chap 2 - cognitive neuroscience methodology

Structural vs. Functional Brain Imaging

• Structural Brain Imaging:

  • Visualize anatomical structure of the brain

  • i.e., size, specific regions, tumors

• Functional Brain Imaging:

  • Visualize active processes in the brain

  • (e.g., neural activity, blood flow)

  • visualizing pathways

Spatial Resolution

• Amount of detail you can see in the image

• structural imaging:

  • Amount of neuroanatomical detail you can see

  • ↳ ex: distinguish gray from white matter

• functional imaging:

  • Precision with which you can localize activation to specific brain regions

  • ↳ ex: how precisely can you identify the region

Temporal (“timing”) Resolution

• Relevant for functional imaging (and single-cell recording)

• Refers to the precision with which you can localize brain activation to a specific point in time → when in time, did the brain activity occured

  • Poor temporal resolution –minutes

  • Good temporal resolution –milliseconds

Single-cell recording

• Record neural activity directly.

• Place a thin electrode in or near a neuron (invasive).

• Count the number of action potentials per second.

• Determine which experimental manipulations change the neural response.

• How is the spatial & temporal resolution?

  • spatial → excellent because you know exactly which neuron is firing

  • temporal → excellent bc record changes right at the neuron

• insert into neuron (sometimes group) & record activity

• limited to animals bc its so invasive

• ex:

  • Record action potentials per second during a delayed-matching task in a monkey

    • wm task

      • have to retain info in wm

        • saw more activity in DLPFC

          • more neuron firing

C(A)T: Computed Axial Tomography

• Basically an “x-ray absorption map”

• Different tissues absorb different amounts of x-ray radiation → look different on the image

  • bone is highly dense & absorbs lots of radiation → why very white

  • CSF is low dense → why black bc not rlly absorbing radiation

  • brain matter is in the middle (density-wise) → why has intermediate color

• poor spatial resolution (compared to MRI)

  • can’t see a lot of anatomical detail

• Less expensive than MRI

• More often used in clinical settings (than in research)

• ex: ct scans

  • Can use CT scans to identify lesion (dead tissue) sites (and areas of overlap) in stroke patients

    • stroke → certain parts weren’t getting blood/oxygen so they died

Magnetic Resonance Imaging (MRI)

• A structural MRI is basically a “proton density map”

• Different tissues have different proton densities and look different on the map

  • protons → normally in the form of water in biological life

    • tissues then have different amounts of water

• How is the proton density map created?

  • Protons have magnetic properties

  • These magnetic properties are measured by the scanner

  • Tissues with different proton densities have different magnetic properties → appear different on an MRI image

    • measuring different magnetic properties

What are some advantages of MRI over CT?

• more detailed

  • better spatial resolution

• can identify specific brain regions

• answer more specific anatomical questions

• doesn’t involve any radiation

• have fMRI & uses same machine

• Ex MRI:

  • Raz et al. (2010) used MRI to assess regional changes in structural volume of different brain regions with increasing age

• Ex MRI (Graphs):

  • Different brain regions showed different structural volume trajectories over time

    • hippocampus:

      • showed age-related decline

      • not pront to remember old info

    • pericalcarine cortex (in the OL)

      • did not show structural decline → bc visual info is used everyday

  • “use it or lose it”

Diffusion Tensor Imaging (DTI)

• Method to measure white matter tract

  • Myelinated axons have a lipid boundary so water must diffuse along length of axon rather than in any direction

    • have lipid bilayer → water can’t diffuse normally

      • diffusion of water must follow path of axon

• still use MRI machine

• Example: Diffusion Tensor Imaging (DTI)

  • Related the microstructural integrity of the superior longitudinal fasciculus (“white matter tract”) to neuropsychological performance in adolescents

• Example: Diffusion Tensor Imaging (DTI) (graph)

  • Greater microstructural integrity of the superior longitudinal fasciculus was related to better set-shifting (executive control)

  • when water is diffusing in a concentrated direction → more myelinated axon

  • relate white matter integrity of SLF in EF task → being used in task

“Active” Brain Regions

• The whole brain is always active

  • some regions may be used more

• Neuroimaging activation maps display the relative differences in brain activity between two or more conditions

  • e.g., compare baseline to visual stimulus

  • subtracting

• Regions showing greater relative activity during a particular cognitive task are labeled with an arbitrary color

PET: Positron Emission Tomography

• A PET image is basically a “blood flow map”

• Areas with more active neurons receive more oxygenated blood

• Poor spatial resolution (compared to fMRI)

• Worst temporal resolution of functional techniques (compared to fMRI and to EEG/ERP)

• How is the blood flow map created?

  • radioactive substance injected into bloodstream

  • Substance travels to brain; Radioactivity measured by detectors around the head

  • Areas where there is more blood flow (i.e. areas of the brain that are more active) emit more radioactivity

• Can be used to measure:

  • resting brain activity

  • brain activity associated with doing a specific task

  • levels of substances in the brain (e.g., neurotransmitters, proteins) that bind to radioactive tracers

    • what its normally used for

    • for: dopamine, amyloid, tau

    • only technique to do it

• Example: PET

  • Rodrigue et al. (2012) used PET to assess the deposition of amyloid plaques (protein-related to alzheimer) in middle-aged and older adults (w/o alzheimer’s)

    • then assess cognition

      • more amyloud = worse cognitive performance

fMRI: Functional Magnetic Resonance Imaging

• Colored areas = Functional image

• Functional data overlaid on a structural MRI (black & white)

  • or atlas image

• doesn’t show structure → that's why overlapped to then distinguish

• A fMRI image is basically a “blood oxygenation level map”

• More oxygenated blood in regions of increased neural activity

  • neurons need more oxygen bc they were active

• Oxygenated blood has different magnetic properties than de-oxygenated blood (comparing)

  • Measure the Blood Oxygen Level Dependent (BOLD) response

• Used to measure brain activity associated with doing a specific task or during resting state

  • fMRI

  • no task is being performed

  • can compare across different populations to see what area tend to be

• Best spatial resolution of all functional techniques (compared with PET)

• Good temporal resolution (compared with PET)

  • ~ 2 sec

• Example: fMRI

  • Nee et al. (2007) identified brain regions associated with performing a directed forgetting task

    • intentional forgetting

    • Left DLPFC → more active when performing task

How does fMRI work? Blood Oxygen Level Dependent (BOLD) signal

1. increased neural activity

2. increased blood flow (to area)

3. comparing:

   1. baseline

   2. active

      1. surplus of freshly oxygenated blood

      2. oxy-deoxyhemoglobin ratio increases

4. increased BOLD signal

• DV = BOLD response → indicator of more active areas

• Interpreting the BOLD signal:

  • Regions activated in fMRI experiments are associated with a task.

    • Correlational

    • we can infer, not conclude

    • tends to be active

      • but may not be essential →

  • However, they may NOT be ESSENTIAL for performing that task

  • Thus, lesion methods are needed to determine whether a brain region makes an ESSENTIAL contribution to performing a task.

    • i.e., w/ a stroke

      • (naturally occuring)

        • when area is damaged, what happens?

EEG: Electroencephalography

• Electrical potentials recorded by electrodes placed on the scalp

  • measuring electrical signals (activity) at the surface of the head

  • has BEST temporal resolution

  • mapping electrical activity of the brain

  • mapping electrical activity of the brain

  • “brain”

ERP: Event-Related Potentials

• Measures neural activity related to a particular event

  • An event can be a sensory stimulus, movement, etc.

• uses EEG

• patterns of electrical activity related to a specific event

• normally repeated → to get avg. of pattern

• Example ERP:

  • Do people preferentially attend to their own name or face?

    • Measured ERPs to onset of their own, famous, or unknown names/faces

    • Examined attentional P300 ERP component

      • signature of electrical activity

• Example ERP: Name ERPs

  • self: earlier & higher freq.; preferentially

• ERP has the best temporal resolution of all of the neuroimaging techniques

  • use most with timing question

• ERP has the worst spatial resolution of all of the neuroimaging techniques

  • don’t know which brain regions were active to produce the electrical activity

    • don’t know specificially

ERP vs. fMRI/PET

• ERP is good for figuring out the timecourse of cognitive events

• fMRI (and PET) are good for understanding where in the brain these cognitive events occur

• “When vs. where”

Transcranial Magnetic Stimulation (TMS)

• Pulses of electromagnetic field from coil induce an electric field in the brain

  • not imaging technique

• Can either:

  • Excite the cortex (induce movement, sensation, etc.)

    • trigger activation

    • single-pulse TMS

  • Or disturb its function (temporary “lesion”)

    • repetitive TMS

    • temporarily making it inactive

    • good for not relying on natural occuring

    • help answer cause & effect question

      • how does this effect performance

• Example: TMS

  • TMS of the left IFG and posterior middle temporal gyrus impaired gesture-speech integration

  • overall: disrupted performance

    • reduction in IFG & pMTG

  • only able to TMS cortical areas

    • not subcortical