rod and cones, bipolar cells, horizontal cells, amacrine cells, ganglion cells, spatial summation and resolution, retinal sampling, spatial tunning, receptive fields of retinal neural cells, retinal potentials, Ricco's law, spatial antagonism, mach band, fourier analysing of visual system

1. Physiologic
Model of
Spatial Vision
and Retinal
Sampling
Dr Gauri Sr Shrestha
MMC, IOM, TU

2. Course
overview

3. Learning outcomes
• Understand basic organisation of retinal neural cells and
their processing.
• Understand the spatial summation and resolution
• Understand spatial frequency tunning and neural channels
at the retinal level
• Understand the receptive field characteristics of the neurons
that represents retinal processing of visual information
(photoreceptors, horizontal cells, bipolar cells, amacrine
cells and ganglion cells)

4. Organisatio
n of neural
cells in the
retina

5. Retinal photoreceptor
cells
Rod spherule Cone pedicle
Ellipsoid
Myoid
Outer rod
fibres
Inner rod
fibres
Ellipsoid
Myoid
Inner cone
fibres

6. Rod cells
• 40 – 60 µm long
• Rod outer segment
• is cylindrical, highly refractile and contains visual pigments (visual purples)
• Consists of lipid protein lamellar discs stacked one over the other (600 – 1000
discs/ rod) and are surrounded by cell membrane.
• Rod inner segment
• Is thicker than the outer segment and has two regions.
• Ellipsoid (the outer portion): contains abundant number of mitochondria.
• Myoid (the inner portion): contains the glycogen and usual organelles.

7. Rod cells
• Peak density of rods occurs 20o
from fovea: 150,000 rods/mm2
• No rods are present at the fovea (0.35 mm, 1.25o
of VF
• Total Number of rods in the retina are 120 million
• Visual purple: Rhodopsin
• Rhodopsin most readily absorbs wavelengths of 507 nm.
• One molecule of rhodopsin absorbs one quanta of light, it is
‘bleached’
• Bleached: the molecule is not capable of capturing another
quantum
• Spontaneously become ‘unbleached’
• 50% recover within 5 minutes

8. Cone cells
• 40 – 80 µm long (longest at the fovea and shortest at the periphery)
• Cone outer segment
• is conical in shape and contains iodopsin
• Consists of lipid protein lamellar discs stacked one over the other (1000 –
1200 discs/ cone) and are surrounded by cell membrane on the one side only.
• Cone inner segment
• Is similar to the rods and has two regions.
• Ellipsoid (the outer portion): is very lumpy and contains abundant number of
mitochondria.
• However, outer fibre is absent and has a stout inner fibre.

9. Cone cells
• Cones are most densely packed at the fovea: 150,000 cones/mm2
• Only 4% of total cones are foveal
• Total Number of cones in the retina are 6 million
• 3 types of cone photopigments:
• Erythrolabe: maximum absorption at 565 nm (L-cones, red-light sensitive cones)
• Chlorolabe: maximum absorption at 535 nm (M-cones, green-light sensitive
cones)
• Cyanolabe: maximum absorption at 430 nm (S-cones, blue-light sensitive cones)
• Recover from bleaching more rapidly than rhodopsin
• 50% of cones will recover within 1.5 minutes

10. Distributio
n of rods
and cones
in the
retina

11. Recording of electrical activity in the
retina
• The retina analyses the optical images fell on it and encodes
them into a complex neural signal to transmit the signal to
higher visual centres.
• Isolated electrical activity is recorded through
• Extracellular recording (in-vitro) – for both action potential and
graded (slow potential)
• Intracellular recording (in-vivo) – for graded (slow potential)
Receptive field: area that influences the neural activity of the cell.
Excitation as an increase in the frequency of action potentials (indicated by plus sign)
Inhibition as a decrease in the frequency of action potential (indicated by minus sign)

12. Graded potential versus Action
potential
Features Graded Action
Type of signal • Input signal
• Depolarising or hyperpolarising
• Output signal
• depolarising
Travel Conveys over small distance
(membrane potential decreases
with distance from the stimulation)
Transfer over long distance
(propagate along entire membrane
surface without decreasing its
strength
Strength of signal Variable, depends on the
magnitude of stimulus
Always same (all or none)
Threshold No threshold is required to initiate Stimulus reaches threshold level to
generate AP
Site Dendrites and cell body Axon

13. Receptive field of retinal
nerve cells
• The various retinal cells show different
receptive field properties (refer to the
picture on the right)
• Photoreceptor and horizontal cells have diffused
receptive field
• Bipolar, amacrine and ganglion cells have centre-
surround organisation in their receptive field.
Receptive
field
Cell
type
Receptive field Stimulus
Photoreceptor diffused light
Horizontal cell a spot of light
Bipolar cells A linear light (a bar of light)
Ganglion cell spatial gratings (alternate light and dark

14. Receptive field of retinal nerve
cells
• In centre surround
organisation, the electric
potential generated is
either excitatory or
inhibitory.
• Light falling on the
receptive field centre has
the opposite effect of light
falling on the surrounding
area of the receptive field.

15. Ganglion
cell
receptive
field

16. Spatial resolution and spatial
summation
• Dark band of stimulus
as well as light band of
stimulus should
stimulate ganglion
cells separately.
• Spatial summation
describes the eye’s
ability to summate
quanta over a certain
area (critical diameter).

17. Ricco’s law
• For small stimuli (below a critical
diameter), the total luminous energy
(intensity × area) must reach a
constant threshold for detection (k).
• A dim but large stimulus can be
detected just as easily as a small
but bright one
• In parafoveal area (4 – 7°), critical
diameter = 30 mins of arc
• At an eccentricity of 30°, the critical
dimeter = 2° of arc
L.An
= k
n = 1, total spatial summation
n = 0, no spatial summation

18. Spatial tuning
• Spatial tuning refers to the ability of
neurons in the visual system to respond
selectively to stimuli of specific sizes,
shapes, or locations within the visual
field
• When a grating is presented to the
ganglion cells, the spatial tunning will be
maximum when
• The dark area fall on the inhibitory surround
• The light areas fall on the excitatory centre
• The response will be very strong.
a more optimal stimulus
for the ganglion cell

19. Spatial tunning

20. Spatial
tunning

21. Spatial
antagonism
• Bipolar cell is the first cell
to show spatial antagonism
to a bar of light
• A spatial grating is a strong
stimulus for a ganglion cell.
• Light bar falls on excitatory
centre and
• Dark bar falls on the
inhibitory surround
a more optimal stimulus
for the ganglion cell
the large bright bar of the low-frequency
grating falls both on the receptive field’s
centre and surround causing lateral inhibition

22. Mach Bands
• Refers to an optical phenomenon
(sometimes called optical illusion) from
EDGE ENHANCEMENT (a relative
enhancement of high spatial
frequencies) at the borders of gradual
physical transition.
• The edges of dark objects next to light
objects appear darker and vice versa at
the junction of a gradual change of
bright to dark regions.
• Gradual transition consists of low
frequencies
• We have a low sensitivity to these
frequencies (lateral inhibition)

23. The Visual System as
a Fourier Analyzer
• The contrast sensitivity function
of our visual system consists of
several independent narrower
spatial frequency channels.
• breaks the visual world into
spatial frequency components
• consists several narrower channel
(no single channel exists)
• again, reassembles components
and forms our spatial perception

24. The Visual
System as a
Fourier Analyzer
• e.g., A subject is
adapted to a
sinusoidal grating (6
cycle/ degree) for
about a minute prior
to determination of
the CSF.
• There is a discrete
reduction in
sensitivity at the
adapted frequency (6
cycle/ degree)

25. The Visual System
as a Fourier
Analyzer
• Now the subject is adapted
to a square wave grating
of 6 cycle/ degree.
• Our visual system breaks
the Complex stimulus
(square-wave) into its
components (fundamental
and harmonic frequencies)
• There would be decrease in
sensitivity at those
frequencies.
Blakemore and Campbell (1969)
6 cycle/degree
18 cycle/degree

26. Photoreceptors
• Photoreceptors convert light energy into
electrical activity.
• Photoreceptors are in the state of
depolarisation (-50mV) in the dark relative
to other neuron cells.
• When exposed to light, they hyperpolarise
(-70mV).
• Potential produced by photoreceptors are
graded.
• 10% rhodopsin bleaching closes Na+
channel, no rhodopsin further
hyperpolarises (rod saturation)
Dark
current

27. From light reception to
receptor potential

28. Horizontal cells (HCs)
• Several photoreceptors (rods and
cones) distributed over a large area
of the retina synapse with the widely
dispersed dendrites of a single
horizontal cell.
• This interconnection contributes to
the spatial summation.
• HCs shows sign-conserving synapses
(Invaginating synapse) with
photoreceptors.

29. Horizontal cells (HCs)
• HCs hyperpolarise in
response to the light.
• HCs produce graded
potential (no action
potential).
• Process luminosity.
• Two types of horizontal cells
• H1: primary input from the M-
and L-cones, and little input
from S-cones
• H2: Strong input from S-cones
and also receives input from
M- and L-cones.

30. Spatial summation and lateral
inhibition facilitated by HCs
A. HCs collect information from
photoreceptors in the receptive
field surround (or centre) and
feedback on to photoreceptors
in the receptive field centre (or
surround) to generate the
antagonistic receptive field
surround (or centre) of the
bipolar cells
B. The reciprocal synapse between
cones and horizontal cells
mediates negative feedback

31. Bipolar (BP) cells
• Bipolar cells are the FIRST retinal cells
to mediate spatial antagonism
• Bipolar cells mediates graded potential
(no action potential)
• S-cones receive input from a distinct
class of bipolar cells (S-cone bipolar
cells)
• Dendrites of BP cells synapse with
photoreceptors and horizontal cells
• Axons of BP cells synapse with
ganglion and amacrine cells
• Neurotransmitter released by BP cells -
glutamate
Sign
conserving
(invaginating)
synapse
Conventional
flat synapse

32. Functional
interaction of
retinal
neurons (please
refer to the
Bipolar cells)
A: Circuitry responsible for generating
center responses of retinal ganglion cells.
B: Circuitry responsible for generating the
receptive field surrounds of retinal
ganglion cells.
A B

33. On- and Off-centre bipolar cells
Features On-centre Off-centre
Synapses with
Photoreceptors
Sign conserving Flat
Light on centre of the
receptive field
Excitation (depolarisation) Inhibition
(hyperpolarisation)
Light on surround of the
receptive field
Inhibition (hyperpolarisation) Excitation (depolarisation)
Synapses Inner sublayer of IPL Outer sublayer of IPL
Glutamate released by
photoreceptors
Inhibitory (reduction in
glutamate released by
photoreceptors causes
depolarisation (excitation)
Excitatory (reduction in
glutamate released by
photoreceptors causes
hyperpolarisation

34. Midget and diffuse bipolar cells
Features Midget Diffuse
Morphology Less tissue volume and less
dendritic branches
More tissue volume and more
dendritic branches
Centre and
midperiphery of the
retina
The receptive field centre receives
input from single M- or L-cones
(high VA, colour opponent)
The receptive field receives input
from 5 – 10 cones (more than
one cone type (M or L).
Periphery of the retina The receptive field centre receives
input from several photoreceptors
(Low VA).
Receptive field
surround
Receives input from single H1 HCs Receives input from several H1
HCs
Colour opponency Present for both on- and off-centre Non-colour opponent

35. Organisati
on of the
primate
retina

36. Amacrine cells
• Location: between INL and GCL of the retina
• Presynaptic connection: Bipolar cells
• Post-synaptic connection: Ganglion cells
• Shows centre surround organisation
• Is the FIRST cell to display action potential.
• Shows time-related characteristics of neural
response
• Responds briefly and transiently at the
stimulus onset and off-set (helps ganglion
cells to detect change in light intensity)

37. Amacrine cells
• Functions of Amacrine cells:
• Plays a role in coding movement
• Bridge the connection between rods and cones in scotopic vision for high sensitivity
• Adjust light sensitivity for photopic and scotopic vision
• Provides synaptic feedback to BP cells
• Types by response to the light
1. Sustained amacrine cells
2. Transient amacrine cells
• Types by cell types:
1. Sublaminar cell A: contains bipolar axons and off-centre ganglion cell connections
(hyperpolarising response to light)
2. Sublaminar cell B: contains bipolar axons and on-centre ganglion cell connections
(depolarising response to light)

38. Ganglion cells (GCs)
• GCs are the largest retinal neurons
• Conveys information from other retinal neurons to the dorsal lateral
geniculate nucleus (dLGN) in the brain.
• Axons of the ganglion cells become myelinated at the optic disc.
• Generates action potential
• Despite no stimulus, ganglion cells have spontaneous generation of action
potential (maintained discharge – background action potential)
• Types
• Midget ganglion cells (retinal parvo cells): both on- and off-centre; receives synapse
from midget bipolar cells (parvocellular pathway in dLGN)
• Parasol ganglion cells (retinal magno cells): both on-and off-centre; receives synapse
from diffuse bipolar cells (magnocellular pathway in dLGN)
• Small bistratified cells: both on and off-centre; receives synapse from s-cone bipolar
cells (Konio pathway in the dLGN). Has large receptive field

39. Midget and Parasol ganglion cells
Features Midget Parasol
Receptive field Small Large
Dendritic trees Small volume Large volume
Centre and
midperiphery of the
retina
The receptive field centre receives
input from single midget bipolar cells
(low spatial summation)
The receptive field receives input
from one or more diffuse bipolar
cells.
Periphery of the
retina
The receptive field centre receives
input from several bipolar cells (high
spatial summation).
The receptive field receives input
from more several diffuse bipolar
cells.
Colour opponency Present for both on- and off-centre Non-colour opponent
Rod input Low Extremely high
VA and colour vision Excellent Poor
Response to light Sustained (input from sustained Transient (input from transient

40. Receptive field organisation of the
ganglion cells
• The receptive field
of the ganglion
cells overlap.
• A small spot of light
can excite or inhibit
many ganglion cells

41. Other types of ganglion cells
• Ganglion cells projecting to the superior colliculus
(midbrain): controls eye movements
• Melanopsin containing ganglion cells: projects to the
suprachiasmatic nucleus of the hypothalamus and is
responsible for the circadian rhythm.
• Intrinsically photosensitive (retino-hypothalamic track)
• Ganglion cells projecting to pulvinar region: visual attention
and motion processing.

42. Linearity of ganglion
cell receptive field
• Ganglion cells act like shift-
invariant linear systems.
• The input is stimulus
contrast. The output is firing
rate.
• Add light intensities at each
point weighted by positive or
negative centre and surround
subregions of the receptive
field, their response changes
to the stimulus linearly.

43. Summary
• The retinal neural cells transfer signals vertically from photoreceptors – bipolar
cell – ganglion cells.
• The lateral communication between retinal neural cells is mediated by horizontal
and amacrine cells.
• The spatial summation takes place within the critical diameter of the retina and
also the area of summation varies with the retinal location.
• Various retinal cell exhibits various types of receptive fields. The first cells to
exhibit lateral inhibition is bipolar cells and to generate action potential is the
amacrine cells.
• Our visual system shows several independent narrower spatial frequency
channels.
• Neural convergence is mandatory (Photoreceptors = 126 million, ganglion cells =
1 million)

44. Thank you
• Further reading
• neurotransmitters in the retina
(https://www.ncbi.nlm.nih.gov/books/NBK11546/).