Elsevier Editorial System(tm) for Hearing Research Manuscript Draft ...

the EEGLAB developers: “Applied to simulated, relatively low dimensional data sets for
which all the assumptions of ICA are exactly fulfilled, all three algorithms (infomax, jader,
and fastica) return near-equivalent components. We are satisfied that Infomax ICA
(runica/binica) gives stable decompositions with up to hundreds of channels (assuming
enough training data are given, see below), and therefore we can recommend its use,
particularly in its faster binary form (binica()). Note about jader: this algorithm uses 4thorder
moments (whereas Infomax uses (implicitly) a combination of higher-order moments)
but the storage required for all the 4th-order moments become impractical for datasets with
more than ~50 channels. Note about fastica: Using default parameters, this algorithm quickly
computes individual components (one by one). However, the order of the components it finds
cannot be known in advance, and performing a complete decomposition is not necessarily
faster than Infomax.”
21) Pages 7-8, Lines 188-189: ".equivalent current dipole modeling."
It would be nice to have a better explanation of this choice of modeling. It seems that one
advantage of searching for CI stimulus artifact is that we should know exactly where the
source is (i.e., the cochlea, and the from the location of the return electrode). Why not use
this information to generate the forward models? Does the ECD modeling return an accurate
estimate of this type of source? Also, the authors earlier mention that the activity from
artifact ICs is "less dipolar and, partly reveal information about the location of the internal
components of the CI device, as can be seen in Figure 1, top row." (page 5, line 109). To
what extent are they "less dipolar"? Also, the dipole location in Figure 1 is not very clear in
terms of 3D localization, and is difficult to really interpret in terms of location.
It seems that from an engineering view, these activities would represent highly dipolar point
charges, with information flow toward the return electrode. Is this not the case? If not, what
kind of activity is the implant generating? Could this less dipolar-ness be due to volume
conduction effects (the cochlea is very deep in the skull). Further, if such a large activation
is not precisely modeled from its known source, could this provide information about the
utility or limits of ECD modeling more generally?
Reply: We agree that our explanation might be misleading. By biophysics, coherent activity
across a small patch of cortex will have a near-dipolar projection pattern on the scalp. To
estimate the location of the equivalent dipole for an IC scalp map, therefore, we can apply
standard inverse source modeling methods to the IC map, as implemented in DIPFIT plugin.
It is not known which parts of the CI device contribute most to the artifact seen in the EEG
recordings. Other authors have pointed out that is likely that the RF transmitter is one of the
main sources (Debener, et al., 2008; Henkin, et al., 2008; Martin, 2007) of the artifact but
other parts may also contribute. We are not aware of any study where the activity of the
implant has been modeled with an ECD at the sensor level. Here we model the IC scalp maps
with an ECD with the main purpose of excluding the ICs likely to represent brain activity
from the pool that will be evaluated. Another criterion is then applied to the remaining ICs to
further investigate if these ICs represent CI artifacts. In Figure-II we show IC scalp maps and
the respective residual variances obtained after dipole fitting. The fact that all ICs
representing CI artifacts have residual variances larger than 30% indicates that the likelihood
that the generators of such scalp patterns are cortical sources is low. Whether this is related to
the curved positioning of (simultaneously active) electrodes in the cochlea we do not know
for certain (but this could be tested easily). Over the past few years we have analysed several
dozen EEGs, recorded from CI users in different labs, with the ICA method and did not notice
a single case where the artifactual ICs had a clear dipolar pattern and unambiguous source
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