"Electroencephalographic Topography Measurements of Experienced Utility", emphasis on experienced. Pedroni A. et al, The Journal of Neuroscience, 20 July 2011, 31(29): 10474-10480. The response they measured unexpectedly increased disproportionately increasing reward, i.e. it did not demonstrate diminishing returns but rather the opposite.
A measure of the mismatch between decision and reward utility, and understanding its biological basis and how it differs between individuals, would be excellent for psychology as well.