final exam Flashcards
Nuclear Magnetic Resonance
MRI
energy at specific radio frequencies is absorbed and reemitted by nuclei with non-zero spin
(electromagnetic energy but same concept as resonance)
w=yB
w=resonant (Larmor) frequency -> 85mHz typ.
y=gyromagnetic ratio
B=magnetic field strength
MRI signal
radio frequency pulse (at Larmor frequency to be absorbed by hydrogen in brain) goes into brain
excites atoms in brain and then energy is released
measures density of hydrogen atoms but pulse of certain freq. only absorbed by H atoms, measure how much is absorbed then released again
Gradients to encode space
- parts of brain have slightly different magnetic field strengths s
- resonance frequency for hydrogen will be different due to magnetic field
Encodes energy using frequency and phase (3 gradients to get separate measurements to create 3D image)
K space
represents density of hydrogen but changes timing to measure other things
records how much energy comes out of brain in each point in time during recording
convert k-space to real space
2D Fourier transformation (mathematical)
information needs to be decoded
different parts of brain represented by different frequencies and phases
Blood oxygenation level dependent (BOLD) contrast
key to fMRI
oxygenation of blood gives hint of how active areas of brain are
- cells require energy source when metabolically active so they use up oxygen and other resources provided by blood
Oxygenated hemoglobin
diamagnetic
zero magnetic moment
–> has iron in it, one of the more magnetically active elements
Deoxygenated hemoglobin
paramagnetic
significant magnetic moment
greater magnetic susceptibility
- causes faster T2 decay
timing from hemoglobin
spike of oxygenated blood rushes in to sweep away deoxygenated hemoglobin
(deoxygenated from active use)
this dip in deoxygenated blood (from replacement) takes about 6s and the response is much more spread out in time
*fMRI does not have very good temporal resolution (cannot see direct neural activity but slower smeared out response for circulatory system)
fMRI data
subjects –> run (sequence on trials and tasks) –> volume (allows to measure where changes happen) –> slices –> voxel
fMRI signal/data
in order to collect frames every 2 seconds, resolution is sacrificed
images are a lot blurrier for fMRI for quick image collection
doesn’t affect too much cause changes are what are being recorded (local areas serviced by certain blood vessels)
BOLD signal is really weak, most of signal is structure but BOLD changes sit on top of main structural signal
fMRI data preprocessing
1) slice timing correction
2) realignment
3) coregistration
4) normalization
5) smoothing
Slice timing correction
data collected slice by slice and it takes time to collect each slice so the time you collected data at the bottom is different from the top
takes 2 seconds to do whole brain
**data has to be interpolated
Realignment
cant assume a voxel equals same bit of brain across time (think small movements, few mms make a difference)
typical
- tries to correct for translations and rotations of the brain
uses 6 parameter rigid body transformation (3 translations 3 rotations)
Reslicing
after realignment, need to replace to match up voxels
Coregistration
align function images to structural image
- uses an affine transformation (translation, rotation, scale, shear)
collects functional and also clear structural images but using different techniques to make them comparable
Normalisation
warp each subject’s brain into a standard space using deformations by linear combination of smooth basis functions
- every brain is a snowflake but there is a standard brain other images are matched up with
segments into grey matter, white matter and CSF using tissue probability maps and Bayesian estimation
Smoothing
deals with remaining inconsistencies by smoothing/blurring images
Blocked design
trials are grouped into blocks
hemodynamic response is estimated per block
e.g. testing reactions to faces, first test response to fearful faces, then happy, then neutral
Event-related design
mix different trial types together and show them in random sequence
uses jittering between conditions
neural activity to each one of the trials will highly overlap with response to other trials (try to control for with jittering)
Mixed design
blocks of separated conditions within which there are separated temporally jittered events of interest
Mass-univariate general linear model (GLM) analysis
univariate analysis of each voxel but for every voxel
- data collected from each voxel in brain over time
- predicted time course for activity related to each event/condition of interest (try to explain data in terms of what’s going on in the experiment)
- look at what was recorded in brain for each voxel and look at pattern of activity for what voxels respond to
- estimates are made for how much each event contributes to signal in each voxel
Familywise error rate (FWE)
correction for multiple comparisons
family-wise error correction gets rid of false positives but also gets rid of most of the signal
False discovery rate (FDR)
correction for multiple comparisons
still has a pretty good job of detecting signal and getting rid of false positives (compromise)
e.g. 5% of active voxels are false positives instead of each test being 5% wrong
Long term memory (LTM)
processes by which information is encoded, stored and retrieved
- encoded within connections between neurons, affected by how many connections, frequency, neurotransmitters, etc.
- massive timescale (minutes, hours, days, years)
- massive capacity (words, faces, events, skills, etc.)
- common underlying representation (# and strength of synaptic connections)
Types of LTM
semantic
episodic
procedural
LTM processes
1) encoding
2) consolidation
3) storage
4) retrieval
5) ~~ reconsolidation?
Encoding LTM
initial creation of memory traces in brain from incoming information
Consolidation LTM
continued organization and stabilization of memory traces over time
Storage LTM
retention of memory traces over time
Retrieval LTM
accessing/using stored information from memory traces
Reconsolidation LTM
possible reorganization and re-stabilization of memory traces after retrieval
Episodic memory
events
specific personal experiences from a particular time and place
- medial temporal love, middle diencephalon, neocortex
subset of declarative/explicit memory (LTM)
Semantic memory
facts world knowledge object knowledge language knowledge conceptual priming middle temporal lobe, middle diencephalon, neocortex
subset of declarative/explicit memory (LTM)
Procedural memory
skills (motor and cognitive)
basal ganglia
skeletal muscle
subset of non declarative (implicit) memory
Recognition memory paradigm
Study phase presents words, images, pairs, etc.
Delay is active or passive
test phase presents test items and response = “old” or “new” item
sometimes asks for confidence, “remember vs. know” etc.
Recollection (remember)
conscious memory for seeing the item during study phase
familiarity (know)
intuitive feeling of having recently encountered the item
DRM paradigm
Deese, Roediger, McDermott
present list of semantically related words
recognition memory test
- words on list
- unrelated distractors
- semantically-related distractors (lures)
lures are not on the list but similar in meaning which are the ones we incorrectly remember as being on the list
DRM paradigm
false memories for lures
lures affirmed almost as often as words actually on the list
high confidence in accuracy if assessed
high level of “remembering”
happens even if informed about effect
Implications from DRM
memory is not replayed like a video but reconstructed based on multiple pieces of information
- what actually happened
- one’s knowledge, experience and expectations
*false memories are possible, likely and vivid in right circumstances
Cabeza et al. 2001 study
18 thematic and categorical 14 word lists
thematic: water, ice, dark, wet, freeze
categorial: cucumber, pea, potato, onion, corn
words presented every 2s with interval of several seconds between lists
participants instructed to remember words and who presented them
Cabeza et al. 2001 study
task/test phase
Recognition tested
- each word presented for 3s
- old vs. new judgment with button press
- 9.5 second pause before next trial (SLOW EVENT RELATED DESIGN)
-for each list: 1 old, 1 lure (often answered as old but should be new), 1 unrelated word
6 blocks of 6 lists each –> 36 old words, 36 lures, 36 unrelated new words
Asked whether female or male presented the word
Cabeza
Bilateral (anterior) hippocampal regions
activation for lures is almost the same as activation for true words
the brain gets fooled by lures
Cabeza study
Left posterior parahippocampal region
in this brain region, lures do not look like true memories but new items
Cabeza conclusions
Left posterior parahippocampal region
- memory for sensory information (new items, thinking at a lower level and more sensory focused)
Bilateral hippocampal regions
- memory for semantic information = incorrectly thinking lures were on list
True memories activate both
false activate hippocampal but not parahippocampal
new memories activated neither
Lure items = no memory for sensory information but for semantic
Broca
1861
- patient w/ lesion of Inferior frontal gyrus
- deficit of speech production
- damage to Broca’s area leads to Broca’s aphasia
Wernicke
1874
- patients w/ damage to superior temporal gyrus
- deficit of speech comprehension
- damage to Wernicke’s area leads to Wernicke’s aphasia
Aims of cognitive neuroscience
1) identify processes that underlie normal condition
2) to localize these processes to particular neural structures/systems
often uses brain damage and breakdown of function/behaviour