Neurophysiology of Visual System

Question 1

What does neuroanatomy/structure refer to?

Answer 1

• Arrangement of structures – which cell connects to which part -> e.g. get retina structure w/ OCT

Question 2

What does neurophysiology refer to?

Answer 2

• How parts interact w/ varying inputs & measuring physical output to see effect -> e.g. output of retina – electroretinogram (ERG)

Question 3

What does psychophysics refer to?

Answer 3

• How well whole system (human) interacts w/ stimulus
  o e.g. measuring VA – px responds w/ yes/no answer
  o Conscious behaviour

Question 4

What does psychology refer to?

Answer 4

• How higher order processing of visual scene impacts behaviour

Question 5

What is the visual pathway?

Answer 5

• Eye -> subcortical structures (LGN, superior colliculus) -> striate -> extra striate cortex

Question 6

How many nerve fibres are in retina and what does this result in?

Answer 6

• 1 million nerve fibres – most in central vision – disproportionate foveal representation since we see in horizontal plane
• Distorted topographical representation of retina
• Bistable stimuli – 2 images in 1 picture – can only see one at once though – visual illusions

Question 7

What is phototranduction?

Answer 7

• Light energy absorbed by photopigments contained within photoreceptors is converted to biological/biochemical energy that leads to hyperpolarisation of photoreceptors

Question 8

Where does retina gather info from? & what is info about?

Answer 8

• Retina gathers info about spatial and temporal distribution of lights reflected from surrounding objects & to form this info into a meaningful perception of the environment
• Output from retina is a complex transformation of input

Question 9

Describe integration in retina?

Answer 9

• Recording from GCs – retinal GCs are final integrator – light has to go through all layers of retina to hit photoreceptors before light can be used as vision – GCs send it in usable format to LGN in midbrain
• Receptive fields
• Spatial summation – things are gathered together in space
• Temporal summation – things are gathered together in time
• Various dichotomies of retinal GCs
• Scotopic (dark), mesopic (middle light levels), photopic system (bright light)
• Foveal and peripheral vision
• On- and off-centre cells
• Magnocellular Systems– large receptive fields, info is moving everywhere
• Parvocellular Systems– small receptive fields, central vision, detail, target usually stationary

Question 10

What are the stages in perception?

Answer 10

• Light reflected from objects
• Image formation
• Sampling of image – know/recognise shape in peripheral vision w/o fully looking at it
• Transduction to neural activity
• S-cone = short λ, L-cone = long λ, M-cone = λ

Question 11

What is fine tuning in retina?

Answer 11

• Neurons sampling bits of light – central vision sampling is what brain concentrates on but if sees something in periphery which needs attention then will turn head

Question 12

What is the ratio of photoreceptors to GCs in periphery vs fovea?

Answer 12

Many photoreceptors send signals to 1 GC in periphery, but ratio is more 1:1 at fovea

Question 13

Describe integrator neurons –> synaptic pathways & signal modification?

Answer 13

• Rods & cones synapse w/ integrator neurons in OPL. Integrators e.g. horizontal cells (HCs) whose communications are fully in OPL – connect rods & cones with bipolar cells (BCs)
• Simplest & fastest pathway for transmission of signal: cone -> BCs -> GCs
• BCs receive signals from rods & cones directly or via HC mediation
• BC dendrites in OPL – axons extend into IPL where they synapse with other integrators specifically amacrine cells (ACs) and GCs
• ACs mediate signals between BCs, other ACs and GCs
• GCs = final element in chain – receive input from either BCs or ACs
• GC axons make up optic nerve (carries info to brain)
• Connection for rods is different than for cones: rods -> BC -> AC -> GC
  o Synapse w/ GC marks end of intra-retinal processing and beginning of transmission of integrated output into visual cortex
• Lateral connections between 2 rods & cones, BCs etc possible and number of switches in system & possible combos is nearly infinite

Question 14

Describe horizontal cells (HCs)?

Answer 14

• Inhibit other cells – inhibitory output always
• As connected laterally to numerous BCs & rods/cones, HCs can suppress generation of info. along some pathways/routes while not affecting other adjacent to them
  o Selective suppression of single transmission = lateral inhibition -> purpose is to ↑ acuity of vision
• When light hits retina, may illuminate some sensors brightly and others in area around less so. By suppressing output of less-illuminated areas, HCs ensure only highest-intensity output get through -> therefore contrast & visual definition are improved
• Cell bodies are in inner nuclear layer
• Outer plexiform layer contains the processes of neurons
• HCs supress other information to allow you to focus on a target
• If a HC hyperpolarise the signal becomes more negative

Question 15

What does on-centre respond to and off-centre do?

Answer 15

On-centre = responds to light
Off-centre = does not respond to light

Question 16

Describe Bipolar Cells (BCs) & Lateral inhibition?

Answer 16

• BCs can hyperpolarise or depolarise depending on type
• BCs therefore can send ‘positive’ or ‘negative’ signal via GCs and brain has capacity to interpret these 2 levels of signal differently
• This mechanism allows for 2 types of signals to be sent, & it enhances the lateral inhibition phenomenon
  Nothing is mutually exclusive – the very nature of a neuron is that all of it wants to communicate and interact with other neurons

Question 17

What is convergence in relation to cells in retina?

Answer 17

Single HC or BC pulls info from >1 photoreceptor cell

Question 18

Describe receptive fields (RFs)?

Answer 18

• Each neuron in retina “covers” area in field of vision. This area in space where prescence of an appropriate stimulus will modify the activity of this neuron is called receptive field of the neuron.
• RF of single photoreceptor cell, for e.g., can be limited to tiny spot of light within your field of vision that corresponds to this photoreceptor’s precise location on retina
  o In each succeeding layer of retina, RFs become ↑more complex
  o Neurons of visual cortex are most complex

Question 19

Describe receptive fields of Bipolar Cells?

Answer 19

• RFs of BCs are circular. Centre and surround work oppositely.
• BCs are distinguished by way they respond to light on the centres of their RFs
  o ON-CENTRE Cells:
   If light stimulus applied to centre of BC’s RF – excitatory effect on that cell, causing it to become depolarised
   Ray of light falls on surround will have opposite effect – inhibiting & hyperpolarising it
  o OFF-CENTRE Cells: opposite to above

Question 20

Describe Receptive Fields of Ganglion Cells?

Answer 20

• Just like BCs, GCs have concentric RFs w/ centre-surround antagonism
• ON-centre GCs & OFF-centre GCs do not respond by depolarisig or hyperpolarising, but rather by ↑ or ↓ frequency with which they discharge action potentials
• GCs – dampening of firing rate when light falls on surround
• Response to stimulation of centre of RF is always inhibited by stimulation of surround
• RFs of GCs are large as lots of information coming in

Question 21

Describe vertical pathway in retina?

Answer 21

• Rods and cones project to next layer
• BCs: project to GCs
• Midget BCs & GCs provide brain with ultimate in spatial resolution, a single cone that detects colour
• 2 types of cone bipolars:
  o ON bipolars: depolarise whenever the cone it synapses with hyperpolarises – turned on by light
  o OFF bipolars: depolarise whenever the cones they synapse with detect decrements of light – excited by darkness and inhibited (turned off) by light
  o Both sets of cone BCs synapse with separate sets of on & off GCs at 2 levels in Inner Plexiform Layer of retina, a more external off-lamina & a more internal on-lamina
  o These parallel channels, transmitting lightness & darkness from local retinal areas, are maintained throughout visual pathway to visual cortex

Question 22

What is the need for retinal processing?

Answer 22

• To adjust “gain” of retina
• Gain has to be reduced if there is too much light – when go to bright sunlight – 1st step is for pupil to constrict
  o Takes 10-15 mins to get fully dark adapted, gain has to ↑ & pupil has to dilate
  o Rods take more time to adjust and are more sensitive in their response
• To code info for efficient transmission to brain
• As 1st step in analysing the info

Question 23

What happens after the outputs of bipolar celss go to inner plexiform layer?

Answer 23

• Primary interest is in messages sent to brain by retinal GCs
• GCs are responsible for the last stage of processing in eye
• GCs do not absorb light but process neural info
• Like other integrator cells lower down the hierarchy they generate action potentials (APs)– brief electrical discharges
  o APs don’t really hyperpolarise or depolarise – they are more interested in firing frequency

Question 24

Describe bipolar cell’s response to light?

Answer 24

• Light shining on centre of BC’s RF & light shining on surround produce opposite changes in cell’s membrane potential. Picture shows ON-centre BC
• If light shone on centre of this cell’s RF, first change is hyperpolarisation of photoreceptor cell, causing depolarisation of BC, because of inhibitory nature of synapse between them. This depolarisation in turn excites following cell, a GC, causing it to emit APs at a higher frequency
  Summation = additive response – neurons are facilitating the object
  If neurons don’t want to see the object, get inhibition
  Spatial summation = gathered in space – all inputs
  Spatial frequency = contrast sensitivity function
  Temporal summation = gathered in time – will be finished in time

Question 25

What is Spatial Summation?

Answer 25

• Recording from retinal GCs
• 130 million receptors
• 1 million retinal GCs
• 1:1 ratio in central part of retina
• Everything else in periphery goes to multiple rods
• GC will respond when 10 plus photon hits are received within its field
• ↑ sensitivity, ↓resolution

Question 26

What is Ricco’s Law?

Answer 26

• Human’s ability to visually detect targets on a uniform background
• Explains the relationship between a target’s angular area A and its contrast C required for detection when the target is unresolved (too small in field of view to make out different parts of it)

Question 27

What is temporal summation?

Answer 27

• Bloch’s Law – ratio of how much input there is to GC and how many GCs are then sending signals to brain
• In camera, to ↑ depth of field in picture (i.e. to make near & far objects in focus), need to reduce size of input aperture & increase the exposure time. Under ~100 milliseconds (1/1000 of a second) stimulus duration, it is possible to exchange amount of light for the duration & maintain a constant effect.
• R = I x T – R is response, I is intensity and T is time
• ↑ sensitivity, ↓ temporal resolution

Question 28

Describe the ganglion cells: X Cells?

Answer 28

o 80%
o Slow
o Sustained response
o Linear summation – looking at detail in centre – tries to respond to little light but then gives up
o Small receptive fields (x3)
o Mainly central retina
o Colour opponency
o They project to the parvocellular layers of the LGN

Question 29

Describe the ganglion cells: Y Cells?

Answer 29

o 10%
o Fast
o Send signal to midbrain – superior colliculus
o Transient response – responds to little bit of light
o Non-linear summation – as they have a burst then go away
o Larger RFs
o Mainly peripheral retina
o Achromatic – not worried about colour
o Detect low contrast, motion and flicker
o They project to the mag o cellular layers of the LGN
o X GC has a sustained response at optimum freq. of light

Question 30

Describe the ganglion cells: W Cells?

Answer 30

o Cells that do not fit into X or Y categories
o V small
o Mostly in LGN
o Not as organised
o Some are ON-OFF
non-concentric RFs
o More common in lower animals but have been found in humans

Question 31

Describe an IP Ganglion Cell?

Answer 31

• Sitting in GC layer
• Melatonin type cells – cells responding to cycles of sleep & waking – won’t respond like rods/cones to light – they are interested in differences of time of appearance of darkness and lightness – circadian rhythm
• Sensitive to non-visual effects of light
• They are involved in circadian rhythms
• These cells signal to brain that there is a difference – if you take melatonin then it can rebalance these cells

Question 32

Describe the response to light from cells in the retina?

Answer 32

• Centre-surround structure of RFs of BCs is transmitted to GCs via synapses located in inner plexiform layer
• Thus, some synapses connect ON-centre BC to ON-centre GCs
• While others connect OFF-centre BCs to OFF-centre GCs
• The accentuation of contrasts by centre-surround RFs of BCs is thereby preserved & passed on to GCs and ultimately the visual cortex
• Human vision depends largely on ability to discern contrasts between objects & backgrounds behind them
  o Establishment of parallel pathways starting in the retina is one of the mechanisms that makes this discrimination possible

Question 33

Describe the distinctions in Ganglion Cells within the retina?

Answer 33

• Image shows everything GCs have to think about
• Parvocellular – detail
• Magnocellular – movements – can see form but not detail
• Fovea: M & P type GCs – both these systems are predominately cone driven
• Periphery: predominately rod driven (some cone input) so these retinal GCs fire in both high and low illumination. Retinal GC RF gets larger further from fovea