[Causes of the slowing-down of cognitive processes in aging: the noise hypothesis]

A typical phenomenon accompanying aging is the slowing down of cognitive processes. This slowing down may be considered to affect only specific aspects of information processing or it may be looked upon as a general property. As yet there exists no generally accepted explanation of this phenomenon. There is disagreement about the exact localisation and the physiological basis of the slowing down. However, a number of hypotheses has been formulated. These hypotheses have in common that the slowing down is attributed to a form of 'noise', either at neuronal level or at psychological level. We discuss several of these noise-related theories and their predictions and review the empirical basis for each of them. It is concluded that the different hypotheses do not exclude each other and that slowing down of cognitive processes can be observed at different levels of information processing.