Attention is a fundamental prerequisite for other mental functions, because if people find it difficult to concentrate on the stimuli in their environment, these are not perceived correctly, evaluated as irrelevant and thus not stored in memory. The result can be forgetfulness, which is not a fundamental malfunction of the actual memory system. Without attention, abstract or more demanding thought processes cannot be carried out, such as learning new skills, remembering important information or planning activities. Attention encompasses many individual attentional functions, such as selective attention, in which individual stimuli are given special focus while others are neglected, sustained attention, i.e. attention that is continuously maintained over a longer period of time, or visual-spatial selective attention, which enables people to orient themselves in space. In attention, many processes and many areas of the brain interact to enable active attention that is controllable, focused and comprehensive enough.