The rewarding efficacy of brain stimulation and its modulation by dopaminergic drugs in young adult and old BN F344F1 rats.

In old age there is evidence of waning motivation, and possibly lowering of mood. Physiologically there is a decline in the levels of brain neurotransmitters such as acetylcholine and dopamine, and loss of myelin. These changes might be expected to impair the functioning of brain circuitry for reinforcement, and to lead to impaired motivation.

Growth of brain stimulation reward as a function of duration and stimulation strength.

The strength of a train of rewarding brain stimulation required to support a criterion level of operant performance declines hyperbolically as the duration is increased. This finding has been attributed to a process of leaky integration. However, the rate at which integration approaches asymptote has been shown to depend on stimulation strength, a finding that differs from the behavior of a simple leaky integrator.