Between bodily changes and performance: some possible reasons for slowing with age.

A number of models are outlined which attempt to account for relationships between various bodily, and especially neural, changes and the widespread slowing of performance observed in old age. Muscular and neuromuscular changes can account for some types of slowing, but most types are attributable to central mechanisms. The accumulation of small increments of time in a long chain of cells from sensory input to motor output is shown to be an inadequate explanation. Correlations between reaction time, EEG alpha cycle-time and age are in line with the widespread tendency for increases of reaction time with age to be proportional, but do not take account of errors. A powerful model which does consider both time and errors is derived from the fact that signals from sense organs and within the brain have to be distinguished against a background of random activity ("noise") both in the signals themselves and in the central nervous system, and slowing of performance with age results from a fall in signal-to-noise ratios. This leads in some circumstances to absolute and in others to proportional increases of time taken by older people. Interacting with these factors are tendencies by older people to adopt higher criteria for responding, and to spend more time monitoring their actions. Certain cases in which there is a disproportionate increase with age of time taken appear to occur when memory is involved in holding data temporarily while other data are gathered to arrive at a decision. The several stages of information processing from input to output are affected by slowing with age differently in different tasks, so that detailed study of various aspects of tasks is needed to account for overall effects observed.