On the choice list above the cursor functions is "Field".
When 'Field' is selected a map of the potential field over the whole head is displayed in the position usually occupied by the montage diagram.
This map shows the potential field at the instant indicated by the simple cursor. If no cursor is displayed then no field display appears until a specific instant is indicated by clicking on the display.
The map is quite crude; the positions of the 15 electrodes of the 10-20 system are indicated intervening positions being interpolated. Potentials are calculated by the 'Average Reference' method.
When 'Field' is selected
A given montage will consist of a number of channels. Each channel will show a trace derived from one or more inputs. Usually an 'input' corresponds to the fluctuating potential recorded from an electrode but may, for example, be derived from a respiration transducer. EEG channels will mostly be bipolar; showing the difference between two inputs.
The Gain, TC and HF controls will, in general, apply only to EEG and similar traces (these are referred to as 'standard'). In the case of routine Electroencephalograms all, or the majority, of traces will have the same gain and filtering but there may be, for example, an EKG trace which might have a gain of 1 mV/cm (10.0 pixels per mV) and a TC of 1.0 second. Such a channel is 'non-standard' and the select function provides a way of manipulating the appearance of such channels.
In addition standard channels can be converted to non-standard and the derivations of individual channels may be altered if needed.
On the choice list above the cursor functions is "Select".
When this mode is chosen dotted triangles appear to the right of the traces indicating any channels which are not "standard". These channels are not ordinarily affected by operation of the Gain, TC and HF controls.
In order to alter the appearance of a channel it is 'selected' by clicking on the trace (or on the dotted triangle of a non-standard channel). When this is done the selected channel is marked by a black filled triangle. The controls will now affect this channel only.
Once the setting of a channel has been altered in this way all the settings are fixed and can be altered only while it is selected; when another channel is selected (or the select mode is left and then reentered) the 'fixed'channel is indicated by a larger shaded triangle.
When a channel is selected the trace may be 'dragged' upwards or downwards. (Drag the trace not the triangle - clicking on the black triangle will 'deselect': see below.) This may be useful where large (often assymmetric) waveforms tend to obscure adjacent channels.
For example
In this case 6 channels are shown from a 10 channel record. The lowest channel (10 - the 'Photic marker') shows nothing while the ECG channel above is of rather low amplitude (except for the extrasystoles).
First select Ch10 and drag downwards as far as it will go to get this channel out of the way. Then select Ch9, increase the gain and drag slightly downwards so that the complexes are clearly seen.
When a channel is selected the montage choice control is replaced by two controls. The left hand one displays the electrode to which the 'black' (lead 1) input of the channel is connected while the right one displays the electrode to which the 'white' (lead 2) electrode is connected. These connections can be altered in the usual way.
NOTE: THE ALTERED CONNECTIONS ARE NOT REFLECTED IN THE MONTAGE DIAGRAM.
An example of where this facility might be useful is shown:
The record (Ch5 to Ch8 - left side, bipolar Parietal) shows repetitive spikes arising in the occipital region which might be an unusual ECG artefact. By reconnecting the inputs of Ch8 to the ECG electrodes and adjusting the gain and time constant appropriately, we can demonstrate that despite occasions (ringed in red) where there appears to be a good correlation, overall there is no relationship between the occipital spikes and the ECG.
Clicking on the filled black triangle of a selected channel restores all controls Gain, Time Constant and HF cut; to their default values - i.e. the values originally assigned when the montage was loaded as well as restoring montage connections and baseline level (if these have been altered).
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