Ruff, D. A., Marrett, S., Heekeren, H. R., Bandettini, P. A., & Ungerleider, L. G. (2010). Complementary roles of systems representing sensory evidence and systems detecting task difficulty during perceptual decision making. Frontiers in Neuroscience, 4: 190. doi:10.3389/fnins.2010.00190.