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PHGY 314 > Shmuel_Resting Brain > Flashcards

Shmuel_Resting Brain Flashcards

1
Q

What are the main components of an MRI scanner?

A
  1. Main Coil
  2. RF (radiofrequency) coil → transmit RF energy to the tissue, and secondly to receive the weak nuclear magnetic resonance signals generated by the tissue during a scan
    - 3 gradient coils (X, Y, Z) → allow spatial encoding of the MR signal
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2
Q

What are the different physiological changes follow the exposure to a stimulus? (visual for ex)

A
  1. Stimulus
  2. Neuronal Activity
  3. Change in Cerebral Blood Flow (CBF) due to neuron-vascular coupling
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3
Q

What are 3 non-invasive imaging methods for the brain?
Which has the best spatial resolution?

A

MRI = Magnetic Resonance Imagine (BEST spatial resolution)
PET = Positron emission tomography
NIRS OI = Near-infrared spectroscopy optical imaging

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4
Q

What are 2 methods used for intracellular recordings?

A

Minority of studies to intracellular recordings to measure transmembrane electrical events

  1. Intracellular micro-electrode
  2. Patch clamp
    *Need ground/reference electrode in both (always when measure potential)
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5
Q

What type of recordings do the majority of electrophysiology studies use?
What are the concepts behind this?

A

Extracellular recordings
- A neuron is considered to be embedded in an extracellular medium that acts as a volume conductor (ECM has ions)

  • When the membrane potential is different between 2 separate region of a neuron → flow of current in neuron occurs
    This flow is matched by return-current through extracellular path
    *If some charges move on way, opposit charges move the opposit way to equilibrate (?)

*Extracellular recording is always measured with respect to a distant neutral site (ground)

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6
Q

What are the 2 components of the mean extracellular potential?

A

*Obtained by adding filter cut-off

  1. MUA = Multi-Unit Activity > 400Hz
  2. LFP = Local Field Potentials < 150Hz
    - Can further be classified to frequency bands used in EEG → delta (1-4Hz), theta (5-8Hz), alpha (8-13Hz), beta (12-30Hz), Gamma (30-150Hz)
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7
Q

What does the Multi-Unit Activity signal/recording represent?

A

MUA is the Component of the Mean Extracellular Potential > 400Hz

Single event duration ~ 1ms
Spatial summation radius of 100-200 microns (very small)
Represents:
1. Activity of the projection neurons that form the OUTPUT of a cortical area (Action Potentials)
2. Local intra-cortical processing (very local as ~80% of synapses occur within 1-2mm of the neuron sooma)

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8
Q

What does the Local Field Potential recording represent?

A

LFP is the Component of the Mean Extracellular Potential < 150Hz

Single event duration ~ 10-100 ms
Spatial summation radius of 1-2 mm (larger than MUA)
Represents:
1. Population Synaptic Potentials (EPSPs, IPSPs)
2. Voltage-gated membrane oscillations
3. INPUT to a given cortical area
3. Local intra-cortical processing (very local as ~80% of synapses occur within 1-2mm of the neuron sooma, excitatory and inhibitory neurons)

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9
Q

What allows us to state that fMRI signal is an indicator of overall activity of very many neurons and processes ?

A
  1. Because of the density of neuron in the cerebral cortex → 12x10^4 /mm^3

The voxel of fMRI is 2 x 2 x 2 mm^3 → there is 1 million neuron in each voxel

  1. Because of the density of synpases in the cerebral cortex → 9x10^8 /mm^3
    → This means 7.2 x 10^9 synpases/ Voxel (of 2x2x2mm3)
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10
Q

What is the voxel used in MRI?

A

Voxel = measurement of volume in an image
In fMRI → 2x2x2 mm^3 → about 1 million neuron/voxel

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11
Q

What is the BOLD response?

A

Blood Oxygenation Level Dependent signal
→ It reflects the content of Deoxy-Hb in blood vessels
*measured by MRI signal

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12
Q

What can be direct causes of a change in [Deoxy-Hb]?

A

Neuronal activity → Hemodynamic response ∆[Deoxy-Hb]

Can be due to:
- ∆ Blood Flow (CBF)
- ∆ Blood Volume (CBV)
- ∆ O2 Consumption (CMRO2)

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13
Q

How is a BOLD signal measured as fMRI signal?

A
  • BOLD is an indirect measure of changes in neuronal activity
  • fMRI relies on magnetic properties of hemoglobin → Deoxy-Hb = para-magnetic (weak magnet, acts as contrast agent) vs Oxy-Hb = not magnetic (way too weak)
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14
Q

What is the contrast agent in BOLD functional fMRI?

A

Deoxy-Hemoglobin → para-magnetic (disrupts the homogeneity of the magnetic field)
- An increase in DeoxyHb → decrease in homogeneity → decrease in BOLD signal
- A decrease in DeoxyHb → increase in homogeneity → increase in BOLD signal

*BOLD = oxygenation level

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15
Q

Where does the BOLD signal originate from?

A

Originates from cortical blood vessels → control blood flow which affects the content of Deoxy-Hb in the capillaries and veins
Control sites:
1. Smooth muscles (arterioles)
2. Pericytes (processes around small vessels at the arterio-capillary junction)

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16
Q

What are the X values of Deoxy-Hb and Oxy-Hb?

A

Deoxy-Hb: X ~ 1.6
Oxy-Hb: X ~ -0.3

17
Q

What are the physiological parameters influencing BOLD signal?
How do they influence it (increase/decrease)?

A
  1. Increase CMRO2 → increase DeoxyHb → decrease BOLD
  2. Increase CBF → decrease DeoxyHb → increase BOLD
  3. Increase CBV → increase DeoxyHb → decrease BOLD
18
Q

In what order do physiological parameters influencing BOLD change?
(Start from when stimulus in shown)

A
  1. Stimulus shown
  2. Increased Neural activity
  3. Increase oxygen consumption (initial dip)
    (neurovascular coupling)
  4. 2-3 fold higher increase in CBF coming from arteries (overcompensation) → increase in CBV
  5. Increased Oxy-Hb from fresh blood rushing in
  6. Increase in MR signal / Positive Bold response
19
Q

What explains the undershoot (negative dip in BOLD signal) which follows a positive BOLD response?

A

The CBV takes time to come back down, but the there is not as much blood flow (fresh blood) so the Deoxy-Hb goes up a bit which decreases a bit the MR signal

20
Q

Which blood vessels can the BOLD signal be recorded from?

A
  • Capillaries (gas exchange)
  • Venules and veins (drain deoxy-Hb)

*Not arteries and arteriols as they do not carry deoxy-Hb

21
Q

How can optical imagin be used in a similar way as fMRI?
*To measure Intrinsic signals

A

*Another way to measure BOLD signal

  • Relies on absorption of 605 nm by deoxy-Hb (being much greater than absorption of 605nm by Oxy-Hb)

At baseline → 50/50 deoxy/oxy → ∆R/R = 0
Initial dip → more deoxy-Hb than Oxy-Hb → ∆R/R < 0 because more light is absorbed and less is reflected back to the camera
Positive BOLD → more OxyRBC from blood rushing (increase in CBF) → less absorption and more reflection of the light back to the camera → ∆R/R > 0

22
Q

What are Voltage sensitive dyes ?
How does imaging work?

A

Voltage sensitive dyes have a portion inside the cell and a portion outside the cell membrane

Voltage-senstiive dyes change their fluorescence in response to voltage changes
- Changes in neuronal activity → changes in membrane potential → changes in fluorescence
- Capable of providing linear measurements of firing activity of single neurons or large neuronal populations

Mechanism:
1. Shine 630nm excitation light
2. Fluorophore are excited and emit light back
3. This light encounters 665nm dichroic mirror (wv > 665nm go straight to Ultima camera // wv < 665nm are deflected to Dalsa camera)

23
Q

What is the timing between BOLD signal and neurophysiological response?
What data did they compare to assess this lag?

A

BOLD signal lags behind the neurophysiological response

Voltage-Sensitive dyes vs BOLD (optical imaging)

24
Q

How did they answer the following question: Is there a quantitative relationship between the BOLD signal and the firing rate? (How much does the BOLD signal change for 1 AP?)

A

Compared the positive BOLD response in human V1 exposed to grating stimuli vs Average firing rates in monkey V1 in response to the same stimuli

Conclusion: Positive BOLD responses in human V1 are proportional to AVERAGE FIRING RATES in monkey V1

*Also CBF vs LFPs in the cerebellar

This raises the question: Is the BOLD signal caused by spiking activity? (NO, but it is correlated)

25
Q

What did they see when they compared Cerebellar blood flow vs LFPs?
What conclusions were pulled from this experiment?

A
  1. Electrically stimulated the climbing fibers or the parallel fibers in the cerebellum
  2. Measured CBF

Results:
Linear increase relationship between the changes in LFP and the increase in %CBF in climbing fibre stimulation

Sigmoidal relationship between the changes in LFP and the increase in %CBF in parallel fibre stimulation

Conclusion:
1. The hemodynamic response (BOLD) increases monotonically with increasing neurons responses
2. The hemodynamic response is in many instances ~ linear (proportional) with the underlying neuronal spike activity
3. Non-linearities between the hemodynamic response and nuerophysiological activity have been observed (ex: parallel fibres of cerebellum)

26
Q

Which type of neuronal activity causes BOLD responses: spikes or synaptic activity?
How can we find out?

A

*Use cerebellar properties to dissociate spiking activity vs synaptic activity

Look at divergence of spike rate and blood flow during parallel fibre stimulation

  1. Record activity from purkinje cells
  2. Stimulate parallel fibers which inhibit the purkinje APs
  3. Still see increase in BOLD signal (even in firing of inhibitory neurons)

Activity-dependent CBF increases evoked by stimulation of cerebellar parallel fibres are dependent on synaptic excitation (including excitation of inhibitory inter-neurons)
- Net activity of Purkinje cells in unimportant for the vasculature response (purkinje cells are principal excitatory neurons of cerebellar cortex)

CONCLUSION: Cerebellar Blood Flow does NOT depend on spiking activity

27
Q

What physiological response does the BOLD response reflect exactly?
What are important aspects to take into account?

A
  1. BOLD response reflects a local increase in neural activity assessed by the mean Extracellular Field Potential signal
  2. BOLD response reflects changes in LFP/synaptic activity /input to- and local processing in a region
    - Not MUA/output of a region (APs)
    *Dissociation of MUA and BOLD-signal → In long stimulus exposure, the MUA showed spike at the start and decreased whereas the BOLD response was higher during the whole duration of stimulus (just delayed a bit)

Signal to Noise Ratio (SNR) of the neural signals is much higher than that of the BOLD fMRI signal → Thresholding methods are likely

28
Q

What was observed when looking at the Signal to Noise Ratio of BOLD vs Neural signal?

A

*Measured amplitude in units of standard deviation

Neural response to a stimulus showed ~60 SD units increase in neural response

BOLD response to this stimulus showed a delayed ~ 2 SD units increase in BOLD signal

Conclusion: SNR of the neural signals is much higher than that of the BOLD fMRI signal → Thresholding methods are likely to underestimate a great deal of actual neural activity related to the stimulus or task

29
Q

What are the frequencies of spontaneous fluctuations?

A

SLOW termporal scale → 0.01 Hz - 0.1 Hz (1 cycle = 10s - 100s)

Large amplitude fluctuations in spontaneous fMRI signals in the resting state in human cerebral cortex

30
Q

What is functional connectivity in the resting state?

A

Functional connectivity corresponds to 2 areas which have correlated function fMRI signals at rest

Analysis is based on Pearson’s correlation (-1 <= r <= 1) → coefficient estimates similarity of time-courses of signals

Areas that are functionally connected in resting state are called RESTING STATE NETWORKS

31
Q

What is anatomical/structral connectivity vs functional connectivity?

A

Structural connectivity = axonal projections from 1 area (A) to another (B)
*Inject tracer and follow the signal

Functional connectivity = statistical dependent (ex: correlation) between the time-courses of activities in areas A and B

*2 areas that show functional connectivity are not necessarily anatomically connected

32
Q

How many resting networks are known in the human brain? What are they?

A

10 different resting state networks:
- Medial visual
- Occipital pole visual
- Lateral visual
- Default mode network
- Cerebellum
- Sensory-motor
- Auditory
- Executive control
- Fronto-parietal (perception-somesthesis-pain)
- Fronto-parietal (language cognition)

33
Q

What is the Talairach system?

A

*Stereotactic coordinates

  • Talairach coordinate system is defined by rotating the brain such that the anterior and posterior commissures crossings with the mid-sagittal place from a horizontal line
  • Distances are measured from the anterior commissure which is defined as the origin of the Talairach coordinate system (0,0,0)
  • This coordinate system makes it possible to spatially scale or wrap an individual brian MRI image onto a model prototypical brain

*This model prototypical brain is made by averaging many Talairach brain models

34
Q

What are the 2 columns/different planes of each resting state networks?
What is BrainMap?

A

Left column = RSN → 36 subject resting fMRI dataset

Right column = BM → 26,671-subject BrainMap activation database
- a corresponding network from BrainMap, shown superimposed on the MNI152 standard space template image

  • 3 most informative orthogonal slices
  • Networks were compared by computing spatial cross-correlation: mean correlation = 0.53

BrainMap = database of brain coordinates that are activated by specific tasks
*in Talairach coordinates to be able to compare

35
Q

What is the spatial cross-correlation value between the resting state functional networks and the active state of that same network?

A

r = 0.53 → these 2 networks are related
*Brain functional networks during resting state and activation do overlap

36
Q

What are default mode networks?
What experiment allowed to observe these regions?

A

Regions of the brain regularly observed to DECREASE ther activity during attention compared to active state
*a set of regions more active during passive tasks than tasks demanding focused external attention

Meta-analysis of 9 functional PET brain imaging studies. In each of the studies included, the subjects processed a particular visual image in the task state and viewed it passively in the control state → decreased activity (blood flow) in task state

37
Q

What is the PCC?

A

PCC = posterior cingulate cortex, a central part of the default mode network

There is an intrinsic correlation between a seed region in the PCC and all other voxels in the brain part of the default mode network

Anti-correlated activity with IPS as IPS is part of the dorsal attention networks

Conclusion: the brain is intrinsically organized into dynamic, anti-correlated functional networks

38
Q

Which 2 functional networks are anti-correlated in the brain? What does it show?

A

The default mode network and the dorsal attention network show decreases and increases, respectively, during tasks; they also show anti-correlated spontaneous fluctuations.

Conclusion: the brain is intrinsically organized into dynamic, anti-correlated functional networks

39
Q
A

PHGY 314 (9 decks)

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