The necessity of diverging/converging connections

The execution of brain processes often requires hundreds of milliseconds. Even a simple reaction-time exercise (in which the subject is required to press a button each time a sound is heard) has a central delay of about 100 ms. As the task becomes more complex (e.g., by requesting the subject to respond only to one of several possible sounds), the central delay becomes longer. The long central delay is accounted for by assuming that information processing is done through neuronal activity that must traverse through many stations in tandem. This type of processing often is visualized as being carried through a chain of serially connected neurons, such as that shown in Figure 6.1.1A. However, this arrangement is faulty, because if one of the neurons in the chain is damaged, the entire chain (composed of n cells) will become inoperative. The cortex is subject to a process by which its neuronal population is continually thinned out. Comparisons of neuronal densities in the brains of people who died at different ages (from causes not associated with brain damage) indicate that about a third of the cortical cells die between the ages of twenty and eighty years [Gerald, Tomlinson, and Gibson, 1980]. Adults can no longer generate new neurons, and therefore those neurons that die are never replaced.