Lecture 3: Observation & Measurement of Neural Activity

Question 1

CT scan

Answer 1

Cranial (or Computed) Tomography

Structural Analysis Only

X-Ray looks at tissue only

Used most often for detection of brain damage

Question 2

MRI

Answer 2

Magnetic Resonance Imaging

Structural Analysis Only

Have the option to look at gray matter (VBM) or white matter (DTI)

Patients not suitable for MRI:
certain dental work disrupts the signal
metal in body [though surgical steel is okay]
homemade tattoos – this is a rare issue
patients with claustrophobia, movement disorder, kids unable to stay still, anxiety

Question 3

Voxel Based Morphology (VBM)

Answer 3

MRI Analysis (structural)

A voxel is a 3-D pixel, usually 3x3x3mm (sometimes 1x1x1)

The MRI measures the amount of gray matter in each voxel, usually to determine if there has been a reduction gray matter

Question 4

Diffusion Tensor Imagery (DTI)

Answer 4

MRI Analysis (structural)

Looks at white matter “tracks” – bundles of axons which resemble a riverbed

Establishes connection, but not direction of communication – can either be one or two way, but cannot determine sender versus receiver

Essentially just a map

Question 5

PET

Answer 5

Positron Emission Tomography

The system detects gamma rays emitted by a radioactive tracer, which is introduced into the body on a biologically active molecule

Structural:
Radioactive tracer
e.g. amyloid β to discover plaque associated with Alzheimer’s

Functional:
Glucose tracer – what’s occurring during cognition?
Monitors the ATP-ADP cycle– areas of the brain using more glucose – inference is that these areas are at work

Question 6

ATP-ADP Cycle

Answer 6

Adenine Triphosphate – “energy molecule”

All brain activity requires energy, ATP gets used up, turns to ADP (Triphosphate “T” turns to Diphosphate “D”)

Glucose and Oxygen is required to convert ADP back to ATP

Question 7

Energy Use in the Brain

Answer 7

**47% of energy use of brain:
Return to resting state via sodium-potassium pumps

**34% of energy use of brain:
Clear out neurotransmitters via reuptake, enzyme breakdown, or astrocyte absorption

Energy also needed for:
Neurotransmitter release
Action Potential more ions

“We use a lot of energy to maintain a resting state”

Question 8

fMRI

Answer 8

Functional Magnetic Resonance Imaging

BOLD – Blood Oxygen Level Dependent
Changes in blood flow caused by changes in metabolism (ATP) caused by increasing neural activity

Glucose needed for ADP-ATP + Oxygen

Oxygen Extraction Fraction: Ratio of oxygenated hemoglobin: deoxygenated hemoglobin

• deoxygenated hemoglobin is paramagnetic, which means it affects the fMRI signal, making the signal less clear. Paradoxically, instead of seeing less activity, the opposite occurs as fresh oxygenated blood “floods” the area

Question 9

fMRI Block Design

Answer 9

Subject performs same cognitive task during “on” blocks and “off” blocks of equal lengths of time

Advantage:
Higher statistical power– longer period of time – more of blood flow to the area

Disadvantage:
Same task, subject may become bored
Inattention may affect results

Question 10

fMRI Event Related Design

Answer 10

Isolated trials, varied presentation

More likely to keep the subject engaged
Hopefully different cognitive processes are employed

General Linear Model of Regression used to interpret and tease apart fMRI data

Question 11

fMRI, advantage/disadvantage

Answer 11

spatially impressive–we’re able to view 3x3x3mm voxels without physically opening the skull

temporally, “even further out from neural activity”
18 seconds of blood flow occurs for one second presentation of picture in an event related design, even though the thought took much less than one second

Question 12

Cognitive Subtraction Model

Answer 12

Experimental design involving comparison of two conditions are brain states that differ in only one discrete feature (i.v.)

e.g. visual recognition of faces vs. houses hopes to establish of cognitive piece/process that differs

Question 13

EEG

Answer 13

Electroencephalography

Measure of electrical activity, relatively safe, no magnets, no radioactive agents

high temporal resolution
poor spatial resolution

Measures Post Synaptic Potentials [*not action potentials]

• action potentials are too fast and move across axon
• post synaptic potentials have stable location and do not reset as fast

Electrodes placed on scalp with conductive gel

• -the signal is limited as it has to travel through several layers (meninges, pia mater, skull, skin, hair)
• • sometimes skin will be scraped off before attaching electrodes

Question 14

delta wave

Answer 14

1-3Hz
base level of wave
deep sleep

Question 15

ERP

Answer 15

Event Related Potential

Using the same EEG equipment, General Linear Model analyzes EEG signals into electrical changes that occur immediately after an item is presented (event)

GLM effectively averages activity across events of the same type (e.g. faces)

ERP looks at changes in frequency or amplitude
E.g. change in frequency due to ambiguous words causing slight delay

ERP looks for minor changes even though you can’t tell where the changes happen

• also looks for alpha waves in other areas to see whether these areas are being suppressed to facilitate activity elsewhere

Notation:
N Negative
P Positive
Single digit: which initial fluctuation
3-digit: time [milliseconds] at which fluctuation usually happens

Question 16

gamma wave

Answer 16

30-100Hz

active cognition, binding representations

Question 17

theta wave

Answer 17

4-8Hz
effort
error related negativity

Question 18

alpha wave

Answer 18

8-12Hz
active suppression of activity – response inhibition
similar to meditative state
restful (awake with eyes closed)

Question 19

beta wave

Answer 19

12-30Hz

attention, looking, engaging with stimulus, movement, motor cortex activity