Learning

Pavlov was the first to observe that old dogs classically condition more slowly than young dogs. This initiated the very fruitful investigation of classical conditioning and normal aging. First in Germany and later in the United States, investigators moved from studying the slower autonomic nervous system responses such as salivation to assessment of conditioning in the somatic nervous system using the eyeblink response. The standard format for the presentation of stimuli in classical conditioning was named the "delay" procedure because there is a delay between the onset of the conditioned stimulus (CS) and the onset of the unconditioned stimulus (US). A neutral stimulus such as a tone or light is the CS, and it is presented for a duration of around half a second. While it is still on, the reflex-eliciting US air puff is presented, and the CS and US end together 50 to 100 msec later. Learning occurs when the organism responds to the CS before the onset of the US. This learned response is called the conditioned response (CR). Many additional classical conditioning procedures are used, but most studies of aging have used the delay procedure discussed here.

Differences between classical eyeblink conditioning in young and elderly nursing home residents in a single seventy-to-ninety-trial conditioning session were first observed in the early 1950s by the Russian scientists and reported in the United States by Edward Jerome in the first Handbook of the Psychology of Aging in 1959. These results were replicated and extended to normal, community-residing older adults. The main and striking result was the relative inability of the older subjects to acquire CRs. Several studies in Diana Woodruff-Pak's laboratory demonstrated that age differences in eyeblink conditioning do not begin in old age, rather the deficits begin to appear in mid-life by the age of fifty years.

The neural circuitry underlying eyeblink classical conditioning in all mammals including humans has been almost completely identified. The essential site of the changes that occur during learning reside in the cerebellum, and the hippocampus, while not essential, can affect the rate of conditioning. Significant changes occur in the cerebellum around the age of fifty years. Anatomical (volumetric) brain magnetic resonance imaging (MRI), delay eyeblink conditioning, and extensive neuropsychological testing were carried out in Woodruff-Pak's laboratory in healthy older subjects. The correlation between the volume of the cerebellum and eyeblink conditioning performance was exceedingly high. Hippocampal volume and total cerebral volume were also measured, but neither hippocampal nor total cerebral volume correlated with eyeblink classical conditioning. A similar high correlation between cerebellar volume and eyeblink conditioning was found in young adults. These volumetric MRI results add to the increasing evidence in humans demonstrating a relationship between the integrity of the cerebellum and eyeblink classical conditioning.

Whereas age-related changes in eyeblink classical conditioning do not impact the daily life of older adults, these changes focused researchers on aging in a brain structure that has been relatively overlooked in gerontological investigations of cognition. Recent findings have demonstrated a role for the cerebellum in such cognitive domains as attention, working memory, visuospatial processing, and language. Age-related changes in the cerebellum may play a role in the aging of these cognitive abilities.