Academic journal article Hong Kong Journal of Psychiatry

Can Brain Mapping Help Psychiatry?

Academic journal article Hong Kong Journal of Psychiatry

Can Brain Mapping Help Psychiatry?

Article excerpt

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INTRODUCTION

Until the 1980s, EEG technology had remained virtually unchanged since the time of plans Berger. It was conventional for the changes in potential measured on the surface of the scalp to be written out as a voltage fluctuation on a sheet of moving paper. The advantages of this method are that it is simple, and that there is a permanent record of all that has gone on during the EEG recording session. The disadvantage is that the analysis that is available is limited to obtaining measurements from the paper trace. It is not surprising that there has been a search for a method of transforming the EEG waveform into a more flexible format for further analysis.

With the advent of the digital computer in the 1960s, early attempts were made to convert the EEG into numbers and these numbers were then used in statistical calculations to analyse the EEG (Reymond 1967, Lehman 1471). Since these early days, the field of EEG analysis has grown dramatically, and the computer analysis of EEG traces is no longer restricted to specialist units. Modern advances in computing technology and the reduction in unit cost, has led to the development of digital EEG machines. This advance is in the process of revolutionising EEG recording, analysis and display. It has also led to a split amongst those working in the field. This split is mainly by age. Those with a long established EEG practice prefer the traces written out on paper, and argue that any other method of display is liable to mislead. The younger amongst us recognise the power of the digital computer and believe that although there is still a place for the permanent recording of EEG traces on paper, these traces can usually be held in a computer memory and reviewed visually. They also maintain that modern computers come with a graphics package and that this allows for different formats of display to highlight different aspects of the signal.

The debate about brain mapping can thus be seen to be artificial. The near generation of EEG machines will automatically have a graphics package and thus Will of necessity provide the opportunity for an infinite variety of displays. I suspect that within another five years all new EEG machines will be digital, and so all new machines will have the ability to brain map and the added facility of transforming EEG data according to the whim of the investigator.

MAKING EEG MAP

TURNING EEGS INTO NUMBERS

Before the raw EEG can be processed by computer, it has to be turned into numbers. This process is called analogue to digital conversion (A/D). This conversion process samples all the EEG channels at an instant in time, and records their voltage levels. These voltage leVelS are then transformed into numbers and stored in the computer. This process is repeated at the next instant in time and so a continuous series of numbers is built up in the computer memory. It is usual to sample the EEG 128 times a second, which will give a good pictorial resolution of the signal up to a frequency of about 25 Hz. However, for special purposes the sampling rate may need to be increased to 256 samples per second: or occasionally, on very special situations (such as recording the brain stem auditory response) up to 10 kHz per channel. Large amounts of data are thus rapidly collected and so there must be adequate storage within the computer.

THE GRAPHICS PACKAGE

Once the EEG is turned onto numbers, then it is possible to illustrate any aspect of it. Brain mapping machines initially display the EEG channels on the screen in the conventional format. However, because the traces are stored as numbers, there is a flexibility which cannot be achieved in a conventional EEG machine. The traces may be expanded or reduced, they can be highlighted, overlaid one on another, turned upside downs or manipulated in any number of ways. …

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