Clark Hull (standing with visor) supervising a classical conditioning experiment at Yale University in the 1920s. The seated graduate student is being conditioned to blink at the sound of a tone in anticipation of a slap to the face. Given that there is a brief interval between the tone and the slap to the face, this is an example of trace conditioning. Actually this particular graduate student, Ernest Hilgard, went on to become one of the most famous psychologists of the twentieth century.
Courtesy of Elizabeth Jecker (daughter of Ernest Hilgard)