20.3 Vision Flashcards

Question

Describe myopia and its correction

Answer 1

* Light is focused in front of the retina * The eyeball is too long (not due to the curvature of the cornea) * Corrected by negative power concave lenses

Answer 2

* Light is focused behind the retina * Corrected by postive power - convex lenses * The lens is convex so can accommodate for it by contraction of the circular ciliary muscles that increase refraction * Accommodation means it may not manifest until later in life

Answer 3

* Astigmatism = curvature of the cornea in different axis * Results in different focuses in different lanes of light * **Horizontal** point of focus = **further back** * **Vertical** point of focus = **in front** * Corrected with spectacles that have a **cylindrical component** on their surface curvature

Answer 4

* Failure of lens accommodation with age as the lens becomes stiffer and less elastic * Near sighted vision becomes more difficult

Answer 5

* Lens become cloudy * Higher incidence with dehydration * Perhaps due to change in salt conditions * Surgically removed and replaced with an artificial lens

Answer 6

* Vision in the day * Rods * Bleached and saturated at -65mV * Cones * Active

Answer 7

* Between night and day (indoors - low) * Rods * Active * Cones * Active

Answer 8

* Vision in the dark * Rods * Active * Cones * Not bleached as easily

Answer 9

* The process by which **light** hitting the retina is **converted** into an e**lectrical, graded potential by photoreceptors** * Results in **photoreceptor hyperpolarisation** in light * Due to an **intracellular biochemical signalling cascade**

Answer 10

*POT PHS* 1. Photopigment 2. Opsin 3. Transducin 4. PDE 5. Hyperpolarisation 6. Synaptic transmission

Answer 11

**P**OT PHS * Light energy (electromagnetic radiation or photon particle) is absorbed by photopigments embedded in the membrane of the photoreceptor outer segment discs * In **rods**, the pigment is **rhodopsin**, a complex of the GPCR opsin and the covalently-linked (prebound) agonist retinal * In **cones**, the pigment is **photopsin** a complex made of one of three opsins (**L, M, S**) and **retinal** * **RGB =** **LMS** * Each type of cone produces a different variant of the opsin protein that distinguishes their absorption wavelength * **Red light excites L the most** * **Green light excites M the most** * **Blue light excites S cones the most**

Answer 12

P**O**T PHS * Each photpigment contains **retinal** and **opsin** * **Retinal** is a small molecule in which the **C₁₁-C₁₂** double bond assumes the **cis configuration** in the dark * Absorption of one photon by retinal converts the bond to the **trans** configuration, changing its molecular conformation * Activates the **opsin** to **metarhodopsin II** * Process is known as **bleaching** of the photopigment because the pigment changes colour from **purple** to **yellow**

Answer 13

PO**T** PHS * The activated opsin (metarhodopsin II) stimulates the **G-protein transducin** which then stimulates **photphosdiesterase** * Transducin activated through the exchange of **GDP** for **GTP** * The **alpha-GTP** subunit diffuses laterally through the disc membrane and activates the membrane bound **phosphodiesterase (PDE)** * The G protein activity is increased by **GAPs (G-protein activating proteins)** * The more metarhdosopin II, the greater the amplification

Answer 14

POT **P**HS * PDE degrades cytosolic **cGMP** to **5'-GMP** leading to a fall in [cGMP]_i

Answer 15

POT P**H**S * The reduction in [cGMP]i causes **closure of non-selective cation channels** in the photoreceptor plasma membrane, reducing the inward Na⁺ current (the dark current) and hyperpolarising the membrane * Going from **-35 mV to -65 mV** * This slows the release of neurotransmitter from the photo receptor * In the dark, **[cGMP]_i is high** thus channels are **open** and transmit a **larger inward Na⁺ current (the dark current)** * The resting E_m of the photoreceptor is therefore **-35mV** and stimulation by light drives the E_m in the negative direction to a peak of around **-65 mV**

Answer 16

* Hyperpolarisation of the photoreceptor membrane (from **reduces Na⁺**influx) **reduces Ca²⁺influx** through the non-specific cation channel thereby **decreasing the rate of glutamate** secretion from the synaptic terminal * In the dark, glutamate is constituvely released becuase the Em is held at a depolarized value (-35 mV)

Answer 17

* Mechanisms (RIT) * **Reduction in calcium increases affinity of cGMP channel for cGMP** * **Inactivation of metarhodopsin 2** * **_Transducin GTPase activity_** * Importance * **Allows the photoreceptor to be sensititive to another photon of light**

Answer 18

* Metarhodopsin II is inactivated by **rhodopsin kinase** which phosphorylates it * This is followed by the binding of **arrestin 2**- blocking the interaction its interaction **transducin** (preventing **PDE** activation)

Answer 19

* Active transducin has **intrinsic GTPase activity** * Converts the bound **GTP** to **GDP** * Releases the **GDP** and recombines with its **beta-gamma subunits** * cGMP concentration is then restored by **guanylate cyclase** that converts **GTP** to **cGMP** * The [cGMP] rises, the cGMP channels open and the Na^₊ current is resumed, membrane back to depolarised dark potential, **glutamate** is released again

Answer 20

* Reduction in the [Ca²⁺] in the light results in: * Start - Accelerated **rhodopsin kinase** phosphorylation of **metarhodopsin II** * Middle - Accelerated **guanylate cyclase activity** (producing cGMP from GTP) * End - Increased affinity of the cGMP channel for cGMP * Calcium channels enter the cell via the **cGMP** channel but are quickly pumped back out by membrane transporters * Thus during the dark, the [Ca²⁺] in the cell are higher than in thelight when cGMP channels close

Answer 21

* In the light, rod photo pigment is bleached * -65mV * Trans-retinal * **_Dark Adaptation_** * When moving to the dark, the photopigment is able to **regenerate** and the rods become more sensitive to light * **Metarhodopsin kinase** regenerates **metarhodopsin** * Trans-retinal back into cis-retinal * This means overtime, more rods are available to detect the light and transduce it * Dark adaptation takes around **1 hour** to reach maximal sensitivity * **_Light Adaptation_** * The huge increase in light sensitivity in **dark adaptation** means there is a temporary saturation of photoreceptors upon re-entry into light * All the **rhodopsin** is **bleached** and ganglion cells discharge action potentials, leading to everything appearing white * After this, light adaptation occurs and the cones become adapted to the light level

Answer 22

* In the dark, the cones are inactive * In the light, the cones become activated and adapted to the level of light * The use of **rods** and **S** **cones** is shifted to R/G cones in photopic light = Purkinje shift * tendency for the peak luminance sensitivity of the eye to shift toward the blue end of the color spectrum at low illumination levels as part of dark adaptation)

Answer 23

* An X-linked disorder affecting the pigments of **L/M** as they are encoded on the **X chromosome** * Mutations lead to defective pigment production thus the retina is less sensitivie to light of that wavelength **(red and green)** * **The cones are not lost thus overall vision is not affected** * **Protanopia** - loss of L cones (red) * **OPN1LW** * **Deuteranopia** - loss of M cones (green) * **OPN1MW**

Answer 24

* The pathology occurs between the **deep pigment epithelium of the retina** and the underlying **Bruch's membrane** (innermost layer of the choroid) * types: * **Wet** - due to **angiogenesis** * **Dry** - debris **accumulation (drusen)** * **Treatment** * In the early stages, may be slowed down by: * **Antioxidants** * **Minerlal supplements** * **Wet** can be slowed down by: * **VEGF antibodies** * vascular endothelial growth factor) antibodies * **Decoy VEGF receptor**

Answer 25

* In **scotopic** and **mesotopic** conditions * Several photoreceptors increasing **converging inputs** onto a rod bipolar cell * This increases the sensitivity of the rod pathway

Answer 26

* Photopic and mesotopic conditions * The cone vertical pathway divides into 2 parallel pathways: * ON * OFF

Answer 27

* **_Dark_** * Photoreceptor is depolarised * Glutamate release from **mGLUR6** (metabotropic glutamate receptor 6) * **Glutamate binding closes the cation channel** and **hyperpolarises the bipolar cell** * **No glutamate released from bipolar cell** * No activation of ON ganglion cells * **_Light_** * Photoreceptor is hyperpolarised * No glutamate release from mGLUR6 * **No glutamate binding results in the channel remaining open and depolarisation of the bipolar cell** * Glutamate released from bipolar cell * Activation of the ganglion cell, action potential generated

Answer 28

* **Scotopic conditions** * Photoreceptor is depolarised * Glutamate release from photoreceptor * AMPA - ionotropic receptor at the bipolar cell * Glutamate binding opens the ion channel and depolarises the cell * Glutamate release * Activation of the OFF ganglion cell - activation generated * **Phototopic conditions** * Photoreceptor is hyperpolarised * No glutamate released * AMPA - inotropic receptor at the bipolar cell * No glutamate binding keeps the channel closed and the membrane is not depolarised * No glutamate released * No activation of OFF ganglion cell

Answer 29

* ON * Photoreceptor is depolarised * Glutamate release from the photoreceptor * Bipolar cell receptor is mGLUR6 * Glutamate binding closes the cation channel and hyperpolairses the bipolar cell * No glutamate release from bipolar cells * No activation of ON ganglion cells * OFF * Photoreceptor is depolarised * Glutamate release from the photoreceptor * AMPA - inotropic receptor at the bipolar cell * Glutamate binding opens the ion channel and depolarises the cell * Glutamate is released * Activation of OFF ganglion cells - action potential generated

Answer 30

* ON * Photoreceptor is hyperpolarised * No glutamate release from photorecpetor * Bipolar cell receptor is mGLUR6 * No glutamte binding results in the channel remaining open and depolarisation of the bipolar cell * Glutamte released from the bipolar cell * Activation of the ganglion cell - action potential is generated * OFF * Photoreceptor is hyperpolarised * No glutamate release from the photoreceptor * AMPA - inotropic receptor at the photoreceptor * No glutamate binding results in the channel being closed and the membrane is not depolarised * No glutamate released * No activation of OFF ganglion cells

Answer 31

* They studied the retina of **mudpuppies (species of salamander)** using electrophysiological intracellular **electrode recordings** to measure the graded potentials in ON and OFF bipolar cells * Under control conditions, light depolarised the **ON bipolar cell** and hyperpolarized the **OFF bipolar cell** as would be expected * When the **mGLUR6 agonist 2-amino-4-phosphonobutyric acid** was added to the solution in which the retina was bathed, upon exposure to light, there was no depolarisation of the ON cell (it remained hyperpolarised) but the OFF cell response was hyperpolarised * It provides evidence for: * **Two different recpetors on ON/OFF cells for glutamate** * **Changes to one pathway did not affect the other, thus they are independent**

Answer 32

* The retina processes the information transmitted to condense the information and extract changes in the visual scene * Involves the detection of: **(CST)** * **Colour opponency** * ****Colour detection * **Spatial contrast** * Edge/shape detection * **Temporal filtering** * ****Movement detection * **127 million photoreceptors --\> 1 million ganglion cells**

Answer 33

* Anatagonistic centre surround receptive fields * Lateral inhibition

Answer 34

* The area of the retina for which a neurone is responsive to changes in light * For bipolar cells it refers to photoreceptors that feed into it and for a ganglion cell, it is the bipolar cells that feed into it

Answer 35

* Receptive fields where the centre has the opposite response to light as the surround * This arrangement means: * **The overall illumination does not change the baseline firing of the ganglion cell but the firing changes when there is contrast (different levels of illumination in the centre and surround)** * This allows the visual scene to be **economically encoded** by the retina as information about the edges is transmitted while the areas of uniform illumination are not * The **temporal pattern** of firing by the ganglion cell and in the optic nerve encodes to higher centres where in the visual field light is

Answer 36

* Central receptive field * Activated in response to light * Photoreceptors synapse onto ON bipolar cells which synapse onto ON ganglion cells - sublamina A * Increased firing frequency * Sign inverting * Surrounding receptive field * Inactivated in response to light * Photoreceptors synapse onto OFF bipolar cells which synapse onto OFF ganglion cells - sublamina B * Decreased firing frequency * Sign preserving

Answer 37

* Central receptive field * Photoreceptors inactivated in response to light * Photoreceptors synapse onto OFF bipolar cells which synapse onto OFF ganglion cells (hyperpolarisation) * Surrounding receptive field * Photoreceptors activated in response to light * Photoreceptors synapse onto ON bipolar cells which synapse onto ON ganglion cells (depolarisation)

Answer 38

* The receptive field size of a ganglion cell is determined by the number of bipolar cells that feed into it which corresponds to the size of the ganglion cell dendritic tree * **Midget ganglion cells** have a **small dendritic tree** and are found at the **fovea --\> synapse with the parvocellular cells of LGN** * **Parasol ganglion cells** have a **larger dendritic tree** and are found in the **peripheral retina --\> magnocellular pathway at the LGN** * Fovea * 1:1 mapping * 1 photoreceptor to 1 bipolar cell to 1 **Midget ganglion cell** --\> **Parvocellular cells** * This arrangement results in **high acuity** with **low sensitivity** * Peripheral retina * Convergent input * Multiple photoreceptors to 1 **parasol type** ganglion cell --\> magnocellular cells of the **LGN** * This arrangement results in **low acuity** and **high sensitivity**

Answer 39

* Soma size * Midget cells are small whereas parasol cells large * Dendritic field size * Midget cells are small whereas parasol cells are large (parasol cells are connected to lots of bipolar cells) * Concentration * Midget cells are concentrated in the fovea whereas the parasol cells are non-foveal * Colour * Midget cells detect opponent light whereas parasol cells are unselective * Summation * Midget cells use **linear spatial summation** whereas parasol cells use **non-linear summation** * Longetivity of AP burst * Midget cells have sustained bursts whereas parasol cells are transient * Projections * Midget cells have projections to the parvocelluar pathway of the LGN whereas parasol cells have projections to to the magnocellular pathway of the LGN * Receptive field size * Midget cells have a small receptive fields (higher acuity) whereas parasol cells have a large receptive fields (lower acuity) * Conduction velocity * Midget cells have a moderate conduction velocity whereas parasol cells have a rapid conduction velocity * Pathway to the LGN * Midget cells utilise the parvocellular pathway whereas parasol cells utilise the magnocellular pathway

Answer 40

* Spatial contrast detected by centre surround antagonism set up by the ON/OFF bipolar cells is enhanced by **lateral inhibition** by **horizontal cells** via **GABAergic** innervation * The horizontal cells act as an indirect pathway between the photoreceptors and bipolar cells * Each horizontal cell has multiple dendrites that synapse with the terminal of a photoreceptor and the dendron of a **bipolar cell** to form a three-way synapse (triad) in the **outer plexiform layer** * *Lateral inhibition also occurs at the level of the amacrine cells that synapse with the bipolar cells and ganglion cells but is less well characterised than the horizontal cell pathway*

Answer 41

* Horizontal cell input * Horizontal cells receive input from its photoreceptors in the centre in a sign preserving manner * Glutamate in the dark depolairses the horizontal cells * No glutamate in the light hyperpolarises the horizontal cells * Horizontal cell output * The horizontal cells inverts the signal it receives from the photoreceptors and feeds it back to adjacent photoreceptors connected to bipolar cells in the surround * This means they have to have the opposite response to the centre and **enhances contrast**

Answer 42

* Human visual system interprets information about color by processing signals from cone cells and rod cells in an **antagonistic manner** * The reitnal circuits are able to process information on colour through **colour opponent ganglion cells** that exhibit **centre-surround receptive fields** sensitive to different colours based on the cone feeding into the pathway * Rods are not colour sensitive so there is no coloured vision in scotopic conidtions * It is important to note that only **miget (parvocellular)** and **non-M, non-P type** ganglion cells are colour opponents * Magnocellular cells have a non-colour specific response as multiple cone types feed into their centre and surround

Answer 43

* **_Red-green_** * 4 types * ON centre (R/G) * OFF centre * **_Blue-yellow_** * Yellow refers to both the red and green cones found in the centre/surround * 4 types * ON centre (B/RG)

Answer 44

* It allows the ganglion cells to compare the different colours of light within its receptive field and send a **comparative signal** to the brain **rather than separately encoding each colour** * **This compresses the information from the visual scene** * **127 million photoreceptors to 1 million optic nerve fibers**

Answer 45

* Movement is encoded through the **convergence of the retinal circuits** * There are no movement specific cells in the retina as there are colour specific cones, instead * In phototopic conditions, multiple cones converge onto a smaller number of ON/OFF bipolar cells that converge onto a smaller number of ON/OFF M cells respectively * In mesotopic conditions, there is the activation of both rods and cones * M cells have **transient** bursts action potentials when light moves across their receptive fields * The rapidly adapting response of the cells allows for movement to be encoded in a **temporal** (**M cell firing frequency** ) and **spatial** (**firing of M cells across the retina)** pattern * Amacrine cells are thought to contribute to the rapid adaptation of M cells

Answer 46

* Detection of movement is important to allow the body to **respond to changes in the visual scene and alert higher centres of potential danger** * then result in the **movement of the eye** and the **head** in the direction of hte stimulus so it can be focused in the centre of the gaze where resolution is high One feature of movement encoding is that it has **poor resolution** * Large convergence that occurs from the photoreceptor to the ganglion cell * In these situations, the **colour or form of the stimulus is not as important as altering higher centres quickly**

Answer 47

* Only the fibres originating from the nasal half of the retina decussate * The nasal halves of each retina view the temporal (lateral) aspects of the visual scene for each side * (The axons of retinal ganglion cells form the optic nerve that leaves the eye at the optic disc and enter the orbit through the optic canal. * Both tracts meet at the optic chiasm where there is partial decussation of the optic nerve fibres

Answer 48

* View the nasal half of the visual field * Send information to the same side of the brain * The central aspect of the visual scene is seen by both temporal retinas = binocular vision

Answer 49

* View the temporal half of the visual field * Send information to the opposite side of the brain

Answer 50

* Loss of the peripheral vision (temporal half of the visual field) * Usually due to compression of the optic chiasm by an **enlarged pituitary gland**

Answer 51

* From the chiasm axons form the optic tracts which run around the **cerebral penducles** * Targets of the optic tract * **Thalamus** * Most of the fibres of the optic tract project to the LGN at the dorsal thalamus * **Hypothalamus** * A small number of axons form synaptic connections with the hypothalamus * **Midbrain** * 10% of fibres form connections at the midbrain with the **superior collicuus** and **pre-tectal areas** * Regulate the **pupilliary light reflex**

Answer 52

* Left optic tract carries information from the **left temporal retina** and **right nasal retina** * **Left temporal retina** views right **visual field centre** * **Right nasal retina** views right **visual field periphery** * Transection of left optic tract = loss of the **right visual field** *

Answer 53

* The right optic tract carries information from the right temporal retina and from the left nasal retina * **Right temporal retina** views **left central visual field** * **Left nasal retina** views **left peripheral visual field** * Transection results in the loss of the left visual field

Answer 54

* The lateral geniculate nucleus is a laminated structure in the dorsal thalamus and the main target for the optic tracts. * The LGN has 6 layers which receive differential inputs that keep the information from both eyes separate * Thus the LGN neurones are **mononuclear** (receive input from one eye) * This anatomical variation supports **parallel processing** * **Right lateral geniculate nucleus** * Information carried from the **right optic tract** * Views the **left visual field** * Carries information from the: * Right temporal retina * Left nasal retina * **Left lateral geniculate nucleus** * Information carried from the **left optic tract** * Views the **right visual field** * Carries information from the: * Left temporal retina * Right nasal retina

Answer 55

* 2, 3 and 5

Answer 56

* 1, 4 and 6

Answer 57

* Midget cells (parvocellular cells) * Small receptive fields * Light/dark receptive fields * Require high contrast such as those from Snellen chart * Colour opponent receptive fields * Fire with sustained increases in action potential frequency * Parasol cells (magnocellular cells) * Large receptive fields * Light/dark receptive fields * Not colour opponent * Fire with a transient increase in action potential frequency * Non-P, non-M cells (koniocellular cells) * Light/dark receptive fields * Colour opponency

Answer 58

* Other parts of the thalamus * Brain stem * Modules the LGN input to **S1** * Activity relates to alertness and attentiveness * Primary visual cortex * 80% of excitatory synapses in the LGN are from **S1** * However, wwe usnure as to what this large input does

Answer 59

* **_The primary visual cortex_** * LGN thalamo-cortical neurones that project through the retrorenticular part of the internal capsule * Pass around the concave part of the ventricle and into a broad sheet - superior and lateral to the lateral ventricles * Inferior fibres project rostrally to the superior aspect of ht einferior horn of the lateral ventricle * Terminates in the **primary visual cortex** in the **occipital lobe**

Answer 60

* Loss of one quadrant of the visual field due to a lesion of one of the divisions of the optic radiation of one side

Answer 61

* Loss of input from the upper right temporal retina and left nasal retina resulting in the loss of the upper left quadrant

Answer 62

* Lesion in the bottom left divisions of the optic radiation carrying information from the bottom left nasal retina and bottom right temporal retina

Answer 63

* The central processing of vision is that which takes place in the visual and extra-striate cortices * It is important for the perception of **form** (fine and gross details), **colour**, **motion** and **stereoposis** (depth) * This occcurs by the increasingly more complex processing of information from the **visual cortex** to the **extra-striate cortex** * **Perceptive opening** * Humans rely on vision for sensing the environment and adapting to change * The importance of visual information is evidenced by the large proportion of the **neocortex dedicated to vision** * **20% of the neocortex** forms the **visual cortex** * **40% of the neocortex** being involved in **visual processing** * ****This is after phototrasndcution, parallel processing in the retina and LGN has taken place * The LGN projects to the striate cortex via the optic radiation

Answer 64

* Striate cortex * V1

Answer 65

* **Pulvinar** – a thalamic nucleus involved in visual attention. * **P**ulvinar = **p**ay attention * **Superior colliculus** – saccadic eye movements * **S**uperior colliculus = **s**accadic

Answer 66

* **Spiny stellate cells** * Spine covered dendrites from the cell body * **4C alpha** and **4c beta** extend across towards V5 * Excitatory * **Pyramidal cells** * Single thick apical dendrite that branches as it extends towards the pia mater * Multiple basal dendrites that extend horizontally * **All layers except 4C alpha and beta** (4A and 4B) * Extend axons out of V1 * **Inhibitory neurones** * Throughout the cortex * Local connections

Answer 67

* Parvocellular * Layer 4C beta * Small receptive fields * Colour opponent * Magnocellular * Layer 4C alpha * Large receptive fieds * Not colour opponent * Koniocellular * Layers 1 & 2 cytochrome oxidase blogs * Varied receptive fields * Colour opponent

Answer 68

* **4C alpha** * **Not colour opponent** * **Magnocellular LGN** * Projects to **layer 4B** * **4C beta** * **Small receptive fields** * **Colour opponent** * Parvocelllular LGN * Projects to **2 & 3 interblob region**

Answer 69

* Neuronal columns rich in **cytochrome oxidase** are layers **2, 3, 5** and **6** * Receive the input from: * **Koniocellular pathway of the LGN** * **Parvocellular/magnocellular input from layer 4C** * Between the blobs are interblob regions with the blobs centred on the ocular dominance column

Answer 70

* A column of the cortex that receives dominant input from one eye * Occurs as the inputs from the lateral geniculate nuclues (LGN) to layer 4C are in a sereis of alternating bands/columns for each eye * Neurones connected to the L/R eyes are distinct in layer 4 as they are in the LGN * In the centre of the columns, one eye is dominant due to radial input from layer 4C stellate cells to layers 4B and 3 * This alternates for dominance between each eye

Answer 71

**V1 = visual cortex** * Respond selectively to colour contrast * **The blob cells are the "color" processing cells of V1** * This pathway is known as the **parvoblob pathway.** * ** Blob pyramidal cell in layers II/III receive input from the parvocellular pathways in layer IV and directly from the single opponant koniocellular layers of the LGN.** * Properties of the neurons found in the blob areas: * Neurons with similar properties found in clusters known as blobs * Each blob is centred on an ocular dominance column * Wavelength sensitive. Double colour opponent circular receptive fields for detection of colour contrast. They are used in the perception of colour, and in colour discrimination. * 4 classes of double opponent cells characterised by their preferred stimuli * Insensitive to achromatic contrast * Monocular (respond to stimulation of one eye only) * Insensitive to orientation or direction

Answer 72

* Neurones with a receptive field in each eye that corresponds to the same bond in the contralateral visual scene (contralateral to the **V1 lobe**) * Essential for binocular vision that allows us to perform fine motor tasks that require stereoscopic vision * perception of death and 3D structure

Answer 73

* The uniting of inputs from both eyes in **layer 4B of V1** provides the basis for **stereopsis:** the sensation of depth that arises from viewing nearby objects with two eyes instead of one * Since the two eyes look at external environment at sligtly different angles, there is **binocular disparity** * **Difference in the image location of an object seen by the R/L eyes** * As this object is moved gradually to the plane of focus, the binocular disparity is reduced until a single object is perceived * ****The small distance each side of the plane of fixation at which single image is still obvserved is interpreted as **depth** (depth of focus)

Answer 74

* The receptive fields of these neurones in V1 and other visual cortical areas are disparity-selective: * The receptive fields of these neurones is driven by the left and right eyes are in sightly different horizontal positions on the two retinae * This makes the enurone maximally responsive to stimuli that fall on non-corresponding regions of each retina and therefore the neurones respond best to single objects at specific distances

Answer 75

* Far cells are stimulated by retinal disparities arising from points beyond the plane of fixation

Answer 76

* Stimulated by retinal disparities arising from points **in front** of the plane of fixation

Answer 77

* Respond selectively to points that lie on the plane of fixation

Answer 78

* AKA lazy eye * One eye is only partially open / congenital squint * This leads to overrepresentation of one eye in the visualc ortex * Occlusion therapy of the dominant eye retrains the cortex, this is important in the **critical period**

Answer 79

* Unlike in the retina and LGN, the receptive fields outside of layer 4C are no longer circular * **Orientation selective** * Hubel and Wiesel found that many neurones in V1 respond best when there is bar of light moving across their receptiv efields * The neurones are however, **orientation selective** * **Orientation column** * ****Within a column of the cortex, all the cells across the layers have the same preferred orientation * Across the cortex, the preferred orientation shifts progressively - a 180 degree shift transverses 1mm of the cortex in layer 3 * This allows for the analysis of object shape

Answer 80

* The receptive fields similar to that of the LGN and retina as they **also have ON/OFF regions** but the **arrangement** of these differ * The orientation selective and binocular neurones of V1 can be: * **Simple cells with simple receptive fields** * ON and OFF regions are segregated - this allows them to be orientation selective * **Complex cells with complex receptive fields** * ON and OFF regions appear to be throughout the receptive field * Respond to elongated stimulus at the **correct angle** in their receptive field * Hypercomplex cells * Even larger receptive fields that act as angle detectors that are sensitive to the length, orientation and motion of the bar of light that moves through their receptive field * Defined by the property of end stopping - decrease in firing strength with increasingly larger stimuli

Answer 81

* A subset of orentation selective cells is also direction selective and receive input from the magnocellular LGN pathway * Direction selectivity * Neurones respond to the direction of movement of a bar of light in their receptive field * The optimal movement is perpendicular to the orientation of the bar * Specialised for the analysis of object movement * **Latency model** * States that when stimulus moved in a "preferred" direction or neurone, there is a **gradual decrease in excitatory response latency** such that for a stimulus moving this way, all excitatory inputs sum at the same point in time (**latency leads to temoporal summation**) leading to a **greater response** * **Asymmetrical inhibitory input model** * Excitatory/inhibitory inputs are asymmetrically distributed such that when stimulus moves in a "preferred" direction, it first stimulates the excitatory side before the inhibitory (which has a longer latency) whereas in the "null direction", the stimulus first passes through the inhibitory region and its latency is such that it will inhibit later excitation

Answer 82

* The receptive fields of the blobs and interblob neurones are not well characterised and are controversial * Interblobs are **binocular** whereas blobs are **monocular** * Recent studies, suggest similar sensitiviity for colour and orientation/direction selective

Answer 83

* **Magnocellular pathway** * As many 4B neurones are direction selective, it is thought this pathway is involved in: * Guidance of motor actions * Analysis of **object motion** * **Parvo-interblob pathway** * Neurones in this pathway have small, orientation selective receptive fields so it thought they are involved in: **analysis of fine object shape** * **Blob pathway** * Many blob neurones are colour selective so thought to be involved in: analysis of **object colour**

Answer 84

* Outputs from the striate cortex are mainly by pyramidal cells (except from 4B) * The main output is to the extra-striate cortex * Layers, 2, 3 and 4B * Axons to the extra-striate cortex * Layer 5 * Axons to the superior colliculus * Layer 6 * Anal projections to the LGN

Answer 85

* V1 to parietal lobe * Function * Analysis of visual motion * Control of visual action * Magnocellular pathway * The cortical areas forming the dorsal stream show some progression in the development of more complex visual representations * The stream runs from V1 to V2, V3, and V5 to the parietal lobe * V5 is the most well studied area in the dorsal stream and processes linear motion in the visual field * Location * **Middle temporal lobe** * Direct input * 4B cells of V1 * Indirect input * Retinotopically organised input from V2 & V3 * Output * V3, parietal cortex * Cell sensitivity * Almost all cells are direction selective within a narro wrange * Respond to types of motion that are not good stimuli for other celsl such as drifting spots of light * Columins * Direction of motion columns analogous to orientation columns in V1 * Motion perception * Presumed that the perception of motion at any one point is by comparison of activity across the columns spanning a full 360 degrees of preferred direction

Answer 86

* V1 to temporal lobe, limbic system and connections with dorsal stream * Function * Perception of the visual world * Recognition of objects * Main V1 component * Blob pathway * Parvo-interblob pathway * The ventral stream progresses from V1 to V2, 3 and 4 towards the temporal lobe for the analysis of visual attributes other than motion * V4 * V4 is one of the most studied areas in the brain * Location * Anterior to V2, anterior to the posterior inferotemporal lobe (PIT) * Input * Interblobs and blobs via V2 * V3, inferotemporal cortex * Output * V2, V3 inferotemporalc ortex * Cell selectivity * Orientation selective * Colour selective * Functions * Plays a role in the perception of shape and colour

Answer 87

* It is the task of identifying and assigning meaning to objects in space * It is clear it requires the concerted effort of many cortical areas but we are yet to know: * Which neurones in which cortical areas determine what we perceive * How the simultaneous activity of widely separated cortical neurones is integrated and where this takes place

Answer 88

* The magnocellular pathway: * colour blind and involved in the analysis of **moving stimuli**, **control of gaze** and **stereopsis** (depth perception) * **Layer IVC** alpha receives input from **M** layers of the **LGN** forming simple cell receptive field * Layer IVC alpha projects to layer IVB which is still largely made up of simple cells. * Properties of the neurons found in the motion sensitive area IVB: 1. Exhibit **orientation specificity** and **direction sensitivity** 2. **Large receptive fields** unable to discriminate fine detail, but sensitive to low contrast achromatic stimuli 3. **Wavelength insensitiv**e thus unable to discriminate colours 4. **Binocular** 5. M pathway is involved in visual attention and gaze reflexes via layer V outputs. * **Pulvinar** – a thalamic nucleus involved in visual attention. * **Superior colliculus** – saccadic eye movements

20.3 Vision Flashcards

(115 cards)