From the context, it sounds like the purpose of the contacts is to drain away or divert undesirable currents (to prevent interference to the sensor).

Such an isolation function is often called a 'guard' function in English.

sample ref (there admittedly aren't many):

Switching power stages are the reason for carrier injection into the substrate. By means of transient 3D device simulations the resulting potential distribution can be evaluated on full chip level. The dynamic potential spread in the substrate will be presented by short movies. The influence of different substrate dopings and guard-contact configurations will be demonstrated.
www.imse.cnm.es/esd-msd/WORKSHOPS/ESSDERC2000/presentations...