No announcement yet.

Coherency Measures

This topic is closed.
  • Filter
  • Time
  • Show
Clear All
new posts

  • Coherency Measures

    To all subscribers,

    We recently have been looking closely into the use of frequency analysis of
    continuous time series biological data and have (apparently) come across
    some what of a misconception/misinterpretation within the movement domain
    concerning the use of the term "coherency".

    Our understanding is that coherency represents a measure of the relation
    between the cross-spectral power (squared) and the auto-spectra of each
    signal. In most movement control articles examining coherency, it has been
    likened to an analogous form of correlation in the frequency domain, and it
    is generally inferred (we think incorrectly) that this measure does not tend
    to include any details about the phase relation inherent in the data. This
    latter part of this statement, concerning the phase involvement, appears to
    be a general assumption bourne by a direct comparison of this technique to
    correlation techniques in the time domain.

    Under this assumption (with no phase relation), one would expect two signals
    which retain the same modal frequency to be reasonable coherent. However,
    this is not shown to be the case in many situations (For a particularly good
    example of this, see the figure in; Marsden et al, (1969) Electrenceph Clin
    Neurophys, pg 181). In these cases, two signals which oscillate at the same
    frequency, do NOT have high coherency, indicating that some part of the
    calculation of these varibles which may up the coherency value probably
    includes details about the phase relation between the signals. For example,
    two processes may oscillate at the same frequency (and hence have a "high"
    cross-spectral arrangement between the signals) but the degree of coupling
    (in phase/out of phase or no consistent phase relation) would effect the
    overall coherency value.

    Unfortunately, most software packages do not provided enough information as
    to how the coherency/phase/power spectral denisty values are calculated so
    one often has to assume that they are following the general formula
    guidelines (set out in such texts as Jenkins and Watts, (1968) & Glaser and
    Ruchkin, (1976)).

    The question we would like to resolve is whether the phase relation is an
    important component to consider when interpreting analysis of the coherency

    As usual, I will summerise all replies.


    Steven Morrison
    Dept Of Kinesiology
    Penn State University
    State College, PA 16802

    Phone: (814) 865-9544