,IMAGE FORMATION IN THE HUMAN EYE

-sharpness of images depends on 2 factors:
1) optical power of cornea & crystalline lens -> determined by accommodation ability
2) eyeball length from front to back

-Thomas Young: light behaves as if it consisted of waves -> light is divergent => HOWEVER: divergent light cannot be
focused  something must reverse the divergence: convex lens
 after hitting convex lens, 2 waves progressively come closer  eventually they converge at focal point
 for perfect vision: retina should be hit at focal point => emmetropic = eye having the perfect distance

SIGHT PROBLEMS

1) if eye is too short/if cornea bents light not sharply enough -> far-sighted = hypermetropia/hyperopia
=> focal point not yet reached at retina  NOTE: accommodation process can solve mild distance-errors, BUT it
involves muscular effort & makes tired/leads to headaches quickly
 alternative: placing a convex lens (glasses) in front of cornea, so that divergence is counterworked already

2) being near-sighted = myopia -> focal point reached somewhere before retina
=> NOTE: accommodation can’t help here  2 different types:
1) refractive myopia = abnormal shape of cornea -> light is bent too sharply
2) axial myopia = eyeball is too long -> too much time & space for the light to converge again
 solution: concave lens, which leads to light diverging more

3) ability to refocus one’s lens relates to age -> very bad when we’re born & decreases after reaching age of 16y (due
to inner layers of lens dying resulting from refractory error (light-bending/focusing error) = presbyopia (old-sighted)
=> leads to increasing the near point distance (how close an object has to be brought to the eye until it can be no
longer held in focus)  MEANING: older people need to hold books abnormally far to be able to focus on the letters

4) astigmatism = cornea is more sharply curved along 1 axis than it is along the other 1
-> result: cornea can’t sharply focus 2 different line orientations simultaneously on retina
=> can be corrected by lenses, which provide equal, opposite distortion

 LASIK-surgery (re-sculpting cornea) is now an option for correcting everything but presbyopia

5) strabismus = eyes are not aligned with each other -> 1 is on-target, the other one is off-target
=> can lead to amblyopia = 1 eye is supressed & does not develop vision

6) Retinitis Pigmentosa (hereditary condition) = photoreceptors do not work like they are supposed to and a bundle
of tissue develops on your retina -> leads to blindness

7) age related macular degeneration = occurs most often in elderly people & constitutes of damage to the macula
-> results in a loss of vision in the centre of our visual field (NO total blindness) => genetic aspect involved

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4) Neural Responses to Light (only read 4 & 5, but rely on 6)

RECEPTIVE FIELD OF A VISUAL NEURON

-info of a single ganglion cell represents the combined activity of a large number of rods & cones
-> THUS, a single ganglion cell responds to a light incident on a larger region of the retina (that cell’s receptive field)
=> MEANING: each ganglion cell processes info coming from a substantial zone of receptor cells in retina

, -sizes of receptive fields vary dependent on retinal location
-> very small close to the macula & becoming larger when moving further towards periphery of retina
=> with increasing eccentricity (deviation from the centre (fovea)) receptive fields increase in size
 HOWEVER, the receptive field size can also vary for the same eccentricity (local variation)

-Hartline & Kuffler: when a single spot of light is displayed, a particular ganglion cell may react in 3 different ways:
1) on response -> burst of neural impulses right after onset of the stimulus
2) off response -> burst of neural impulses right after offset of the stimulus
3) on-off response -> burst of neural impulses right after onset & offset of the stimulus
=> particular ganglion cell response tends to vary depending on location of stimulus

*organization of receptive field: some are on-centre receptive fields -> HOWEVER, off-centre receptive fields exist
too (approximately 50/50) => organization shown in graphic below

- the antagonistic regions of a receptive field (centre & surrounding)
compete with each other = lateral inhibition
=> enables to summarize the messages from many photoreceptors in
just 1 statement: “I have detected a light/dark boundary”

PARVO & MAGNO GANGLION CELLS

-classification system regarding ganglion cells’ size & shapes has been
established: most important difference = size -> parvo vs magno cells

*differences between parvo & magno cells:
1) parvo: as long as the same amount of illumination is present in centre & around, parvo cells do not distinguish
between different locations of illumination => neural response stays the same
 responsible for stationary pattern analysis
2) magno: if location of same amount of illumination differs somehow (caused by object-movement), the neural
response varies  responsible for motion detection

**other differences:




-sometimes, researchers add K cells to the categorization (others: P cells (80%) & M cells (10%))
-> relatively little research is done on them, BUT they more closely resemble P cells
=> HOWEVER, K cells have much better spatial resolution & react more sensitive to contrast compared to P cells

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5) Perceptual Consequences of Centre/Surround Antagonism