Introduction to Brain Imaging techniques and other methods

	Introduction to Brain Imaging Techniques and other methods	A number of techniques are available to investigate the question of how and where in the brain particular perceptual and cognitive processes occur. Tasks or tests can be devised that place varying levels of demand on the cognitive, sensory or motor capacities of the subject being tested. Performance of these tasks is then correlated with physiological measurements, and on the basis of these results, we may go on to ascribe functions to areas of the brain.

	Testing brain damaged subjects.	Deficits in stimulus processing are often reported by people who have suffered some kind of brain damage. The damaged areas are a good indicator of where particular stimuli are normally processed. Problems: Specific deficits in processing are rarely found without the occurrence of other deficits. Observation is of course after the event and therefore lacks adequate experimental control.
	Lesion Studies	The removal of part of the brain. Comparison is made between performance before and after the lesion and consequent deficits are noted. Problems: In practice, it is difficult to make a brain lesion that entirely removes one part of the brain whilst leaving the rest entirely intact. Lesions may damage other systems which happen to pass through the lesion site. .
	Stimulation.	This involves feeding a signal (chemical or electrical) into some part of a neural circuit and measuring its consequences at some other point. Problems: Difficulties involve delivering stimulation at an intensity that mirrors the level of activity that spontaneously occurs in the brain and determining which structures have been affected by the stimulation
	Single-cell recording	. Microelectrode recordings indicate specific neuronal networks dedicated to processing particular stimuli. (e.g. bars of a certain orientation, movement in a particular direction, particular objects like faces). For much of the 1950's physiologists probed the visual cortex using this technique.
	CT Scan	. This technique takes advantage of the fact that X-rays reflect the relative density of the tissue through which they pass. If a narrow X-ray beam is passed through the same object at many different angles, it is possible to use computational techniques to construct a visual image of the brain
	PET	- Positron Emission Tomography - This involves introducing a low activity, short-lasting radioactive label to compounds like glucose or oxygen in the brain. The radioactive labels decay in a characteristic way, giving off sub-atomic particles (positrons). By surrounding the subject's head with a detector array, connected to a suitable computer, it is possible to build up images of the brain showing different levels of radioactivity, and therefore, cortical activity. Problems: Expense, inaccessibility, lack of temporal (40 seconds) and spatial (1 cm) resolution.
	FMRI	Functional Magnetic Resonance Imaging - This involves measuring changes in oxygen levels in the brain which is an indicator of blood flow, which is also a property of cortical activity. The amount of oxygen carried in the blood affects the bloods magnetic properties. FMRI can detect the functionally induced changes in blood oxygenation. Spatial resolution is about 2mm. Problems: Expense, lack of availability and poor temporal resolution (2 seconds).
	EEG	Electroencephologram and Event-related Potentials (ERPs). Electro-chemical signals are the basis of communication between nerve cells and these can be recorded at the scalp. The first recordings from the human brain were published in 1924. The amplitude of normal EEG varies approximately between -100 and + 100 microvolts. The EEG recorded at the scalp is only a gross measure of the activity of large numbers of neurones but several frequency patterns may be distinguished in the EEG. Waves which occur in normal EEG include the delta rhythm at 1-4 Hz, the Theta rhythm at 4-8 Hz, the alpha rhythm at 8-12 Hz and the Beta rhythm at 13-20 Hz. Two measurements are commonly used to analyze an EEG record. The amplitude, or size of the waves, and the number of waves per second. Broadly speaking the more relaxed a person is the greater the amplitude and the lower the frequency of the waves. The lower the amplitude and the greater the frequency then the more likely it is that the person is in an excited state. About 20 electrodes are normally applied placed according to the '10/20' system developed by Jasper (1958). In this system each electrode is placed in terms of its proximity to particular brain regions - Frontal, Central, Temporal, Parietal and Occipital. Sites are given an odd number when on the left side of the head and an even number on the right, and midline electrodes are labeled 'z'. The most common form of ERP recording is made between a scalp electrode located at a site of interest and a reference electrode usually placed at a site which is relatively uninfluenced by the electrical activity of experimental interest.. Recordings are based on the difference in voltage between each exploring electrode and the common reference electrode. Eye and jaw movements can cause fluctuating electrical fields across the scalp thus subjects are requested to remain still and to minimise eye blinks/movements. Eye movement is also recorded with the EEG so that trials on which there are gross movements can be eliminated or corrected from the analysis. Current systems can record from up to 128 sites simultaneously using a 'geodesic dense array sensor net'. By making maps of the ERPs at different times after the stimulus event the relative times at which certain brain areas become active in processing information can be determined. Problems: More advantages than problems in that temporal resolution is excellent, it is relatively inexpensive, and involves no health risk.
	University Home \| Psychology Home \| Admissions \| Teaching \| Research \| People \| Events Contact \| A to Z
	© , School of Psychology, University of St. Andrews