Trace eyeblink conditioning in rabbits demonstrates heterogeneity of learning ability both between and within age groups

Abstract

conditioning in rabbits demonstrates heterogeneity of learning ability both between and within age groups. NEUROBIOL AGING 17(4) 619-629, 1996.-Rabbits 2 to 41 months of age were conditioned in the 500 ms trace eyeblink paradigm to cross-sectionally define the age of onset and the severity of age-associated impairments in acquisition of this relatively difficult hippocampally dependent task. Using a strict behavioral criterion of 80% conditioned responses (CRs), age-associated learning impairments were significant by 24 months of age. Among rabbits that successfully reached this criterion, impairments in acquisition plateaued at 30 months of age. However, the proportion of severely impaired rabbits (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 severe impairment (failure to reach criterion) were rare. Additional controls 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 criterion of 80% CRs when switched from a trace to a delay conditioning task that is not hippocampally dependent. The results are discussed in terms of operationally defining and predicting behavioral effects of aging, hypothetical neural mechanisms, and efficient experimental design.

Trace Eyeblink Conditioning Increases CA1 Excitability in a Transient and Learning-Specific Manner

“…It was shown that physiological reductions of I AHP increased the stability of stored patterns, whereas large increases reduced the correlation between network output and the stored pattern, i.e., memory and the recall of that memory (Berner, 1991). Such observations are intriguing because reduced AHPs have been observed in CA1 neurons in vitro after learning and because enhanced AHPs have been observed in CA1 neurons from aged animals (Landfield and Pitler, 1984;Moyer et al, 1992;Disterhoft et al, 1994a) that are significantly impaired in many learning tasks, including trace eyeblink conditioning (Thompson et al, 1996a). Another model based on empirical data demonstrated how general increases in postsynaptic excitability coupled with presynaptic, pathwayspecific modulation can prevent storage of incoming information from interfering with recall of previously formed associations (Hasselmo and Bower, 1993).…”

“…Subjects were New Zealand albino rabbits (Oryctolagus cuniculus), purchased from Hazelton rabbitry (Denver, PA), and maintained in accordance with guidelines established by the USDA and approved and managed by Northwestern University's Animal Care Committee. Rabbits received 500 msec trace eyeblink conditioning or pseudoconditioning as described previously (Moyer et al, 1990;Thompson et al, 1992Thompson et al, , 1996a. Briefly, rabbits were fitted with restraining headbolts and trained in pairs in individual sound-attenuated chambers for daily 80 trial sessions (mean intertrial interval, 45 sec).…”

“…Slow-learning rabbits were a small subset of six rabbits that showed Ͻ30% CRs after at least 15 training sessions. Three rabbits from this population were randomly selected and shifted to the easier delay conditioning task Thompson et al, 1996a) and rapidly acquired the task within 320 trials (data not shown), indicating no apparent sensory deficits. In addition, there were no significant differences in learning rates among any of the seven groups of trace-conditioned rabbits ( Fig.…”

The role of awareness in delay and trace fear conditioning in humans

“…SCRs were slightly larger on unperceived trace CS than on unperceived trace CS trials, and perhaps significant differences would have been observed had it not been for other factors that can influence conditioning. For example, trace procedures appear to be more difficult than delay conditioning, and increasing task difficulty often interferes with CR acquisition (Carter et al, 2003;Solomon & Groccia-Ellison, 1996;Thompson, Moyer, & Disterhoft, 1996). In addition, the duration of the trace CS was shortened to maintain the same interstimulus interval between CS onset and UCS presentation for the delay and trace procedures.…”

Aging and Learning-Specific Changes in Single-Neuron Activity in CA1 Hippocampus During Rabbit Trace Eyeblink Conditioning

“…In this paradigm, each trial consists of a neutral auditory conditioned stimulus (CS) followed by a stimulus free trace period, then a nonnoxious eyeblink-eliciting airpuff unconditioned stimulus (US) presented to the cornea. After repeated trials, young animals and humans learn to associate the CS and US, whereas a subset of aged animals and humans show poor learning on this task (Finkbiner and Woodruff-Pak 1991;Thompson et al 1996;Woodruff-Pak et al 1999). This ability to separate aging-impaired from -unimpaired subjects has made trace eyeblink conditioning a very effective tool in animal and human research on aging.…”