  We use a maximum entropy technique to obtain the electron-boson spectral
density from optical scattering rate data across the underdoped region of the
Bi_2Sr_{2-x}La_xCuO_6 (Bi-2201) phase diagram. Our method involves a
generalization of previous work which explicitly include finite temperature and
the opening of a pseudogap which modifies the electronic structure. We find
that the mass enhancement factor \lambda associated with the electron-boson
spectral density increases monotonically with reduced doping and closer
proximity to the Mott antiferromagnetic insulating state. This observation is
consistent with increased coupling to the spin fluctuations. At the same time
the system has reduced metallicity because of increased pseudogap effects which
we model with a reduced effective density of states around the Fermi energy
with the range of the modifications in energy set by the pseudogap scale.
