Placing REF and BIAS frontally simplifies the montage but degrades common mode on posterior channels. Why BIAS is a driven-right-leg, not a ground.
On a compact cap, the temptation is to group the reference and the active ground frontally, where the skin is clear and stable. This placement choice has a direct cost on the quality of the most distant channels.
What the chain actually measures
The amplifier does not measure one electrode against earth, it measures a difference relative to the reference. With SRB2 used as a pure reference and SRB1 disabled, each channel sees REF as its common point. At x24 gain, the slightest common-mode imbalance ends up amplified.
A frontal REF forces occipital channels into a long common-mode path across the scalp. Mains noise picked up differently at the two ends of the skull is no longer rejected as well.
BIAS is not a passive ground
BIAS is a driven-right-leg: it actively reinjects the inverted common mode to cancel it. Frontally, it drives the nearby zone effectively but loses authority over posterior channels, where residual common mode rises.
The takeaway: REF and BIAS define the geometry of the rejection, not just the wiring. Placing them at the center of the montage, rather than at one end, balances performance across all channels.