Third, we need more research focused on understanding why object-based attention is more dominant in everyday life than under laboratory conditions.
One possibility is that we have an incentive to attend to objects in everyday life but this is often missing in the laboratory.
responded rapidly to a light. The light was preceded by a central cue (arrow pointing to the left or right) or a peripheral cue (brief illumination of a box outline). Most cues were valid (indicating where the target light would appear), but some were invalid (providing inaccurate information about the light’s location).
KEY TERM
Covert attention
Attention to an object in the absence of an eye movement towards it.
Responses to the light were fastest to valid cues, intermediate to neutral cues (a central cross) and slowest to invalid cues. The findings were comparable for central and peripheral cues. When the cues were valid on only a small fraction of trials, they were ignored when they were central cues but influenced performance when they were peripheral cues.
The above findings led Posner (1980) to distinguish between two systems:
1 An endogenous system: it is controlled by the individual’s intentions and is used when central cues are presented.
2 An exogenous system: it automatically shifts attention and is involved when uninformative peripheral cues are presented. Stimuli that are salient or different from other stimuli (e.g., in colour) are most likely to be attended to using this system.
Corbetta and Shulman (2002) identified two attention systems. First, there is a goal-directed or top-down attention system resembling Posner’s endogenous system. It consists of a fronto-parietal network including the intraparietal sulcus and is the dorsal attention network. This system is influenced by expectations, knowledge and current goals. It is used if observers receive a cue predicting the location or other feature of a forthcoming visual stimulus.
Research activity:
Attention-grabbing adverts
Second, Corbetta and Shulman (2002) also identified a stimulus-driven or bottom-up attention system resembling Posner’s exogenous system. It is often described as the ventral attention network. This system is used when an unexpected and potentially important stimulus (e.g., flames appearing under the door) is presented. This system (consisting of a right-hemisphere ventral fronto-parietal network) has a “circuit-breaking” function, meaning visual attention is redirected from its current focus.
What stimuli trigger this circuit-breaking? We might imagine salient or distinctive stimuli would be most likely to attract attention. However, Corbetta et al. (2008) argued that distractors closely resembling task stimuli are more likely than salient stimuli to attract attention from the ventral attention network.
The two attention systems or networks often influence and interact with each other. As we will see, the intraparietal sulcus is involved in such interactions. Corbetta et al. (2008) spelled out some of the interactions involved. First, signals from the top-down system suppress distractor information in the stimulus-driven system so goal-directed processing can proceed unimpeded. Second, when the stimulus-driven system detects stimuli irrelevant to the current goal, it sends signals to disrupt processing occurring within the goal-directed system.
The existence of two attention systems makes much sense. The goal-directed system (dorsal attention network) allows us to focus attention on stimuli directly relevant to our current goals. However, if we only had this system, our attentional processes would be dangerously inflexible. It is also important to have a stimulus-driven attentional system (ventral attention network) leading us to switch attention away from goal-relevant stimuli in the presence of an unexpected threatening stimulus (e.g., a ferocious animal).
Findings
Corbetta and Shulman (2002) provided evidence for their two-network model by carrying out meta-analyses of brain-imaging studies. In essence, they argued that brain areas most often activated when participants expect a stimulus that has not yet been presented form the dorsal attention network. In contrast, brain areas most often activated when individuals detect low-frequency targets form the ventral attention network.
Subsequent research has clarified which brain areas are associated with each network (Corbetta & Shulman, 2011; see Figure 5.6). Key areas within the goal-directed or dorsal attention network are as follows: superior parietal lobule (SPL), intraparietal sulcus (IPS), inferior frontal junction (IFJ), frontal eye field (FEF), middle temporal area (MT) and V3A. Key areas within the stimulus-driven or ventral attention network are as follows: inferior frontal junction (IFJ), inferior frontal gyrus (IFG), supramarginal gyrus (SMG), superior temporal gyrus (STG) and insula (Ins) (see Figure 5.6). There is much evidence that the temporo-parietal junction is also involved in bottom-up attentional processes (e.g., Shomstein et al., 2010).
Hahn et al. (2006) tested Corbetta and Shulman’s (2002) theory by comparing patterns of brain activation when top-down and bottom-up processes were required. As predicted by the theory, there was practically no overlap between the brain areas associated with top-down and bottom-up processing. In addition, the brain areas involved in each type of processing corresponded reasonably well to those identified by Corbetta and Shulman.
Figure 5.6
The brain areas associated with the dorsal or goal-directed attention network and the ventral or stimulus-driven network. The full names of the areas involved are indicated in the text.
From Corbetta and Shulman (2011). © Annual Reviews. With permission of Annual Reviews.
Talsma et al. (2010) confirmed previous research showing that different brain areas are associated with top-down and bottom-up processing. In addition, however, they found top-down and bottom-up processes both made use of a common network of parietal areas.
Neuroimaging studies cannot establish that any given brain area is necessarily involved in stimulus-driven or goal-directed attention processes.
Relevant evidence on this issue can be obtained by the use of transcranial magnetic stimulation (TMS; see Glossary) to create a temporary “lesion”. Chica et al. (2011) did this. TMS applied to the right temporo-parietal junction impaired the functioning of the stimulus-driven but not top-down attentional system. TMS applied to the right intraparietal sulcus impaired functioning of both attention systems.
Evidence from brain-damaged patients (discussed more fully later) is also relevant to establishing which brain areas are necessarily involved in goal-directed or driven attentional processes. Shomstein et al. (2010) had brain-damaged patients complete two tasks. One task required stimulus-driven attentional processes while the other required top-down processes. Patients having greater problems with top-down than with stimulus-stimulus-driven attentional processing typically had brain damage to the superior parietal lobule (part of the dorsal attention network). In contrast, patients having greater problems with stimulus-driven attentional processing typically had brain damage to the temporo-parietal junction (often regarded as forming part of the ventral attention network).
Chica et al. (2013) reviewed research on the two attention systems and identified 15 differences between them. For example, stimulus-driven attention is faster than top-down attention and is more object-based. In addition, it is more resistant to interference from other peripheral cues once activated. The existence of so many differences strengthens the argument that the two attentional systems are separate.
Indovina and Macaluso (2007) tested Corbetta et al.’s (2008) assumption that the bottom-up attentional system is affected more by task-relevant than
salient distractors. Participants reported the orientation of a coloured letter T in the presence of a letter T in a different colour (task-relevant distractor) or a flickering draughtboard (salient distractor). As predicted, the bottom-up system was activated more by task-relevant distractors than salient ones.
Wen et al. (2012) argued that it is important to study interactions between the two visual attention systems. They assessed brain activation while participants responded to target stimuli in one visual field while ignoring all stimuli in the unattended visual field. There were two main findings. First, stronger causal influences of the top-down system on the stimulus-driven system led to superior performance on the task. This finding suggests the appearance of an object at the attended location caused the top-down attention system to suppress activity within the stimulus-driven system.
Second, stronger causal influences of the stimulus-driven system on the top-down system were associated with impaired task performance. This finding suggests activation within the stimulus-driven system produced by stimuli not in attentional focus led to a breaking of the attentional set maintained by the top-down system.
Evaluation
The theoretical approach proposed by Corbetta and Shulman (2002) has several successes to its credit. First, there appear to be somewhat separate stimulus-driven and top-down attention systems. Second, each attention system involves its own brain network. Third, research using transcranial magnetic stimulation has shown that major brain areas within each attention system play a causal role in attentional processes. Fourth, some ways in which the two networks interact have been identified. Fifth, as we will see in the next section, research on brain-damaged patients has provided good support for dorsal and ventral attention networks.
What are the limitations with this theoretical approach? First, it has proved hard to identify the precise brain areas associated with each attention system. There are various possible reasons for this. However, one likely reason is that the brain areas involved depend on the detailed requirements of a current task (Talsma et al., 2010). Second, the model is oversimplified. Attentional processes are involved in the performance of numerous tasks and it is unlikely all these processes can be neatly assigned to one or other of the model’s attention systems. Third, there is more commonality (especially within the parietal lobe) in the brain areas associated with the two attention networks than was assumed theoretically by Corbetta and Shulman (2002). Fourth, much remains to be discovered about how the two visual attention systems interact.
DISORDERS OF VISUAL ATTENTION
We can learn much about attentional processes by studying brain-damaged individuals. Here we consider two important attentional disorders: neglect and extinction. Neglect (or spatial neglect) is a condition in which there is a lack of awareness of stimuli presented to the side of space on the opposite side to the brain damage (the contralesional side).
KEY TERM
Neglect
A disorder involving right-hemisphere damage (typically) in which the left side of objects and/or objects presented to the left visual field are undetected; the condition resembles extinction but is more severe.
The brain damage is typically in the right hemisphere and there is often little awareness of stimuli on the left side of the visual field. This is known as subject-centred or egocentric neglect. It occurs because of the nature of the visual system – information from the left side of the visual field proceeds to the right brain hemisphere. When patients cancel targets presented to their left or right side (cancellation task), they generally cross out more of those presented to the right. When patients try to put a mark through a horizontal line at its centre (line bisection task), they typically put it to the right of the centre.
There is also object-centred or allocentric neglect. This involves a lack of awareness of the left side of objects rather than simply the left side of the visual field (see Figure 5.7). Object-centred neglect is often more important than subject-centred neglect. Gainotti and Ciaraffa (2013) reviewed research on several patients with right-hemisphere damage who drew the right side of all figures in a multi-object scene but neglected the left side of most of them regardless of whether they were presented to the left or right visual field.
There has been much controversy as to whether object-centred or allocentric neglect and subject-centred or egocentric neglect reflect similar or different underlying disturbance to the attentional system. Rorden et al. (2012) obtained two findings strongly supporting the notion that the two forms of neglect are similar. First, the correlation between the extent of each form of neglect across 33 patients was +0.80. Second, there was a large overlap in the brain regions associated with each type of neglect.
Extinction is often found in patients with neglect. Extinction involves a failure to detect a stimulus presented to the side opposite the brain damage when a second stimulus is presented to the same side as the brain damage. It is a serious condition because multiple stimuli are typically present at the same time in everyday life. Extinction and neglect are closely related. However, most of the evidence suggests they are separate deficits (de Haan et al., 2012).
We will discuss neglect in more detail than extinction because it has attracted more research.
KEY TERMS
Extinction
A disorder of visual attention in which a stimulus presented to the side opposite the brain damage is not detected when another stimulus is presented at the same time to the side of the brain damage.
Weblink:
Patients with stroke
Which brain areas are damaged in neglect patients? Neglect is a heterogeneous condition in which the brain areas involved vary considerably across patients. Molenberghs et al. (2012b) found nine brain areas sometimes damaged in neglect patients in a meta-analysis (see Glossary). The main areas damaged are typically in the right hemisphere and include the superior temporal gyrus, the inferior frontal gyrus, the insula, the supramarginal gyrus and the angular gyrus (Corbetta & Shulman, 2011). Nearly all these areas are located within the stimulus-driven or ventral attention network (see Figure 5.6).
This suggests the attentional problems of neglect patients depend on brain networks rather than simply on specific brain areas (Corbetta & Shulman, 2011;
Bartolomeo et al., 2012).
Figure 5.7
Left is a copying task in which a patient with unilateral neglect distorted or ignored the left side of the figures to be copied (shown on the left). Right is a clock-drawing task in which the patient was given a clock face and told to insert the numbers into it.
Reprinted from Danckert and Ferber (2006). Reprinted with permission from Elsevier.
Several brain areas can also be damaged in extinction patients. Brain damage is generally centred on the right hemisphere. The temporo-parietal junction and the intraparietal sulcus are especially likely to be damaged (de Haan et al., 2012). When transcranial magnetic stimulation (TMS; see Glossary) is applied to these areas to produce a temporary “lesion”, extinction-like behaviour is produced (de Haan et al., 2012).