Köhler analyzed the difference between overt and covert behaviours. He argued that the behaviourists focused solely on overt behaviours in order to make inferences concerning human functioning. Using his background in physiology, Köhler suggested that covert behaviours (such as heart rate and blood pressure) could offer additional insight into how we function and interact with the environment. In "Gestalt Psychology", Köhler describes advancements made in physiological research and the tools created to measure covert behaviours. Covert behaviours such as increased heart rate could provide additional insight into how people interact with particular stimuli. The behaviourists, according to Köhler, never adequately utilized these new instruments to make sound inferences on human behaviour.