What is the function of attention and how does it work? - Chapter 7

What is selective attention, and what holds the anatomy of attention?

William James made an astute observation in the late 19th century. He insightfully captured key characteristics of attentional phenomena that are under investigation today. 'It is the taking possession by the mind' that we can choose the focus of our attention, that attention can be voluntary. Since James, knowledge about attention has blossomed, and researchers have identified multiple types and levels of attentive behavior. 

Arousal refers to the global physiological and psychological state of the organism, and it is best thought of on a continuum ranging from deep sleep to hyperalertness. In contrast, selective attention is not a global brain state. Instead, at any level of arousal, it is the allocation of attention among relevant inputs, thoughts, and actions while simultaneously ignoring irrelevant or distracting ones. Selective attention is the ability to prioritize and attend to some things and not to others. This is goal-driven control, steered by an individual's current behavior goals and shaped by learned priorities based on personal experience and evolutionary adaptions. 

Your reaction is stimulus-driven and therefor also stimulus-driven control, which is much less dependent on current behavior goals. The mechanisms that determine where and on what our attention is focused are referred to as attentional control mechanisms. Several cortical areas are important for attention: portions of the posterior superior temporal cortex, as well as more medial brain structures, including the anterior cingulate cortex. 

The superior colliculus in the midbrain and the pulvinar nucleus of the thalamus, located between the midbrain and the cortex, are involved in the control of attention. Damage to these structures can lead to deficits in the ability to orient both overt and covert attention. Overt attention is for instance eye gaze direction, the covert attention holds the attention directed without changing the eyes, head, or body orientation. Also, attention acts on sensory systems, and therefore much work on attention investigates the effect of attention on sensory signal processing. 

What is the neuropsychology of attention?

Much of what neuroscientists know about brain attention systems has been gathered from examinations of patients who have brain damage that influences attentional behavior. Though the best-known disorder of attention, attention deficit hyperactivity disorder (ADHD), has heterogeneous genetic and environmental risk factors, it is characterized by disturbances in neural processing that may result from anatomical variations of white matter throughout the attention network. 

What is neglect?

A patient with neglect may notice you more easily when you are on her right side, etc. And she may deny having any problems. Unilateral spatial neglect, or neglect, is quite common. It results when the brain's attention network is damaged in just one hemisphere, typically as the result of a stroke. More severe and persistent effects occur when the right hemisphere is damaged. The right-hemisphere lesion biases attention toward the right, resulting i n a neglect of what is going on the left visual field. The patient behaves as though the left regions of space and the left parts of objects simply do not exist, and has limited or no awareness of her lesion and deficit. 

Neuropsychological tests are used to diagnose neglect. 

• In the line cancellation test, patients are given a sheet of paper containing may horizontal lines and are asked to bisect them in the middle. Patients with left-sided neglect tend to bisect the lines to the right of the middle. 

• There is also an related test that asks patients to copy objects or scenes. When you ask a patient to copy a clock with a right-hemispheric neglect, the patient shows an inability to draw the entire clock and tends to neglect the left side of the clock. 

Visual field testing shows that the patients are not 'blind' in their left visual field. They are able to detect stimuli normally when those stimuli are salient and presented in isolation. When simple flashes of light or the wiggling fingers of a neurologist are shown at different angles within the visual field of the patient, the patient can see each stimulus. But when you present simultaneously two stimuli, one in each hemifield, the patient fails to perceive or act on the contralesional stimulus. This is known as extinction, because the presence of the competing stimulus in the ipsilateral hemifield prevents the patient from detecting the contralesional stimulus. 

What is the difference between neglect and Bálint's Syndrome?

In contrast to the patient with neglect, a Bálint's syndrome patient demonstrates three main deficits that are characteristic of the disorder:

• Simultanagnosia is a difficulty in perceiving the visual field as a whole scene

• Ocular apraxia is a deficit in making eye movements to scan the visual field, resulting in the inability to guide eye movements voluntarily

• Optic apraxia is a problem in making visually guided hand movements

The patterns of perceptual deficits in neglect and Bálint´s syndrome are quite different, however, because different brain areas are damaged in each disorder. Neglect is the result of unilateral lesions of the parietal posterior temporal, and frontal cortex. It can also be due to damage in subcortical areas including the basal ganglia, thalamus and midbrain. Bálint´s syndrome patients suffer from bilateral occipitoparietal lesions, neglect shows us that disruption of a network of cortical and subcortical areas, especially in the right hemisphere results in disturbances of spatial attention

What are the models of attention?

Attention can be divided into two main forms: voluntary attention, also known as endogenous attention, is our ability to intentionally attend to something, such as a book. It is a top-down, goal-driven process, meaning that our goals, expectations and rewards guide what we attend. Reflexive attention, or exogenous attention, is a bottom-up, stimulus-driven process in which a sensory event - a loud bang, sting of a mosquito - captures our attention. 

It is useful to think that these two attention systems as being in perfect balance, so that we are neither so focused on something like a beautiful flower that we miss the tiger sneaking up behind us.

What is the cocktail party effect?

Imagine yourself at a Super Bowl party having a conversation with a friend. How can you focus on this single conversation while the TV is blaring out and boisterous conversations around you are present? This is called the cocktail party effect. Selective auditory attention enables you to participate in a conversation at a busy restaurant or a party while ignoring the rest of the sounds around you. By selective attending, you can perceive the signal of interest amid the other noises. 

Bottlenecks in information processing - stages through which only a limited amount of information can pass - seem to occur at stages of perceptual analysis that have a limited capacity. There are many stages of processing between the time information enters your eardrum and the time you become aware of what was said. At which stages are there bottlenecks that make attention necessary to favor one signal over another? This question has led to one of the most debated issues in psychology over the past six decades: Are the effects of selective attention evident early in sensory processing or only later, after sensory and perceptual processing are complete? Does the brain faithfully process all incoming sensory inputs to create a representation of the external world biased by the current goals and stored knowledge of your internal worlds. 

Broadbent elaborated on the idea that the information-processing system has processing bottlenecks. The sensory inputs that can enter higher levels of the brain for processing are screened early in the information-processing stream by a gating mechanism so that only the 'most important' or attended, events pass through. Early selection is the idea that a stimulus can be selected for further processing or be tossed out as irrelevant before perceptual analysis of the stimulus is complete. 

Models of late selection hypothesize that the perceptual system first processes all inputs equally, and then selection takes place at higher states of information processing that determine whether the stimuli gain access to awareness, are encoded in memory, or initiate a response. One way to measure the effect of attention on information processing is to examine how participants respond to target stimuli under differing conditions of attention. One popular method is to provide cues that direct the participant´s attention to a particular location or target feature before presenting the task-relevant target stimulus. Endogenous cueing is when the orienting of attention to the cue is voluntary and driven by the participant's goal. When a cue correctly predicts the location of the subsequent target, it is a valid trial. Sometimes, though, because the target may be presented at a location not indicated by the cue, the participant is misled in a invalid trial. Also, the researcher may include some cues that give no information about the most likely location of the impending target - this is the neutral cue. 

According to most theories, a highly predictive cue induces participants to direct their covert attention internally, shining a sort of mental 'spotlight' of attention onto the cued visual field location. 

What are the neural mechanisms of attention and perceptual selection?

Although most of the experiments in this chapter focus on visual attention, this should not be taken to suggest that attention is only a visual phenomenon. Selective attention operates in all sensory modalities.

What is voluntary visuospatial attention?

Visuospatial attention involves selecting a stimulus on the basis of its spatial location. It can be voluntary, such as when you attend to this page, or it can be reflexive, such as when motion at the door of the classroom attracts your attention and you look up. Spatial attention influences the processing of visual inputs: attended stimuli produce greater neural responses than do ignored stimuli, and this process takes place in multiple visual cortical areas. 

Many stages of neural processing take place within the visual area. Different neurons display characteristics receptive-field proper ties: some are called simple cells other are called complex cells. The simple cells exhibit orientation tuning and respond to contrast borders. Researchers found that spatial attention enhanced the responses of the simple cells, but did not affect the spatial or temporal organization of their receptive fields, which remained unchanged over the trials. 

We now understand that visuospatial attention can influence stimulus processing at many stages of cortical visual processing. Are the effects of attention the same at these different stages of processing, or does attention act at different stages of the visual hierarchy to accomplish different processing goals? One prominent model is known as the biased competition model for selective attention. This model may help answer two questions: (1) why are the effects of attention larger when multiple competing stimuli fall within a neuron's receptive field, (2) how does attention operate at different levels of the visual hierarchy as neuronal receptive fields change their properties? In this model the idea is that when different stimuli in a visual scene fall within the receptive field of a visual neuron, the bottom-up signals from the two stimuli compete like two snarling dogs to control these neuron's firing. Attention can help resolve this competition by favoring one stimulus. 

Could attentional filtering or selection occur even earlier along the visual processing pathways - in the thalamus or in the retina? Unlike the cochlea, the human retina contains no descending neuronal projections that could be used to modulate retinal activity by attention. But there are massive neuronal projections that extend from the visual cortex back to the thalamus. These projections synapse on neurons in a portion of the thalamic reticular nucleus (TRN) that surrounds the lateral geniculate nucleus. Research shows that highly focused visuospatial attention can modulate activity in the thalamus. 

What is reflective visuospatial attention?

Sometimes things in the environment attract out attention without our cooperation. This is called reflective attention, and it is activated by stimuli that are salient in some way. The more salient the stimulus, the more easily our attention is captured. So, the question that comes from this is, are reflexive and voluntary attention processes in the same way? To tackle this question, attention researchers have used a variant of the cuing method. These studies examine how a task-irrelevant event somewhere in the visual field, like a flash of light, affects the speed of responses to subsequent task-relevant target stimuli that might appear at the same or some other location. This method is referred to as reflexive cuing or exogenous cuing. Interestingly, when more than about 300 ms pass between the task-irrelevant light flash and the target, the pattern of effects on reaction time is reversed. Participants respond more slowly to stimuli that appear in the vicinity of where the flash has been. This phenomenon is called the inhibition of return (IOR) - that is, inhibition of the return of attention to that location. The recently reflexively attended location becomes inhibited over time such that responses to stimuli occurring there are slowed. 

Our automatic orienting systems has built-in mechanisms to prevent reflexively directed attention from becoming stuck at a location for more than a couple hundred milliseconds. Responses to endogenous and exogenous cues result in attention shifts that enhances the processing of attended sensory stimuli and decrease the processing of unattended stimuli. 

Does spatial attention automatically move freely from item to item until the target is located, or does visual information in the array help guide the movements of spatial attention among the array items? Researchers compared spatial attention and feature attention in a voluntary cuing paradigm. The researchers found that prior knowledge from the cue produced the typical voluntary cuing effect for spatial attention: participants were more accurate at detecting the presence of the target at the cued location compared to when the cue did not signal one location over another. 

When attention is focused on a stimulus, neurons in the visual system that code that stimulus increase their postsynaptic responses and firing rates. How does this happen in a selective fashion so that attended information is routed appropriately to influence subsequent stages of processing? One model suggests that at different stages of visual analysis, neurons that code the receptive-field location of an attended stimulus show increased synchrony in their activity. 

What are the attentional control networks?

As we know now, attention can be either goal directed (top-down) or stimulus-driven (bottom-up). Top-down neuronal projections from attentional control systems contact neurons in sensory-specific cortical areas to alter their excitability. As a result, the response in the sensory areas to a stimulus may be enhanced if the stimulus is given high priority, or attenuated if it is irrelevant to the current goal. Current models of attentional control suggest that two separate cortical systems are at play in supporting different attentional operations during selective attention: a dorsal attention network - concerned primarily with voluntary attention based on spatial location, features, and object properties, and a ventral attention network - concerned with stimulus novelty and salience. The two control systems interact and cooperate to produce normal behavior, and these interactions are disrupted in patients with neglect. 

The dorsal frontoparietal attention network is bilateral and includes the superior frontal cortex, inferior parietal cortex, superior temporal cortex, and portions of the posterior cingulate cortex and insula. The ventral network is strongly lateralized to the right hemisphere and includes the posterior parietal cortex of the temporoparietal junction (TPJ) and the ventral frontal cortex (VFC), made up of the inferior and middle frontal gyri.