The Student's Guide to Cognitive Neuroscience: Chapter 15 - The Executive Brain PDF

C H A P TE R 15 The executive brain CONTENTS Anatomical and functional divisions of the prefro...

C H A P TE R 15 The executive brain CONTENTS Anatomical and functional divisions of the prefrontal cortex 387 Executive functions in practice 390 The organization of executive functions 398 The role of the anterior cingulate in executive functions 411 Summary and key points of the chapter 413 Example essay questions 414 Recommended further reading 414 The executive functions of the brain can be defined as the complex processes by which an individual optimizes his or her performance in a situation that requires the operation of a number of cognitive processes (Baddeley, 1996). A rather more poetic metaphor is that the executive functions are the brain’s conductor, which instructs other regions to perform, or be silenced, and generally coordinates their synchronized activity (Goldberg, 2001). As such, Copyright © 2019. Taylor & Francis Group. All rights reserved. executive functions are not tied to one particular domain (memory, language, perception and so on) but take on a role that is meta-cognitive, supervisory or controlling. Executive functions have traditionally been equated with the frontal lobes, and difficulties with executive functioning have been termed as “frontal lobe syndrome.” More accurately, executive functions are associated with the prefrontal cortex (PFC) of the frontal lobes, and it is an empirically open question as to whether all aspects of executive function can be localized to this region. Primates, relative to other mammals, have disproportionately more gray matter in their frontal lobes (Bush & Allman, 2004) and humans, relative to other primates, have further disproportionately enlarged this region of their brain (Donahue et al., 2018). This is shown in Figure 15.1. The concept of executive functions is closely related to another distinction with a long history in cognitive science—namely, that between automatic and controlled behavior (e.g., Schneider & Shiffrin, 1977). This distinction DOI: 10.4324/9781351035187-15 Ward, Jamie. The Student's Guide to Cognitive Neuroscience, Taylor & Francis Group, 2019. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/leidenuniv/detail.action?docID=5990310. Created from leidenuniv on 2024-09-29 21:47:56.  386 THE STUDENT’S GUIDE TO COGNITIVE NEUROSCIENCE KEY TERM has already been encountered in another context, namely the production of actions. When driving a car, one may accelerate, change gear and so on, in an Executive functions apparently “autopilot” mode. But if the traffic is diverted through an unfamiliar Control processes that route, then one would need to override the automatic behavior and exert enable an individual to optimize performance in online control. This is often assumed to require the use of executive functions situations requiring the (Norman & Shallice, 1986). The same logic may also apply in situations that operation and coordina- lack motor output, i.e., in the online control of thoughts and ideas. This tion of several more ba- provides humans (and possibly other species) with a remarkable opportunity; sic cognitive processes. namely, to mentally simulate scenarios and think through problems “in the mind” without necessarily acting them out. It is hardly surprising, therefore, that some theories of executive function are effectively synonymous with aspects of working memory (Baddeley, 1996; Goldman-Rakic, 1996). Two other general points are in need of mention in this preamble. First, the extent to which behavior is “automatic” (i.e., not requiring executive function) versus “controlled” (i.e., requiring executive function) may be a matter of degree rather than all or nothing. Even when generating words in fluent conversation, some degree of executive control may be exerted. For example, one may need to select whether to say the word “dog,” “doggy,” “Fido” or “Labrador” depending on pragmatic context, rather than relying on, say, the most frequent word to be selected. Second, one must be cautious about falling into the trap of thinking that controlled behavior requires an autonomous controller. This is the so-called homunculus problem: think of a little man inside your head making your decisions, and then imagine another little man in his head making his decisions, and so on. Control may be an outcome of multiple competing biases rather than the presence of a controller. Decisions may arise out of an interaction of environmental influences (bottom-up processes) and influences related to the motivation and goals of the person (top-down processes). The sight of a cream cake may trigger an “eat me” response, but whether one does eat it may depend on whether one is hungry or dieting. This basic idea FIGURE 15.1: Enlargement of frontal cortex shows an evolutionary progression (the brains are not drawn to scale). In humans, this region occupies almost a third of the Copyright © 2019. Taylor & Francis Group. All rights reserved. cortical volume. Adapted from Fuster, 1989. Ward, Jamie. The Student's Guide to Cognitive Neuroscience, Taylor & Francis Group, 2019. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/leidenuniv/detail.action?docID=5990310. Created from leidenuniv on 2024-09-29 21:47:56.  THE EXECUTIVE BRAIN 387 has already been introduced from the perspective of attention (Chapter 9) and action (Chapter 10), and will be explored here from the perspective of performing tasks with novel or complex configurations. This chapter first considers the major anatomical divisions within the prefrontal cortex. The subsequent section outlines the main types of cognitive tests that are believed to depend critically on the functioning of the prefrontal cortex. The chapter then considers different possible functional organizations of the prefrontal cortex: for instance, different functional roles for the lateral versus orbital surfaces; different functional roles for posterior versus anterior portions of the lateral surface; and hemispheric differences. Before discussing executive functions, it is worthwhile to review the anatomy of the prefrontal cortex. A N AT O M I C AL AND FUNCTIONAL DIVISIONS OF T H E P R E F RO N TAL C O R T EX The most basic anatomical division within the prefrontal cortex is that between the three different cortical surfaces (Figure 15.2). The lateral surface of the prefrontal cortex lies anterior to the premotor areas (Brodmann’s area 6) and the frontal eye fields (in Brodmann’s area 8). This surface lies closest to the skull. The medial surface of the prefrontal cortex lies between the two hemispheres and to the front of the corpus callosum and the anterior cingulate cortex. In terms of anatomy, the anterior cingulate is not strictly part of the prefrontal cortex, but it does have an important role to play in executive functions and, as such, will be considered in this chapter. The orbital surface of the prefrontal cortex lies above the orbits of the eyes and the nasal cavity. The orbitofrontal cortex is functionally, as well as anatomically, related to the ventral part of the medial surface (termed ventromedial prefrontal cortex) (Öngür & Price, 2000). The terms orbito- and ventromedial PFC are sometimes used interchangeably when finer anatomical divisions are not necessary. The prefrontal cortex has extensive connections with virtually all sensory systems, the cortical and subcortical motor system and structures involved in affect and memory (Yeterian et al., 2012). There are also extensive connections between different regions of the prefrontal cortex. These extensive connections enable the coordination of a wide variety of brain processes. The lateral prefrontal cortex is more closely associated with sensory inputs than the orbitofrontal cortex. It receives visual, somatosensory and Copyright © 2019. Taylor & Francis Group. All rights reserved. auditory information, as well as receiving inputs from multimodal regions that integrate across senses. In contrast, the medial and orbital prefrontal cortex is more closely connected with medial temporal lobe structures critical for long-term memory and processing of emotion. Aside from these gross anatomical divisions, a number of researchers have developed ways of dividing different regions into separate areas of functional specialization. These correspond approximately, although not exactly, with different Brodmann areas (e.g., Fletcher & Henson, 2001; Petrides, 2000; Stuss et al., 2002). These include areas shown on Figure 15.2 as ventrolateral (including Brodmann’s areas 44, 45 and 47), dorsolateral (including Brodmann’s areas 46 and 9), the anterior prefrontal cortex (Brodmann’s area 10) and the anterior cingulate. These terms are sufficient to capture most of the functional distinctions discussed in this chapter, but it is to be noted that not all researchers regard the prefrontal cortex as containing functionally different subregions. Ward, Jamie. The Student's Guide to Cognitive Neuroscience, Taylor & Francis Group, 2019. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/leidenuniv/detail.action?docID=5990310. Created from leidenuniv on 2024-09-29 21:47:56.  388 THE STUDENT’S GUIDE TO COGNITIVE NEUROSCIENCE Anterior cingulate cortex Lateral PFC Pre-SMA 9 9 24 46 32 10 45 44 10 47 25 11 14 11 10 10 12 47 11 11 47 12 Orbital PFC 13 13 14 Brodmann’s Other names Possible functions Possible functions areas (left hemisphere) (right hemisphere) 44, 45, 47 Ventro-lateral Retrieval and maintenance Retrieval and maintenance prefrontal cortex of semantic and/or linguistic of visual and/or spatial (VLPFC) information information (Areas 44 + 45 on left also called Broca’s area) 9, 46 Dorso-lateral Selecting a possible range Monitoring and checking prefrontal cortex of responses, and suppressing of information held in (DLPFC) inappropriate ones; mind, particularly in manipulating the contents conditions of uncertainty; of working memory vigilance and sustained attention 10 Anterior prefrontal Multi-tasking; maintaining future intentions / goals whilst cortex; frontal pole; currently performing other tasks or sub-goals. (The medial Copyright © 2019. Taylor & Francis Group. All rights reserved. rostral prefrontal portion has been implicated in “theory of mind” – see cortex Chapter 16) 24 (dorsal) Anterior cingulate Monitoring in situations of response conflict and error 32 (dorsal cortex (dorsal) detection Pre-SMA 11, 12, 13, Orbito-frontal Executive processing of emotional stimuli (e.g. evaluating 14 cortex rewards and risks) FIGURE 15.2: The prefrontal cortex has three different surfaces: the lateral surface (top left), the medial surface (top right) and the orbitofrontal surface (bottom). The numbers refer to Brodmann areas that are discussed in the text. Ward, Jamie. The Student's Guide to Cognitive Neuroscience, Taylor & Francis Group, 2019. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/leidenuniv/detail.action?docID=5990310. Created from leidenuniv on 2024-09-29 21:47:56.  THE EXECUTIVE BRAIN 389 THE EXTRAORDINARY CASE OF PHINEAS GAGE One of the most famous cases in the neuropsychological literature is that of Phineas Gage (Harlow, 1993; Macmillan, 1986; Figure 15.3). On September 13, 1848, Gage was working on the Rutland and Burlington railroad. He was using a large metal rod (a tamping iron) to pack explosive charges into the ground when the charge accidentally exploded, push- ing the tamping iron up through the top of his skull; it landed about 30 m behind him. The contemporary account noted that Gage was momentarily knocked over but that he then walked over to an ox-cart, made an entry in his time book and went back to his hotel to wait for a doctor. He sat and waited half an hour for the doctor and greeted him with, “Doctor, here is business enough for you!” (Macmillan, 1986). Not only was Gage conscious after the accident, he was able to walk and talk. Although this is striking in its own right, it is the cognitive consequences of the injury that have led to Gage’s notoriety. Before the injury, Gage held a position of responsibility as a foreman and was described as shrewd and smart. After the injury, he was considered unemployable by his previous company; he was “no longer Gage” (Harlow, 1993). Gage was described as irreverent, indulging at times in grossest profanity … manifesting but little deference for his fellows, impatient of restraint or advice when it conflicts with his desires … devising many plans of future operation, which are no sooner arranged than they are abandoned in turn for others. (Harlow, 1993) After various temporary jobs, including a stint in Barnum’s Museum, he died of epilepsy (a secondary consequence of Copyright © 2019. Taylor & Francis Group. All rights reserved. his injury) in San Francisco, some 12 years after his accident. FIGURE 15.3: The skull of Phineas Gage, Where was Phineas Gage’s brain lesion? This question with tamping iron in situ and a recently was answered by an MRI reconstruction of Gage’s skull, discovered photograph of Gage. Modern which found damage restricted to the frontal lobes, partic- reconstructions suggest that his brain lesion may have been specific to the ularly the left orbitofrontal/ventromedial region and the left medial and orbital surfaces of the anterior region (Damasio et al., 1994). Research suggests prefrontal cortex, sparing the lateral that this region is crucial for certain aspects of decision mak- surfaces. ing, planning, and social regulation of behavior, all of which Damasio et al., 1994. From the collection of Jack and Beverly Wilgus. appeared to have been disrupted in Gage. Other areas of the lateral prefrontal cortex are likely to have been spared. Ward, Jamie. The Student's Guide to Cognitive Neuroscience, Taylor & Francis Group, 2019. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/leidenuniv/detail.action?docID=5990310. Created from leidenuniv on 2024-09-29 21:47:56.  390 THE STUDENT’S GUIDE TO COGNITIVE NEUROSCIENCE The prefrontal cortex is the starting and ending point of various circuits that pass through the basal ganglia and thalamus (Alexander & Crutcher, 1990). These circuits modify activity in prefrontal cortex (both upwards and downwards) such that it ultimately affects the probability of behavior. In motor behavior, these circuits might influence the likelihood of an action and how vigorous it is (see Chapter 10). In executive functions, it is proposed that the corresponding prefrontal loop through the basal ganglia acts as a gatekeeper by, for instance, making the information stored in working memory less stable so that it can be updated (O’Reilly & Frank, 2006). The basal ganglia loop to prefrontal cortex is believed to be crucial for learning of novel tasks and, hence, important for making the transition from controlled behavior to automatic behavior (O’Reilly & Frank, 2006). This is important for procedural learning as exemplified by complex tasks such as driving a car that, eventually, appear effortless. However, note that the basal ganglia are not normally considered to be responsible for directly determining the content of ongoing tasks. This depends on interactions between prefrontal cortex and posterior regions involved in, say, language, perception or emotion. Instead, the basal ganglia circuits are primarily concerned with task efficiency and task learning. EXEC U T I V E FUNCTIONS IN PRACTICE This section considers some concrete situations in which executive functions are needed. Evidence will be presented that the prefrontal cortex (or subregions within it) are important for implementing this kind of behavior. Working memory The prefrontal cortex within the frontal lobes is widely recognized as playing a crucial role in working memory. Most models tend to assume that the main storage site of information is not within the frontal lobes themselves but in the posterior cortex, and that the function of the prefrontal cortex is to keep this information active and/or manipulate the active information according to current goals. In Baddeley’s (1986) model, for instance, the notion of the central executive is effectively synonymous with models of prefrontal functioning. Copyright © 2019. Taylor & Francis Group. All rights reserved. Goldman-Rakic’s (1996) account also regards the prefrontal cortex as implementing a working memory system and draws primarily on animal lesion studies and single-cell recordings. Lesions to the lateral prefrontal cortex can impair the ability to hold a stimulus/response in mind over a short delay (Butters & Pandya, 1969). In one delayed response task, monkeys were presented with a box in a particular location on the screen. The box then disappeared and the monkey was required to hold the location “in mind.” After a delay, they were then required to look at where the target was previously displayed (Figure 15.4). Single-cell recordings from monkeys show that some dorsolateral prefrontal neurons respond selectively during the delay period, suggesting that this is the neural mechanism for holding locations in mind (Funahashi et al., 1989). Ward, Jamie. The Student's Guide to Cognitive Neuroscience, Taylor & Francis Group, 2019. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/leidenuniv/detail.action?docID=5990310. Created from leidenuniv on 2024-09-29 21:47:56.  THE EXECUTIVE BRAIN 391 FIGURE 15.4: Single-cell recordings in the dorsolateral prefrontal cortex show that different neurons respond to (a) studying in a target location, (b) holding it “in mind” during a delay, and (c) responding to the removal of a cue by moving the eyes to that location. From Goldman-Rakic, 1992. Reprinted with permission of Patricia J. Wynne. www.patriciawynne.com. Goldman-Rakic (1996) argued that there is a division between the content of information processed in dorsolateral and ventrolateral regions, but that the same types of process are used for both. Specifically, she suggests that ventral regions support working memory for objects and dorsal regions support spatial working memory (that is, the dorsal and ventral visual stream is manifested at the level of executive functions). Other evidence is inconsistent with this view. Rao et al. (1997) report that individual neurons can change their responsiveness from being object based to being location based as the demands of the task change, irrespective of whether they are located in dorsolateral or ventrolateral regions. Petrides (2000) offers an alternative account of working memory to that of Goldman-Rakic. He argues that the dorsolateral and ventrolateral prefrontal Copyright © 2019. Taylor & Francis Group. All rights reserved. regions should be distinguished by the fact that they are engaged in different types of process and not that they are specialized for different types of material (e.g., spatial versus object based). This is a hierarchical model of working memory (Figure 15.5). In this model, the ventrolateral prefrontal cortex is responsible for activating, retrieving and maintaining information held in the posterior cortex. The dorsolateral prefrontal region is FIGURE 15.5: A hierarchical model of working memory in which responsible when the information held within ventrolateral prefrontal cortex (VLPFC) activates and maintains this system requires active manipulation (e.g., information, and the dorsolateral prefrontal cortex (DLPFC) ordering of information). Petrides and Milner manipulates that information. Ward, Jamie. The Student's Guide to Cognitive Neuroscience, Taylor & Francis Group, 2019. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/leidenuniv/detail.action?docID=5990310. Created from leidenuniv on 2024-09-29 21:47:56.  392 THE STUDENT’S GUIDE TO COGNITIVE NEUROSCIENCE KEY TERM (1982) found that patients with prefrontal lesions were impaired on a test of working memory termed the self-ordered pointing task. The patients Self-ordered pointing were presented with an array of eight words or pictures and, on the first task A task in which partic- trial, required to pick anyone. On the second trial, they were asked to pick a ipants must point to different one from the first; on the third trial, they must pick a different one a new object on each again and so on. As such, they must maintain and update an online record trial and thus maintain a of chosen items. This is shown in Figure 15.6. Similar studies on monkeys working memory for previ- suggest the critical region to be the dorsolateral prefrontal cortex (Petrides, ously selected items. 1995). In a human functional imaging study, Owen et al. (1996) found that maintaining and updating a record of which locations had been marked was linked to dorsolateral PFC activity whereas short-term retention of spatial locations was associated with ventrolateral activity. Task-setting and problem-solving Problem-solving is synonymous with many lay notions of what it is to exhibit intelligent behavior and it is not surprising that executive functions, and the prefrontal cortex, have been linked to intelligence both within and across species. For instance, performance on tests of executive function tends to correlate with each other and also correlates with certain standardized measures of intelligence (Duncan et al., 1997). In the lab, problem-solving is often tested by giving an endpoint (a goal) and, optionally, a starting point (a set of objects) and participants must generate a solution of their own. This kind of open-ended solution is also referred to as task-setting. Patients with lesions to the prefrontal cortex often show clinical symptoms of poor task-setting and problem-solving. To test this formally, a number of tests have been devised. Shallice (1982) reports a test called the “Tower of London,” in which patients must move beads from one stake to FIGURE 15.6: A self-ordered pointing task based on Petrides and Milner (1982). Participants are required to point to a new object on each trial and, as such, must keep an online record of previous selections. Copyright © 2019. Taylor & Francis Group. All rights reserved. Ward, Jamie. The Student's Guide to Cognitive Neuroscience, Taylor & Francis Group, 2019. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/leidenuniv/detail.action?docID=5990310. Created from leidenuniv on 2024-09-29 21:47:56.  THE EXECUTIVE BRAIN 393 KEY TERMS FAS Test A test of verbal fluency in which participants must generate words beginning with a letter (e.g., “F”) in a limited amount of time. Stroop Test FIGURE 15.7: The “Tower of London” task requires beads to be moved from an initial Response interference position to a specified endpoint. Performance can be measured in terms of time to from naming the ink color complete task or number of moves taken (relative to the optimal number of moves). of a written color name From Shallice, 1982. Royal Society of London. (e.g., the word BLUE is printed in red ink and participants are asked another to reach a specified endpoint (Figure 15.7). Patients with damage to to say the ink color, i.e., the left prefrontal cortex take significantly more moves. This implies that they “red”). perform by trial-and-error rather than planning their moves (see also Morris Go/No-Go Test et al., 1997). Functional imaging studies of healthy participants suggest that A test of response inhibi- activity within the dorsolateral prefrontal cortex increases with the number tion in which participants of moves needed to reach the endpoint (Rowe et al., 2001). must respond to a fre- A number of verbal tests also involve finding solutions to problems in quent stimulus (go trials) which there is no readily available answer. In the Cognitive Estimates Test but withhold a response (Shallice & Evans, 1978), patients with damage to the prefrontal cortex are to another stimulus (no- impaired at producing estimates for questions in which an exact answer go trials). is unlikely to be known (“How many camels are in Holland?”) but can be inferred from other relevant knowledge (e.g., camels are only likely to reside in a small number of zoos). In the FAS Test (Miller, 1984), participants must generate a sequence of words (not proper names) beginning with a specified letter (“F,” “A” or “S”) in a 1-minute period. This test is not as easy as it sounds (have a try) and involves generating novel strategies, selecting between alternatives and avoiding repeating previous responses. Patients with left lateral prefrontal lesions are particularly impaired (Stuss et al., 1998). Overcoming potent or habitual responses The classic example of overcoming a habitual response is provided by the Stroop Test (Stroop, 1935). In this task, participants must name the color of the ink and ignore reading Copyright © 2019. Taylor & Francis Group. All rights reserved. the word (which also happens to be a color name), as shown in Figure 15.8. The standard explanation is that reading of words occurs automatically and this generates a salient incorrect response that competes with the less-automatic task of naming colors (MacLeod & MacDonald, 2000). Performance on the Stroop Test has long been linked with the integrity of the prefrontal cortex (Perret, 1974). Go/No-Go Tests involve the participant making a set of responses to some stimuli FIGURE 15.8: The Stroop Test involves naming the color of the (“go” trials) but withholding responses to a ink and ignoring the written color name (i.e., “red, green, yellow, subset of stimuli (“no-go” or “stop” trials). blue, yellow, white”). Ward, Jamie. The Student's Guide to Cognitive Neuroscience, Taylor & Francis Group, 2019. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/leidenuniv/detail.action?docID=5990310. Created from leidenuniv on 2024-09-29 21:47:56.  394 THE STUDENT’S GUIDE TO COGNITIVE NEUROSCIENCE KEY TERMS The no-go trials are often infrequent, so the participant gets into the habit of making a response. No-go rules can be defined in terms of simple rules Impulsivity (e.g., “respond to all stimuli except the letter B”) or more complex rules (e.g., A behavioral tendency “respond to all stimuli except the letter B when it follows another letter B”). to make immediate responses or seek imme- Brain activity during successful no-go trials is normally taken as indexing diate rewards. response inhibition, and the proportion of errors on no-go trials is taken as a Wisconsin Card Sorting behavioral marker of impulsivity (Perry & Carrol, 2008). Test Both the Stroop Test and the Go/No-Go Test are related by virtue of the A test of executive fact that they are typically explained with respect to the concept of inhibition. functions involving rule Inhibition, in terms of neural activity, has a very specific definition (reduced induction and rule use. spiking rate) with a well-characterized mechanism at the synaptic level (more negative post-synaptic membrane potential). Behavioral or cognitive inhibition simply means reducing the likelihood of a particular thought/ action and the mechanism behind it, at the neuronal level, is not clear. Some contemporary models of executive function do not rely on the concept of inhibition at all and rely solely on biasing activation signals, also termed ONLINE RESOURCES “gain” (Stuss & Alexander, 2007). Certainly, tasks such as the Stroop and Go/No-Go are likely to involve a variety of functions such as task-setting To test yourself on the Stroop Test and on and monitoring ongoing performance, in addition to biasing of competing task-switching, visit responses (either via gain or inhibition). the demo test library Contemporary research has suggested that performance on these tasks (www.testable.org/ is related to particular brain regions rather than the “prefrontal cortex” in ward). general. A meta-analysis of functional imaging studies of the Go/No-Go task suggests that a region of the medial prefrontal cortex (specifically the pre- SMA, pre-supplementary motor area) was common across tasks for No-Go stimuli with right lateral prefrontal cortex also implicated in more complex No-Go rules (Simmonds et al., 2008). Studies of patients with damage to the prefrontal cortex confirm that the pre-SMA region and the right lateral prefrontal cortex are important for this task (Picton et al., 2007). With regards to the Stroop Test, a similar picture emerges that highlights the importance of the anterior cingulate cortex and the nearby Reference cards pre-SMA region (Alexander et al., 2007). The specific roles of the anterior cingulate and pre-SMA are returned to later. Task-switching Copyright © 2019. Taylor & Francis Group. All rights reserved. Color Number Form Random choice In the Wisconsin Card Sorting Test, a or complex rule series of cards must be matched against reference cards (Milner, 1963; Nelson, 1976). The cards can be matched according to one of three dimensions, namely color, Response cards number and shape (see Figure 15.9). For example, in the color condition a blue card must be grouped with blue cards and red cards grouped with red cards FIGURE 15.9: In the Wisconsin Card Sorting Test, patients are given (ignoring number and shape). After each a card that can be sorted by a number of rules (matching shape, trial, participants are told whether they are number or color). Sometimes the rule unexpectedly changes and the correct or not. Eventually, they are told patients must adjust their responses to the new rule. that they are incorrect and they must then Ward, Jamie. The Student's Guide to Cognitive Neuroscience, Taylor & Francis Group, 2019. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/leidenuniv/detail.action?docID=5990310. Created from leidenuniv on 2024-09-29 21:47:56.  THE EXECUTIVE BRAIN 395 spontaneously switch task, i.e., start sorting according to number or shape. KEY TERMS Many patients with damage to the prefrontal cortex fail to make this shift and continue to incorrectly sort according to the previous rule, a behavior Perseveration Failure to shift away from termed perseveration. a previous response. The Wisconsin Card Sorting Test has a number of features that make it demanding: the switches are unpredictable and, moreover, the relevant Task-switching Discarding a previous dimensions (color, shape, number) are not given but need to be inferred. schema and establishing This also makes it hard to know why, in cognitive terms, failure on the task a new one. happens. Other task-switching paradigms have been developed that enable Switch cost more fine-grained analysis of the underlying mechanisms. These tend to be A slowing of response used in studies of non-brain-damaged participants using fMRI or TMS. time due to discarding To give an example of a task that involves switches that occur predictably, a previous schema and imagine that you are a participant looking at a square 2 × 2 grid such as that setting up a new one. in Figure 15.10. A digit and/or number pair (e.g., L9) will appear in each part of the grid, moving clockwise, and you must make a response to each stimulus. When the stimulus is in the upper half of the grid, you must decide if the letter is a consonant or vowel using a left-right button press. When the stimulus is in the lower half, you must decide if the digit is odd or even (some participants would get the complementary set of instructions). This produces two types of trial—those in which the task switches and those in which it does not. The reaction times for the switch trials are significantly slower, and this difference remains even though the change is predictable and even if the subject is given over a second to prepare before each stimulus is presented (Rogers & Monsell, 1995). This difference in reaction time between switch and non-switch trials is called the switch cost. The switch cost could either reflect suppressing the old task or reflect setting up the new task. This can be evaluated by considering switches between easy and hard tasks. A greater switch cost from easy to hard would imply a difficulty in setting up the new (harder) task, whereas a greater switch cost from hard to easy would imply a difficulty in inhibiting the old (harder) task. The evidence suggests it is the latter; i.e., the switch cost has more to do with inhibiting the old Copyright © 2019. Taylor & Francis Group. All rights reserved. FIGURE 15.10: When the digit and/or letter pair is in the top half, the subject must decide whether the letter is a consonant or vowel. When it is in the bottom half, the digit must be classified as odd or even. This generates two types of trial—those in which the task switches and those in which it does not. Switch trials are significantly slower even though the switch is predictable and even if participants are given over 1 sec to prepare before the stimulus is shown. Reprinted from Monsell, 2003. © 2003, with permission from Elsevier. Ward, Jamie. The Student's Guide to Cognitive Neuroscience, Taylor & Francis Group, 2019. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/leidenuniv/detail.action?docID=5990310. Created from leidenuniv on 2024-09-29 21:47:56.  396 THE STUDENT’S GUIDE TO COGNITIVE NEUROSCIENCE task than setting up the new one. For example, bilinguals are slower at switching from their second to their first language than from their first to their second language in picture naming (Meuter & Allport, 1999). With Stroop stimuli, people are faster at switching from word naming to color naming (easy to hard) than color naming to word naming (hard to easy) (Allport et al., 1994). Functional imaging studies reveal a variety of prefrontal regions together with the anterior cingulate cortex/pre-SMA to be involved in task- switching, by comparing switch trials with no-switch trials (Ravizza & Carter, 2008) or contrasting the switch preparation time (before the stimulus) with switch execution after the stimulus (Brass & von Cramon, 2002). However, it is not always straightforward to link specific regions with specific cognitive processes because there are often different types of switching mechanism. Most task-switching experiments involve both a switching of response rules and a switching of the stimulus selected. In the study described previously, for example, the left hand switches from responding “consonant” to responding “odd,” and the stimulus selected switches from letter to digit (i.e., multiple aspects of the task are switched). Rushworth et al. (2002) attempted to control for these differences in a combined fMRI and TMS study. They found that the medial frontal lobes (the pre-SMA region) are important for reassignment of stimulus–response pairings (e.g., which button to press), whereas lateral frontal regions may be involved in selection of the current rule (e.g., whether KEY TERM to respond to color or shape in their task). Multi-tasking Carrying out several tasks in succession; Multi-tasking requires both task-switch- Multi-tasking experiments can be regarded as having an element of maintaining ing and maintaining future goals while current goals are being dealt with (Figure 15.12). This is future goals while current related to, but an extension of, task-switching. In task-switching one goal is goals are being dealt with. substituted for another. In multi-tasking several goals are maintained at the same time (but only one executed). FIGURE 15.11: Bilingual speakers are faster at switching from their first to their second language, than from their second to their Copyright © 2019. Taylor & Francis Group. All rights reserved. first language. How can this apparently paradoxical result be explained? Akchamczuk/iStock Ward, Jamie. The Student's Guide to Cognitive Neuroscience, Taylor & Francis Group, 2019. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/leidenuniv/detail.action?docID=5990310. Created from leidenuniv on 2024-09-29 21:47:56.  THE EXECUTIVE BRAIN 397 Patients with lesions to the prefrontal cortex may be particularly impaired at multi- tasking, even though each task in isolation may be successfully performed and even though they perform normally on other tests of executive function, including the Wisconsin Card Sorting Test and FAS Test (Burgess et al., 2000; Shallice & Burgess, 1991). This suggests a possible fractionation of executive functions (assuming it isn’t simply related to task difficulty)—an idea returned to in the next section. In the Six Element Test the participant is given six open- ended tasks to perform within a 15-minute period (e.g., arithmetic, writing out names FIGURE 15.12: How do we perform multi-tasking? Could the anterior prefrontal region hold the key? of pictures). Critically, they are instructed to attempt each task. However, they will be unable to complete all of them in the time allowed, and more points are awarded for earlier items. Constraints are placed on some of the ordering of tests. Patients with prefrontal lesions would often fail to switch tasks, spend too long planning (e.g., taking notes) but never execute the plans and so on. The patients could easily perform the isolated tasks, but their difficulties were only apparent when they had to coordinate between them (Shallice & Burgess, 1991). Evaluation By the mid-1990s there was a generally agreed-upon definition of what the essential features of executive functions were: e.g., allowing flexible or “intelligent” behavior, exerting control via a biasing influence. There was also a general consensus that the prefrontal cortex had a critical role in implementing this, and there were also a set of frequently used tasks that were assumed to be a good indicator of prefrontal functioning (e.g., the Wisconsin Card Sort, the Stroop Test). There was also agreement on the kind of model that could account for this. One simple model of executive functions is the original version of the SAS (Supervisory Attentional System) model— introduced in Chapter 10. This consists of a set of tasks and behaviors (termed schemas) and a biasing mechanism that activated/suppressed these schemas according to the individual’s current goals (Norman & Shallice, 1986). The Copyright © 2019. Taylor & Francis Group. All rights reserved. activation of schemas was conceptualized as a balance between bottom-up processes (cues in the environment, habits, etc.) and top-down processes (task instructions, long-term plans, etc.). Disruption of this balance, for example, by a prefrontal lesion would tend to result in recent or habitual responses being inappropriately elicited (e.g., in the Stroop Test, or Wisconsin Card Sort), poor planning and so on. Although these core ideas and empirical results are as valid today as they were in the 1990s, the contemporary intellectual landscape relating to executive functions is far more detailed and complex. In the mid-1990s there was already some evidence that was hard to accommodate by existing theories. For instance, it was found that some patients with prefrontal lesions could pass the standard tests of executive functions, but yet show significant impairments in organizing their daily life and in their social interactions Ward, Jamie. The Student's Guide to Cognitive Neuroscience, Taylor & Francis Group, 2019. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/leidenuniv/detail.action?docID=5990310. Created from leidenuniv on 2024-09-29 21:47:56.  398 THE STUDENT’S GUIDE TO COGNITIVE NEUROSCIENCE EGAS MONIZ AND THE PREFRONTAL LOBOTOMY The career of Egas Moniz was an eventful one. In politics, he served as Portuguese Ambassador to Spain and was President of the Portuguese Delegation at the Paris Peace Conference in 1918, following the First World War. However, it is his contribution to neurology and neurosurgery that gained him fame and infamy. In the 1920s he developed cerebral angiography, enabling blood vessels to be visualized with radioactive tracers. In 1935, he developed the prefrontal lobotomy/ leucotomy for the treatment of psychiatric illness. Between then and 1954, more than 50,000 patients would have the procedure in the USA (Swayze, 1995) and over 10,000 in the UK (Tooth & Newton, 1961). This brought Moniz mixed fortunes. He was awarded the Nobel Prize for Medicine. However, he had to attend the ceremony in a wheelchair because, some years previously, he had been shot in the spine and partially paralyzed by one of his lobotomized patients. Moniz’s operation was designed to sever the connections between the prefrontal cortex and other areas, notably the limbic system (Moniz, 1937, 1954). This procedure was adapted by oth- ers in frighteningly simple ways. For instance, an ice-pick-type implement was inserted through the thin bony plate above the eyes and waggled from side to side. At that point, there were no pharmacological treatments for psychiatric complaints. Lobotomy was used for a variety of disorders, including obsessive-compulsive disorder, depression and schizophrenia. The measurement of “improvement” in the patients was rather subjective, and the fact that the lobotomized patients tended to be duller and more apathetic than before was not sufficient to halt the appeal of the lobotomy. Formal assessments of cognitive function, if they had been carried out, would undoubtedly have revealed impairments in executive function. Moniz died in 1955. By then, his surgical innovation had been phased out and its success has been left to history to judge. (Eslinger & Damasio, 1985; Shallice & Burgess, 1991). This revealed a potential flaw in the early accounts. However, these observations could still be explained away: for instance, by pointing out that lab tests may not be fully sensitive to deficits apparent in the “real world.” Brain imaging has made a very significant contribution toward moving the debate forward. This has enabled a more fine-grained analysis of the functions of different regions of the prefrontal cortex (and their connectivity) both in Copyright © 2019. Taylor & Francis Group. All rights reserved. studying healthy participants (in fMRI) but also in identifying more precise lesion locations in patients. The next section considers various ways in which executive functions might be organized in the brain. T H E O R G AN I Z ATION OF E XE CUTIVE FUNCTIONS Although there are many different approaches to explaining executive functions, it is important to emphasize that they typically agree on many of the core principles outlined so far. Namely, that they require flexible processing in order to override automatic behavior, switch flexibly between tasks and carry out a current task while holding in mind other goals—and that this is achieved via a biasing influence (they make certain behaviors more or less likely) rather than dictating to the rest of the brain. As for differences Ward, Jamie. The Student's Guide to Cognitive Neuroscience, Taylor & Francis Group, 2019. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/leidenuniv/detail.action?docID=5990310. Created from leidenuniv on 2024-09-29 21:47:56.  THE EXECUTIVE BRAIN 399 between models, one of the key distinctions is the extent to which different models assume that executive function