CLASSICAL-CONDITIONING AND BRAIN SYSTEMS - THE ROLE OF AWARENESS

Classical conditioning of the eye-blink response, perhaps the best stu died example of associative learning in vertebrates, is relatively aut omatic and reflexive, and with the standard procedure (simple delay co nditioning), it is intact in animals with hippocampal lesions. In dela y conditioning, a tone [the conditioned stimulus (CS)] is presented ju st before an air puff to the eye [the unconditioned stimulus (US)]. Th e US is then presented, and the two stimuli coterminate. In trace cond itioning, a variant of the standard paradigm, a short interval (500 to 1000 ms) is interposed between the offset of the CS and the onset of the US. Animals with hippocampal lesions fail to acquire trace conditi oning. Amnesic patients with damage to the hippocampal formation and n ormal volunteers were tested on two versions of delay conditioning and two versions of trace conditioning and then assessed for the extent t o which they became aware of the temporal relationship between the CS and the US. Amnesic patients acquired delay conditioning at a normal r ate but failed to acquire trace conditioning. For normal volunteers, a wareness was unrelated to successful delay conditioning but was a prer equisite for successful trace conditioning, Trace conditioning is hipp ocampus dependent because, as in other tasks of declarative memory, co nscious knowledge must be acquired across the training session. Trace conditioning may provide a means for studying awareness in nonhuman an imals, in the context of current ideas about multiple memory systems a nd the function of the hippocampus.