Bechara A, Tranel D, Damasio H, Adolphs R, Rockland C, Damasio AR (1995). Double dissociation of conditioning and declarative knowledge relative to the amygdala and hippocampus in humans. Science 269: 1115–1118.

The procedure was to show different color slides to the patients and to have one color, for instance blue, be consistently followed by a loud startling noise (a 100dB boat horn). The expectation was that the patient should show a conditioned fear response, measurable as the skin's increased electrical conductivity (Galvanic skin response) when the blue slide is shown later. Thus "conditioning" refers to the fear response, while "declarative knowledge" refers to the ability to remember and identify the blue slide as the one that preceded the loud noise. The authors' summary reads:

"A patient with selective bilateral damage to the amygdala did not acquire conditioned autonomic [fear] responses to visual or auditory stimuli, but did acquire declarative facts about which visual and auditory stimuli were paired with the unconditioned stimulus. By contrast, a [different] patient with selective bilateral damage to the hippocampus failed to acquire the facts but did acquire the conditioning. Finally, a [third] patient with bilateral damage to both amygdala and hippocampal formation acquired neither the conditioning nor the facts. These findings demonstrate a double dissociation of conditioning and declarative knowledge relative to the human amygdala and hippocampus (p. 1115)."

In other words, the patient with the damaged amygdala knew the blue slide had been followed by the loud noise, but did not show a fear response (Galvanic skin response) to it. On the other hand, the patient with the damaged hippocampus did show the fear response to the blue slide, but did not remember that the blue slide had been followed by the loud noise during the experiment. A "double dissociation" is strong evidence that these two brain regions are involved in separate independent functions -- and indeed, that the the two functions ARE independent -- since the abilities lost in each case are separable, and are specific to which region is damaged though they are otherwise preserved.

In terms of classical conditioning of the fear response, the basic four components can be identified as:

US: the loud noise

UR: the Galvanic skin response (fear), automatically elicited by the loud noise without any conditioning

CS: the blue slide (but not the other colors), which was followed by the loud noise

CR: the Galvanic skin response (fear), eventually produced when the blue slide is shown

For comparison, Pavlov's conditioning of dogs' salivation response used US = food, UR = salivation to the food, CS = bell, and CR = salivation to the bell.

(See http://media.pluto.psy.uconn.edu/classical conditioning basic terms.htm)