calculating frequency interaction

[Thomas Thesen]

Hi,

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

then 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 = 4 units
V = 9 units
AV = 36 units