A somatosensory evoked response (SEP.edf)

Description

The main difference between a somatosensory evoked potential and the previous antidromic sensory potential is that usually more than one channel has to be considered. In this case we detect the cortical potential with an electrode placed 2 cm behind vertex (referred to Fz) and the lumbar potential in L1 (referred to Iliac Crest). We use two ordinary signals and one annotation signal.

The header

This is the global header record

Version	0
Patient	X F 17-JAN-1978 X
Recording	Startdate 01-JAN-2002 emg1 jop N1 SEP L_Tibial_Nerve
Startdate	01.01.02
Starttime	11.30.00
Bytes in header	1024
Reserved	EDF+D
Data records	2
Duration	0.100
Signals	3

In this case, the signal header record defines three signals (two conventional and one annotation signal)

	First signal	Second signal	Third signal
Label	EP CZ'-FZ	EP L1-IC	EDF Annotations
Transducer	Ag-AgCl electrodes	Ag-AgCl electrodes
Phys. dimension	uV	uV
Phys. minimum	-100	-100	0
Phys. maximum	100	100	1
Dig. minimum	-9625	-9625	-32768
Dig. maximum	9625	9625	32767
Prefiltering	HP:20Hz LP:3kHz	HP:20Hz LP:3kHz
Samples	500	500	500
Reserved

The traces

The content of the first signal of the first and second data records can be seen in figure 5.1

Figure 5.1: Superimposed plot of records 0 and 1 for the first signal

The content of the second signal of the first and second data records can be seen in figure 5.2

Figure 5.2: Superimposed plot of records 0 and 1 for the second signal

The annotations

The annotations of the data records are

+0[20][20]StimA[20]<EDF_XMLnote><!--co...

+600[20][20]StimB[20]<EDF_XML...

The EDF_XMLnotes include

<EDF_XMLnote>
<!--coded as example-->
  <stimulus label="StimA" mode="constant intensity" 
  duration="0.2 ms" intensity="9.2 mA">Medial Malleolus</stimulus>
  <average mode="accepted">400</average>
  <measurements>
    <latency peak="N22" unit="ms">21.0</latency>
    <latency peak="P37" unit="ms">36.2</latency>
  </measurements>
</EDF_XMLnote>

And the second record

 
<EDF_XMLnote>
  <stimulus label="StimB" mode="constant intensity" 
  duration="0.2 ms" intensity="9.2 mA">Medial Malleolus</stimulus>
  <average mode="accepted">400</average>
</EDF_XMLnote>

Some comments

Coding EP generated by left and right stimulus pose an interesting problem. In general, the recording points are different. So one file can not be used for both sides. In this particular case, left and right stimulation share common recording places. The normative document indicates that even if right and left stimulus alternate (e.g., in some cases of intraoperative monitoring) they are stored in different files, although "Alternatively, if sufficient amplifiers are available, the recording can be stored in one file".

Also, notice that the measurements are not referred to any data record. In clinical practice it is usual to take the measurements on superimposed recordings. Alternatively a measurement could be considered for each trace.