Attention is defined as a readiness on the part of the organism to perceive stimuli that surround it. Attention is sustained concentration on a specific stimulus, sensation, idea, thought or activity, enabling one to use information processing systems with limited capacity to handle vast amounts of information available from the sense organs and memory stores.
Attention involves the selection of some incoming information for further processing. At the same time, we give meaning to the information that is coming in. We process the information in the sensory registers for meaning. A filtering process at the entrance to the nervous system allows only those stimuli that meet certain requirements to pass through. Those stimuli that do get through the filter are compared with what we already know, so that we can recognize them and figure out what they mean.
Many experiments show that performance can be affected in the absence of explicit awareness of the stimulus. Because of this fact, attention does not seem necessary for at least some degree of perceptual processing, but does seem to be necessary for an event to enter consciousness or to be remembered. Contemporary theories of attention have sprung from theoretical framework developed by Donald Broadbent in the 1950s. Current models include selective serial models and parallel models with differential attentional weighting such as race models of selection. No extant model has accounted for the full range of empirical data, but substantial progress has been made.
Clearly attention is a significant process for life. Without the appropriate filtering mechanism we are overwhelmed by the incredible amounts of sensory information in the world. But without being able to focus on particular stimuli, all would be just a blur. Understanding the process of attention, and how it interacts with our view of the world, and thus our behavior in it, is vital to a complete understanding of human nature.
Attention is the selection of some incoming information for further processing. It is the process of selectively looking, listening, smelling, tasting, and feeling. We process the information in the sensory registers for meaning and, at the same time, we give meaning to the information that is coming in. A filtering process at the entrance to the nervous system allows only those stimuli that meet certain requirements to pass through. Those stimuli that do get through the filter are compared with what we already know, so that we can recognize them and figure out what they mean.
Attention is viewed as the cognitive process of selectively concentrating on one aspect of the environment while ignoring other things. Examples include listening carefully to what someone is saying while ignoring other conversations in the room or listening to a cell phone conversation while driving a car (Strayer, Drews, & Johnston 2003). Sometimes attention shifts to matters unrelated to the external environment, a phenomenon referred to as "mind-wandering" or "spontaneous thought." Attention is one of the most intensely studied topics within psychology and cognitive neuroscience.
Overt and covert attention
Attention may be differentiated according to its status as "overt" versus "covert." Overt attention is the act of directing sense organs towards a stimulus source. Covert attention is the act of mentally focusing on one of several possible sensory stimuli. Covert attention is thought to be a neural process that enhances the signal from a particular part of the sensory panorama.
There are studies that suggest the mechanisms of overt and covert attention may not be as separate as previously believed. Though humans and primates can look in one direction but attend in another, there may be an underlying neural circuitry that links shifts in covert attention to plans to shift gaze. For example, if individuals attend to the right hand corner field of view, movement of the eyes in that direction may have to be actively suppressed.
The current view is that visual covert attention is a mechanism for quickly scanning the field of view for interesting locations. This shift in covert attention is linked to eye movement circuitry that sets up a slower saccade to that location.
Changes in spatial attention can occur with the eyes moving, overtly, or with the eyes remaining fixated, covertly. Within the human eye only a small part - the fovea - is able to bring objects into sharp focus. However, it is this high visual acuity that is needed to perform actions such as reading words or recognizing facial features. Therefore, the eyes must continually move in order to direct the fovea to the desired goal. Prior to an overt eye movement, where the eyes move to a target location, covert attention shifts to this location (Hoffman & Subramaniam, 1995; Kowler et al., 1995; Deubel & Schneider, 1996 Peterson, Kramer, & Irwin, 2004). However, it is important to keep in mind that attention is also able to shift covertly to objects, locations, or even thoughts while the eyes remain fixated. For example, when persons drive and keep their eyes on the road, even though their eyes do not move, their attention shifts from the road to thinking about what they need to get at the grocery store. The eyes may remain focused on the previous object attended too, yet attention has shifted (Hoffman, 1998).
Voluntary vs. Automatic Attention
Attention can be directed either voluntarily, also referred to as endogenous control, or automatically, which is also called exogenous or reflexive attention. While endogenous control involves one choosing of their own volition to direct their attention, exogenous control occurs when an external object or event, for example, a bee flying by, grabs attention away from the book one is reading, and attracts it involuntarily. The neural mechanisms in the brain have been shown to produce different patterns of activity for endogenous and exogenous attention (Gazzaniga et al., 2002).
Three stages of attention orienting
Another influential idea came from Posner and Petersen in 1990, breaking orienting of attention into three distinct stages. The concept is that in order for a person to reorient to a new location, they first would have to disengage, or take attention away from where it is currently focusing. Next, the physical shifting of one’s attention would occur from one location to another. And finally, attention would be engaged, or focused onto the new location (Eysenck & Keane, 2005). Current research, regarding neural correlates of these physical shifts of attention, specifically focuses on the areas of covert and overt attention, as well as, voluntary attention and automatic attention shifts.
Attention-deficit hyperactivity disorder (ADHD) is a childhood mental disorder characterized by inattentiveness, inattention, impulsiveness, and hyperactivity. ADHD was once known simply as hyperactivity. The new name reflects the fact children with the disorder typically have trouble focusing their attention in the sustained way that other children do. Instead they are easily distracted, often impulsive and almost constantly in motion. ADHD affects nearly five percent of all school-age children and is much more common among boys than girls.
Many theorists believe that ADHD is present at birth, but becomes a serious problem only after the child starts school. The class setting, practically in all cultures and countries, demands that children sit quietly, pay attention as instructed, follow directions, and inhibit urges to yell and run around. The child with ADHD simply cannot conform to and cope with these demands.
Psychologists do not know much about the causes of ADHD, but most of them assume that biological factors are very influential. Family interaction and other social experiences may be more important in preventing the disorder than in causing it. That is, some exceptionally competent parents and patient, tolerant teachers may be able to teach "difficult" children to conform to the demands of schooling. Although some psychologists train the parents of children with ADHD in these management skills, the most frequent treatment for these children is a type of drug known as a psychostimulant. Psychostimulants do not work by "slowing down" hyperactive children; rather, they appear to increase the children’s ability to focus their attention so that they can attend the task at hand, which decreases their hyperactivity. Psychostimulants often produce only short-term benefits, and their use is controversial.
Research models of attention
William James, in his monumental Principles of Psychology (1890), remarked:
Everyone knows what attention is. It is the taking possession by the mind, in clear and vivid form, of one out of what seem several simultaneously possible objects or trains of thought. Focalization, concentration, of consciousness are of its essence. It implies withdrawal from some things in order to deal effectively with others, and is a condition which has a real opposite in the confused, dazed, scatterbrained state which in French is called distraction, and Zerstreutheit in German (James 1890, 403-404).
In his book Elementary Psychology of Feeling and Attention (1908), Edward B. Titchener described and analyzed attention as an elementary and basic unit for cognition along with sensations and perception.
Attention remains a major area of investigation within psychology and neuroscience. Many of the major debates of James' and Titchener's time remain unresolved. For example, although most scientists accept that attention can be split, strong proof has remained elusive. And there is still no widely accepted definition of attention more concrete than that given in the James quote above. This lack of progress has led many observers to speculate that attention refers to many separate processes without a common mechanism.
Areas of active investigation involve determining the source of the signals that generate attention, the effects of these signals on the neuronal tuning properties of sensory neurons, and the relationship between attention and other cognitive processes, like working memory.
In James' time, the method more commonly used to study attention was introspection. However, as early as 1858, Franciscus Donders used "mental chronometry" to study attention and it was considered a major field of intellectual inquiry by such diverse authors as Sigmund Freud, Walter Benjamin, and Max Nordau. One major debate in this period was whether it was possible to attend to two things at once (split attention). Walter Benjamin described this experience as "reception in a state of distraction." This disagreement could only be resolved through experimentation.
In the 1950s, research psychologists renewed their interest in attention when the dominant epistemology shifted from positivism (behaviorism) to realism during what has come to be known as the cognitive revolution (Harré, 2002). The cognitive revolution admitted unobservable cognitive processes like attention as legitimate objects of scientific study.
Colin Cherry and Donald Broadbent, among others, performed experiments on dichotic listening. In a typical experiment, subjects would use a set of headphones to listen to two streams of words in different ears and selectively attend to one stream. After the task, the experimenter would question the subjects about the content of the unattended stream.
During this period, the major debate was between early-selection models and late-selection models. In the early selection models, attention shuts down processing in the unattended ear before the mind can analyze its semantic content. In the late selection models, the content in both ears is analyzed semantically, but the words in the unattended ear cannot access consciousness. This debate has still not been resolved.
Anne Treisman developed the highly influential feature integration theory (Treisman & Gelade, 1980). According to this model, attention binds different features of an object (such as color and shape) into consciously experienced wholes. Although this model has received much criticism, it is still widely accepted or held up with modifications as in Jeremy Wolfe's Guided Search Theory.
In the 1960s, Robert Wurtz at the National Institutes of Health began recording electrical signals from the brains of macaques who were trained to perform attentional tasks. These experiments showed for the first time that there was a direct neural correlate of a mental process (namely, enhanced firing in the superior colliculus.
In the 1990s, psychologists began using Positron Emission Tomograph (PET) and later, Functional magnetic resonance imaging (fMRI) to image the brain in attentive tasks. Because of the highly expensive equipment that was generally only available in hospitals, psychologists sought for cooperation with neurologists. Pioneers of brain imaging studies of selective attention are psychologist Michael I. Posner (then already renown for his seminal work on visual selective attention) and neurologist Marcus Raichle. Their results soon sparkled interest from the entire neuroscience community in these psychological studies, which had until then focused on monkey brains. With the development of these technological innovations neuroscientists became interested in this type of research that combines sophisticated experimental paradigms from Cognitive Psychology with these new brain imaging techniques. Although the older technique of electroencephalogram (EEG) had long been to study the brain activity underlying selective attention by Psychophysiology, the ability of the newer techniques to actually measure precisely localized activity inside the brain generated renewed interest by a wider community of researchers. The results of these experiments have shown a broad agreement with the psychological, psychophysiological and monkey literature.
Clinical model of attention
Clinical models often differ from investigation models. This is the case of attention models. One of the most used models for the evaluation of attention in patients with very different neurologic pathologies is the model of Sohlberg and Mateer (1989). This hierarchic model is based in the recovering of attention processes of brain damage patients after coma. Five different kinds of activities of growing difficulty are described in the model; connecting with the activities that patients could do as their recovering process advanced.
- Focused attention: This is the ability to respond discretely to specific visual, auditory or tactile stimuli.
- Sustained attention: This refers to the ability to maintain a consistent behavioral response during continuous and repetitive activity.
- Selective attention: : This level of attention refers to the capacity to maintain a behavioral or cognitive set in the face of distracting or competing stimuli. Therefore it incorporates the notion of "freedom from distractibility"
- Alternating attention: it refers to the capacity for mental flexibility that allows individuals to shift their focus of attention and move between tasks having different cognitive requirements.
- Divided attention: This is the highest level of attention and it refers to the ability to respond simultaneously to multiple tasks or multiple task demands.
This model has been shown to be very useful in evaluating attention in very different pathologies, correlates strongly with daily difficulties and is especially helpful in designing stimulation programs such as APT (attention process training), a rehabilitation program for neurologic patients.
Corbetta and Shulman, who are proponents of the belief that separate neural systems exist for endogenous and exogenous control, conducted a meta-analysis of multiple studies showing brain activation due to either of the two attentional processes. Specifically, the dorsal posterior parietal and frontal cortex region are mainly implicated with voluntary attention, while activity is transiently shown in the occipital region. The endogenous mechanisms are thought to integrate previous knowledge, expectations, and goals to voluntarily decide where to shift attention. On the other hand, neural areas involved in reflexive attention are believed to have the purpose of focusing attention on events or objects that stand out in the environment. The temporoparietal cortex and ventral frontal cortex region, particularly in the right brain hemisphere, have shown involvement with reflexive attention (Corbetta and Shulman, 2002). Even though separate regions are thought to be in existence for these two attentional processes, the question still remains on whether these regions interact with one another, indicating more research on this point is still needed (Eysenck & Keane, 2005).
Neural correlates of attention
The environment around us is full of various objects, features and scenes that compete for our attention. The human mind is limited in its ability to process information, and simultaneous processing cannot occur without a substantial cost (Gazzaniga et al., 2002). Therefore, shifting of attention is necessary because it allows us to redirect attention to aspects of the environment we want to focus on. Research has shown that when an object or area is attended, processing operates more efficiently (Posner, 1980; Gazzaniga et al., 2002). We are limited by the size of our visual field. With multiple objects in a scene, only some may show up in our field of vision at one time. Therefore, the eyes, along with one’s attention must constantly be moved and, in a sense, refocused in order to process multiple stimuli. It is this practice of refocusing one’s attention which involves an attentional shift.
Some of the first research into the neurology behind attention shifts came from examining brain damaged patients. First, Posner et al. studied persons affected by progressive supranuclear palsy, a condition wherein it is difficult to exert eye movements voluntarily, particularly vertical movements. Patients were found to have damage present in the mid-brain area and associated cortical areas. Although patients were not able to move their eyes, they were still able to shift attention covertly. However, there was a slowing of the process of shifting attention in these patients, suggesting that the mid-brain and cortical areas must be associated with covert attention shifts. Additionally, previous research has shown support for covert attention shifts being associated with activity in the parietal lobe. On the other hand, research seems to indicate differences in brain areas activated for overt attention shifts, as compared to covert shifts. Previous evidence has shown that the superior colliculus is associated with eye movements, or overt attention shifts (Posner et al., 1982). Additionally, the medial cerebellum has shown activation only during eye movements (Corbetta et al., 1998).
Neural overlap for overt and covert attention
Although, after reviewing Posner’s research, it may seem logical to conclude that covert and overt attention shifts utilize different neural mechanisms, other more recent studies have shown more overlap than not. Multiple studies have shown activity evident in the frontal cortex, concentrating in the precentral sulcus, the parietal cortex, specifically in the intraparietal sulcus, and in the lateral occipital cortex for both overt and covert attention shifts (Beauchamp et al., 2001). This is in support of the premotor theory of attention. While these studies may agree on the areas, they are not always in agreement on whether an overt or covert attentional shift causes more activation.
Utilizing functional magnetic resonance imaging (fMRI) technology, Corbetta et al., found that overt and covert attention shift tasks showed activation within the same areas, namely, the frontal, parietal and temporal lobes. Additionally, this study reported that covert shifts of attention showed greater activity levels than in the overt attention condition. However, it is important to note that different tasks were used for the covert versus the overt condition. One task involved a probe being flashed to the subject’s fovea, while another task showed the probe in the participant’s peripheral vision, making it questionable whether these results can be directly compared (Corbetta et al., 1998). Nobre et al also sought to determine whether covert and overt attention shifts revealed activation in the same brain areas. Once again fMRI technology was utilized, as well as, two separate tasks, one for covert attention and one for overt attention. Results showed overlap in activated areas for overt and covert attention shifts, mainly in the parietal and frontal lobes. However, one area was shown to be specific to covert attention, which was the right dorsolateral cortex; typically associated with voluntary attention shifts and working memory. One should question whether this additional activation has to do with the selected task for the covert condition, or rather if it is specific to a covert shift of attention (Nobre et al., 2000).
Beauchamp et al. more recently attempted to reproduce these same results by performing a study utilizing the same task for both conditions, as well as, across multiple shift rates. Results were in agreement that covert and overt attentional shifts engage the same neural mechanisms. However, this study differed in that overt shifts of attention showed greater activation in these neural areas, and this occurred even at multiple shift rates. Once again, the neural regions implicated in this study included the intraparietal sulcus, the precentral sulcus, and the lateral occipital cortex. This larger activation evident with overt attention shifts was attributed to the added involvement of eye movements (Beauchamp et al., 2001).
Neural overlap for voluntary and reflexive attention
There appear to be agreement that multiple areas of the brain are involved in shifts of attention, however research is not quite as conclusive regarding the amount of overlap evident with voluntary versus reflexive attention. Rosen et al.’s study found a fair amount of overlap between endogenous and exogenous shifts of attention. Both conditions showed activation in the dorsal and parietal premotor areas. However, the voluntary condition also showed activation in the right dorsolateral prefrontal cortex, which did not appear in the reflexive condition. As this area has been shown to be associated with working memory, it may indicate that working memory is engaged voluntarily. The subcortical global pallidus region was also activated only in the voluntary condition. Additionally, the activation shown in the temporoparietal junction (TPJ) was slightly different in both conditions, with the endogenous condition showing more spreading to the lateral, anterior and superior regions. Although these differences did exist, overall there was a lot of overlap demonstrated for voluntary and reflexive shifts of attention. Specifically both showed activations in the dorsal premotor region, the frontal eye field area, and the superior parietal cortex (SPC), although, the SPC exhibited greater activation in the endogenous condition (Rosen et al., 1999).
Attention can be guided by top-down processing or via bottom up processing. Posner's model of attention includes a posterior attentional system involved in the disengagement of stimuli via the parietal cortex, the shifting of attention via the superior colliculus and the engagement of a new target via the pulvinar. The anterior attentional system is involved in detecting salient stimuli and preparing motor responses.
Many neural mechanisms are involved in shifts of attention. While the type of attentional shift can dictate different brain regions becoming active, there is a lot of overlap seen. For instance, with regards to covert and overt attentional shifts, much of the research seems to point to a shared neural network. Although common brain areas may be activated, they do tend to differ in terms of the amount of activation. For endogenous and exogenous attention, research was less clear about the amount of overlap in the neural areas. Voluntary and reflexive attentional shifts may have some overlap, but other studies do not support this. Additionally, even if the same neural areas are being utilized, one should question whether the same processes are being engaged within the same region. Further research, as neuroscience methods are able to gather more detailed and precise information may shed light on this. Finally, research can be reviewed in other areas of attention to give more insight into the shifting of attention. This review concentrated on visual shifts of attention, but it has also been shown that we can shift attention to an auditory target and selectively attend to this stimulus (Eysenck & Keane, 2005).
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