Why does attention “behave” like a curve?
Attentional abilities behave this way because attention is a complex phenomenon, consisting of a set of neuropsychological processes, including:

• Arousal: the physiological preparation to receive stimuli from the external environment.
• Sustained attention: the ability to maintain a high level of concentration over a long period of time.
• Selective attention bottom-up: a phenomenon that occurs when some environmental input captures our attention independently of our will.
• Top-down selective attention: the ability to select certain inputs in order to be able to process them more thoroughly at a later time.