Neuroimaging

Disadvantages of PET scan

-Poor temporal resolution (many minutes) -Poor spatial resolution (several centimeters, worse than fMRI) -Requires injection of radioactive material (Invasive) -Tracer decays quickly -Requires a cyclotron in building or close by due to short half life, very expensive

Describe the process of the PET scan

-Radioactive tracer injected into blood stream (e.g. fluorodeoxyglucose) -Tracer takes 30 seconds to peak -Take into body by a sugar(FDG) and then only the tag is left. -Increased metabolic activity in area leads to decay of radioactive tracer, which produces a positron -When positron hits electron, both are annihilated and gamma ray emitted -PET scanner detects gamma rays, creates map of metabolic activity(which parts of the brain are working)

Advantages of TMS

-Relatively non-invasive method to stimulate brain areas -Can say whether area is used for a specific task, not just involved -May be useful for treatment of depression, schizophrenia, autism ... (maybe)

What is rTMS? What is a fast rTMS? What is a slow rTMS?

-Repetitive TMS, multiple pulses -Stimulate at 1-60 Hz -A fast rTMS excites -A slow rTMS inhibits -Can cause a temporary lesion. Ex: Stimulate over Broca's area and patient can't talk

Transcranial magnetic stimulation (TMS)

-Safer method compared to electrical stimulation, which required the head to be cut open for direct stimulation -Allows transient and safe disruption of local neuronal activity, in effect creating reversible lesions -Only a single pule that can briefly activate an area. Ex: Motor cortex -Magnetic wand is put over the skull to change certain parts of the brain's functions -Skull and scalp is transparent and people have used this to administer brief pulses of stimulation -Typically used to study response of motor system. Ex: Stimulate over M1 hand area of the brain and measure time it takes to move the finger

fMRI

-Similar principles to MRI -Measures changes in blood oxygenation and flow that occur in response to very localized changes in neural activity. -Indirect measure of neural activity -Difference in magnetization(different levels of attraction) of oxygenated and deoxygenated blood -fMRI is based on increases in the blood-oxygen level in discrete parts of the brain. -Blood Oxygenated Level Dependent (BOLD) Response

Advantages of PET scan

-Simple, takes pictures of uptake in the brain -Provides an absolute 0, if no activity level in an area in the brain then it won't show -Can look at neurotransmitters, allows imaging of anything that can be tagged -Quiet (good for auditory studies)

MRI versus CT

-Soft tissue, e.g, the pronounced white matter-grey matter differentiation of the MRI thalamic nuclei, subdural layers not visualized by CT -MRI can see difference between gray and white matter -MRI can image very small structures like nuclei of brain stem -MRI can make out sulci and gyri

What to use in each scenario? If you want to measure... Structure Function or synaptic interactions Processing, where timing is important

-Structure- use CT or MRI -Function or synaptic interactions- use PET, fMRI -Processing, where timing is important- use ERP and/or MEG.

What is Temporal resolution (TR)?

-Temporal resolution (TR) refers to the precision of a measurement with respect to time. -Often there is a tradeoff between temporal resolution of a measurement and its spatial resolution.

What is the difference between the studies of the MRI and the fMRI?

-The MRI studies brain anatomy, tells you about structure and tissue consistency of the brain -The fMRI(Functional MRI) studies brain function

EEG/ERP Disadvantages

-Poor spatial resolution -Can't always be exactly sure where signal coming from -Can't measure some subcortical activity in some parts of the brain like basal ganglia, which will not be able to record reliable responses from the scalp

Limitations of the lesion method

-Higher cortical function is difficult to study in other animals. -Ex: Aphasia is extremely difficult to study or if even possible in animals -Human brains are three times larger in volume than most of other animals

Disadvantages of TMS

-A new procedure and unclear if there are long-term effects -Can only examine one area at a time

Event related potentials (ERP)

-Are voltage potentials that can be recorded from the scalp, using electrodes, that reflect time-locked electrical activity generated in the brain in response to sensory, motor, or cognitive events. -Derived from ongoing EEG (electroencephalography) recordings, and specifically measure neural activity following or preceding the onset of a target event (word, picture, sound, or other visual, auditory, or somatosensory stimulus). -EEG signal is averaged over many events (to reduce effects of random neural firing) and synchronized to some aspect of the event (e.g. onset of stimulus, pressing a button) -The longer you average the more you cancel out what isn't related to the stimulus. -Timing and amplitude of the peaks is related to different aspects of the stimulus and task -Shows in real time, millisecond by millisecond level of how the information is processed

What is the BOLD response?

-Based off the assumption that when a certain brain area is monetarily particularly active (during a math calculation), it consumes more oxygen and, to meet that increased demand, blood flow increases to that area. -When a part of the brain is being used, in order to function it requires oxygen, which is why you have elevated levels of oxyhemoglobin and this thereby causes decreased levels of deoxyhemoglobin. When a part of the brain is being activated, you see it appear due to elevated oxygen levels.

What are some limitations of ERP?

-Big limitation is that you're never exactly sure where the electrical source is -Can make a general estimate/ballpark but cannot really tell the EXACT pinpoint location -Limited spatial resolution meaning you don't know where the activity is occurring -"Inverse problem"

PET vs. SPECT

-Both use tracers -Injected in the blood flow -Neurons that are being used that are activated, see more tracers in areas that are used -SPECT takes longer to decay (8 hours) -SPECT is more low cost and since longer half life, cyclotron is not needed so it is more readily available -Both absolute and have a true 0, can physically measure the actual level of tracers -Both have poor temporal and spatial resolution when compared to fMRI

Advantages of CT scans

-CT more sensitive than standard x-ray -Faster and less expensive than MRI, good for trauma and other acute neurological emergencies -Less sensitive to patient motion during the examination because the imaging can be performed much more rapidly -Easier to perform in claustrophobic or very heavy patients -CT good at distinguishing differences in tissue & detection of calcium and metal foreign bodies

Disadvantages of MEG

-Can not image below cortex -Expensive and not very common(3 million dollars) -MEG can only pick up currents flowing tangential to the scalp(corresponding to sulcal activations), not gyral

Disadvantages Continued.

-Contribution of brain region to a particular cognitive function can go undetected if task can be performed via multiple strategies. Ex: Patients tend to have remarkable capacity to work around deficit and may give misleading results -Fails to capture the temporal dimension of phenomena

Disadvantages of DTI

-Conventional DTI-fiber tracking fails to identify the existence or extent of many white matter connections and will also identify connections that do not exist in reality. -Loud -Claustrophobic

Magnetoencephalography (MEG)

-Current flow in neurons creates local magnetic fields -Detects the magnetic fields produced by electrical activity in the brain, while the EEG detects electrical potential generated by neural currents -MEG measures local magnetic field changes from the surface of the scalp using superconducting coils or SQUIDs -Excellent temporal resolution -Better spatial resolution than ERP

What do people's brains that have ADHD look like?

-Decreased volume of basal ganglia -Decreased volume of dorsolateral prefrontal cortex -Decreased volume of anterior cingulate cortex, caudate -Increased volume of white matter

Diffusion tensor imaging (DTI)-scan

-Determines the organization of white matter tracts -Maps white matter fiber orientation and tracks white matter pathways -Measures diffusion of water molecules through axons in tissues.

What are some uses of an EEG?

-Diagnose brain disorders , such as identifying seizures -Sleep-stage identification -Coherent patterns of activity

In Diffusion tensor imaging (DTI)-scan which direction of has the largest diffusion? Also, where is the scan performed?

-Diffusion tensor imaging (DTI)-scan -Direction of axon has the largest diffusion -Performed in an MRI scanner

What are Dissociations used for?

-Dissociations used to infer the existence of separate mental processes -Logic: If a patient cannot do X, then the execution of X must depend on the lesioned area

Explain Double dissociation and what is an example of it?

-Ex: Lesion A disrupts function X but not function Y. Plus Lesion B disrupts function Y but not function X. -Broca's(inferior temporal gyrus) and Wernicke's(superior temporal gyrus) aphasia are examples of a double dissociation

Disadvantages of MRI

-Expensive -Very loud and claustrophobic -Cannot have any metal implants -Environmental hazards (flying metal objects)

Advantages of MEG

-Great temporal location and timing(milliseconds) and ok spatial resolution (accuracy of approximately 2 mm). -Direct information about the brains activity -Quiet

What is the Lesion method? How did it come about?

-Identify where brain is damaged (before MRI, used postmortem). -Damage in gladiators or in patients shows deficit in behavior after infarction(tissue death). -Would learn about normal function through dysfunction. -Emphasis on deficit measurement. -Determine which regions are necessary for task.

Disadvantages Cont.

-Interdependencies between brain areas may give misleading results -Does not allow direct observation of the function performed by the damaged portion of the brain - reveals how the brain performs without that particular region -Or reveals the effects of disconnection. Brain region may be implicated in a particular aspect of cognition just because the region connects two or more other regions that must interact for accurate performance.

Computed tomography (CT) scan or CAT scan

-Introduced in 1970 -Specialized type of x-ray which builds up a three-dimensional picture -CT scan is an image of a slice, and putting the slices together you can make a 3D image of the brain -More sensitive than standard x-ray -Produce a detailed picture of bone, soft tissue, muscles, internal organs and, crucially, tumors or other irregularities

Magnetic resonance imaging (MRI) scan

-Introduced in 1980s -Good for size and shape information -Extremely powerful magnet causes water molecules in body to orient in particular direction and radio frequency pulses will cause them to spin off axis and time it takes for them to spin back to normal will tell you something about tissue density -Shows anatomy of where everything is, use water molecules

Disadvantages of CT scan

-Involves x-ray, which means you cannot have many within a short period of time due to a small risk of cancer -Some parts of the brain don't image very well, problem with bone artifact (posterior fossa) -Not good at identifying soft tissue abnormalities, inflammation -MRI's pick up inflammation much better due to high resolution -Not good at differentiating similar but not identical tissue -Requires breath holding

MEG vs EEG

-MEG and EEG provide excellent temporal resolution (in milliseconds) and are superior to fMRI or PET. They are inferior, however, to fMRI or PET with respect to inferring the sites of brain activation (spatial resolution). -MEG and EEG both reflect bioelectrical activity on the surface of the brain, not primarily within it. Sources located deep in the brain are thus difficult or impossible to measure with these techniques. -MEG can only pick up currents flowing tangential to the scalp(corresponding to sulcal activations. -EEG can measure both tangential and radial (corresponding to gyral activations) currents.

Volumetric differences in ADHD using MRI

-MRI is good for imaging neurodevelopmental disorders like ADHD -Can trace out area of brain and compare volume of one area to another area or to another person and start to identify subtle difference between people that have ADHD vs people who don't have it.

What scams are 2-D or 3-D? MRI fMRI CT EEG

-MRI=3-d, fMRI=3-D, CT=3-D, EEG=2-D

Positron emission tomography (PET) scan

-Measures local blood flow (rCBF) -Indirect measure of neural activity -Recording deposits of radioactive labels in different parts of brain and making assumption that more radioactivity in an area means it was more active during uptake. -Shows how much radioactivity is in the brain(what parts of the brain are working).

Electroencephalography (EEG)

-Measures patterns of ongoing electrical activity by placing electrodes on the scalp -Use EEG's for looking at sleep, changes in alertness: relaxed vs alert -Sensitive to large groups of neurons, firing in synchrony

Disadvantages of fMRI

-Mediocre temporal resolution, takes a while for it to happen (few seconds) -Very complex -Difficult to interpret in conditions where the coupling between neural activity and blood flow breaks down (ex: medications) -Very loud (120-130 dB SPL), patients need to wear protective earphones during -Expensive and time consuming -Cannot be used on people with implanted metal devices

Hemodynamic imaging

-Neural activity consumes oxygen as well as generating electrical signals -In order to compensate for increased oxygen consumption, more blood is pumped into the active region -PET measures the blood flow in a region, whereas fMRI measures the blood oxygenation -The time taken for this response is slow (several seconds) and so functional imaging has a poor temporal resolution, but a good spatial resolution -This is the complementary profile to ERPs

Advantages of MRI

-No exposure to radiation, can have one taken everyday, no ill effects -Has a great capacity to look at soft tissue changes -No injections -Depicts anatomy in greater detail/great anatomical resolution, and is more sensitive and specific for abnormalities within the brain itself -MRI contrast agents have a considerably smaller risk of causing potentially lethal allergic reaction. -MRI allows the evaluation of tissues that may be obscured by bone in CT images

EEG/ERP Advantages

-Non-invasive -A direct measure of neural(brain) activity -Excellent temporal resolution, able to identify neural activation millisecond by millisecond -Can be used without a required behavioral response. Ex: Babies, or patients with limitations in their ability generate motor or verbal responses because instrumentation does not get in the way -Somewhat portable -Low cost -Can correlate with measures taken directly from cortical surface -Immediacy of recordings and data analysis makes it potentially useful for biofeedback and brain-computer interface.

Advantages of fMRI

-Non-invasive -Not harmful, safe for women and children -Excellent spatial resolution -Well-validated, common

When are CT scans most likely used? Are patients allowed to move?

-Often used in trauma centers for head injuries -Excellent for evaluating skull fractures -Excellent in detecting bleeding in the brain (Subarachnoid hemorrhage) Ex: Natasha Richardson -Motion is less of a problem compared to MRI.

Lesions and Broca's aphasia How many of the patients with chronic Broca's aphasia have lesions in Broca's area? How many of the patients with lesions in Broca's area have a persisting Broca's aphasia?

-Only 85 percent of patients with chronic Broca's aphasia have lesions in Broca's area -Only 50-60 percent of patients with lesions in Broca's area have a persisting Broca's aphasia.

How would seizures look like on an EEG? How long do they tend to last? What type of seizures are there?

-They tend to last a short period of time, less than 3 minutes -Can have seizure that is not manifested in obvious changes in behavior. Ex: Just stare and not move while inside you are having a seizure -Some seizures have convulsions, body movements=tonic seizures -Would show a spike that is very sharp that represents of flurry of uncontrolled activity in the brain that is followed by slow wave that inhibitor influences bring it back to normal and then the spike again and it repeats the up and down reactions. -Large populations of neurons that are firing, have to be oriented toward the same direction, when neurons fire uncontrollably it results in spikes (uncontrolled activity).

Single photon emission computed tomography (SPECT) scan

-Use a radioactive label to measure blood flow or oxygen utilization -Label is attached to substance that travels and deposits in the brain -Label/tracer has a longer half life, around 8 hours -Has poor resolution when compared to fMRI

What is TMS used for?

-Used in research or clinical. -Ex: Depression

Brainstem auditory evoked response (BAER)

-Used to see if children are born deaf in order to diagnose their hearing loss -5 waves that are being produced in different parts of the auditory system -First two waves come from auditory nerve -If you have a brainstem lesion, first few waves will be normal but then the rest will be diminished

Disadvantages of lesion method

-Variability in characteristics of participant population, had no control over pre-morbid(preceding the occurrence of symptoms of disease) mental state. -Had to wait until patient died to identify where the lesion was. -Intervening factors: If you had one stroke you are at risk for another and back then no way to know how many strokes someone had and where the brain was damaged and how long it was damaged.

Who was Hans Berger?

-Was able to detect brain waves with electrodes placed on the scalp. -Like Caton, Berger noted that these voltages could be influenced by external events that stimulated the senses -Called "event related potentials"

What are the five waves of BAER? Not sure if need

-Wave I: The far-field representation auditory nerve action potential in the distal portion of cranial nerve (CN) VIII. -Wave II: Is generated by the proximal VIII nerve as it enters the brain stem -Wave III: Caudal portion of the auditory pons -Wave IV: Superior olivary complex, but additional contributions may come from the cochlear nucleus and nucleus of lateral lemniscus. -Wave V: Believed to originate from the vicinity of the inferior colliculus

Localization of Function: What is the issue of localization?

-Wernicke's area involved in understanding of words, but many areas of the left temporal lobe participate in aspects of semantic/lexical access. -Broca's area is also activated. Involved in phonologic & syntactic processing -Cannot localize one function to just one single area

How is the BAER performed?

-You lie on a reclining chair or bed and remain still. -Electrodes are placed on your scalp and on each earlobe. -The earphones give off a brief click or tone. -The electrodes pick up the brain's responses to these sounds and record them. -You do not need to be awake for this test.

What is true of the fMRI, PET, and ERP scans due to recent advances? NOT SURE IF NEED

-fMRI and PET have excellent spatial resolution (millimeters) but poor latency and slow time constant (seconds) -Although ERP has limited spatial resolution, source modeling techniques are improving

Single versus double dissociations

...

Structural imaging methods

1. CT Scan 2. MRI 3. EEG 4. ERP 5. MEG 6. PET 7. fMRI 8. TMS

If you want to observe deafness in infants, what scan would you use?

Brainstem ERP, like EEG except it time locks on certain stimuli

What if there is bleeding of the brain, what scan would you use?

CT scan

If patient has a skull fracture what scan would you use?

CT, shows tissue density

What scan would you use for a closed head injury?

DTI, looks at white matter tracts

If you want to observe a coma, what scan would you use?

EEG

What scan would use to measure sleep?

EEG

Summary of Scans: EEG MEG PET fMRI

EEG Strengths: Excellent temporal Weakness: Poor special Measures: Electrical currents MEG Strengths: Excellent temporal and special Weakness: Limited regions Measures: Magnetic currents from scalp PET Strengths: Ok spatial Weakness: Poor temporal Measures: Radioactive labels fMRI Strengths: Excellent spatial Weakness: Ok temporal Measures: Blood flow

If you want to measure a patient having seizures, what scan do you use?

EEG scan to measure electrical activity, shows action potentials and if glutamate is excitatory

Explain Single dissociation

Ex: Lesion A disrupts function X but not function Y.

True/false: EEG's are a measure of when cognitive events occur

False, EEG's are not a measure of when cognitive events occur unless time lock to a stimulus (ERP).

True/false: You would use an MRI for an auditory test.

False, would not use MRI for auditory tests because they are so loud

Advantages of DTI

Good spacial resolution

What is the problem with lesions and Broca's aphasia?

Inconsistency in location and extent of lesions since lesions often extend across numerous areas - rarely small enough to ideally assess function-structure relationships

What is spatial resolution?

Inferring the sites of brain activation

What is the Inverse Problem?

Infinite number of different source configurations can produce the measured electric field

When should you get a CT scan compared to a MRI?

MRI scanner will cause metal to move so if you have any metals in the eye or brain, get a CT scan instead

What is there is a structural abnormality (brain size), what scan would you use?

MRI, structural scan

If you are studying someone with Autism and you want to know how many neurotransmitters are being used, what scan would you use?

PET scan since you want to trace certain things

What needs to be kept in extremely low temperatures?

Superconducting coils or SQUIDs, part of MEG's, detect biomagnetic signals

Age Variations: Not sure if needed

What happens during ages 3-6? -Rapid growth in Frontal Circuits: attention, vigilance, alertness What happens during ages 7-15? -Growth Spurt in temporal/parietal lobes: languages, mathematics What happens during ages 16-20? -Tissue loss in Frontal Circuits: self-control, planning, regulate behavior