My understanding is that language acquisition proceeds at the same pace
regardless of the channel (i.e. signed versus spoken) and that children who
learn both sign and spoken will acquire them at the same rate passing through
the milestones for both at the same time. Laura Petito, a researcher at one
of the Canadian universities (can't remember which), does work in this area,
and there is a video module on it included in the Scientific American set
that Worth Publishers provides.
Nancy Melucci
LACCD
