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<font face="Helvetica, Arial, sans-serif">Hi,</font><br>
<br>
<font face="Helvetica, Arial, sans-serif">around two months ago the
same topic came up on CVnet and most people seemed to argue that
the convention originated from clinical neurophysiology, as the
plotters have been wired that way since the 40s. No one seemed to
be able to come up with a reference as to why the original
amplifiers were connected that way to the plotters, but some
people also speculated that it may be due to the EPSP and IPSP
relations to negativity/positivity.</font><br>
<br>
<font face="Helvetica, Arial, sans-serif"><font size="2">For those
interested in technical details, here's a quote from "Clinical
Neurophysiology, Volume 1: EMG, Nerve Conduction and Evoked
Potential" editeb by Binnie CD, Cooper R, Mauguiere F, Osselton
J, Prior PF and Tedman BM (2004, Elsevier - I think most med
school libraries should have this book, in case anyone wants to
look it up themselves), chapter 1.2 on techniques (p.25-26):</font></font><br>
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"At this point it would be appropriate to look at the polarity
of the displayed data. There is considerable ignorance and
confusion about the way that voltages applied to the input
terminals cause the output of the recording system (pen,
oscilloscope trace, etc.) to move (up or down), especially in
evoked potential systems. This is vital in the interpretation of
recordings, so it is prudent to take a little time to understand
it. </font></font><br>
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As we have just seen, the differential amplifiers used in
neurophysiology have two inputs and an earth. These inputs have
been variously known as grid 1 and grid 2 (dating from the days
of thermionic valves), "black and white" because of the colour
of the wires used, "positive and negative" according to the
convention of electrical engineering, and lead 1 and lead 2. Now
imagine lead 1 connected via a switch to the negative pole of
the battery and lead 2 to the positive pole. Let us also imagine
that the potential difference applied to the two inputs is 100
nmicrovolts. This can be obtained using a potential divider as
in Fig 1.2.10.</font></font><br>
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When the switch is closed, the output will be deflected (say) up
(depending on how the manufacturer has done his wiring) (Fig
1.2.10a). If the polarity of the battery is now reversed
(positive to input 1; negative to input 2), the output will be
displaced downwards (Fig 1.2.10b) - this is how a differential
amplifier works. In neurophysiology, where we are trying to
locate sources from the deflections of the display (the inverse
problem to that in Fig. 1.2.10), we can say that, if the
amplifiers are connected as in Fig.1.2.10a, an upward deflection
of the display means that the electrode connected to lead 1 is
negative with respect to that connected to lead 2. This is the
same as describing lead 2 as being positive to lead 1. It is not
possible to be more specific.</font></font><br>
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From 1945 to 1966 there was a generally accepted convention that
if a potential difference was applied to the two inputs of an
amplifier with lead 1 connected to the negative pole, then the
output display (usually pen writers) would be deflected upwards.
The same negative signal applied to lead 2 would deflect the
display downwards. However, with the arrival into clinical
neurophysiology of other techniques than EEG (particularly
evoked potentials), this convention was sometimes changed to
match engineering practice where positivity is usually displayed
upwards.</font></font><br>
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</font></font><br>
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This caused confusion and a straw poll at an international
meeting in 1976 showed that about 60% workers in evoked
potentials displayed a negative signal applied to input 1 as an
upward deflection on the output and 30% a downward deflection
(the rest did not know!). No one was prepared to change their
convention, so both were deemed acceptable provided that the
polarity was clearly indicated on any diagram (Donchin et al.,
1977). The matter was not improved by some manufacturers
labelling their input terminals positive and negative, red and
green or active and passive! It is interesting to note that Grey
Walter and George Dawson deliberately chose "black" and "white"
to avoid any connotation with other conventions. The matter
remains contentious, although in clinical EEG negative at lead 1
(grid 1, black lead) up is the accepted convention (BUN: black
up negative); in evoked potential work it usually depends on the
type of EP being recorded. It really does not matter which way
up the display is - we can all learn to read a map with south at
the top - so long as we know."</font></font><br>
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<br>
<font face="Helvetica, Arial, sans-serif">regards,</font><br>
<font face="Helvetica, Arial, sans-serif">jasna</font><br>
<br>
<br>
<font face="Helvetica, Arial, sans-serif"><font size="2">
Dr Jasna Martinovic</font></font><br>
<font face="Helvetica, Arial, sans-serif"><font size="2">
School of Psychology</font></font><br>
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University of Aberdeen</font></font><br>
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William Guild Building</font></font><br>
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Aberdeen</font></font><br>
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AB24 2UB</font></font><br>
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</font></font><br>
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tel: 01224 272240</font></font><br>
<font face="Helvetica, Arial, sans-serif"><font size="2">
email: j.martinovic @ abdn.ac.uk</font></font><br>
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web: <a
href="https://mail.abdn.ac.uk/owa/redir.aspx?C=c3a81cef73474a5ebe7379f5a81cb70f&URL=http%3a%2f%2fwww.abdn.ac.uk%2f%7epsy527%2fdept%2f"
target="_blank">http://www.abdn.ac.uk/~psy527/dept/</a></font></font><br>
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