THE RISE AND FALL IN INFORMATION-PROCESSING RATES OVER THE LIFE-SPAN

We surveyed studies that measured information-processing durations in groups of experimental subjects (children or elderly adults) and a gro up of college-aged control subjects. Some studies varied the type of p rocessing while keeping the age of a subject group fixed. Process-dura tions in experimental subjects could be described by a multiplicative function of the control durations, regardless of the type of processin g. Other studies varied the age of the subject groups while keeping th e type of processing fixed. Process-durations declined during childhoo d, in a manner that could be described by a negative exponential funct ion of age. Process-durations increased throughout middle- and old-age , in a manner that could be described by a positive exponential functi on of age. The sum of the two exponentials defined a U-shaped function that described process-durations over the life span. The most importa nt studies varied both the type of processing and the age of the subje ct groups. An array of measurements of this kind could be described by a two-dimensional function that combined the multiplicative effect of process-duration and the exponential effects of age. The multiplicati ve effect of process-duration suggested that the execution of a proces sing sequence was conditioned by a single developmental parameter in b oth the experimental subject and the control subject. The exponential components determined the magnitude of the developmental parameter as the age of the subject changed. Given the global character of these ef fects, it seemed to us that the developmental mechanism may operate at a more elementary level than the information-processing stages concei ved by cognitive theories. In a developmental framework, information p rocessing may be reducible to a large number of small steps of a homog eneous duration or reliability, such as might be realized on a neural network. The exponential rate constants may be related to constant-pro bability hazards that act on one or another population of neural eleme nts to create minute defects or incremental improvements. Their cumula tive effects alter the functioning of the network over its lifetime, i n a way that parallels the observed changes in process-durations.