Which german sociologist explained that people living in cities experience intense stimulation and often respond by "tuning out" some of the world around them?

The idea that urban citizens become blase an tune out some of the world around them can be traced to the German sociologies Georg Simmel's 1903 work, "The Metropolis and Mental Life."  
Simmel's idea was that problem presented to the individual by modern life was that of maintaining one's individuality and independent existence in the face of historical and social forces. In Simmel's view the urban environment presented a particularly difficult problem because its constant barrage of stimuli threatened to overwelm the individual. In order to survive, therefore, the individual had to adapt by learning to tune out some of the stimulations of the city environment.