Transcript Powerpoint slides

Vision in
vertebrates
How do we see ?
Aims
 To describe how sensory systems fulfil their
need to
 transduce
energy
 compare inputs
 lead to appropriate behaviour
 using the vertebrate visual system
Main parts of visual
system
 eye
 lens,
retina
 brain - primates
 lateral
geniculate nucleus
 visual cortex
 brain - other vertebrates
 superior
colliculus / optic tectum
Warning
 Not all vertebrates work the same way
 differences
in anatomy
 differences in retinal & CNS function
 even
between closely related species, e.g. frog
and toad
 always
look and see what species the work was
done on!
Visual systems
 frog - optic tectum
 man
Lens
 light is a wave 3 * 108 m/s
 focus light onto the retina
 maps places in outside world to places on
retina in 1:1 fashion
Retina
 structure
 photoreceptors - at back of retina
 layers of cells
 output from ganglion cells - at front of retina
 Blindspot
Structure of retina
 retina at back of eye
ganglion
cells
photoreceptors
Structure of retina
 Electron - micrograph
Structure of retina
 Diagram
light
Photoreceptors
 Rods and Cones
 Transduction
 Can we perceive photons?
 Colour vision
Rods and Cones
 rods 100 * more
sensitive
 most cones at fovea
 rod density highest
around fovea
 Therefore, turn eye to
see in different places
Photoreceptor
 rhodopsin in
membrane
discs inside
outer segment
 membrane
voltage
determined by
cell membrane
light
Transduction
 in the dark, channels are open
Transduction
 Transduction movie
Transduction
 Sensitivity increased by
 gain
in enzymes
 gain in channel
 gain at synapse
 vesicle
ribbon increases number of vesicles
released
 this reduces quantal noise at synapse
Physiological recording
 suction pipette records inward current in outer
segment
bright
dim
Can we perceive
photons?
 Macaque rods able to detect individual
photons
 people can see light flashes when 1 in 100
rods will get a photon with 0.2s
Colour vision
 most common form is red-green deficiency
Colour vision
 Diurnal animals &
birds have colour
vision
 Humans and OldWorld monkeys are
tri-chromatic
 most
monkeys
dichromatic
 May have evolved to
detect when fruit is
ripe
Colour vision
 Diurnal animals/birds have colour vision
 Humans and Old-World monkeys are
trichromatic
 most
monkeys dichromatic
 May have evolved to detect when fruit is ripe
 other mechanisms of color vision exist
 oil
droplets in amphibians, turtles
 gene homology
Colour vision
 gene duplication on X chromosome
Summary so far
 At retina,
 world
is spatially mapped
 light level is encoded by current
 color is used (but not in all animals)
 very sensitive
Retina is Layered
 Diagram
light
light
Physiology
 Dowling - Necturus
(mudpuppy)
light
Physiology
 receptor is inhibited
by light
 Sign conserving
/reversing synapses
 horizontal cells
mediate lateral
inhibition
light
Physiology
 ganglion cells signal
to brain
 difference in light
between adjacent
receptors
 amacrine cells signal
on or off
 Not light level
On-Off
responses
 ganglion cells
usually respond to
changes in light
 results from lateral
inhibition
Lateral inhibition
 Hermann Grid
 common to
vision, touch,
hearing...
Summary so far
 At retina,
 world
is spatially mapped
 light level is encoded by current
 color is used (but not in all animals)
 very sensitive
 At ganglion cells
 on/off
& surround /center
 not a 1:1 relation between light level and signal
 this enhances dynamic range
Blindspot
 axons of ganglion cells run over surface and
turn to give optic nerve
Ganglion cells project
to LGN
 Lateral geniculate
nucleus
Mapping of cells
 visual field
mapped spatially
 different ganglion
cells project to
different layers
LGN sensitive to lines
 ganglion cells
respond to
spots
 LGN to lines
 different line
orientations for
each LGN cell
LGN projects to Visual
cortex
 visual = striate
cortex = V1
Orientation selectivity
in cortex
 LGN orientation is maintained, with a
pinwheel pattern
Ocular dominance
 LGN kept data from
the eyes separate
 in visual cortex,
data converges.
 Some cells have
dominant input from R, some from L
 cells in same column have same dominance
V1 Cortex
 orientation and
ocular dominance
work together
 grey
is
contralateral eye
Depth perception
 Use both eyes to calculate how far away
objects are
 Hypothesis 1: rangefinder
 Hypothesis 2: measure overlap of images
Disparity...
 1) rotate eyes
 2) compare
signals from
different parts
of the retina
 2 wins out
Depth perception
 Muller - Lyer
 Ponzo
which lines are longer ?
Hering
Blindsight
 loss of visual cortex may show evidence for
blindsight
 patient cannot “see” but can follow targets
with their eyes
 patient can discriminate words
 projection to superior colliculus may be
responsible
Deconstruction of
signal
 Visual system does not froward a direct
“photographic copy”
Reconstruction ?
 Kanizsa illusion
 temporal lobe
Summary
 transduction well understood; high gain
system
 retina well understood; lateral inhibitory
mechanism
 LGN and V1 cortex fairly well understood;
lateral & temporal inhibition; binocular vision
 further processing still to be elucidated