To accompany Baars & Gage - Chapter 2

1

Chapter 4: Methods.

Elsevier web materials. • Teaching

materials. • Powerpoints

with movies, figures, and major chapter points.

• Study Guide• Quiz items

To accompany Baars & Gage - Chapter 2

2

The brain generates an electromagnetic field.

• Very large arrays of neurons in the brain generate significant electrical and magnetic fields, which can be detected fairly easily. That is how the EEG was discovered in 1929, simply by placing an electrode on the scalp and amplifying the electrical signal.

Above, the first alpha rhythms at 7-12 Hz, discovered by Hans Berger in 1929 by placing an electrode over the back of the scalp --- the occipital lobe. The regular waves below are artificially generated to provide a timing trace at 10 Hz.

To accompany Baars & Gage - Chapter 2

3

The brain generates an electromagnetic field.

• Today, EEG is recorded by an array of electrodes placed on the scalp. Medical EEG makes use of about 20 electrodes, but research EEG machines may have as many as 256, to get the highest spatial resolution possible. (Left, bottom)

Delta waves, regular, high-voltage - <2.5 HzUnconscious non-REM sleep.

Fast, complex, irregular low-voltage waves - waking state and REM dreams.

Alpha waves, regular - 8-12 Hz, working memory, visual imagery,

and eye closure.

To accompany Baars & Gage - Chapter 2

4

MEG picks up the magnetic component of the electromagnetic field

around the brain. • MEG has much better spatial

resolution than EEG. Both MEG and EEG have instantaneous temporal

resolution.

To accompany Baars & Gage - Chapter 2

5

Single-cell studies: A small needle electrode, or a grid of tiny electrodes, can pick up

electrical activity from cells (or small sets of cells) inside the brain. • EEG and MEG signals reflect tens of billions of

neurons, generally near the surface of cortex. To look more closely at specific neurons or clusters of neurons anywhere in the brain, needle electrodes (or tiny electrode grids) are placed in the brain itself. Single-cell studies are fundamental in cognitive neuroscience. They often show large-scale functions at the smallest level of analysis.

• Most single-cell recording are done in animals, like the monkey (upper left). These implants are not painful, since the brain itself lacks pain receptors.

• However, epileptic patients are often studied with implanted electrodes, to determine the location of the epileptic "focus" (which triggers seizures) to be removed by surgery. It is also vital to avoid cutting areas of the brain that are needed for normal functioning, like language cortex.

• Scientific studies can sometimes be done under ethical guidelines in medical patients with electrode

implants.

Electrodes in a monkey brain. The spiking graph is

shown on the right, stimulated by a visual slanted bar.

Typical placement of depth electrodes in humans.

To accompany Baars & Gage - Chapter 2

6

Averaged evoked potentials (EPs) allow us to simplify massive,

and highly complex brain activity. • EPs (also called Event-Related Potentials, or ERPs)

occur when a fast, intense stimulus is presented, like a loud clap, a light flash, or an electrical shock to the hand.

• The result is a series of massive waves going through the entire cortex over a one-second period, like a tsunami; so large that it can be isolated from the background wave activity of the brain. These voltages are evoked, hence "evoked potentials."

• When EPs are averaged over multiple trials, a beautifully regular pattern emerges out of the noisy complexity of spontaneous EEG. (MIddle left) The peaks and valleys are labeled "Negative 1" or "Positive 2," by convention.

• EPs are highly sensitive to personal or cognitive significance. E.g., compare the subject's own name vs. other names on the lower left. Notice that both the height and timing of the large wave wave has

shifted.

To accompany Baars & Gage - Chapter 2

7

Complex EEG or MEG can also be analyzed into component sine waves. • With the advent of high-powered

computers, it also became possible to analyze very complex EEG/MEG into its components --- sine waves. (Using Fourier Analysis).

• The top graph shows raw EEG; the middle shows different colors for different frequencies; and the bottom shows the distribution of the electrical power in each frequency

range.

Regular rhythms can be extracted from the back, front and middle of the head. These appear to have important cognitive functions. For example, theta is believed to be involved in

episodic memory retrieval.

To accompany Baars & Gage - Chapter 2

8

We can also use electrical or magnetic energy to stimulate neurons

in the brain. • Electrical stimulation of a human brain during

surgery. Electrodes stimulated points in the cross-hatched regions. (Penfield & Roberts, 1957).

• TMS (Transcranial Magnetic Stimulation) of the intact human head stimulates specific brain regions. TMS is generally safe, and may have medical applications as well. Notice the number-8-shaped magnetic field generator. It stimulates a small patch

of cortex. (Lower left).

To accompany Baars & Gage - Chapter 2

9

EEG, MEG and single-neuron recording are direct measurers of electromagnetic activity in the brain. MRI, fMRI, and PET are indirect measures of brain metabolic

activity or blood-flow. • PET was the first indirect brain imaging

method to become practical. A radioactive glucose or oxygen "tag" is injected into the bloodstream, and as nutrients are metabolized in the brain, safe levels of radiation are released. The are recorded by radiation detectors situated around the head, and processed by computer to reconstructed slices of the brain. (Slice=tomos, PET = Positron emission tomography).

• A great deal of important work has been done using PET. (Left) However, it is expensive, invasive (because of the tracer injection) and slow (exposure times are about 20-30 minutes). A more popular method currently is MRI (and fMRI), based on somewhat different principles.

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

To accompany Baars & Gage - Chapter 2

10

MRI physics.

• (a and b) Protons in water --- the H's in H2O--- have random magnetic spin. Under a high magnetic field, protons all line up with the same "spin" orientation.

• (c, d and e) When a radio-frequency pulse is provided and the proton spin is "relaxed," the spin orientations become random again and the target emits an electromagnetic signal.

• The signal is picked up by the donut of detectors surrounding the

patient's head.

To accompany Baars & Gage - Chapter 2

11

MRI measures brain structure; fMRI measures brain function.

(MRI is tuned to proton spin; fMRI is tuned to the oxygen spin in hemoglobin.) • A famous MRI study: Brain structure in

London taxi drivers. Notice that we are not looking at brain activity directly, but at the relative size of a part of the brain, the hippocampus, which is known to be involved in spatial navigation. Notice the computer-constructed brain slices in the upper right-hand corner.

• An fMRI study: Looking at left-hand stimulation. We are looking at brain activity due to neurons firing in the right cortical region for the hand (somatosensory cortex). Notice that Rest and Task periods alternate every forty seconds or so. We can see the results of brain activity (but not as fast as the activity itself.)

To accompany Baars & Gage - Chapter 2

12

MRI and fMRI measure the atomic spin of molecules in a high

magnetic field. (Magnetic Resonance Imaging)

• (A) Neurons and the BOLD response. Spiking neuron trace in the upper graph shows fast electrochemical firing several seconds before the blood oxygen signal begins to change (lower graph).

• BOLD reflects a population of neurons. The BOLD (blood-oxygen-dependent) signal occurs when oxygen is used by active neurons (small dip), followed by a surge of new oxygen supply to the region (large peak), a momentary overshoot (second small dip), and a return to homeostatic balance. Notice that the BOLD response takes place over a period of several seconds, while the neuronal firing burst occurs much sooner. That is why fMRI is said to be an indirect measure of neuronal activity.

• (B) Another view of the same process. Red dots indicate oxygenated blood cells, with high levels of

oxygenated hemoglobin.

(A)

(B)

Neuronal firing

BOLD response

To accompany Baars & Gage - Chapter 2

13

Signal subtraction is needed to get meaningful results in PET/MRI/fMRI. • Because the experimental effects typically

constitute a small signal in a great deal of background activity, PET and fMRI use signal-averaging at each point in space. Two very similar experimental conditions are used, differing in only one crucial feature. Notice that the yellow brain scans (upper left) look very similar, but when one is subtracted from the other, a very clean signal remains. In this case it shows significant activity in the occipital lobe.

• Notice that we are looking down on a horizontal

(or axial) slice of the brain. (Below)

To accompany Baars & Gage - Chapter 2

14

A typical fMRI design: Faces vs. non-faces. • Notice the similarities

between the faces and non-faces conditions in this experimental comparison.

• The results are shown in the yellow and red regions of activation in the brain diagrams below, which include the face recognition areas of the inferior

temporal lobe.

To accompany Baars & Gage - Chapter 2

15

Tractography: White matter tracts (bundles of axons) are traced by

water flow.

• Since the axons that make up neural pathways are tiny tubes, water molecules tend to flow in the direction of the tube.

• MRI is tuned to the hydrogen molecules in water, and the H2O flow is transformed into artificially colored patterns, as above.

• These beautiful tractography scans reveal an immense highway system

inside the white matter of the brain.

To accompany Baars & Gage - Chapter 2

16

Summary: Picking up both direct and indirect brain signals.

• Direct brain signals are usually electromagnetic. They include EEG, MEG, and single-cell electrical recording. We can also pump in electromagnetic energy to stimulate neurons in the brain.

• Indirect signals are produced by brain metabolism and blood flow (glucose and oxygen for MRI/fMRI), and radioactive tracers (PET).

• All brain recording methods

have their pros and cons. (Left)

Spa

tial s

cale

Time scale