PSYCHO-PHYSIOLOGICAL MEASURES

Electro-dermal Measures

Electro-dermal measures measure the electrical activity of the skin surface in relation to various motivational-emotional situations.
Skin conductance is obtained by applying weak electric current to the skin (usually the palm of the hand), and measuring the skin’s resistance to the flow of the current.
The resistance to flow of current decreases (conductance increases) when a person becomes more aroused or alert.
This is also called galvanic-skin response (GSR).

Skin potential may also be used to study electro-dermal activity. It is obtained by placing two electrodes at a distance from each other on the skin, and measuring the difference in potential between two points on the skin.




Measures of the Circulatory system

Heart-rate and Blood-pressure are measures of circulatory system that give information about the arousal levels of the body in different conditions, and thus the functioning of the sympathetic and parasympathetic nervous systems.

Techniques for Studying the Brain Functioning

• Lesioning
– irreversible lesions
– reversible lesions
• Stimulation
–  electrical
– chemical
• Recording
• Scanning/ Imaging




Electroencephalography (EEG)

Electroencephalography (EEG) is the study of “brain waves”.
In an EEG, about twelve electrodes are placed at key points over the surface of the head, to record nerve activity.
Four main types of brain activity can be seen, classified by the frequency of the waves (from slowest to fastest – delta, theta, alpha and beta).
WAVE FREQUENCY CHARACTERISTIC
DELTA   0 – 4 Hz– Normally found in deep sleep and represent the activity of deeper brain structures.
THETA   4 – 7 Hz –Occur frequently during sleep and are abnormal if present in excess in an awake adult.
ALPHA  8 – 12 Hz — tend to disappear with mental concentration, particularly if vision is involved.
BETA    12 – 30 Hz– Are smaller and seen frequently in all age groups.

Electroencephalography is particularly useful in picking up abnormal brain activity that might be associated with seizure disorders like epilepsy, head injury, brain tumours, infection and inflammation of the brain, chemical disturbances and some sleep disorders.
The test can quickly be carried out with the person awake or asleep, or recorded over a longer period as the person goes about their daily routine (via telemetry).




Brain Imaging Techniques
Brain structural imaging can be at the tissue level, for example differentiating between grey and white matter or beyond even the microscopic level, for example exploring receptors present on different nerve types.
Brain functional imaging can provide information about blood flow to different parts of the brain and so give some idea of “activation”.

X-RAY (German physicist Wilhelm Röntgen, 1895)
X-ray can be used in revealing the blood vessels of the brain – angiography.
An abnormal expansion of a blood vessel, an aneurysm can be detected. In this way, the source of certain types of brain haemorrhage can be found, and some therapeutic methods like attaching a clip over, or inserting a coil within, the aneurysm, may be used, to prevent recurrent problems.

Pneumoencephalography and Ventriculography

Pneumo-encephalo-graphy included injecting air in the spinal cord. It used the fact that replacement of the serebro-spinal fluid (CSF) with air would provide a contrast medium, since the air would be much less dense than CSF to X-rays.
Ventriculo-graphy involved injecting air directly into the brain cavities, through small holes in the skull.

Computed axial tomography (CAT) – the CT scan

Sir Godfrey Newbold Hounsfield, an English electrical engineer, and Allan MacLeod Cormack , a South African
physicist, were independently credited with the invention that began a revolution in the study of the brain, and were awarded the Nobel Prize.
The CT scanner still used X-rays, but the detector was mounted on a rotating frame, with the person’s head in the middle of the “dough-nut”, so that the absorption of X-rays passing through the head could be measured at a series of angles.
The next development came in the late 1980s, with the invention of spiral CT. In this, the X-ray camera rotates spirally downwards and can encompass data from an entire organ within half a minute.
CT is better than any other brain scan at detecting blood in or around the brain soon after a brain haemorrhage (it appears white on the image) and also small fractures and other abnormalities of the skull bones.
Although CT can help identify large abnormalities of the brain tissue, such as tumours, the resolution of the scan is not good enough to pick up smaller ones.

Magnetic resonance imaging (MRI)

In the 1950s, it was discovered that different materials resonated at different magnetic field strengths.
The invention is credited to the British physicist Peter Mansfield and the American chemist Paul Lauterbur, who were awarded the Nobel prize for their efforts. Another American scientist, Raymond Damadian, is said to have produced the first MRI images of the human body.
Research into MRI began in the 1970s and the first scanners were tested on patients in 1980.
It works on the principle of nuclear magnetic resonance (NMR).
Magnetic resonance imaging detects protons like hydrogen. Because hydrogen is a principal constituent of water (H2O), which forms seventy per cent of the human body, MRI is particularly good at imaging the human body.
Applying a magnetic field of sufficient strength causes protons to align themselves in the direction of the magnetic field.
In the person lying inside the scanner, the protons within their brain tissues all line up in the same direction. Then, bursts of radio waves are generated, which causes the protons to resonate and generates the “MR signal”.
When the external radio waves are stopped, the time taken for the protons to line up with the magnetic field again depends on the characteristics of the tissue.
The detection part of the scanner picks up the released energy from protons throughout the brain as the radio waves are switched repeatedly on and off and this is reconstructed as a three-dimensional black and white map.
The most abundant odd-numbered nucleus in the brain belongs to hydrogen.
The rate of realignment of the hydrogen axis is determined by its immediate environment, a combination of both the nature of the molecule of which it is a part and the degree to which it is surrounded by water.
Hydrogen nuclei within fat realign rapidly, and hydrogen nuclei within water realign slowly. Hydrogen nuclei in proteins and carbohydrates realign at intermediate rates.
Routine MRI studies use three different radiofrequency pulse sequences.
The two parameters that are varied are the duration of the radiofrequency excitation pulse and the length of the time that data are collected from the realigning nuclei.




T1
Because T1 pulses are brief and data collection is brief, hydrogen nuclei in hydrophobic environments are emphasized. Thus, fat is bright on T1, and CSF is dark.
The T1 image most closely resembles that of CT scans and is most useful for assessing overall brain structure.
T1 is also the only sequence that allows contrast enhancement with the contrast agent gadolinium-DTPA.
As with the iodinated contrast agents used in CT scanning, gadolinium remains excluded from the brain by the blood-brain barrier, except in areas where this barrier breaks down, such as inflammation or tumour.
On T1 images, gadolinium-enhanced structures appear white.
T2
T2 pulses last four times as long as T1 pulses, and the collection times are also extended, to emphasize the signal from hydrogen nuclei surrounded by water.
Thus, brain tissue is dark, and CSF is white on T2 images. Areas within the brain tissue that have abnormally high water content, such as tumours, inflammation, or strokes, appear brighter on T2 images.
T2 images reveal brain pathology most clearly.
The third routine pulse sequence is the balanced sequence.
In this sequence, a short radio pulse is followed by a prolonged period of data collection, which equalizes the density of the CSF and the brain and allows distinction of tissue changes immediately adjacent to the ventricles.
MRI magnets are rated in teslas (T), units of magnetic field strength.
MRI scanners in clinical use range from 0.3 to 2.0 T.
In research settings for humans, magnets as powerful as 4.7 T are used; for animals, magnets up to 12 T are used.
Unlike the well-known hazards of X-irradiation, exposure to electromagnetic fields of the strength used in MRI machines has not been shown to damage biological tissues.

Functional Magnetic resonance imaging (fMRI)

Neuronal activity within the brain causes a local increase in blood flow, which in turn increases the local haemoglobin concentration. The net effect of neuronal activity is to increase the local amount of oxygenated haemoglobin.
This change can be detected essentially in real time with the T2 sequence, which thus detects the functionally active brain regions.
This process is the basis for the technique of fMRI.
Thus, the fMRI is actually a measure of blood flow in areas of the brain, rather than brain activity.
This can be combined with various tasks, for example looking at pictures that provoke strong emotions, to reveal which parts of the brain are activated.

Positron emission tomography (PET)

The scientist Michael Phelps is cited as the major contributor to development of the first PET camera, built in 1973 at Washington University, St Louis, USA.
The first commercial whole-body PET scanner however appeared at the end of 1976.
Positron emission tomography relies on a radioactive phenomenon, “positron decay”.
Certain radioactive materials will release positively charged particles, called positrons, as they decay.
Because a positron is a positively charged electron, it is the “antimatter” counterpart of the normal and abundant negatively charged electrons surrounding all atoms.
When a positron collides with its “matter” equivalent (a negatively charged electron), energy is released in two beams, at 180 degrees to one another; a ring of detectors around the person can record this and establish where the collision happened.
Millions of such collisions happen after the injection of the tracer.
The theory is that, if blood containing the tracer is preferentially taken up in one brain area during a specific task, or the tracer itself binds briefly to a receptor on a group of nerve cells, then “hot spots” will show up where relatively more collisions were detected.
Thus, a picture of the activity of the brain or the location of nerve receptors is built up.
A particle accelerator, or cyclotron, bombards chemical elements at high speed, to create radio-active materials that decay by positron emission. Chemists then attach these molecules to the substance that will be traced.
Since glucose is the major source of energy for the brain, glucose can be used as a tracer to look at brain activity.
PET has been used increasingly to study normal brain development and function as well as to study neuropsychiatric disorders and tumours.

Single photon emission computer tomography (SPECT)

Manufactured radioactive compounds are used in SPECT to study regional differences in cerebral blood flow within the brain.
This high-resolution imaging technique records the pattern of photon emission from the bloodstream according to the level of activity in different regions of the brain.
As with fMRI, it provides information on the cerebral blood flow, which is highly correlated with the rate of glucose metabolism.