Transcript bcs508_lecture_week1

Goals of the class
Get a broad overview of various aspects of cognition and
perception, and how they can be studied in the brain
• What insights do brain images give us, beyond behaviour?
Read articles, present them
• Practice presenting ideas, explaining them clearly,
discussing them with each other
Bounce ideas around, maybe even come up with ideas for
new experiments to run
Get given a grade
• This is grad school. Grades not very important any more
Today's class, and discussion
• What is Cognitive Neuroscience, anyway?
• Can brain imaging tell us anything about the mind,
or does it just give us pretty pictures?
• Aspects of CogNeuro to maybe be skeptical about
• Aspects of CogNeuro which look potentially
promising
• Some tips on how to find open research
questions, and how not to feel swamped by the
literature
What is Cognitive Neuroscience?
Note: there does not exist a single, universally
agreed-upon answer to this question
In fact, that’s true for almost all Qs related to the
brain
• Which is why it’s good to ask simple Qs, express
uncertainty, be ok with not knowing the answers
• Probably nobody else knows the answer either!
• But probably there are some interesting ideas out
there about how to look for an answer
What's a mechanism?
Eye movement demo
What's a mechanism? (continued)
From outside the head, we can figure out
something about what's going on inside the
head
So, if we look inside the head, we must have
a royal road to finding mechanisms, right?
Shermer: Why you should be
skeptical of brain scans
Shermer, M. (2008). Why You Should Be Skeptical of Brain Scans.
Scientific American Mind, 19(5), 66-71.
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Unnatural environment for cognition
Indirect measure of brain activity
Colours exaggerate the effects in the brain
Brain images are statistical compilations
Brain areas activate for various reasons
Networks, not modules
Problems with pretty pictures
Caricature of a bad brain-imaging study:
• Put people in a scanner, get them to do some task X
• See which part of the brain lights up the most
• Declare this is “the part of the brain responsible for X”
Modules
What does it mean for something to be
“modular” ?
Biological modules: organs in the body
• Liver, kidneys, heart etc.
More distributed
• Blood circulatory system, immune system
Is the brain modular?
• Fodor (1983): “Modularity of Mind”
• Functional modules vs. physically separated
Most brain areas have multiple functions.
Each mental function is supported by multiple areas
Saying
• “Which is the part of the brain responsible for X?”
• “Aha, it's this bit!”
…is a bit like saying
• “Where in the USA is the economy?”
• “Aha, it's in Wall Street”
Need to look at broad patterns of neural activation,
distributed across multiple brain areas
MRI
Magnetic Resonance Imaging
• Takes a 3D picture of the inside of
body, completely non-invasively
• One picture, just shows the structure
http://www.coppit.org/brain/
fMRI
functional Magnetic Resonance Imaging
• Shows brain activity (indirectly)
• Takes a series of pictures over time,
e.g. one every three seconds
• The “f” in fMRI means functional,
i.e. you get a movie of brain
function, not a still image of brain
structure
http://www.fmrib.ox.ac.uk/image_gallery/av/
What are we actually measuring with fMRI?
• An MRI machine is just a big magnet (30,000 times
stronger than Earth's magnetic field)
• The only things it can measure are changes in the
magnetic properties of things inside the magnet: in this
case, your head
• When neurons are active, they make electrical activity,
which in turns creates tiny magnetic fields
• BUT far too small for MRI to measure (100 million
times smaller than Earth's magnetic field)
• So, how can we measure neural activity with MRI?
What makes fMRI possible:
Don't measure neurons, measure blood
Two lucky facts make fMRI possible
• When neurons in a brain area become active,
extra oxygen-containing blood gets pumped to
that area. Active cells need oxygen.
• Oxygenated blood has different magnetic
properties than de-oxygenated blood.
Oxygenated blood gives a bigger MRI signal
End result: neurons fire => MRI signal goes up
This fMRI method is known as BOLD imaging:
Blood-Oxygenation Level Dependent imaging.
Invented in 1992.
But neurons do the real work, not blood.
Neurons represent and process information
Individual nerve cells (neurons) represent information
• Sensitive to “preferred stimuli”, e.g. /ba/
• These stimuli make them active
• Firing activity: send electrical spikes to other neurons
/ba/
/ba/-sensitive neuron
Populations of neurons
process information together
Information is distributed across
large populations of neurons, and
across brain areas
There's no “grandmother cell”:
the one single cell that
recognizes your grandmother
To really understand the brain,
we'd need somehow to read the
information from millions of
individual neurons at once!
The basic design of an fMRI experiment
Aim:
• Find which brain areas are active during a given task
• E.g. discriminating speech sounds, producing speech
Typical design:
• Present blocks, e.g. 30s of task, 30s of rest
• Measure fMRI activity regularly every few seconds
• Look for brain areas which are more active during the
task periods, compared to rest periods
Example time-courses
Time-course of task versus rest periods
Task
Rest
Task
Rest
Rest
MRI signal from voxel that correlates well with task: Active
Signal from voxel that does NOT correlate with task: Inactive
TIME
What are those little coloured blobs, actually?
Colour represents
statistical significance of
how well the voxel's
activation correlates with
the task.
The hi-res grayscale
anatomical picture
underneath the coloured
blobs is a completely
different type of image,
from a different type of
scan. Shows the anatomy
at the spot where the
significant voxel's timecourse was recorded.
The key problem
Interpreting what brain activation means
Reverse inference
Why it's hard to infer processing from activation:
Brain areas are multi-functional
???
Attention
Intention
Spatial reasoning
Numerical magnitude
Parietal cortex
The false “logic” of reverse inference
If cognitive process P is happening, then
brain area A becomes active
Aha, look, brain area A is active!
Therefore, cognitive process P must be
happening
A famously horrible example
“You love your iPhone, literally”
http://www.nytimes.com/2011/10/01/opinion/you-love-your-iphone-literally.html
“But most striking of all was the flurry of activation in the insular
cortex of the brain, which is associated with feelings of love and
compassion. The subjects' brains responded to the sound of
their phones as they would respond to the presence or proximity
of a girlfriend, boyfriend or family member.
In short, the subjects didn't demonstrate the classic brain-based
signs of addiction. Instead, they loved their iPhones.”
Reverse inference in real life
• Forward inference (valid): If you have Bubonic
Plague, then you will start by experiencing flu-like
symptoms
• Observation: I have flu like symptoms
• Reverse inference: “Oh no, I have flu like
symptoms. I must have Bubonic Plague!”
Problem:
• Lots of other things cause flu-like symptoms too
• Similarly, lots of other cognitive processes can
activate brain area A
It’s not just that different processes
can activate the same area
Brain areas aren’t just multi-functional, they
are multi-representational too
A single area contains lots of different neural
representations, all participating in the same
cognitive function
Distinct representations may produce
same overall activation
/ba/-sensitive
population of neurons
Speech area
Subtraction:
/ba/ minus /da/ = zero
/da/-sensitive
population of neurons
Representations that standard fMRI can handle:
Localised and segregated
If you do a PubMed search for
representations and fMRI,
you find figures like these:
Hand
Lips
Houses
Faces
Representations that are more difficult:
Distributed and overlapping
Distinct but overlapping representations:
same average activation,
but different local patterns
Stimuli A and B activate the same neural population,
both activating shared neurons to differing degrees,
so they elicit different activation patterns
Stimulus A
B
Average local activation is the same for both stimuli
Ok, reverse inference is bad.
Can we make any inferences from
fMRI about cognitive processes?
Mather, M., Cacioppo, J. T., & Kanwisher, N. (2013).
How fMRI can inform cognitive theories. Perspectives
on Psychological Science, 8(1), 108-113.
From a special issue of Persp on Psych Sci:
20 Years of fMRI - What Has It Done for
Understanding Cognition?
http://pps.sagepub.com/content/8/1.toc
Mather, Cacioppo & Kanwisher:
How fMRI can inform cognitive theories
They suggest fMRI can help to answer four types of Qs
• Which (if any) functions can be localized to specific
brain regions?
• Can markers of Mental Process X be found during
Task Y?
• How distinct are the representations of different
stimuli or tasks?
• Do two Tasks X and Y engage common or distinct
processing mechanisms?
A closer look at Mather et al.’s four Qs
“1. Which (if any) functions can be localized to specific
brain regions?”
•“The brain, and hence the mind, contains specialized
mechanisms for particular mental processes”
•E.g. Fusiform Face Area, Parahippocampal Place Area,
Extrastriate Body Area
•This is a very controversial claim! See Shermer’s
arguments against modules
A closer look at Mather et al.’s four Qs
“2. Can markers of Mental Process X be found during Task Y?”
•Maybe you can infer a mental process from brain activation, if
you are careful enough
Example processes:
•Suppression of to-be-ignored stimuli
•Attentional selection of entire objects, not just locations or
features
A closer look at Mather et al.’s four Qs
“3. How distinct are the representations of
different stimuli or tasks?”
•Does the brain treat A and B as the same or
different?
Example methods:
•fMRI adaptation
•Multivoxel-pattern analysis (MVPA)
•E.g. Does a given brain area treat a photo the
same way as a line-drawing?
A closer look at Mather et al.’s four Qs
“4. Do two Tasks X and Y engage common or distinct
processing mechanisms?”
•“If conducted properly, experiments showing overlapping
brain activation for the two tasks, with appropriate control
conditions and within individual subjects, can provide
evidence for common mechanisms.”
Example:
•Remembering the past and imagining the future activate
similar circuits.
•Generates hypothesis: both part of same mechanism?
A valid argument?
“Activation of same region implies
the same cognitive process”
Consider:
When people talk, they use their mouth
When people eat, they use their mouth
Therefore, talking and eating are the same process
Ok, that doesn’t work.
But does that mean that such reasoning never works?
Searching PubMed and Google Scholar
PubMed:
• The “review” tag
• Boolean searches
Google Scholar:
• The wonder of the “Search within citing articles”
checkbox
A useful tip (taught to me by Dave Kleinschmidt)
• Type “.ezp.lib.rochester.edu” into any journal URL
• E.g. www.sciencedirect.com.ezp.lib.rochester.edu
Speak up!
There's no such thing as a bad question
• Simple questions often get at deep issues
• “Sophisticated” questions often rest upon lots of unexamined
assumptions, may miss deeper issues
• There are lots of open questions, waiting for us to explore
Nobody understands how the brain works
• So, do not be alarmed if you don't either
Class will be a welcoming atmosphere, say whatever you like.
Things which are unfair, but true of the world
• Loudmouths get heard. This is America. Be loud.
• If you don't say anything then people (falsely) assume that
you don't have anything to say
• Part of your grade will be for participation in class discussions
I wish those other people would address
my interests and concerns!
If your work is outside of neuroscience:
• E.g. in behaviour or computational modeling,
• Do most neuroscience studies fail to speak to the Qs
that actually interest you?
• How might neuro studies be made to speak more
directly to your questions?
If your work is within neuroscience:
• Often non-neuro people bemoan how neuro work
strikes them as uninteresting or missing the point.
• How might you convince them that neuro work is
asking cognitively interesting questions after all?