Photoreceptors Contain pigment disks In the dark, receptors are
depolarized and glutamate is released constantly. This inhibits
some target cells and activates others During illumination,
pigments deteriorate and receptors hyperpolarize Thus, glutamate
flow decreases, and activation/inhibition of target cells reverses
Rods Cones 20 : 1
Slide 10
Photoreceptors
Slide 11
Rods Grayscale Long integration time ( ~ 12 Hz) Detect 2%
contrast change React to 1 photon Outside fovea Converge on
bipolars Dim light Cones Colour Short integration time ( up to
55Hz) Detect 10% contrast change Require several photons Mainly in
fovea Often 1 cone to 1 bipolar Precise shape and colour
The bipolar layer Transforms and compresses input from
photoreceptors to retinal ganglion cells Two layers of lateral
interaction: Bipolar cells receive direct excitatory input from 1
cone or several rods and contact retinal ganglion cells Horizontal
cells are activated by a large number of photoreceptors and inhibit
bipolar cells Amacrine cells are activated by many bipolar cells
and inhibit retinal ganglion cells/bipolars These lateral
interactions produce e.g. fast gain control
Slide 14
Lateral inhibition 0.1 5 mm!
Slide 15
Mexican hat function Inhibition slower than excitation because
of transduction through interneurons! How much slower depends on
depth in the bipolar cell layer!
Slide 16
The bipolar layer The bipolar layer has at least 10 sub-layers,
10 types of horizontal cells and over 30 types of amacrine cells
Each cell type has different spatial extents (0.1 to several mm)
and different temporal properties (transient to sustained) Deeper
layers are usually faster As a result, bipolar cells have a graded,
linear response with On/Off center and selectivity for stimuli of
different spatial/temporal extent
Slide 17
Photoreceptor layer Bipolar cell layer Ganglion cell layer ~1
mm Phototransduction Center-surround interactions, filtering for
different spatial and temporal frequencies, gain control The
emergence of the spike
Slide 18
Retinal ganglion cells
Slide 19
Retinal ganglion cells Magno- and parvocellular pathway Magno
Large receptive fields Transient responses Grayscale Stimulus
movement! Parvo Small receptive fields More sustained responses
Colour selectivity
Slide 20
The network of retinal ganglion cells- Synchrony Slow scale (40
100 ms): Shared input from photoreceptors Medium scale (2 40 ms):
gap junctions from amacrine cells Fast scale (< 1 ms): gap
junctions between ganglion cells Meister & Berry (1999)
Slide 21
Summary II The retina is one of the most successful circuits in
evolution is one of the best-understood examples of a neuronal
network contains a large number of different cell types tuned to
different spatial and temporal frequencies, including
On/Off/On-Off, magnocellular and parvocellular as well as direction
selective ganglion cells Contributes crucially to gain control and
motion processing favours interdependence and synchrony of
individual discharges