Five-year retention of the classically conditioned eyeblink response in young adult, middle-aged, and older humans.

Human participants who 5 years earlier participated in studies of acquisition of the classically conditioned eyeblink response to a tone conditioned stimulus (CS) and an air puff unconditioned stimulus (UCS) returned to the laboratory to test for retention of the conditioned response (CR). Retention consisted of 20 tone CS-alone presentations. Young adult participants (23-31 years of age at the time of retention testing) showed good retention of the CR (45%), middle-aged participants (45-52 years) showed reduced retention (28%), and older participants (69-78 years) showed little evidence of retention (< 5%). Retention testing was followed by reacquisition of the CR in which the CS and the UCS were again paired. The ability to reacquire the CR also showed a decline with age. The data suggest that the CR can be retained over long intervals and that the degree of retention is age dependent.