Transcript Sensory Coding

Biological Bases of Behaviour.
Lecture 12: Sensory Coding.
Learning Outcomes.
 By the end of this lecture you should be able to:
 1. Describe the processes involved in sensory coding.
 2. Describe the components on the visual system
 3. Explain how coding takes place in the retina and cortex.
Principles of Sensory Coding.
 When any stimulus (e.g. light) reaches a receptor (e.g. a
retinal cell) the following events take place:
 1. Reception: Specialised cells called receptors absorb the
physical energy.
 2. Transduction: Physical energy is converted into
electrochemical energy represented by the firing pattern of
different neurons.
 Each receptor is specialised to absorb and transduce only
one kind of energy i.e. visual receptors in the retina are not
activated by sound.
 The strength of the receptor potential (a bright spot of light
or a dim spot of light) determines how strongly the
receptors are activated.
3. Coding.
 This refers to the one-to-one correspondence between
some aspect of the physical stimulus and some aspect of
neural activity.
 A key aspect of sensory coding is that a given frequency of
impulses in one neuron may mean something different than
the same frequency of impulses in another neuron.
 This is the law of specific nerve energies, (Müller, 1838).
 This means that an action potential always conveys the
same kind of information, i.e. the brain 'sees' the activity of
the optic nerve but 'hears' the activity of the auditory
nerve.
 Flashes of light can be seen when the eye is pressed - the
brain interprets any stimulation to the retinal receptors as
'light' even when there is none present.
Qualifications.
 a) Cells with a spontaneous firing rate may signal one kind
of stimulus by increasing their firing rate, and a different
kind of stimulus by a decrease in their firing rate.
 b) In some cases, information depends upon the timing of
action potentials and not just their total number. E.g in the
retina, neuron A firing just before neuron B may signal
movement.
 c) The exact meaning of an impulse in a single neuron
depends upon which other neurons are active, so the
activity of a visual neuron may contribute to the sensation
of green or yellow depending upon the activity of other
neurons.
Synaesthesia.
 If it were possible to swap the auditory and optic nerves
then we may 'see’ sounds and 'hear' lights.
 A neurological disorder called synaesthesia may actually
indicate that such rewiring is possible.
 Here, a stimulus presented in one sensory modality (sound)
triggers sensations in another sensory modality (vision).
 A synaesthete may describe the colour of someone’s voice;
taste shapes, or describe music in visual terms (BaronCohen et al., (1987).
 Baron-Cohen et al., (1993) argued that this condition may
be caused by connections between colour and hearing
modules that have not died off (in normal development
regions share many interconnections which are then
pruned).
4. Awareness.
 Most stimuli that are received, transduced and coded are
then perceived.
 E.g, when smelling a flower, scent molecules strike
olfactory receptors in the nose (reception).
 This produces a chemical reaction that depolarizes the
resting potentials of the olfactory receptors, they fire
(transduction) and this information is passed via the
olfactory nerve to the olfactory bulb at the base of the brain
(coding).
 The olfactory bulb then sends connections to various parts
of prefrontal cortex where smells are recognised
(awareness).
Example: The Visual System.
 Light enters the eye through an opening in the centre of the
iris called the pupil.
 The light is focused by the cornea and lens and projected
onto the retina - the light sensitive cells that line the rear of
the eyeball.
 Light from the top left of the visual scene strikes the
bottom right of the retina and vice versa so the visual
image on the retina is upside down and reversed.
 The centre of the retina is called the macula and this is the
most sensitive part of the retina used for resolving fine
detail.
 The most precise region of visual analysis takes place
within the macula at the fovea, a small region where
receptors are tightly packed.
Anatomy of the Eye
fovea
iris
macula
pupil
cornea
lens
Kalat (2001) p154
retina
Optic
nerve
Retinal Receptors.
 The photoreceptors lie on the inner surface of the retina
facing away from the light source. There are two types:
 Rods: Highly sensitive to dim light, and found at the
periphery of the retina. They are highly sensitive to
movement but have little colour coding capabilities.
 Cones: Are found mainly in the fovea, are highly sensitive
and used for precise vision. There are three types (red, blue
and green) which are maximally responsive to these
colours. They do not work well in dim light.
 Rods and cones contain chemicals that release energy
when struck by light (photopigments). They consist of a
derivative of vitamin A called 11-cis-retinal, which is stable
in the dark, but is converted to all-trans-retinal by light.
Other Retinal Cells.
 Rods and cones connect to the bipolar cells, which receive
support from the horizontal cells and the amacrine cells. In
turn the bipolar cells induce action potentials in the
ganglion cells, of which there are two types:
 Magnocellular (M) cells: These are large and are found
mainly in the periphery of the retina, and so receive their
input mainly from rods. They are thus sensitive to light and
movement, but not to colour.
 Parvocellular (P) cells: These are smaller, and are found
mainly in the fovea. They receive their input mainly from
cones and so are sensitive to colour and fine detail.
 The axons of both M and P cells form the optic nerve, which
leaves the retina at the optic disc or blind spot where there
are no receptors.
Retinal Cells.
Carlson (2001) p165
The Retina.
Blood vessels
Blind spot
Horizontal cell
Optic
nerve
Amacrine
cell
Ganglion cells
Bipolar cells
Rods and cones
Kalat (2001) p155
Coding in the Retina.
 The human retina contains around 120 million rods and 6
million cones but we do not individually process 126 million
bits of information.
 Each ganglion cell has a receptive field whose size and
sensitivity depends upon how many rods or cones converge
upon it.
 In the macula only a few cones converge upon each
ganglion cell so visual acuity is enhanced.
 In the periphery, many rods converge upon each ganglion
cell so sensitivity is reduced.
 The receptive fields of the ganglion cells converge to form
the receptive fields at the next neural level and so on.
The Ganglion Cells.
 In the 1930's Hartline discovered that the retina contains 3
types of ganglion cells:
 On cells: these respond when a light strikes the retina.
 Off cells: these respond when the light is removed.
 On/off cells: these respond briefly when the light is on and
also again briefly when the light is switched off.
 In the 1950's Kuffler recorded the activity of retinal
ganglion cells and discovered that their receptive fields are
in the form of a central region surrounded by a concentric
circle.
 Stimulation of the centre or the surround had different
effects depending upon the type of the cell:
Activity of Ganglion Cells.
 On-centre: Light falling on the centre of the receptive field
stimulates the cell, but light falling in the surround inhibits
the cell.
 Off-centre: Light falling on the surround stimulates the cell
but light falling on the centre inhibits the cell.
 At rest all ganglion cells fire spontaneously at a low rate,
but when light falls across their receptive fields 'on cells'
signal an increase in illumination, 'off cells' signal a
decrease.
 Receptive fields overlap which ensures that a small spot of
light will excite or inhibit many ganglion cells - this is how
we determine shapes.
 It also ensures that the visual system is primed to perceive
edges - even where none actually exist.
On/Off Centre Cells.
Carlson (2001) p171
On/Off Cells in Action.
Look at the centre
of the grid - can
you see black
blobs at the
intersections?
Kalat (2001) p167
Perception of Edges.
The ganglion cells in the retina enhance
the contrast between edges, the squares
appear to be very different in tone but
actually are not
Carlson (2001) p171
Visual Pathways.
 The optic nerves from both eyes meet at the optic chiasm.
 Here fibres from both visual fields in each eye cross over to
be represented in opposite hemispheres.
 All the P ganglion cell axons and some of the M ganglion
cell axons project to the lateral geniculate nucleus (LGN) a
part of the thalamus specialised for visual perception.
 The LGN then sends projections to the visual (striate)
cortex.
 This pathway is called the geniculostriate system.
 Remaining M ganglion cell axons connect to the superior
colliculus (of the tectum) then to part of the thalamus
called the pulvinar, and then on to visual regions in
temporal and parietal cortices.
 This pathway is called the tectopulvinar system.
The Visual Pathways
Striate cortex
Superior
colliculus
Optic chiasm
Retina
Lateral
geniculate
nucleus
Optic nerve
Kalat (2001) p165
Visual Cortex.
 Most information from the
LGN goes first to primary
visual cortex (V1), which
is responsible for the basic
elements of processing.
 This region then sends
information to:
 Secondary visual cortex
(V2).
 Visual association areas
(V3-V5).
 Temporal
and
parietal
cortices.
V3
V1
V2
V4
V5
Kolb & Whishaw (2001) p293
Coding in Visual Cortex.
 Individual cells in V1 receive input from many ganglion
cells and so their receptive fields are large, they do not thus
respond just to 'light on' or 'light off', but respond to bars
of light oriented in a particular direction - they are thus
orientation or feature detectors.
 In the 1950's Hubel and Wiesel discovered three types of
cell:
 Simple cells: these responded to the presence of a bar of
light at a particular orientation and position.
 Complex cells: these responded to bars of particular
orientations moving across the retina.
 Hypercomplex cells: these also responded to moving bars
but also had a strong inhibitory region at their end.
Coding in Visual Cortex Continued.
 The 3 types of cell have
different tunings, some
respond maximally to a thin
bar of light, others to a
thicker bar etc.
 The cells are organised into
columns within which
neurons respond to similar
features.
 Neurons in adjacent columns
respond to slightly different
features.
 Information from each eye is
split between adjacent
columns referred to as ocular
dominance columns.
Carlson (2001) p171
Simple cell
Complex cell
Hypercomplex cell
Coding in Other Cortical Regions.
 Cells in striate cortex only provide basic visual analysis (shape, orientation), perception (awareness) take place in
additional regions specialised for different processing.
 Mishkin et al., (1983) described two distinct visual
pathways which emerge from primary visual cortex:
 Dorsal stream (‘where’ pathway): This travels to the
parietal lobes and is important for perceiving where an
object is located. Damage to it impairs the ability to locate
objects even though they are perceived.
 Ventral stream (‘What pathway’): This travels to the
temporal lobes and is important for perceiving what an
object is. Damage to it impairs visual object recognition
(agnosia); e.g. a patient may be able to describe an object
but not to be able to recognise what it is.
‘What’ and ‘Where’ Pathways.
Parietal lobe
Visual
cortex
Temporal
lobe
Kolb & Whishaw (2001) p290
References and Bibliography
 Baron-Cohen, S., Wyke, M., & Binnie, C. (1987). Hearing words
and seeing colours: an experimental investigation of a case of
synaesthesia. Perception, 16: 761 - 767.
 Carlson, N.R. (2001). Physiology of Behaviour.
 Baron-Cohen, S., Harrison, J., Goldstein, L.H., & Wyke, M. (1993).
Coloured speech perception: is synaesthesia what happens when
modularity breaks down? Perception, 22: 419 - 426.
 Kalat, J.W. (1995). Biological Psychology.
 Kolb, B., & Whishaw, I.Q. (2001). Fundamentals of Human
Neuropsychology.
 Mishkin, M., Ungerleider, L.G., & Macko, K.A. (1983). Object vision
and spatial vision: two cortical pathways. Trends in
Neurosciences, 6: 414-417.