calculating frequency interaction (now with the complete message)

Thomas Thesen tnt at PHYSIOL.OX.AC.UK
Sun Aug 14 17:05:18 CEST 2005

SORRY, I send the last message too early by mistake. Please find the whole
text below




I have a conceptual question which I'd like to post to the list.

It has been the practice in multisensory research to define the integration
of two inputs from different sensory modalities (e.g. auditory (A) and
visual (V)) by calculating the interaction [AV - (A+V)], where AV is the
simultaneous presentation of both unisensory stimuli. There, it is assumed
that any non-linear summation denotes the interaction of the two sensory
streams, and hence, is a measure of multisensory integration.

For example, assume the following response profile
A = 2 units
V = 3 units
AV = 6 units

integration effect = 6-(2+3)= 1

Here, the response to the bimodal presentation AV cannot be predicted by the
summation of the unimodal inputs alone, hence the difference is thought to
be related to multisensory integration.
This has been applied to both fMRI and ERP data (from MEG & EEG). (There
are, of course, some confounds associated with this method, such as
attention, etc, but that's a different matter.)

I am interested in how this interaction effect can be calculated with
frequency data. Frequency power estimates are done by squaring the amplitude
of the bandpassed response, so there is already a "non-linear" step involved
in the process.
Calculating the interaction as above could then result in erroneous
estimates of the integration effect:

A = 3 units; squared = 9
V = 3 units; squared = 9
AV = 6 units; squared = 36

integration effect = 6^2-(3^2+3^2) = 18

even though it is quite evident that the neuronal response to AV is a direct
summation of A and V.

Any suggestions on how to solve this problem would be greatly appreciated.


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