1
Q
Function of the Eye
A
Collect, focus, sense light stimulus
- Beginning of visual processing
2
Q
What is Light?
A
Form of EM radiation that travels as a wave.
Physical Light Properties: Wavelength, frequency, amplitude, purity
3
Q
Evolution of Colour Vision in Primates
A
- Cones are well suited at distinguishing red/yellow against green background
- Added contrast between objects and background
- Beneficial when foraging for food
4
Q
Physical Light Property: Amplitude
A
Height of each peak.
- Indication of perceived brightness
- Greater amplitude = More light emitted
Psychological Perception: Brightness
5
Q
Physical Light Property: Wavelength & Frequency
and the EM spectrum!
A
- Varying wavelength affects colour
- 360 nm (violet) –> 750 nm (red)
* “Visible spectrum” is a small part of the EM spectrum - Different species can see different wavelengths of light
- Frequency is INVERSELY RELATED to wavelength
Psychological Perception: Colour
6
Q
Physical Light Property: Purity
A
- Pure light has a single wavelength (SATURATED)
- Natural light has more than one wavelength (DESATURATED)
Psychological Perception: Saturation
7
Q
Cornea
A
Curved transparent window at the front of the eye.
- First site where light passes through
- Beginning of focusing process
8
Q
Pupil
A
- Black dot in the middle of your eye
- Size controlled by the iris
9
Q
Iris
A
- Coloured part of eye
- Controls size of pupil
- Band of muscles controlled by brain
- Low light level –> pupil dilation
- High light level –> pupil constriction
10
Q
Lens
A
- Flexible transparent tissue
- Final focusing of light onto the retina
- Curvature causes image to land on retina upside down, reversed, smaller
11
Q
Retina
A
- Neural tissue that lines the back of the eye
- Recieves distorted image from the lens
- Begins the translation fo light into neural impulses
12
Q
Accommodation
A
Lens accomodates for the distance of objects in focus.
Close focus: Rounder lens
- Clearer image
Far focus Elongated lens
- Focuses image to back of eye
13
Q
Vitreous Humour
A
- Clear jelly like substance that comprises the main chamber inside the eye
14
Q
Order of the Passage of Light in the Eye
A
Cornea –> Pupil –> Lens (distortion) –> Vitreous Humour –> Retina
15
Q
Why is the Retina Arranged “Inside-to-Out”?
A
Photoreceptor layer gets its nutrients from the retinal pigment epithelium (RPE) which is situated at the back of the eye.
- Lack of RPE causes photoreceptors to die
- Hence, light must travel to photoreceptors –> bipolar –> ganglion
16
Q
3 Layers of the Retina
A
Note: Inside out process: Light inside 3 layers first, hits photoreceptors, then travels out
- Light sensitive photoreceptors
- Bipolar cell layer
- Ganglion cell layer
- Translates physical stimulus of light into neural signal relayed to brain
17
Q
Comparing the 2 Types of Photoreceptors
A
Rods: Designed to operate at low light intensities
- 125 million
- No colour
- Poor visual acuity
- Concentrated in periphery (peripheral vision!)
- Responsible for night vision due to rhodopsin (photopigment that is light sensitive)
Cones: Designed to operate at high light intensities
- 6 million
- Responsible for day vision due to iodospin (photopigment that is less light sensitive)
- Good visual acuity
- Concentrated in the fovea (Tiny spot in the middle of the retina)
- COLOUR: 3 main types (blue, red, green) sensitive to respective wavelength to help perceive colour
18
Q
Ganglion Cells
A
Recieves input from rods and cones; relays signals to visual centres in the brain.
- Axons of ganglion cells converge on one point, the optic disc, to leave the eye and join the optic nerve
19
Q
Optic Disc
A
Site where ganglion axons converge
* An exit hole in the eye for ganglion axons
* No photo receptors… a blindspot!
20
Q
Main Function of Photoreceptor
A
Converts light to neural signal
21
Q
Horizontal and Amacrine Cells
A
Cells in the retina that allow areas within a retinal layer to communicate with eachother.
- Carries out a part of convergence: Signals from multiple photoreceptors to flow into 1 bipolar cell, multiple bipolar into one ganglion cell
- Information put into groups, groups converted into neural signal
22
Q
Fovea
A
- Centre of retina
- Highest visual acuity
- Smaller and more precise receptive field
- Horizontal and amacrine cells converge groups of one photoreceptor –> one neural signal, hence, more detail is preserved!
23
Q
Receptive Field of a Ganglion Cell
A
Collection of rods and cones in the retina that, when stimulated, affects the firing of a particular ganglion cell.
Center–surround antagonism: Stimulating the surround elicits the opposite response as stimulating the centre.
- Processing of information before signal reaches brain!
- Encode differences in light across space / contrast
23
Q
Order of Signal from Retina to Brain
A
Photoreceptors –> Bipolar cells –> Ganglion cells –> Optic Nerve
24
Visual Field Input in Cortex
Nasal region of __
* Left eye --> Left visual field --> Right hemisphere/visual cortex
* Right eye --> Right visual field --> Left hemisphere/visual cortex
Temporal region of __
* Right eye --> Left visual field --> Right hemisphere/visual cortex
* Left eye --> Right visual field --> Left hemisphere/visual cortex
*Each hemisphere receives information from both eyes.*
25
Optic Chiasm
Point at which optic nerves from nasal region of each eye cross over to the opposite hemisphere.
26
Post Optic Chiasm
* Optic nerve fibres split and travel along two pathways
LEFT and RIGHT optic tract
* Optic tracts made of AXONS of ganglion cells!
* Contains information from **contralateral** visual field (left visual tract --> right visual field)
* Ganglion cell axons synapse with the **lateral geniculate nucleus**
27
Lateral Geniculate Nucleus
| LGN
Thalamic relay center that receives input from retinal ganglion cells and sends visual information to the primary visual cortex (V1).
* Ganglion synapse with LGN after crossing
* Part of thalamus (relay station, gateway to cortex)
* Visual signal sent to areas in occipital lobe (visual cortex)
* Contains six distinct layers of cells
* P-cells and M-cells (colour/form, motion - respectively) organize and process information
* Integrates information inputs from higher-order brain structures (brain stem, thalamus, etc.)
* Slightly larger receptive field than retina
28
V1, Primary Visual Cortex
Located on occipital lobe.
AKA. **Striate cortex**
* Includes line of Gennari (long stripe in cortex)
* Where information goes after LGN (where many LGN cell receptive fields converge to single V1 cell receptive field)
* Maintains topographic representation of visual space (neighbouring photoreceptors project to neighbouring visual cortex locations)
29
Extrastriate Cortex
Visual cortex outside the striate cortex/V1
* V2 - V5
* Subserves functions related to colour, motion, object recognition
30
Dorsal and Ventral streams
Processed information from V1 gets sent to extrastriate cortex and separated to either dorsal or ventral streams.
**Dorsal Stream:** The "where" pathway
* Processes depth, motion
* Extrastriate cortex --> Parietal lobe
**Ventral Stream:** The "what" pathway
* Processes colour, form
* Extrastriate cortex --> Temporal lobe
31
**Convergence** of Input Down Visual System
*Some number of neurons receiving input and relaying signal onto fewer neurons.*
Many photoreceptors
-->
Receptive field made of many retinal ganglion cells
-->
Combine to form receptive field of a single LGN cell
-->
Multiple LGN cells combine to form receptive field of single V1 cell
32
Topographic Arrangement/Mapping
Neighbouring regions in retina correspond to neighbouring regions in primary visual cortex
33
Five Stages of Eye Evolution
Lazy Cats Prefer Crunchy Apples
1. Light sensitive patches
2. Curved eye shape
3. Pinhole
4. Crude lens
5. Adjustable lens
34
Light Sensitive Patches
1st step.
* Able to detect the presence or absence of light
35
Curved Eye
2nd step.
* Light sensitive patch formed slight depression
* Allows to detect the direction of light
36
Pinhole Aperture
3rd step.
* Allows organisms to resolve detail of an image
* Changes amount of light that enters the eye
37
Crude Lens
4th step.
* Development of a lens, cornea, spherical shape
* Relatively solid and shape changes little / NOT ADJUSTABLE!
* Best suited for animals who do not rely heavily on visual acuity
* Increase focusing!
38
Adjustable lens
5th step.
* Development of an adjustable lens for acommodation
* Process things at varying distances.
* Found in vertebrates
39
Cumulative Selection
An evolutionary process whereby new adaptations are layered upon old adaptations.
* Small changes were made to the existing eye on each successive step
* Gradually increasing sophistication!
40
High Metabolic Cost of Adaptation
Advantages of modification must outweigh metabolic cost to system of growing and maintaining modification.
* Survival and reproductive advantages to eye evolution outweighed cost!
* Think: Predator-prey relationships
41
Factors affecting EYE VARIANCE across species
* Degree of light in habitat
* Direction of food source (above? below?)
* Movement, shape, colour of prey
42
Simple Eyes
Found in vertebrates, humans, mollusks
* Standard eye w/ eyeball, lens (crude & accomodating), retina
43
Environmental Factors Determine WHAT Characteristics of the eye?
* Shape of pupil
* Size of eye
* Where eye is placed on head
* Where on the retina photoreceptors are located
44
Compound eyes
* Anthropods (insects, crabs, etc.)
* **Ommatidium:** Individual tubular units that gather light from slightly different directions
* Each ommatidium produces a different signal that are put together to create a single image in the eyes
* Good at detecting movement at close distances
45
Two functions of the eye
* Resolution (acuity): Ability to detect stimuli at a distance
* Sensitivity (ability to get enough light)
46
What size of eyes have better acuity?
BIG BIG BIG!
* Bigger eyes observed in species that require better eyesight / more acuity
* More cones = more acuity
* More rods = more nightvision
47
Modification in Predator Eyes
* Larger pupil size: Allow more light to enter retina
* Greater number of rods in retina: Well suited for low light
* Front facing eyes for depth perception
48
Trade-Off in Big Eyes
* Either has more rods for night vision, or more cones for acuity
* Evolution does not select for both
* Examples: Humans and hawks vs. cats and owls
49
Trade-off in Eye Placement/Direction
* Depth perception vs. total view of environment animal can see
* Laterally-Directed Eyes = Wide total view
* Front-Facing Eyes = Depth perception
50
Laterally Directed Eyes
Eyes placed at the sides of the face.
* Wide total view
* Essentially two separate fields of view
* Less binocular overlap = Poor depth perception
**Adapted by prey!**
* Allows them to continually scan environment for predators
51
Front-Facing Eyes
Eyes directed towards the front.
* Narrow total view
* Essentially a single field of view
* More binocular overlap = excellent depth perception
**Adapted by predators!**
* Depth perception is key to successful hunting.
52
Prenatal Development of Eyes
2nd prenatal month: Eyes are formed
6th prenatal month:
* Eyes are capable of reacting to light
* Random firing of prenatal cells
53
Least developed sense at birth...?
| and why?
Visionnn!!!! (who would've guessed)
* Relies heavily on visual input
* This can only occur after the baby is born... of course
54
Postpartum Development of Eyes
Newborns:
* Weak lens muscles
* Inconsistent pupil reactions (blurred image)
3 months:
* Almost adult-like focusing
55
Development of Retinal Ganglion Cells in Early Childhood
Newborn:
* Low cell density in retina
* Cells are immature, especially fovea
11 years:
* Visual brain area development complete
56
Secondary Visual Pathway
Retina → Superior colliculus (SC) → Pulvinar nucleus of thalamus → Extrastriate visual cortex
57
Emotional Blindsight
* Emotional reaction to visual stimuli even though you don't consciously see it
* Occurs when V1 is damaged.... secondary visual pathway used
58
Action Blindsight
* Able to interact with visual stimuli even though you don't see it
* Occurs when V1 is damaged... secondary visual pathway used
59
Process of Transduction
Transformation of the physical characteristics / senses of our world into electrical signals
60
Process of Sensation
Sensory systems gathering raw data from physical environment
61
Process of Perception
Outcome of the brain's processing of raw data
* Brightness, saturation, colour
62
Hyperopia and Myopia
| Reasons for these abnormalities
**Hyperopia:** Farsightednes
**Myopia:** Nearsightedness
*Reasons for these abnormalities:*
* Abnormal eye length (long in myopia, short in hyperopia)
* Abnormal curvature of cornea
* Disrupted accommodation
63
Convergence of Rods and Cones & It's Implications
~120 **rods** converge onto 1 ganglion cell
* Promotes better vision in dark: small responses combine to become great
* Poorer visual acuity: Light from rods come from various areas on visual field
~5 **cones** converge onto 1 ganglion cell
* Higher visual acuity: More precise locations on visual field
64
Dark Adaptation
*The transition from a bright environment to a dark environment involving both rods and cones.*
Where "threshold" is the minimal intensity of light needed to be perceived....
* Note: Obviously, as the threshold decreases, perception rates will increase.
1) Cone adaptation
* Occurs 0:00 - 5:00 min
* Threshold for activation of cones LOWERS.
* Results in INCREASE in sensitivity to light
* Stays constant after a while
2) Rod adaptation
* Occurs after 5:00 - 10:00 min, following the rod-cone break
* Threshold of rods LOWERS considerably
65
Parvocellular Cells / P-cells
Retinal ganglion cells of the LGN.
* Smaller cell bodies
* Perception of colour, pattern, form, texture, and depth
* Layers 3, 4, 5, and 6 of LGN
65
Magnocellular Cells / M-cells
Retinal ganglion cells of the LGN.
* Larger cell bodies
* Perceives movement
* Layers 1 and 2 of LGN
66
V1 Feature Detector Cells
**Simple:** Sensitive to the orientation of bars of light
* Do not care about direction of movement
* DO have on region and orientation
**Complex:** Sensitive to orientation and movement
* Do not care about region
* DO have an orientation and movement
**Hypercomplex:** Sensitive to orientation, movement, and length/size
* Cares about all three
67
Columnar Layout of V1
*Cells in a set column tend to share similar properties.*
**Ocular dominance columns:** Columns of neurons in V1 that respond preferentially to input from either the left or right eye.
**Orientation columns:** Columns of simple cells that respond preferentially to input from different angles.
68
Depth Perception
## Footnote
+ Two Types of Depth Cues
*The process of the visual system involving taking 2D image on retina and interpreting it in a 3D manner.*
* Depth perception is innate AND learned
* Evolved in predator-prey relationships
**Monocular Depth Cues:** Cues that only need to be captured by one eye
**Binocular Depth Cues:** Cues that require both eyes to capture same regions of visual fields from different positions.
69
Monocular Depth Cues
Estimates distance/depth with information sent from just one eye
Types of Cues...
* Accommodation
* Motion-based:
* Motion parallax: Differences in perceived speed of objects
* Optic flow: Perceived motion of visual field resulting from ONES OWN movement
* Pictoral depth: Interposition (overlap) and linear perspective (small objects in distance)
* Aerial perspective: Distant objects appear bluer
* Shading
70
Binocular Depth Cues
Estimates distance/depth with information from both eyes
* **Binocular Disparity:** Amount of difference between images formed on each retina, due to eyes being a few cm apart
**Stereopsis:** Perceiving depth using binocular disparity
71
Colour Blindness
*Lack of sensitivity to certain colours due to partial or complete loss of functioning of one or more of the 3 cone types.*
* Trichromat: Normal-sighted person
* Dichromat: A person w/ only two functional cone types
* Limited range of perceived colours
## Footnote
Protanopia (loss of red), deuteranopia (loss of green), tritanopia (loss of blue)
^ Not going to be tested (I hope) but cool to know :)
72
Preferential Looking Paradigm
*A technique to measure infant visual acuity.*
* Infant presented w/ two cards: plain and w/ stripes
* Good acuity --> Infant lookes at the stripes (complexity!)
* Poor acuity --> Infant unable to differentiate and show a preference
73
Function of Bipolar Cells
| Newsflash: They are not useless!!!! (as in the module..)
* Relays signal from photoreceptors to ganglion cells
* ON bipolar cells increasing activity of downstream ganglion cells in response to light
* OFF bipolar cells decreasing the activity
Psych 1XX3
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