Can GGPLOT make 2D summaries of data?

I wish to plot mean (or other function) of reaction time as a function of the location of the target in the x y plane. As test data:

library(ggplot2)
xs <- runif(100,-1,1)
ys <- runif(100,-1,1)
rts <- rnorm(100)
testDF <- data.frame("x"=xs,"y"=ys,"rt"=rts)

I know I can do this:

p <- ggplot(data = testDF,aes(x=x,y=y))+geom_bin2d(bins=10)

What I would like to be able to do, is the same thing but plot a function of the data in each bin rather than counts. Can I do this?

Or do I need to generate the conditional means first in R (e.g. drt <- tapply(testDF$rt,list(cut(testDF$x,10),cut(testDF$y,10)),mean)) and then plot that?

Thank you.