[R] 'asymmetric span' for 2D loess?

[Tokuyasu, Taku]

Hello,

I am interested in performing a 2D loess smooth on microarray data, i.e.
log2 ratios on a 2D grid, using different spans in the horizontal and
vertical directions (the immediate reason being that replicate spots are
laid out in the horizontal direction).  Is it possible to do this in R?
Functions like loess(stats) seem to apply the same span for all
predictors, which carries over to functions like ma2D(marray).

As an elementary second question, are there circumstances where one
expects to see a substantial difference in the fits between say loess(y
~ x1 + x2) and loess(y ~ x1 * x2) with an interaction term (and if so,
what are they)?

Many thanks,

_Taku

Taku A. Tokuyasu, PhD
UCSF Helen Diller Family Comprehensive Cancer Center
San Francisco, CA 94143-0808