A taxonomy of attention

In 1890, William James famously declared in The Principles of Psychology that “Everyone knows what attention is”. Attention has become a catch-all term for how the brain processes its own information, and its effects can be measured through behaviors, electrophysiology, and brain imaging methods. However, attention is so ubiquitous and has such multiple forms that it is unwieldy to study. When this happens, a taxonomy proves useful in the sense that organizing and categorizing large collections of finding might bring clarity and understanding. This is exactly what Chen et al. (2011) had in mind when writing the review “A taxonomy of external and internal attention”.

The review aims for a big-picture synthesis focusing on global issues and features at the expense of detail and serves as a portal for the attention literature, which might stimulate new research and more integrative theories. The review begins by defining and giving clarifying examples of core properties that are well-known terms in attention research such as limited capacity, selection, modulation, and vigilance.

A taxonomy of external and internal attention

They propose that the core characteristics of attention are shared across multiple systems, and their main goal is to organize them into a taxonomy. To do that, they categorize attention according to the types of information that attention operates over. In other words, the taxonomy is based on the targets of attention, encompassing all of its mechanisms and properties.

Sitting at your desk you can focus on the information on the computer screen, a conversation in the office next-door, or the taste of the tea in your cup. These are examples of external attention that can be distinguished from how you could instead be focusing your attention on your thoughts, trying to remember the author of a paper you just read or trying to decide at what time to go to lunch (examples of internal attention), all while staring at your computer screen with conversations going on in the hallway.

Therefore, the taxonomy makes a primary distinction between external and internal attention, and defines the terms as follows:

  • External attention (or perceptual attention) refers to the selection and modulation of sensory information, generally in a modality-specific representation and often with episodic tags for spatial locations and points in time.
  • Internal attention (or reflective attention) refers to the selection and modulation of internally generated information, such as the contents of working memory, long-term memory, task sets, or response selection.

Moreover, they also mention another distinction of attention between stimulus-driven  (exogenous, bottom-up) attention from goal-directed (endogenous, top-down) attentional control.

Figure 1. Mechanics of attention. From SenseandSensation.

Figure 1 provides a schematic overview of the proposed taxonomy. External and internal attention are on opposite ends of an axis. Each box within this space represents a target of attention. Along the orthogonal axis, goal-directed (top-down, endogenous) attention and stimulus-driven (bottom-up, exogenous) attention characterize how different levels interact. For reasons explained in the paper, goal-directed attention can target any of the levels in the taxonomy, whereas stimulus-driven attention is by definition a property of external attention.

Figure 2. Schematic overview of external and internal attention. Each box represents a target of attention. From Chun et al. (2011).

External Attention

External attention can be directed to spatial locations, time points, or modalities alone, or it can be directed to features or objects that can be selected across space, time, and modality.


Attention serves to select and modulate processing within each of the five modalities (i.e., vision, hearing, touch, smell, and taste), and it directly impacts processing within relevant sensory cortical regions.

  • Multiple signals coming from the same modality interfere more with each other than do signals coming in from across different modalities.

Spatial attention

Spatial attention concerns how to prioritize spatial locations in the environment, and it is especially important for understanding attentional deployment when there are multiple objects and events in the visual field.

  • Spatial attention is often compared to a “spotlight” (i.e., a single and limited focus), although attention can be split across multiple locations or spread across space with reduced effectiveness.
  • Spatial attention facilitates processing at attended locations and inhibits neighboring attended locations and items. Cueing improves target detection and discrimination.
  • Resolution deteriorates dramatically moving out into the periphery, affecting the ability to individuate two targets appearing close to another. Surprisingly, two targets close to each other are more difficult to perceive than two targets spaced far apart.

The Posner Task is a well-known spatial orientation/cueing task. You can read more about it and run a demo of the task in PsyToolKit.

Temporal attention

Attention can also be focused on stimuli appearing at different time points even in the same location. Temporal attention can be studied by asking subjects to search for targets among distractors appearing in the same location in rapid succession. You can train your ability to retain and report targets presented in rapid serial visual perception by running the demo of an Attentional Blink Paradigm.

Features and objects

Features are points in modality-specific dimensions, such as color, pitch, saltiness, and temperature. One of the primary mechanisms for selection is via saliency (i.e., unusual or extreme values) in these features dimensions. Nonetheless, attention can be directed not just to features but to whole objects. When objects are selected, all of its features are selected together with the object, including information about the identity and history of the object.

  • It is easier to select two features coming from the same object that to direct attention to two features that span across two objects.

Internal Attention

Whereas external attention relies on selecting perceptual information coming through the senses, much of cognition involves regulating our internal mental life, such as trying to remember what is in the fridge, or planning what to eat for dinner on the drive home from work. Internal attention includes cognitive control processes and operates over representations in working memory, long-term memory, task rules, decisions, and responses. Indeed, Chun et al. define internal attention as the set of operations that are focused on internally cognitive representations.

Response and task selection

When asked to make two simple responses or choices in succession, the ability to execute the second response is delayed when it appears within a few milliseconds of the first. This delay is known as the psychological refractory period. You can experience how it is like by running a demo of the Psychological Refractory Period Paradigm (PRP).

  • Observers are slower to switch from one kind of task to a different task, as compared to simply repeating the same task.

Moreover, response and task selection require inhibition of competing options. This is especially important for simple behaviors such as withholding a response or saccade when a stop signal appears. It is also true in automated tasks such as the Stroop interference, where naming the color of a word is slowed by the difficulty of suppressing the written word when it is the name of a different color. The Wikipedia web site gives a good description of the effect, and Psytoolkit provides a demo of the Stroop Task.

Response and task selection are clearly internal processes, they suffer from the limited-capacity problem (read the section “Basic characteristics and functions of attention” from the paper for further information), and are challenged to sustain vigilance. Indeed, these processes seem to be affected in disorders such as attention-deficit hyperactivity disorder (ADHD).

Long-term memory

Attention helps determine which information is encoded into long-term memory and how it is retrieved. It has been found that elaborative encoding, which involves actively associating new information with context and other information in the mind, enhances memory.

Reflecting back on a recent perceptual experience also benefits long-term memory, although the act of retrieving one item increases the likelihood of later forgetting other unretrieved items that share associative links. This results from the strengthening of associations during retrieval combined with the weakening of associations for unretrieved items. Notwithstanding, such competitive interactions during retrieval may be essential for learning.

  • Forgetting typically arises from memory retrieval failures rather than the loss of the information per se.

Working memory

Working memory enables the maintenance and manipulation of information in the absence of sensory support (of what is no longer externally available). Perceptual selection is critical because working memory is limited in capacity and serves as a filter that determines entry into working memory for maintenance.

  • In the case of vision, the capacity of working memory is about four objects, whereas in verbal working memory (i.e., phonological loop) the capacity is about seven chunks.

Internal attention includes a cognitive control that prioritizes which perceptual information to encode and maintain in working memory while suppressing distraction. Maintainance of information in working memory biases attention for similar kinds of information and correspondingly guides eye movements.


M.M. Chun, J.D. Golomb, and N.B. Turk-Browne. “A Taxonomy of External and Internal Attention“. Annual Review of Psychology, 2011. 62:73-101.



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