Transcript Components of memory - University of Leicester

Topic 2
Methods of Cognitive Neuroscience
PS3002: Brain & Cognition
John Beech
School of Psychology
University of Leicester
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Topic 2. Methods of Cognitive
Neuroscience
Cognitive
Neuroscience
draws on
approaches at
many levels in
trying to explain
function with
respect to structure
• Neurophysiology
• Lesion techniques
• Structural image
methods
• Functional image
methods (activity)
• Cognitive
psychology
• Computer modelling
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Electrical brain mapping
If people have to undergo
surgery for epilepsy, one
procedure it to use
electrical brain mapping in
order to guide surgery. In
Stage 1 the surgeon
makes an opening in the
skull exposing the brain’s
surface onto which
electrodes are placed.
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Electrical brain mapping
• The scalp is then closed and the patient returns to
bed and is closely monitored. This extraoperative
brain mapping is done while the patient is awake and
conscious. Several days later in Stage 2 the surgeon
performs the operation removing the abnormal tissue
using this information from the brain map.
• Another procedure, usually connected with motor
areas takes place intraoperatively – during the
operation – in which locations of the brain are
stimulated to identify areas of movement, language,
sensation or vision.
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Electrical brain mapping
• But the patient needs to be able to
actively participate in this and for this
purpose is woken from anaesthesia and
given enough medication to stop pain.
• The risks involved are small and most
patients find the process interesting –
e.g. their arm can suddenly lift without
effort, or a tickle might be felt, or a
specific visual image evoked, or they
involuntarily start laughing.
• But outside the sensory and motor areas
the picture is not so clear. The temporal
lobes, for example, evoke memories, but
the areas are not too consistent.
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Neurophysiology: Single cell recording
• The fluctuating potential during discharge makes it possible in
principle to record the firing of a single cell. However this is
technically difficult, and impaling the cell may damage it.
• Thus more commonly the activity of a small group of neurons in the
vicinity of the microelectrode tip is recorded - field potentials. The
recording must be made within 100 micrometres* of the neuron.
• Computer algorithms can be used to resolve this pooled activity into
single neurons.
• Baseline activities (rate/frequency of firing) can range from 1 to
100Hz**.
• Neuroscientists look at what alters rate (in either direction)
• Hence, can record during a task, or as the animal moves around the
cage.
(*One micrometre = one millionth of a metre. **Hz = standard measure
of frequency is Hertz and means “cycles per second”, so 1 Hz means
one firing per second and 100 Hz means firing @ 100 times a
second.)
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Neurophysiology: Single cell recording
• Virtually all recordings on humans are done in the
context of operations for epilepsy.
• Can record from visual cortex; firing correlates with
only a subset of visual stimuli having particular
properties of place, shape, colour or movement.
• Retinotopic maps. When an object produces an
image on the retina, it similarly activates higher
retinotopic areas in the brain, keeping the object’s
shape. This image mapping is 'retinotopic mapping‘.
This is a point-for-point copy of the topography of
the retina. But the “brain image” activation doesn’t
“look like” the original object.
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Neurophysiology:
Single cell recording
Similarly in the
somatosensory
cortex there is
a topographical
map. The
figure on the
right shows
how parts of
the body are
mapped in the
somatosensory
cortex. It can
be see that
these body
maps are quite
orderly,
although there
is no obvious
reason why
they exist in
precisely this
way.
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Neurophysiology: Single cell recording
• Tonotopic (i.e. cochleotopic) maps can be found for the auditory
system.This means that the acoustic frequencies in the auditory
cortex are mapped with a similar topology in an associative area
of the brain.
• Often there are responses in multiple brain areas. This raises
the question of which is primary. Is one driving the other?
Accurate timing of the responses may help to establish this. As
an example consider the relationship between the cerebellum
and the motor cortex…
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• Patients with damaged
cerebellums (CBs) have balance
problems like someone who’s
drunk. Alcohol depresses
cerebellum activity.
• The CB first gets information
about intended movement from
the sensory and motor cortexes.
Then the CB returns information
to the motor cortex about
direction, force and duration of
the movement.
Neurophysiology:
Single cell recording
• Cells in the motor cortex and the
cerebellum (CB) are reciprocally,
but indirectly, connected. They are
activated prior to a movement, but
which comes first?
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• In fact (cf Thach, 1975) cells in
the lateral (side of) CB fire before
those in the motor cortex, and
thus are likely to be involved in
movement planning.
• Cells in the intermediate CB
region fire after those in the motor
cortex, and thus are likely to be
involved in ongoing movement.
• This approach can be extended to
simultaneous recordings of many
cell groups - Wilson and
McNaughton recorded
simultaneously from 150 cells in
the rat’s hippocampus.
Neurophysiology:
Single cell recording
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Lesion techniques
• Much information has come from the study of the
effects of lesions. However, caution is needed in
attributing causal explanations.
• For human studies we are limited to studying
naturally occurring lesions (although lesions are also
induced surgically or chemically as part of a
treatment).
• In animals we may also study the effects of induced
lesions. Much of this animal work in the past has
involved physical removal of (usually small) parts of
the brain.
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Lesion techniques
• This can be made more local by using the passage of
current at an electrode tip, producing a very small
sphere of damage.
• However such lesions are non-selective in type.
• More selective lesions can be induced by local
injections of drugs or chemicals such as kainic acid.
• Chemically selective lesions may also result from
self-administration of drugs in humans.
• That is how MPTP, which selectively lesions
dopamine neurons, was discovered - as a
contaminant in synthetic heroin. MPTP mimics the
same neurological damage that afflicts those with
Parkinson’s Disease.
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Lesion techniques
• A student, Barry Kidston, in 1976 mistakenly manufactured
MPTP thinking it was something else and within 3 days of
taking it had severe Parkinsonian symptoms. About 2
years on he committed suicide and postmortem showed
neural damage in the substantia nigra – the same as found
in Parkinsons patients.
• MPTP has since been useful in animal research to
simulate the effects of Parkinson's Disease and hence
search for a cure (e.g. by stem cells).
• “Ecstasy” 3,4-methylenedioxymethamphetamine (MDMA)
also induces chemically selective lesions in serotonergic*
systems in animals, which have behavioural effects.
These lesions appear to be permanent according to much
animal work (e.g. Fischer et al. 1995), but others dispute
this.
* (capable of liberating serotonin, esp. in transmitting nerve impulses)
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Lesion techniques
• Thus “rave culture” is actually a massive natural
human experiment.
• Transient, reversible, functional lesions can be
induced by local injection either of antagonists*, or of
local anaesthetics, which will wear off, or by local
cooling, which can also be reversed.
[ON - OFF - ON states]
*(Antagonist – a chemical substance that interferes with
the physiological action of another, esp by combining
with and blocking its nerve receptor.)
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Natural lesions - the Neurological approach
• As noted before, neurologists have often been
interested in the behavioural effects of particular
lesions, whatever their source.
• There has been an increase in the use of in-depth
psychological testing of neurological patients.
• There is a wide variety of causes of neurological
disorders:
- vascular*(strokes); tumour; degenerative (may be
genetic) and infectious (e.g. viral)
- trauma (wounds)
*(Vascular = relating to mainly blood vessels.)
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Natural lesions: Neurological approach
This interest in neuropsychology springs from three
sources:
1. from the increasing sophistication of psychological
testing;
2. from the rapid development of in vivo* tools for the
accurate localisation and assessment of brain lesions;
3. because these changes enable us to learn from the
study of small groups, or even of individual patients,
rather than requiring large groups, who are often
heterogeneous. One case can be enough to show it
can happen, and the methods show the detail.
*(in vivo = in the living organism.)
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In vivo tools: Structural imaging
Functional neurosurgery: can look at functional
consequences of surgery (often needed to treat
intractable epilepsy) – can involve the removal of a
lobe, or produce a functionally split brain, removal of
the hippocampus, etc.
Principles of CAT or CT (Computerized axial
tomography): Tomography in which computer
analysis of a series of cross-sectional scans. These
are made along a single axis of a bodily structure or
tissue and is used to construct a 3D image of that
structure. The technique is used in diagnostic studies
of internal bodily structures, as in the detection of
tumours or brain aneurysms. It relies on X-rays.
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In vivo tools: Structural imaging
Principles of MRI (Magnetic
resonance imaging)
• This involves the use of a nuclear magnetic
resonance spectrometer to produce
electronic images of specific atoms and
molecular structures in solids, esp. human
cells, tissues and organs. It goes into
magnificent detail. Thus it uses magnetism
and radio waves and doesn’t involve any Xray radiation.
• Scanning also important in the evaluation of
surgery, and in relation to transplants.
• Scanning is painless – apart perhaps from
the claustrophobic sensation that can occur.
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MRI
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How does an MRI scan differ from a CT
scan?
In an MRI one can take pictures from any
angle, but a CT only takes pictures
horizontally. As mentioned, there are no X
rays involved with producing an MRI.
MRIs are usually more detailed as well.
The differences between abnormal and
normal tissues are often clearer in an MRI.
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Imaging magnetic activity of the
brain. Functional imaging
Magnetoencephalography
(MEG) also useful – it
detects magnetic
variations due to currents
in the brain. Limited to
fields parallel to skull
surface, but yields more
information on
localisation than EEG ERP
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PET (positron emission tomography)
depends on localising the source of
emissions from positrons - i.e. inject a
compound labelled with an unstable
isotope*, and monitor the arrival of that
compound in the brain, and its
subsequent decline. PET measures the
changes in blood flow associated with
brain function.
(To explain the 3 terms on the right, imagine
a horizontal section – this is AXIAL, note
the white cross – the vertical bar of the
cross corresponds to the SAGITTAL
plane – a vertical longitudinal plane
separating into left and right halves – and
the CORONAL plane corresponds to the
horizontal bar of the cross separating
“ear-to-ear” into front and back.)
Functional
imaging: PET
*(isotope = radioactive form of an element.)
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Functional imaging of the brain
• This radioactive compound can be a drug which
binds to particular receptors, so we can see the
distribution of these receptors; also can indicate the
displacement from these receptors.
• It can be labelled water, or Xenon (Xe), an inert gas,
or a heavy isotope* of oxygen gas.
• These give measures of local blood flow, which is
useful to know, given that increased local metabolic
activity of the brain is accompanied by increased
local blood flow.
*(isotope = radioactive form of an element.)
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Functional imaging of the brain
Thus besides looking at the difference between
something and nothing (visual fixation +/stimulus) we can compare with more
sophisticated controls (shadowing words vs
creating a word association).
fMRI enables us to make the same sort of
measurements - uses variation in the signal
due to differences in the haemoglobin signal
reflecting increased oxygenation of blood in
metabolically more active areas.
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Functional imaging: fMRI
• A volunteer preparing for an fMRI
study on face recognition
• fMRI unlike PET, doesn’t need
radioactive materials and produces a
higher resolution image.
• This involves a giant magnet around
the head that induces hydrogen atoms
in the brain to give out radio signals.
These increase when the blood
oxygen increases, showing which
areas are most active.
• Hundreds of scans can be done as the
method is not invasive and it yields
very detailed information.
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Functional imaging of the brain
• Can detect decreases, as well as increases, from baseline or
control, in blood flow.
• Although spatial resolution is limited due to speed of sampling
required, can project results onto a detailed structural picture
obtained in the same session.
• There are individual differences in size and shape of the brain,
and of local areas, but this can be compensated by
“normalising” the image to a “standard” brain. This can be done
within the processing that the computer is doing anyway.
• Trade-off of time resolution and spatial resolution.
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Imaging electrical activity of the brain: the
electroencephalograph (EEG)
• The bonnet contains
electrodes that are “hooked”
to the scalp and these sense
impulses from the brain.
• The picture below illustrates
the recorded data.
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Imaging electrical activity of the brain
• Instead of looking at “damaged”
brains, we can correlate activity in the
healthy brain with mental processes.
• EEG - global
– evoked potentials
-- signal averaging
-- sensory processing deficits
• In the cortex, it tells you when not
where.
• Problem of localising the generators
(sources). Can predict surface from
generators, but not vice versa,
especially if you don’t know how
many generators there are.
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ERPs (event-related potentials)
An example of use
The ERP is produced by averaging
signals/traces from the EEG over
say 100 trials, which reflect neural
activity associated with a cognitive
event.
In the example on the right responses
to standard and deviant sounds are
recorded.
P1 effects usually occur about 100 ms
after a stimulus, this is followed
rapidly by an N1 effect.
(The upper part of the signal represents
the negative part, hence “N1”,
whereas the lower area is the
positive part, hence “P1”.)
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Cognitive psychology
• Concerns mental representations and transformation.
- Giving some illustrative examples here.
• Object sensation perception memory action.
• Not simply a linear sequence because memory
influences perception.
• For some time there have been efforts to identify areas of
brain for each type of process.
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The Posner task
• Posner (1978) presented two letters together (e.g. AA) and
people had to say if they were “same” or “different”. RTs
varied according to the type of categorisation. For
example AA vs AB involves simple pattern matching; but
if you are required to respond that Aa has the same letter
sound as AA (vs Ab is different), you have to check that
both letters have the same sound /a/ (as in “cake”).
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The Posner task
• If the stimuli (e.g. Aa) are presented successively (i.e.
A followed by a), with a variable delay, the longer the
delay, the less difference there is between the
physically identical, and phonetically identical, cases.
• This is because processing has been going on
meanwhile.
• Hence by appropriate manipulations we can find out
about categorisation of stimuli.
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The Posner task
• As an illustration of the use of the technique, consider
Barraclough & Beech (1995) used the Posner task to look
at the effects of caffeine comparing these two different
levels of processing (AA vs Aa) within the left and right
hemispheres.
• Under conditions without caffeine we found that patternmatching was faster in the right hemisphere and
phonological processing in the left hemisphere. However,
under the influence of caffeine there was a reversal in this
pattern.
• This is an example of the use of a cognitive task to
examine underlying biological processes.
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Saul Sternberg
Saul Sternberg’s classic
experiments (1966-9): Ss shown
series of numbers, each for 1.2 s.
E.g. given 9 3 5 6 2 and then
they were given probe no. (e.g.
5) and had to say if present in
the set of numbers in their STM.
Each trial gave them a new set
with a different size of 1-6 items.
See fig for results.
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Sternberg
• Sternberg found
that RT increased
with set size.
Processing rate
was 38
msec/item. Also
no difference in
slope for the ‘yes’
or ‘no’ items.
• Strange result as
suggested
scanning is serial
and exhaustive
rather than selfterminating.
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Sternberg
• One would expect that when a “Yes” required that when
found S would immediately make a response. If this had
happened the slope would have been half that for the “N”
responses. For the “N” responses, all items would have to
be scanned.
• But this is not what Sternberg got. He could conclude that
scanning STS is serial and that it is exhaustive.
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Stroop effect (review in McLeod, 1991).
• Task: report the ink colour of words: blue red green
• Delay when those words are actually the names of
other colours (e.g. green). The effect is more
marked for a verbal response (name each word –
“green”), than for a key-press. (Is the colour of the
word congruent/incongruent with the colour it
represents? In this case the “incongruent” key
pressed.)
• Representation of the colour (hue) associated with
reading the word is also activated so that it interferes.
• Stroop effect is not reduced by a concurrent task
judging pitch, but is reduced by a concurrent task
monitoring a word list for targets, i.e. this interferes
with the verbal encoding of the colour name.
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Computer modelling
• A model or theory must be explicit with respect to its
operations, although the outcome may not be known
beforehand.
• Altering connectivity can alter behaviour directly; it
does not necessarily imply that the system has a
representation, or an “understanding” of the situation
• Beware anthropomorphism – saying a machine “is
attracted to the light” or “avoids the light”
• Problem of the homunculus – the conscious being
who sits inside the machine – may return in disguise
as the internal executive controller, or the supervisory
attentional system.
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• Representations - input and
output layers - hidden units.
• Look like neuronal arrays
(although the elements are
much less complicated in
function than neurons)
• Show graceful degradation
• Can examine the effects of
“lesions”, e.g. pruning
(selective or random) of the
network, and compare with
what is seen in human
pathology.
• Testable predictions can be
generated.
Computer modelling
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Computer modelling
Connectionist modellers have had a number of criticisms:
• There is not much testing of one model against another – usually
testing is sufficient only to establish that a model can account for this,
rather than that the brain does actually do it this way.
• Related to this, in neuroscience a major endeavour is to theorise
about how neurons or collections of neurons operate. These theories
are then tested. But it does not appear that these findings are
presently used in any way by connectionists. There is a big gap
between connectionist networks and real neural nets.
• There is sometimes the idea that if a theory can be simulated, then
this means that the theory is consistent. But this does not follow as
the modelling itself may in its deeper areas be acting inconsistently.
• Another problem is that connectionist models can simulate virtually
anything; so they don’t really get us any further about choosing
between alternatives.
• Finally, characteristically the descriptions of these models are too
fuzzy to get a meaningful conception of what is going on.
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Converging methods
• When examining cognitive deficit and brain damage –
the clinician/experimenter analyses how behaviour is
changed, using multiple tasks, rather than loss of
single function (e.g. reading requires the interaction
of many components)
• Many areas interact. Better to use more refined
testing with contrasting conditions, to probe for single
deficits
• Double dissociation important because it eliminates
possibility that one task is harder than the other…
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Converging methods: single and
double dissociations
Tasks (% correct)
Group
Recency memory
Single dissociation
Temporal lobe damage
90
Controls
90
Double dissociation
Temporal lobe
90
Frontal lobe
60
Controls
90
Familiarity
memory
70
95
70
95
95
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Converging methods: single and
double dissociations
• The previous table is a hypothetical example (from
Gazzaniga et al.) in which two types of memory are
distinguishable – recency memory concerned with us
knowing how long ago we learned something and
familiarity memory – concerned with how familiar a
piece of information is.
• In single dissociation the temporal lobe damage group
show problems only in familiarity memory.
• Whereas in double dissociation one patient group is
impaired in one type of memory while the other group is
impaired only in the other type of memory.
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Converging methods
• Merits of group versus individual studies.
• Examining individual cases has a strong tradition in the fields of
learning/conditioning and in neuropsychology.
• Inevitably it is good to have confirmation within a group studied
together, but individuals vary in the precise localisation of their
lesions.
• However, reconstruction software allows determination of
degrees of overlap across a group from imaging data.
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Conclusion
• This was an overview of methods used in cognitive
neuroscience, which as you can see covers most of
the methods used in neuroscience itself.
• The aim is to give you background, so that you will
have some familiarity with these methods.
• These methods, often in varying combinations, are
used to look at specific cognitive functions, in order to
build up a more integrated understanding of cognitive
brain function.
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