TRACE EYEBLINK CONDITIONING IN RABBITS DEMONSTRATES HETEROGENEITY OF LEARNING-ABILITY BOTH BETWEEN AND WITHIN AGE-GROUPS

Rabbits 2 to 41 months of age were conditioned in the 500 ms trace eye blink paradigm to cross-sectionally define the age of onset and the se verity of age-associated impairments in acquisition of this relatively difficult hippocampally dependent task. Using a strict behavioral cri terion of 80% conditioned responses (CRs), age-associated learning imp airments were significant by 24 months of age. Among rabbits that succ essfully reached this criterion, impairments in acquisition plateaued at 30 months of age. However, the proportion of severely impaired rabb its (that failed to reach the 80% criterion) continued to increase age dependently. Using an easier criterion of 8 out of 10 CRs, behavioral impairments were not detected until 30 months of age, and cases of se vere impairment (failure to reach criterion) were rare. Additional con trols demonstrated that the deficits observed were not attributable to nonassociative changes that might have artifactually skewed the data. Even severely impaired 36-month-old rabbits were able to reach a crit erion of 80% CRs when switched from a trace to a delay conditioning ta sk that is not hippocampally dependent. The results are discussed in t erms of operationally defining and predicting behavioral effects of ag ing, hypothetical neural mechanisms, and efficient experimental design .