The salience (also called saliency) of an item - be it an object, a person, a pixel, etc. - is its state or quality of standing out relative to neighboring items. Saliency detection is considered to be a key attentional mechanism that facilitates learning and survival by enabling organisms to focus their limited perceptual and cognitive resources on the most pertinent subset of the available sensory data. Saliency typically arises from contrasts between items and their neighborhood, such as a red dot surrounded by white dots, a flickering message indicator of an answering machine, or a loud noise in an otherwise quiet environment. Saliency detection is often studied in the context of the visual system, but similar mechanisms operate in other sensory systems.
When attention deployment is driven by salient stimuli, it is considered to be bottom-up, memory-free, and reactive. Attention can also be guided by top-down, memory-dependent, or anticipatory mechanisms, such as when looking ahead of moving objects or sideways before crossing streets. Humans and other animals cannot pay attention to more than one or very few items simultaneously, so they are faced with the challenge of continuously integrating and prioritizing different bottom-up and top-down influences.
The hippocampus participates in the assessment of salience and context using past memories to filter new incoming stimulus; placing those that are most important into the long term memory. The entorhinal cortex is the pathway into and out of the hippocampus and is damaged early on in Parkinsons disease.