[Eeglablist] ERSP Conversion: dB to Microvolts-squared

[Zachary Moran]

Hi all,

I have been trying to convert ERSP estimates from dB units to uV or uV^2 but
have so far been unsuccessful.  I saw that a number of people have posted on
this topic, but I haven't seen any conclusive replies and thought I'd send
out another attempt.

Based on my own experience and the posts of others, the trouble seems to
relate to the following:

1)  There is a command - 'outputformat','old' -  that is supposed to make
this conversion (and is also said to convert ITC arrays into non-complex
numbers) that I haven't gotten to work.  The problem may be in the
programming - matlab requires a "10.^" rather than the dot-less "10^" to do
element-wise exponents on non-square matrices, and the below error suggests
that this may be missing:

??? Error using ==> mpower
Matrix must be square.

Error in ==> newtimef at 1750
    mbase = 10^(mbase/10);

2)  Someone has suggested that the 'baseline','NaN' command may provide a uV
output, though when I use it I get plots in units of uV/Hz.  It was also
suggested that these values must then be further converted to get absolute
power or volts but I'm unsure as to how that would be done.

3)  I considered the possibility of converting from dB to uV^2 directly in
Matlab by using a formula for converting from dB to Volts.  However, given
that dB here are used as logarithmic voltage ratios (I think?), I don't know
what value I would use as a reference voltage in the equation.  Would I use
the values given in the 'powbase' vector as reference values for the
conversion?

Any help or insight into conversion of ERSP and ITC arrays would be very
greatly appreciated!

Many thanks,

Zach Moran
Graduate Student
University of California-Los Angeles
Lab for Clinical Affective Psychophysiology
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