(i) The determinants of selective attention are classified into external and internal factors. External factors relate to the features of stimuli. Important determinants include size, intensity, and motion of stimuli. Large, bright, and moving stimuli easily catch our attention. Stimuli which are novel and moderately complex also easily get into our focus. Studies indicate that human photographs are more likely to be attended to than photographs of inanimate objects. Similarly, rhythmic auditory stimuli are more readily attended to than verbal narrations. Sudden and intense stimuli have a wonderful capacity to draw attention. Internal factors relate to the characteristics of individuals.

(ii) Selective attention differs from sustained attention in several ways. While selective attention is mainly concerned with the selection of stimuli, sustained attention is concerned with concentration. Selective attention involves selecting a limited number of stimuli or objects from a large number of stimuli. Our perceptual system has limited capacity to receive and process information, so it can deal only with a few stimuli at a given moment. Sustained attention refers to our ability to maintain attention on an object or event for longer durations. It is also known as "vigilance". Air traffic controllers and radar readers provide good examples, as they have to constantly watch and monitor signals on screens for many hours.