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

photoreceptors

ganglion cells

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 segmentbright

dim

Can we perceive photons?

people can see light flashes when 1 in 100 rods will get a photon with 0.2s

Macaque rods able to detect individual photons

Colour vision

most common form is red-green deficiency

Colour vision

Diurnal animals & birds have colour vision

Humans and Old-World 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

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

light

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