Saliency is the perceptual quality that makes something stand out from its environment, causing an object, sound, or thought to capture our attention. This phenomenon influences what we notice and remember. For example, a single red flower in a field of yellow ones is salient due to its distinctiveness. This quality is not inherent to the item but arises from its contrast with its surroundings.
The Brain’s Attention System
The brain employs two primary systems to determine what is salient, enabling us to navigate a world full of sensory information. These systems work together, allowing us to react to the unexpected while staying focused on our objectives. This interaction dictates how we allocate our limited cognitive resources.
One system is stimulus-driven, or “bottom-up,” saliency. This automatic process is where the physical properties of a stimulus grab our attention. Examples include a loud car horn, a bright flash of light, or a fast-moving insect. These stimuli are noticeable due to their intensity or novelty, ensuring we can react quickly to potentially important events.
The other system is goal-driven, or “top-down,” saliency. This form of attention is intentional, guided by our internal goals, knowledge, and expectations. For instance, if you are looking for a friend’s red coat in a crowd, the coat becomes salient because of your goal. Similarly, when searching for keys on a cluttered desk, your focused search makes them stand out.
These two attention systems are supported by a large-scale brain network called the salience network, composed of the anterior insula and the dorsal anterior cingulate cortex. This network acts as a filter, detecting salient events and switching between the brain’s other networks: the default mode network (internal thought) and the central executive network (external tasks). Functional magnetic resonance imaging (fMRI) studies confirm these brain regions become active when a person’s attention is captured by an external event or internal goal.
Visual Saliency
Visual saliency refers to the physical properties of an object that cause it to visually “pop out” from its background. This pop-out effect is a clear demonstration of the bottom-up attention system, where certain features automatically draw our gaze.
Several visual features contribute to this effect. Contrast is a powerful one; a black object on a white background is salient due to its luminance contrast. Color is another dominant feature, such as a single green apple in a bowl of red ones.
Properties like orientation and motion also play a significant role. A single diagonal line within a pattern of vertical lines will stand out due to its unique orientation. Likewise, a stationary object becomes salient the moment it begins to move, especially if everything else in the scene is static.
This process can be observed in the early stages of visual processing. Recordings from the primary visual cortex (V1) show that neurons respond differently to a stimulus that is distinct from its neighbors. This neural response provides a biological basis for the perceptual pop-out effect, explaining why our attention system prioritizes certain visual information.
Saliency in Technology and Media
The principles of saliency are widely applied in technology and media to direct human attention. Designers and marketers strategically manipulate visual elements to guide where people look, leveraging the brain’s natural attentional mechanisms to influence engagement and behavior.
In marketing and advertising, saliency makes products or calls-to-action stand out. A brightly colored “Buy Now” button is a classic example, using high-contrast color to make it the most salient element and increase clicks. Size and placement are also manipulated, as larger or centrally-placed ads are more likely to be seen.
This concept is also foundational to user interface (UI) and user experience (UX) design. Designers create a clear visual hierarchy to guide users by making important information or interactive elements, like login buttons and notification icons, more salient. This helps users navigate interfaces efficiently.
Artificial intelligence, especially in computer vision, also uses this concept. AI models can be trained to predict what humans find interesting in an image by creating “saliency maps.” These are grayscale images where brighter regions correspond to more salient areas. This technology is useful for automatic image captioning, object recognition, and helping autonomous vehicles identify objects like pedestrians.
When Saliency Goes Wrong
The brain’s saliency detection system can become dysfunctional, leading to challenges in perception and mental well-being. When this system is altered, the brain may struggle to filter stimuli appropriately or may assign importance to irrelevant information, changing how an individual experiences the world.
In neurodevelopmental conditions like Sensory Processing Disorder or Attention-Deficit/Hyperactivity Disorder (ADHD), the brain’s filtering mechanism is often affected. This can cause everyday sensory inputs, like the hum of fluorescent lights or the texture of clothing, to feel overwhelmingly salient and distracting. The brain may fail to habituate to these stimuli, making it difficult to focus or engage in social situations.
In conditions such as schizophrenia, the issue can manifest as “aberrant salience,” where significance is incorrectly attributed to neutral stimuli. This can lead to delusions, as an individual tries to make sense of why ordinary events feel intensely meaningful. fMRI research has shown altered activity in brain regions like the insula and striatum in individuals with psychosis during tasks involving salience attribution.