In daily life, people are exposed to different stimuli. These stimuli often evoke conflicting responses: some are relevant to our goals, while some are totally irrelevant. Studies of attention look into how people coordinate these various stimuli to achieve their desired goals. The study of attention plays an important role in cognitive psychology and cognitive neuroscience. Since the emergence of psychology, people have been studying attention. Ribot and Titchener were two of those researchers. There are five distinct periods in the historical overview of research on attention.

Which philosophers were important for the field of attention?

Interest in attention began in the field of philosophy, prior to the founding of psychology as a scientific discipline. Philosophers claimed that attention was central to human perception and thinking. Moreover, they stated that a person could only focus his attention on one thing at the same time. Important themes were the role of attention in awareness and whether attention was an automatic or controlled process.

Important philosophers were:

• Vives: mainly studied attention and memory;
• Malebranche: claimed that attention is needed to understand things;
• Leibniz: came up with the term "apperception", which refer to the processes that are needed to consciously perceive an event;
• Hamilton: claimed that a person can handle more than one thing at the same time;
• Herbart: developed mathematical models for research.

Who were the first psychological researchers and what did they investigate?

Due to methodological advances (mathematical models), at the end of the 19th century, it was possible to conduct psychological experiments. Wundt, who is credited with establishing the first laboratory devoted to psychological research, studied the speed of mental processes. This speed was measured based on reaction times. His conclusion was that attention could be consciously directed. Therefore, Wundt called his theoretical system 'voluntarism'. The first real psychological studies (no longer based on philosophy) were conducted into the relationship between perception and attention. The findings showed that attention was essential for visual perception, that the capacity of attention was limited and that attention could be consciously directed (Helmholtz).

Other important ideas about attention came from James. He believed that attention leads to clearness. With the use of attention you can withdraw from certain stimuli, so that you can deal with other stimuli more efficiently. Researchers at the beginning of the 20th century debated how this increased clearness was obtained. Does attention have a direct influence on clearness (Mach, Stumpf), or does attention have an indirect influence on clearness because other obscuring influences are inhibited (Wundt, Kulpe)?

James suggested several ways in which attention could be classified:

• Attention is directed either to a) physical or b) non-physical objects (ideas)
• Attention is either a) immediate or b) derived
• Attention may be either a) passive, reflex, non-voluntary, effortless (exogenous control) or  b) active and voluntary (endogenous control)

Most psychologists focused on perception. Some researchers, however, also looked at muscle movements that have to do with attention. In the period up to 1909, attention was central in psychological studies and the effect of attention were studied. Until then, the mechanisms behind attention were unknown.

What were the findings? 

During the period 1910-1940 there was not much research into the mechanisms behind attention. During this period, the focus of studies was behavior. Nevertheless, a number of important discoveries have been made about attention. These discoveries are described below.

• Task-switching costs: it appears that when people perceive multiple ways for doing something, people take longer compared to when there is only one option.
• 'Psychological refractory period effect': it appears that after stimulation, a neuron becomes less sensitive to the next stimulus. The time that the neuron is insensitive, is called the refractory period.
• Effects of irrelevant information in the Stroop test: for example, if red is written in blue, people have a lot of trouble saying that the word is blue, while people experience no difficulties in reading the word 'red'. 

Why was the cognitive revolution important?

From 1950 to 1974, people were interested in human information processing. This led to research into the mechanisms behind attention. Research was mainly done into the interplay between theory and technical applications. It was also emphasized that selection is essential in attention. Two different movements emerged, each with its own view on selection: the 'early selection theories' and the 'late selection theories'.

Broadbent developed the first complete model of attention: the filter theory. According to this theory, information is held in a preattentive temporary store, and only sensory events that have some physical feature in common (for example, spatial location) are selected to pass into the limited capacity processing system. So, Broadbent's filter theory implies that the meaning of unattended messages is not identified. He also stated that there is an amount of time which is needed to shift the filter from one channel of events to another. Treisman later reformulated the filter theory into what is called the 'filter-attenuation theory'. According to this theory, early selection by filtering still precedes stimulus identification, but the filter attenuates the information only on unattended channels. 

The 'late selection theories' describe that all messages are identified, and that selection takes place later (hence the name). 

Through the creation of these theoretical models, research into attention became popular in the middle of the 20th century. Before the 70s, people focused mainly on auditory stimuli, while after the 70s they focused on visual stimuli. The research into visual stimuli led to the 'unitary capacity' or 'resource theory'. According to this theory, attention is a single resource that can be divided among different tasks in different amounts. The available supply of this resource varies as a function of arousal and task demands, and voluntary allocation strategies determine the tasks and processes to which the resource should be devoted when the demand exceeds the supply. This theory also serves as an explanation for dual-task methodologies. The basic idea behind dual-task methodologies is that multiple tasks should produce interference when they compete for the limited capacity resources.

After 1970, it was possible to study people through 'event-related-potentials' (looking at brain activity). Previously, research was mainly conducted on animals. 

What are contemporary theories of attention?

After 1975, a number of fields within the attention spectrum received attention.

• Multiple resource models: these models state that it is easier to perform two tasks when these tasks are different (when different modalities are needed): for example a task where hearing is central and a task where vision is central.
• Space-based approaches: state that attention is determined by the location in the visual field.
• Object-based approaches: state that attention is determined by the object. 
• Selection-for-action view: state that attentional limits should not be attributed to limited capacity resources or mechanisms. Instead, they view the limitations as byproducts of the need to coordinate action and ensure that the correct stimulus information is controlling the intended responses.
• Two fields of application became important: ergonomics (human-machine interaction, improvements in working environment etc.) and clinical neuropsychology.