Week 9- Vision

Question 1

Vision purpose

Answer 1

-Identify, locate and react to things in environment
-Communication

Question 2

Vision: a process of inverse optics

Answer 2

-The source of light is seldom the ‘message’ of interest, the reflection of light off objects is more relevant

-The amount of light reflected off an object gives us a perception of lightness

-The pattern of light reflected gives us a perception of shape, texture

Question 3

Stimulus for vision. Particle versus wave.

Answer 3

-Light

-Can behave as a particle or wave

-Particles vibrating back and forth generate electromagnetic force- this radiates away from particles as a wave

Question 4

Photon

Answer 4

Discrete “packet” of a wave’s energy (i.e. one wavelength)

Question 5

Faster vibration=
Slower vibration=

Answer 5

-Shorter wavelength photon (greater energy)

-Longer wavelength photon (lower energy)

Question 6

Visible light spectrum

Answer 6

-Photons come in a range of wavelengths, most of which are invisible to us (but not all animals)

-The portion of the electromagnetic spectrum we can perceive (380-750nm) is known as the visible spectrum

Question 7

Light interacts with matter in a number of ways…

What is the most important for our vision?

Answer 7

-Absorption

-Diffraction (light bends when it hits the medium)

-Reflection (think of a mirror) = most important for vision we see the light that is reflected off a surface!

-Refraction (light bends both when entering and exiting the medium)

Question 8

Intensity as a stimulus property of light…

Answer 8

-Perceptual submodality= brightness

-Measure= power(watt) or emitted light (lumen) or luminous intensity (candelas)

Question 9

Wavelength as a stimulus property of light…

Answer 9

-Perceptual submodality= colour

-Measure = nm (~380- 750nM= visible spectrum)

Question 10

Purity as a stimulus property of light…

Answer 10

-Perceptual submodality= saturation

-Measure= How many different wavelengths are being mixed together?

Question 11

How is lightness as a perceptual submodality of reflected light measured?

Answer 11

-Candelas

Question 12

What areas of the eye work to focus light on the retina?

Answer 12

-Lens
-Cornea
-Pupil
-Iris

Question 13

What areas of the eye are associated with the ‘output’?

Answer 13

-Retina
-Macula
-Fovea
-Optic nerve

Question 14

The retina is the site of….

Answer 14

-Stimulus detection and signal transduction (light converted from a chemical signal to a neural signal)

Question 15

The pupil. What is it influenced by?

Answer 15

-Can accommodate (dilate, constrict) to allow amount of light entering the eye

-Influenced by:
1) light levels

2) autonomic nervous system (e.g. fear, excitement, attraction)

3) Drugs e.g. opioids, cholinergic

4) Age (lose the ability to accommodate)

Question 16

Visual focus

Answer 16

-Incoming waves of light are refracted (bending as enter a medium) onto the retina

-This happens first at the cornea and then at the lens

-In addition to refraction, the eyeball must be the right shape and size so the focused image falls onto the retina exactly

-Light is concentrated (focused) onto the fovea (center point of retina- dip here)

Question 17

The lens (function and changes with age)

Answer 17

-Can accommodate (make more/ less convex) to improve refraction of light

-Lens accommodation ability decreases with age

-And with non-use

Question 18

Consequence of lack of accommodation of the lens

Answer 18

Normally the ciliary muscles either side of the lens extend or compress to alter lens shape in response to the demands of various stimulus in the visual field.

As the ciliary muscles extend the lens become more circular and this brings the near point closer

As the ciliary muscles compress the lens becomes more ‘almond shaped’ and this increases the far point.

So when lens accommodation does not occur ability for the near and far point to adjust is lost- vision becomes more fixed i.e. the ability for an individual to focus on both near and far objects is lost.

Question 19

Lens losing transparency

Answer 19

-Fancy term: opacification. Medical term: cataracts

-Congenital (from birth), or from UV exposure

-Can replace the lens with a synethic lens but you’ll lose accommodation and thus are stuck with a medium near/ far point

Note: there is now an accommodating synthetic lens that allows for ‘natural’ focus but it’s rarely used: very much still in the developing stage.

Question 20

If we didn’t have the lens what would you have?

Answer 20

-Lens focuses light

-Without the lens would have hyperopia (far-sightedness: difficulty seeing things close)

Question 21

Is the lens fixed in size?

Answer 21

-No, the lens keeps growing as we age (from 0-90, thickness increase ~4x)

-This means, a battle as we age between thickness of the lens and accommodation ability

Question 22

The retina

Answer 22

-A thin layer of tissue that lines the inside of the back of the eye

-Contains photoreceptor cells: rods and cones

-These synapse onto bipolar cells

-Which synapse onto ganglion cells

-Whose axons from the output nerve

Question 23

3 ‘weird things’ going on in the retina…

Answer 23

1. Rods and Cones are at the back of the retina and so light must pass through other cell layers first
2. A huge hole in the middle where the optic nerve exits the eye- means a portion of the retina is missing and so doesn’t collect light (blind spot)
3. Light doesn’t excite rods and cones, it inhibits them- stops them from releasing neurotransmitter.

Question 24

Rods and Cones difference….

Answer 24

-Specific excitation spectra i.e. peaks at different wavelengths of light. For rods= ~500nm. For cones= ~440, 530, 560nm

-Differencing sensitivity. For rods= activated by 1 photon therefore good for low-light conditions (scotopic vision). For cones= need 100s of photons therefore used in higher light conditions (photopic vision).

-Rods saturate easily so low acuity, cones on the other hand have high acuity vision.

Question 25

Colour vision in the retina

Answer 25

-Starts with the cones.

-3 different cones activated by different wavelengths (Blue cones: s, Green cones: m, Red cones: l).

-Combination of the B, G, R cones gives us access to a range of colours across the visible spectrum

Question 26

Colour blindness

Answer 26

8% of the population, mostly men, missing one type of cone- colour variant vision

Question 27

Tetrachromats

Answer 27

Extra cone means they see a range of colours that others don’t (greater sensitivity)

Question 28

Adaptation to the dark: rods and cones

Answer 28

-Involves sensitization of both cones and rods

-Dark adaptation curve: cones are the more sensitive photoreceptor during the first 10 minutes of darkness. Then after 10 minutes rods become the more dominant photoreceptor (the swap from cone to rods is known as the rod-cone break).

-When rods are dominant = low acuity vision, and reduced colour vision. Note: there is a point where rods increasing sensitivity with increased time in the dark levels of.

Question 29

Rods and cones: difference in input

Answer 29

-Rods= hundreds of rods per bipolar cell and then ganglion cell. This can amplify a dim light source as rods ‘work together’. This amplification means high sensitivity but you lose acuity as information can’t be pinpointed to a single rod.

-Cones= as few as one cone per bipolar cell and then per ganglion. Low sensitivity due to lack of amplification but high acuity- can pinpoint where information came from.

Question 30

Rods and Cones: Number

Answer 30

-91 million rods
-4.5 million cones

Question 31

Rods and cones: Specific distribution

Answer 31

-Rods are located away from the fovea (direct line of focus) as are sensitive but have lower acuity: peripheral vision!

-Cones are are enriched/ highly concentrated at the fovea as are less sensitive but have high-acuity: central vision.

-Explains why anecdotally we see a lot of detail directly in front of us but at the sides vision is blurry.

Question 32

What is the fovea a part of? What does degeneration of this area cause?

Answer 32

-Fovea is part of the macula

-Macula degeneration= loss of visual acuity. Followed by blindness (leading cause in 50+ year olds)

Question 33

Rods: inhibited by light…. How…

Answer 33

For rods= no light = means depolarisation (excitation- release of neurotransmitter), which activates bipolar cells, which activates ganglion cells.

Question 34

Cones: inhibited by light… How…

Answer 34

-For cones= light= means hyperpolarization & neurotransmitter release stops.

-Important to understand: glutamate release has various downstream effects in that it can be excitatory or inhibitory. This is because glutamate acts on many receptors with some receptors opening ion channels and some closing them.

-If the lack of glutamate release when light hits a cone inhibits downstream bipolar cells these are known as ‘OFF bipolar cells’.

-If the lack of glutamate release when light hits a cone activates downstream bipolar cells these are known as ‘ON bipolar cells’.

-This means 2 distinct channels of information being relayed to the ganglion cells. An OFF bipolar cell will not excite ganglion cells. An ON bipolar cell will increase the firing rate of a subsequent ganglion cell.

Question 35

Blind spot

Answer 35

-‘hole’ in the back of each eye where the optic nerve exits

-This means a point where there is no photoreceptors and therefore a ‘blind spot’ for each eye

-The blind spot is in the nasal portion of the retina

Question 36

Why is it good to have two eyes…

Answer 36

• Gives a complete visual field. The visual field extends approximately 60% nasally and 107% temporally from the vertical meridian, and 60% above and 80% below the horizontal median.
  Note: specific numbers aren’t important here just know that the visual field extends more temporally than nasally and more below than above.

-Aids in depth perception (stereopsis). Convergence involves reducing the interocular angel and allows you to focus on near objects. Angel of convergence provides a binocular cue that the brain can interpret to gauge depth/ distance.

Note: not soley binocular cues that result in depth perception. Monocular cues such as relative size of an object also work to achieve this.

Question 37

The visual field maps to the retina in reverse

Answer 37

-Light from inferior visual field go to the superior retina. Light from superior visual field go to the inferior retina

-Light from temporal visual field go to the nasal retina. Light from the nasal visual field go to the temporal retina

Question 38

What is the visual pathway from the eye to the cortex?

Answer 38

-Retina-geniculate-striate pathway

Geniculate= refers to the lateral geniculate nucleus of the thalamus

Striate= another name for the cortex.

Question 39

The nerve partially decussates…

Answer 39

-Optic nerve= cranial nerve 2

-This partially decussates at the optic chiasma such that all incoming information from the left visual field now heads to right hemisphere, and all information from right visual field now heads to left hemisphere.

-Note: when say left visual field not referring to the view of the left eye but the left field of both the left eye and the right eye. Same thought process with the right visual field.

Just draw the diagram in the slides: way easier to understand than written.

Question 40

In the retina-geniculate-striate pathway is information from the retina binocular or monocular?

Answer 40

Binocular

Question 41

Percentage spilt of protections

Answer 41

-20% of protections relayed to the superior colliculus: responsive to sudden shifts in light

-80% of projected directed tot he lateral geniculate nucleus of the thalamus

Question 42

After the thalamus what level are we at in the retina-geniculate- striate pathway?

Answer 42

-Cortex level AKA striate

Question 43

Ganglion cells have receptive fields…Lateral inhibition… Center surround architecture…

Answer 43

-Their axons fire in response to light signals coming from specific potion of the visual field.

-Also decrease in activation when there is activity in an adjacent area of the visual field= lateral inhibition

-Ganglion cells have what is known as a centre-surround receptive field.

-On-centre means that when light comes from the centre of the field and no light from the surround the cell increase it’s firing rate.

-Off centre means that when light comes from the centre of field, and no light from the surround, the cell decreases firing.

-So and on-centre ganglion cell will fire a lot if a white dot was against a black background and off-center ganglion cell will dire a lot if a black dot is against a white background.

Question 44

How does the on/ off centre ganglion cell receptive field architecture relate to bipolar cells and in turn the cones?

Answer 44

Note change in naming. Previously known ON/OFF bipolar cells are actually ON centre/ OFF centre bipolar cells.

-Cones in the center of a receptive field that is light will activate an ON center bipolar cell and in turn an ON center ganglion cell will increase it’s firing rate.

-Cones in the center of a receptive field that is dark will activate and OFF center bipolar cell and in turn an OFF center ganglion cell will increase it’s firing rate.

This means there are distinct info streams relayed onto ganglion cells!

Question 45

Retina ganglion cells + response to edges…

Answer 45

-The distinct info streams that are relayed onto ON/ OFF centre ganglion cells means a unique pattern of activity across the array of ganglion cells lining the retina.

-This is important as it means ganglion cells respond to patterns of light surrounded by dark or vice versa and thus are good for distinguishing the edges of an object.

-In an image for example, an grey area= no response, a white area= large response from ON-center ganglion cells. Black=large response by OFF-center ganglion cells.

Question 46

Raw data received by rods & cones is overwhelming…

Answer 46

-Processing information from each and every rod and cone would be overwhelming

-Instead the retina responds to differences in the data i.e. light levels across the visual field

-This means edges/ borders/ contrast is computed and this allows identification of shapes, features and in turn objects

The key here is we get a general gist and don’t need to process all information coming in.

Question 47

Are ganglion cells purely for edge detection… 3 main types…

Answer 47

-No, there are 20 different types of retinal ganglion cell that have varying functions

-3 main types accounting for 80-90% of all ganglion cells are

Midget cells
Parasol cells
Bistratified cells

Question 48

Differential roles of the midget, parasol and bistratified cells…

Answer 48

Bistratified cells= blue/ yellow

Midget cells= red/ green

Parasol cells= light vs dark (i.e. broadband, achromatic)

Together these cells convey patterns of light/ dark colour signals from the thalamus.

Question 49

Spectra opponency

Answer 49

-Cones react to specific wavelengths of light

-Thus, ganglion cells respond to light depending on the mixture of cones in its receptive field

-If all three cones equally present, no selectivity

-If R/G/B present to greater degree, the responds more to light from corresponding portion of spectrum (and won’t respond to light from underrepresented potion).

Question 50

Midget cells versus parasol cells (high acuity and low acuity vision)

Answer 50

-Midget cells: Slow, sustained responses: good for high-acuity, slow vision (i.e. still objects)

-Parasol cells: Fast, transient responses: good for conveying information about object motion, low acuity.

Question 51

Two populations of cells in the LGN of the thalamus? How do these relate to the midget and parasol cells?

Answer 51

Two populations of cells in the LGN of the thalamus….
-Parvocellular cells: small bodies, respond to colour, fine detail, still or slow-moving objects.

-Magnocellular cells: big cell bodies, respond to object in motion.

How do these relate to the midget and parasol cells…
-P cells get input from the more central potion of the retina and the consequently the midget cells (high acuity vision)

-M cells get input from the periphery of the retina and consequently the parasol cells

Question 52

What layers of the LGN do the distinct populations inhabit? How do these layers project differentially to the primary visual cortex?

Answer 52

-1,2- Magno
-3-6= Parvo

These layers project to different areas of the primary visual cortex. Different info streams maintained from retina to rear of the brain.

-Parvocellular stream: sustained response, sees colour, low contrast gain, high spatial resolution, slower

-Magnocellular pathway: transient response, monochrome, high contrast gain, lower spatial resolution, faster.

Question 53

What about the bistratified RGC? What info do they convey to they convey to cortex and how?

Answer 53

Third stream= koniocellular stream, very complex and not well understood.

Question 54

V1 and the retinotopic map

Answer 54

-Signals from LGN sent to V1 (primary visual cortex)

-Retinotopic map is organised according to input from retina (via LGN)

-Columns of neurons devoted to specific areas of retina and thus visual field.

-Note: cortical magnification- largest chunk of the cortex is devoted to central high acuity vision despite it being the smallest area of the visual field.

Question 55

Hierarchal arrangement of the visual cortex…

Answer 55

-Hierarchy of processing in the cortex from V1 to V2 to V3 etc.

-but, information does not just flow linearly along this hierarchy. Information can be bidirectional (concurrent processing). I.e. channels feedback to previous area e.g. from V2 to V1

But….everything has to go through V1initially right?

Question 56

Blindsight patients + What do they suggest about the role of V1

Answer 56

-Blindsight patients = damage to V1 so would expect no vision at all (as everything initially goes through V1?)

-But despite blindsight patients not reporting conscious vision they can still ‘see’ e.g. negotiate a new environment and avoid obstacles when eyes are open.

-How? There is a direct projection from V1 to V3 that allows blindsight.

Question 57

Role of V1

Answer 57

-The first stage of cortical processing

-The neurons in V1 have the smallest receptive fields of any region of visual cortex (and thus highest resolution)

-V1 neurons are responsive to colour, orientation, spatial frequency.

-Note: some cells responsive to motion, but not all.
-Processes edges, boarders, contrast, shading= contour enhancement and feature detection

-Another alternative explanation/ hypothesis for V1 is that it a “salience detection”- highlighting important features of visual scene to guide shifts of attention and gaze.

-V1 receives wavelength information but this is not where colour conception happens. V1 “sees: in black and white.

Question 58

V2 + V3 function

Answer 58

Responsive to orientation, spatial frequency and wavelength (i.e. the same things as V1)

-But not responsive to colour. Being responsive to wavelength doesn’t mean perceiving colour.

Question 59

Where does colour vision occur and how do we know?

Answer 59

V4
-Known because of cerebral achromatopsia= an acquired condition involving impaired colour perception after damage to V4 (typically after a TBI). This often has severe consequences as colour perception gives contrast and delineates objects.

NOTE: previous areas such as V1, V2, V3 are responsive to wavelengths but V4 is the first area we have conscious perception of colour!

Question 60

Object perception

Answer 60

-V1-3 does the basics of simple form analysis (orientation, spatial frequency, wavelengths)

-V4 adds colour and some analysis of complex contours e.g. convex and concave (this is because neurons here are responsive to various degrees of curvature as well as orientation). = beginnings of shape representation

-But, how do we get object perception from this. Idea of hierarchal processing i.e. simple features from each ‘areas contribution’ combine to form more and more complex percepts at each step.

-This idea could explain why different views can give the same/ similar object perception i.e. a coffee mug can be recognised from multiple angles. But not all objects can be decomposed into simple elements in this way

-Solution to this problem: object recognition is not soley bottom up there is an element of top down processing. I.e. based on experience (template/ memory of objects in our brain that we can match).

Question 61

Evidence against pure bottom up processing of objects…

Answer 61

-Would be very slow and we can recognise objects fast

• We don’t always need every single bit of info to perceive an object e.g. closure- triangle illusion

-We can perceive the same object as being two different things e.g. reversible figures.

Question 62

Top down processing theories emphasize…

Answer 62

-Pattern recognition
-Automated recognition of patterns and regularities in data e.g. continuity, closure, figure ground relationships, proximity.

Question 63

Gestalt psychology and top down processing

Answer 63

-The idea that the whole is different from the sum of its parts

-I.e. the brain creates a percept that is more than simply the sum of available sensory inputs

-Simple sum of sensory inputs= bottom-up approach to vision.

Question 64

What are the neural mechanisms of top down processing?

Answer 64

-Poorly understood. Likely to involve reciprocal connections to help us attend to certain stimuli.

-Higher order neurons boost/ suppress lower order neurons e.g. once neurons encode a ‘pattern’. Boost relevant lower order neurons that feed that pattern, but suppress those that don’t. Evidence of these reciprocal connections have been found i.e. higher cortical areas can boost/ suppress cells in V1-V3.

Question 65

Motion perception

Answer 65

-Some V1 neurons are tuned to motion direction.

-But there’s a projection from V1 to area MT (middle temporal areas, aka V5). Longer route through V1 encodes slower motion.

-And also a direction projection from thalamus to MT that does not go through V1 (could relate to blind sight). Which encodes for fast motion.

-We can see the role of V5/MT in a case study of patient LM: 48 y/o who suffered stroke and damage to this area. Patient LM had perfectly fine colour and object perception but impaired motion perception—- known as akinetopsia.

-Also know the role of MT/V5 through fMRI: MT/ V5 responds to objects in motion. All neurons are motion-selective. Most are direction-selection. Input from V1 comes specifically from motion-sensitive neurons.

-MT/V5 is needed to bind feature motion into global motion!!!

Question 66

Area MST and specialized motion

Answer 66

-MT projects to medial superior temporal area (MST)

-MST neurons encode changes in optic flow (flow of entire visual stream as you travel).

-MST neurons have a large receptive field and are tuned to respond to different speeds in different areas of the visual field (locations)

• Distant objects move slowly, near objects move quickly so you need cells that respond to these phenomena

Question 67

What if someone is moving in our visual scene: one last specialization of motion perception

Answer 67

-Evolutionary advantage to knowing this- threat, social perception

-Neurons in the superior temporal sulcus (STS) respond selectively to biological motion and inhibiting STS function inhibits biological motion perception.

-This is specific to conspecifics i.e. human motion is detected by human STS. Monkey motion detected by monkey STS. Is STS the social perception centre???

Question 68

Dorsal and ventral streams in vision?

Answer 68

-Dorsal= where= retina to LGN to V1 to V5 (i.e. motion pathway.

-Ventral= what= Retina to LGN to V1 to V2 to V3 to V4

-Lesions ‘match up’ with these stream functions.
I.e. Lesion to the dorsal stream= deficits in object localization/ orientation
Lesion the ventral stream= deficits in object identification

-But… this model may be too simplistic. “where” info can inform “what” information. (e.g. human motion, figure-ground segmentation); areas of both streams are connected, so no real segregation (e.g. both “what” and “where” info required for some tasks, involving reaching/ grasping. There needs to be cross-talk between these areas!!!

Question 69

I’m so sorry Tilly

Answer 69

That was god awful