Early stimulation studies: Deficit studies leave open the possibility that the lesion under study may not be the locus of the function itself but of some minor component of it. If function is dependent upon several areas, a loss in one area may not abolish but only modify the function.
Gustav Fritsch and Eduard Hitzig (Germany, 1870) by electrical stimulation of cortex of dog found muscular responses in various parts of the opposite side of the body.
A few years later (deficit study) David Ferrier in England found that ablation of the prefrontal areas of the cortex led to apathy/ lack of curiosity and active intelligent exploration.
Later stimulation studies: Wilder Penfield (Penfield and Rasmussen, 1950) in developing procedures for brain surgery, using tiny elctrodes produced detailed maps of the motor and somato-sensory cortex (disproportionate representation of various body parts - see figures in Stillings or other books). Found that stimulation of areas of temporal lobe produced detailed memories (more detailed than in voluntary recall).