(t.o.v.a.) tower of london test trail-making test (tmt) or trails a & b wisconsin card sorting test (wcst) symbol digit modalities test experimental evidence the executive system has been traditionally quite hard to define, mainly due to what psychologist paul w. burgess calls a lack of "process-behaviour correspondence". that is, there is no single behavior that can in itself be tied to executive function, or indeed executive dysfunction. for example, it is quite obvious what reading-impaired patients cannot do, but it is not so obvious what exactly executive-impaired patients might be incapable of. this is largely due to the nature of the executive system itself. it is mainly concerned with the dynamic, "online" co-ordination of cognitive resources, and, hence, its effect can be observed only by measuring other cognitive processes. in similar manner, it does not always fully engage outside of real-world situations. as neurologist antonio damasio has reported, a patient with severe day-to-day executive problems may still pass paper-and-pencil or lab-based tests of executive function. theories of the executive system were largely driven by observations of patients with frontal lobe damage. they exhibited disorganized actions and strategies for everyday tasks (a group of behaviors now known as dysexecutive syndrome) although they seemed to perform normally when clinical or lab-based tests were used to assess more fundamental cognitive functions such as memory, learning, language, and reasoning. it was hypothesized that, to explain this unusual behaviour, there must be an overarching system that co-ordinates other cognitive resources. much of the experimental evidence for the neural structures involved in executive functions comes from laboratory tasks such as the stroop task or the wisconsin card sorting task (wcst). in the stroop task, for example, human subjects are asked to name the color that color words are printed in when the ink color and word meaning often conflict (for example, the word "red" in green ink). executive functions are needed to perform this task, as the relatively overlearned and automatic behaviour (word reading) has to be inhibited in favour of a less practiced task – naming the ink color. recent functional neuroimaging studies have shown that two parts of the pfc, the anterior cingulate cortex (acc) and the dorsolateral prefrontal cortex (dlpfc), are thought to be particularly important for performing this task. context-sensitivity of pfc neurons other evidence for the involvement of the pfc in executive functions comes from single-cell electrophysiology studies in non-human primates, such as the macaque monkey, which have shown that (in contrast to cells in the posterior brain) many pfc neurons are sensitive to a conjunction of a stimulus and a context. for example, pfc cells might respond to a green cue in a condition where that cue signals that a leftwards fast movement of the eyes and the head should be made, but not to a green cue in another experimental context. this is important, because the optimal deployment of executive functions is invariably context-dependent. one example from miller & cohen involves a pedestrian crossing the street. in the united states, where cars drive on the right side of the road, an american learns to look left when crossing the street. however, if that american visits a country where cars drive on the left, such as the united kingdom, then the opposite behavior would be required (looking to the right). in this case, the automatic response needs to be suppressed (or augmented) and executive functions must make the american look to the right while in the uk. neurologically, this behavioural repertoire clearly requires a neural system that is able to integrate the stimulus (the road) with a context (us or uk) to cue a behaviour (look left or look right). current evidence suggests that neurons in the pfc appear to represent precisely this sort of information. other evidence from single-cell electrophysiology in monkeys implicates ventrolateral pfc (inferior prefrontal convexity) in the control of motor responses. for example, cells that increase their firing rate to nogo signals as well as a signal that says "don't look there!" have been identified. attentional biasing in sensory regions electrophysiology and functional neuroimaging studies involving human subjects have been used to describe the neural mechanisms underlying attentional biasing. most studies have looked for activation at the 'sites' of biasing, such as in the visual or auditory cortices. early studies employed event-related potentials to reveal that electrical brain responses recorded over left and right visual cortex are enhanced when the subject is instructed to attend to the appropriate (contralateral) side of space. the advent of bloodflow-based neuroimaging techniques such as functional magnetic resonance imaging (fmri) and positron emission tomography (pet) has more recently permitted the demonstration that neural activity in a number of sensory regions, including color-, motion-, and face-responsive regions of visual cortex, is enhanced when subjects are directed to attend to that dimension of a stimulus, suggestive of gain control in sensory neocortex. for example, in a typical study, liu and coworkers presented subjects with arrays of dots moving to the left or right, presented in either red or green. preceding each stimulus, an instruction cue indicated whether subjects should respond on the basis of the colour or the direction of the dots. even though colour and motion were present in all stimulus arrays, fmri activity in colour-sensitive regions (v4) was enhanced when subjects were instructed to attend to the colour, and activity in motion-sensitive regions was increased when subjects were cued to attend to the direction of motion. several studies have also reported evidence for the biasing signal prior to stimulus onset, with the observation that regions of the frontal cortex tend to come active prior to the onset of an expected stimulus. connectivity between the pfc and sensory regions despite the growing currency of the 'biasing' model of executive functions, direct evidence for functional connectivity between the pfc and sensory regions when executive functions are used, is to date rather sparse. indeed, the only direct evidence comes from studies in which a portion of frontal cortex is damaged, and a corresponding effect is observed far from the lesion site, in the responses of sensory neurons. however, few studies have explored whether this effect is specific to situations where executive functions are required. other methods for measuring connectivity between distant brain regions, such as correlation in the fmri response, have yielded indirect evidence that the frontal cortex and sensory regions communicate during a variety of processes thought to engage executive functions, such as working memory, but more research is required to establish how information flows between the pfc and the rest of the brain when executive functions are used. as an early step in this direction, an fmri study on the flow of information processing during visuospatial reasoning has provided evidence for causal associations (inferred from the temporal order of activity) between sensory-related activity in occipital and parietal cortices and activity in posterior and anterior pfc. such approaches can further elucidate the distribution of processing between executive functions in pfc and the rest of the brain. bilingualism and executive functions a growing body of research demonstrates that bilinguals might show advantages in executive functions, specifically inhibitory control and task switching. a possible explanation for this is that speaking two languages requires controlling one's attention and choosing the correct language to speak. across development, bilingual infants, children, and elderly show a bilingual advantage when it comes to executive functioning. the advantage does not seem to manifest in younger adults. bimodal bilinguals, or people who speak one oral language and one sign language, do not demonstrate this bilingual advantage in executive functioning tasks. this may be because one is not required to actively inhibit one language in order to speak the other. bilingual individuals also seem to have an advantage in an area known as conflict processing, which occurs when there are multiple representations of one particular response (for example, a word in one language and its translation in the individual's other language). specifically, the lateral prefrontal cortex has been shown to be involved with conflict processing. however, there are still some doubts. in a meta-analytic review, researchers concluded that bilingualism did not enhance executive functioning in adults. in disease or disorder the study of executive function in parkinson's disease suggests subcortical areas such as the amygdala, hippocampus and basal ganglia are important in these processes. dopamine modulation of the prefrontal cortex is responsible for the efficacy of dopaminergic drugs on executive function, and gives rise to the yerkes–dodson curve. the inverted u represents decreased executive functioning with excessive arousal (or increased catecholamine release during stress), and decreased executive functioning with insufficient arousal. the low activity polymorphism of catechol-o-methyltransferase is associated with slight increase in performance on executive function tasks in healthy persons. executive functions are impaired in multiple disorders including anxiety disorder, major depressive disorder, bipolar disorder, attention deficit hyperactivity disorder, schizophrenia and autism. lesions to the prefrontal cortex, such as in the case of phineas gage, may also result in deficits of executive function. damage to these areas may also manifest in deficits of other areas of function, such as motivation, and social functioning. future directions other important evidence for executive functions processes in the prefrontal cortex have been described. one widely cited review article emphasizes the role of the medial part of the pfc in situations where executive functions are likely to be engaged – for example, where it is important to detect errors, identify situations where stimulus conflict may arise, make decisions under uncertainty, or when a reduced probability of obtaining favourable performance outcomes is detected. this review, like many others, highlights interactions between medial and lateral pfc, whereby posterior medial frontal cortex signals the need for increased executive functions and sends this signal on to areas in dorsolateral prefrontal cortex that actually implement control. yet there has been no compelling evidence at all that this view is correct, and, indeed, one article showed that patients with lateral pfc damage had reduced erns (a putative sign of dorsomedial monitoring/error-feedback) – suggesting, if anything, that the direction of flow of the control could be in the reverse direction. another prominent theory emphasises that interactions along the perpendicular axis of the frontal cortex, arguing that a 'cascade' of interactions between anterior pfc, dorsolateral pfc, and premotor cortex guides behaviour in accordance with past context, present context, and current sensorimotor associations, respectively. advances in neuroimaging techniques have allowed studies of genetic links to executive functions, with the goal of using the imaging techniques as potential endophenotypes for discovering the genetic causes of executive function. more research is required to develop interventions that can improve executive functions and help people generalize those skills to daily activities and settings